effect of changed thrust line.

[[email protected]]

How does a person determine what the proper height of an engine should
be when building an airplane? If a particular engine design mandates
the prop is 4 inches, say, lower than where it would be with the
engine originally installed, what effect will it have on handling, and
what changes in downthrust might be advised?

We are building a Pegazair, and my Corvair engine would need to have
the cowl higher than ideal to keep the crank centerline at the same
hight as say, an O200. Weight wize, the engines are just about
identical as equipped Have not determined the center of gravity of the
engine yet, to determine the overall length of the mount.

For those unfamiliar with the plane it is a highwing STOL 2 placer
roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan,
)

[Morgans[_2_]]

> wrote

> How does a person determine what the proper height of an engine should
> be when building an airplane? If a particular engine design mandates
> the prop is 4 inches, say, lower than where it would be with the
> engine originally installed, what effect will it have on handling, and
> what changes in downthrust might be advised?
>
> We are building a Pegazair, and my Corvair engine would need to have
> the cowl higher than ideal to keep the crank centerline at the same
> hight as say, an O200. Weight wize, the engines are just about
> identical as equipped Have not determined the center of gravity of the
> engine yet, to determine the overall length of the mount.
>
> For those unfamiliar with the plane it is a highwing STOL 2 placer
> roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan,

Doubtful that you would ever notice a difference in a 4" change, but if it
originally has downthrust, and you move the prop lower, you would take
downthrust out, but not much. 1/2 degree, perhaps? Hardly enough to be
able to change, I would think.

Also, don't forget the use of washers. (shims) With it being experimental,
that is one of the fun things to "have to" figure out. Dang, hours flying
around, doing tests! What a way to spend your time, right? <g>

Tough life, but someone's got to do it! <g>
--
Jim in NC

[Morgans[_2_]]

"Morgans" > wrote

> Doubtful that you would ever notice a difference in a 4" change, but if it
> originally has downthrust, and you move the prop lower, you would take
> downthrust out, but not much. 1/2 degree, perhaps? Hardly enough to be
> able to change, I would think.

Dang, it, I got it backwards. It would theoretically need downthrust
_added_.

Think about it. A seaplane with an engine up on a pylon has up thrust, so
when you add power, that the nose does not go down as much.

Wit the center of drag higher than the engine, you have down thrust so added
power does not lift the nose as much, so going higher, take down out; lower,
add down.
--
Jim in NC

[Morgans[_2_]]

> wrote

> For those unfamiliar with the plane it is a highwing STOL 2 placer
> roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan,

Good to see you around, again. :-) Howz the project going?

Did you have a web page or blog with your project on it? I lost everything
with a total computer melt-down a while back.
--
Jim in NC

[Alan Baker]

In article >,
"Morgans" > wrote:

> "Morgans" > wrote
>
> > Doubtful that you would ever notice a difference in a 4" change, but if it
> > originally has downthrust, and you move the prop lower, you would take
> > downthrust out, but not much. 1/2 degree, perhaps? Hardly enough to be
> > able to change, I would think.
>
> Dang, it, I got it backwards. It would theoretically need downthrust
> _added_.
>
> Think about it. A seaplane with an engine up on a pylon has up thrust, so
> when you add power, that the nose does not go down as much.
>
> Wit the center of drag higher than the engine, you have down thrust so added
> power does not lift the nose as much, so going higher, take down out; lower,
> add down.

Read this. Read it again and again until you get it:

Don't take moments about anything other than the centre of mass.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[cavelamb himself[_4_]]

Alan Baker wrote:
> In article >,
> "Morgans" > wrote:
>
>
>>"Morgans" > wrote
>>
>>
>>>Doubtful that you would ever notice a difference in a 4" change, but if it
>>>originally has downthrust, and you move the prop lower, you would take
>>>downthrust out, but not much. 1/2 degree, perhaps? Hardly enough to be
>>>able to change, I would think.
>>
>>Dang, it, I got it backwards. It would theoretically need downthrust
>>_added_.
>>
>>Think about it. A seaplane with an engine up on a pylon has up thrust, so
>>when you add power, that the nose does not go down as much.
>>
>>Wit the center of drag higher than the engine, you have down thrust so added
>>power does not lift the nose as much, so going higher, take down out; lower,
>>add down.
>
>
> Read this. Read it again and again until you get it:
>
> Don't take moments about anything other than the centre of mass.
>


Why?

I mean specifically, how else would you even FIND the center of mass?

What difference does it make whether it's center of mass or center of
the axles?

Or leading edge of the wing?

Or a point 1" in front of the tip of the spinner???

The numbers for the moments will certaintly look different, but they
all relate to the same spot on the aircraft...




--

Richard

(remove the X to email)

[Alan Baker]

In article >,
cavelamb himself > wrote:

> Alan Baker wrote:
> > In article >,
> > "Morgans" > wrote:
> >
> >
> >>"Morgans" > wrote
> >>
> >>
> >>>Doubtful that you would ever notice a difference in a 4" change, but if it
> >>>originally has downthrust, and you move the prop lower, you would take
> >>>downthrust out, but not much. 1/2 degree, perhaps? Hardly enough to be
> >>>able to change, I would think.
> >>
> >>Dang, it, I got it backwards. It would theoretically need downthrust
> >>_added_.
> >>
> >>Think about it. A seaplane with an engine up on a pylon has up thrust, so
> >>when you add power, that the nose does not go down as much.
> >>
> >>Wit the center of drag higher than the engine, you have down thrust so
> >>added
> >>power does not lift the nose as much, so going higher, take down out;
> >>lower,
> >>add down.
> >
> >
> > Read this. Read it again and again until you get it:
> >
> > Don't take moments about anything other than the centre of mass.
> >
>
>
> Why?
>
> I mean specifically, how else would you even FIND the center of mass?
>
> What difference does it make whether it's center of mass or center of
> the axles?
>
> Or leading edge of the wing?
>
> Or a point 1" in front of the tip of the spinner???
>
> The numbers for the moments will certaintly look different, but they
> all relate to the same spot on the aircraft...

First: yes, any *fixed* point will do, which the centre of drag is not.

Second, the math is easiest when you pick the point that is actually the
one about which the body will rotate.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[cavelamb himself[_4_]]

Alan Baker wrote:
> In article >,
> cavelamb himself > wrote:
>
>
>>Alan Baker wrote:
>>
>>>In article >,
>>> "Morgans" > wrote:
>>>
>>>
>>>
>>>>"Morgans" > wrote
>>>>
>>>>
>>>>
>>>>>Doubtful that you would ever notice a difference in a 4" change, but if it
>>>>>originally has downthrust, and you move the prop lower, you would take
>>>>>downthrust out, but not much. 1/2 degree, perhaps? Hardly enough to be
>>>>>able to change, I would think.
>>>>
>>>>Dang, it, I got it backwards. It would theoretically need downthrust
>>>>_added_.
>>>>
>>>>Think about it. A seaplane with an engine up on a pylon has up thrust, so
>>>>when you add power, that the nose does not go down as much.
>>>>
>>>>Wit the center of drag higher than the engine, you have down thrust so
>>>>added
>>>>power does not lift the nose as much, so going higher, take down out;
>>>>lower,
>>>>add down.
>>>
>>>
>>>Read this. Read it again and again until you get it:
>>>
>>>Don't take moments about anything other than the centre of mass.
>>>
>>
>>
>>Why?
>>
>>I mean specifically, how else would you even FIND the center of mass?
>>
>>What difference does it make whether it's center of mass or center of
>>the axles?
>>
>>Or leading edge of the wing?
>>
>>Or a point 1" in front of the tip of the spinner???
>>
>>The numbers for the moments will certaintly look different, but they
>>all relate to the same spot on the aircraft...
>
>
> First: yes, any *fixed* point will do, which the centre of drag is not.
>
> Second, the math is easiest when you pick the point that is actually the
> one about which the body will rotate.
>


Some people find it "easiest" to avoid the negative numners and use the tip
of the spinner :)

--

Richard

(remove the X to email)

[Stealth Pilot[_2_]]

On Thu, 13 Nov 2008 22:20:25 -0500, wrote:

>
>
>How does a person determine what the proper height of an engine should
>be when building an airplane? If a particular engine design mandates
>the prop is 4 inches, say, lower than where it would be with the
>engine originally installed, what effect will it have on handling, and
>what changes in downthrust might be advised?
>
>We are building a Pegazair, and my Corvair engine would need to have
>the cowl higher than ideal to keep the crank centerline at the same
>hight as say, an O200. Weight wize, the engines are just about
>identical as equipped Have not determined the center of gravity of the
>engine yet, to determine the overall length of the mount.
>
>For those unfamiliar with the plane it is a highwing STOL 2 placer
>roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan,
>)

suck it and see.
your elevator should have enough authority to control the resulting
couple.
you should experience some upthrust but tweaking back the throttle
should control it.

your question in the first line....
take the centre of mass as the pivot point.
you have 4 force couples.

lift vs moment arm.
elevator down thrust vs moment arm
wing drag vs moment arm
thrust vs moment arm.

they will summate to zero in equilibrium but
you probably wont know any of the values o those forces.
alan baker will probably give a guestimate.
suck it and see.

I think you'll need the same side thrust but a little more down thrust
as the cessna 150..

Stealth Pilot

[Alan Baker]

In article >,
Stealth Pilot > wrote:

> On Thu, 13 Nov 2008 22:20:25 -0500, wrote:
>
> >
> >
> >How does a person determine what the proper height of an engine should
> >be when building an airplane? If a particular engine design mandates
> >the prop is 4 inches, say, lower than where it would be with the
> >engine originally installed, what effect will it have on handling, and
> >what changes in downthrust might be advised?
> >
> >We are building a Pegazair, and my Corvair engine would need to have
> >the cowl higher than ideal to keep the crank centerline at the same
> >hight as say, an O200. Weight wize, the engines are just about
> >identical as equipped Have not determined the center of gravity of the
> >engine yet, to determine the overall length of the mount.
> >
> >For those unfamiliar with the plane it is a highwing STOL 2 placer
> >roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan,
> >)
>
> suck it and see.
> your elevator should have enough authority to control the resulting
> couple.
> you should experience some upthrust but tweaking back the throttle
> should control it.
>
> your question in the first line....
> take the centre of mass as the pivot point.
> you have 4 force couples.
>
> lift vs moment arm.
> elevator down thrust vs moment arm
> wing drag vs moment arm
> thrust vs moment arm.
>
> they will summate to zero in equilibrium but
> you probably wont know any of the values o those forces.
> alan baker will probably give a guestimate.
> suck it and see.
>
> I think you'll need the same side thrust but a little more down thrust
> as the cessna 150..
>
> Stealth Pilot

The math isn't all that hard.

Assuming the designed thrust line goes relatively close to the centre of
mass, then if you want to keep the torque created by thrust close to the
same, you need to change the angle of the engine by arctan(h/l), where h
is the amount you're moving the engine up or down and l is the distance
between the propellor and the centre of mass.

I suck at ASCII art, but:


<-----T' (new thrust line)
^
|
h |
|
<-----T--(old thrust line)--------------------------------C
l (CoM)

h/l is equal to the tangent of the angle TCT'

If the thrust line is not aligned with the CoM to begin with, then
situation isn't quite as simple, but for small misalignments the effect
is small and for larger misalignments the overall change in torques is
smaller in comparison.

Draw few diagrams of the situation and you'll see what I mean. I'm not
even going to try to draw that situation here. Basically, if the thrust
line was already above the CoM and you move it up, then the change is
smaller than the arctangent of h/l and if it was below the CoM the
change is a little greater than the arctangent.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article
>,
Alan Baker > wrote:

> In article >,
> Stealth Pilot > wrote:
>
> > On Thu, 13 Nov 2008 22:20:25 -0500, wrote:
> >
> > >
> > >
> > >How does a person determine what the proper height of an engine should
> > >be when building an airplane? If a particular engine design mandates
> > >the prop is 4 inches, say, lower than where it would be with the
> > >engine originally installed, what effect will it have on handling, and
> > >what changes in downthrust might be advised?
> > >
> > >We are building a Pegazair, and my Corvair engine would need to have
> > >the cowl higher than ideal to keep the crank centerline at the same
> > >hight as say, an O200. Weight wize, the engines are just about
> > >identical as equipped Have not determined the center of gravity of the
> > >engine yet, to determine the overall length of the mount.
> > >
> > >For those unfamiliar with the plane it is a highwing STOL 2 placer
> > >roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan,
> > >)
> >
> > suck it and see.
> > your elevator should have enough authority to control the resulting
> > couple.
> > you should experience some upthrust but tweaking back the throttle
> > should control it.
> >
> > your question in the first line....
> > take the centre of mass as the pivot point.
> > you have 4 force couples.
> >
> > lift vs moment arm.
> > elevator down thrust vs moment arm
> > wing drag vs moment arm
> > thrust vs moment arm.
> >
> > they will summate to zero in equilibrium but
> > you probably wont know any of the values o those forces.
> > alan baker will probably give a guestimate.
> > suck it and see.
> >
> > I think you'll need the same side thrust but a little more down thrust
> > as the cessna 150..
> >
> > Stealth Pilot
>
> The math isn't all that hard.
>
> Assuming the designed thrust line goes relatively close to the centre of
> mass, then if you want to keep the torque created by thrust close to the
> same, you need to change the angle of the engine by arctan(h/l), where h
> is the amount you're moving the engine up or down and l is the distance
> between the propellor and the centre of mass.
>
> I suck at ASCII art, but:
>
>
> <-----T' (new thrust line)
> ^
> |
> h |
> |
> <-----T--(old thrust line)--------------------------------C
> l (CoM)
>
> h/l is equal to the tangent of the angle TCT'
>
> If the thrust line is not aligned with the CoM to begin with, then
> situation isn't quite as simple, but for small misalignments the effect
> is small and for larger misalignments the overall change in torques is
> smaller in comparison.
>
> Draw few diagrams of the situation and you'll see what I mean. I'm not
> even going to try to draw that situation here. Basically, if the thrust
> line was already above the CoM and you move it up, then the change is
> smaller than the arctangent of h/l and if it was below the CoM the
> change is a little greater than the arctangent.

Did a quick little check:

As an example, a Cessna 150 is about 25 feet long and from looking at
wikipedia's little jpeg, the centre of mass should be about 5 feet
behind the propellor disc.

So if you raise the thrust line 4 inches, you need to angle the engine
up an additional 3.8 degrees; arctan(4/60).

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[[email protected]]

On Nov 14, 8:25 am, Alan Baker > wrote:

> Did a quick little check:
>
> As an example, a Cessna 150 is about 25 feet long and from looking at
> wikipedia's little jpeg, the centre of mass should be about 5 feet
> behind the propellor disc.
>
> So if you raise the thrust line 4 inches, you need to angle the engine
> up an additional 3.8 degrees; arctan(4/60).
>

Don't bother with center of mass. It's not really relevant.
Angling the engine up 3.8 degrees would lead to trouble. That's a lot
of angle. Most engines are aligned with the longitudinal axis or
parallel to it (the waterline) or angled *down* a bit (Ercoupe has
lots; Cherokee and its brethren have some, 172 has none at all) and
some are angled to the side a bit as well to control P-factor.

Thrust works against the center of DRAG, which is much harder
to locate than CG. Lowering the thrust line would tend to raise the
nose more on powering up, which would require more nose-down trim to
control, which would lead to a bigger drop in attitude when the power
is removed.
But I don't think four inches lower is going to be a big
deal. The loss of ground clearance, OTOH, is significant for a STOL
airplane.

Dan

[cavelamb himself[_4_]]

wrote:
> On Nov 14, 8:25 am, Alan Baker > wrote:
>
>
>>Did a quick little check:
>>
>>As an example, a Cessna 150 is about 25 feet long and from looking at
>>wikipedia's little jpeg, the centre of mass should be about 5 feet
>>behind the propellor disc.
>>
>>So if you raise the thrust line 4 inches, you need to angle the engine
>>up an additional 3.8 degrees; arctan(4/60).
>>
>
>
> Don't bother with center of mass. It's not really relevant.
> Angling the engine up 3.8 degrees would lead to trouble. That's a lot
> of angle. Most engines are aligned with the longitudinal axis or
> parallel to it (the waterline) or angled *down* a bit (Ercoupe has
> lots; Cherokee and its brethren have some, 172 has none at all) and
> some are angled to the side a bit as well to control P-factor.
>
> Thrust works against the center of DRAG, which is much harder
> to locate than CG. Lowering the thrust line would tend to raise the
> nose more on powering up, which would require more nose-down trim to
> control, which would lead to a bigger drop in attitude when the power
> is removed.
> But I don't think four inches lower is going to be a big
> deal. The loss of ground clearance, OTOH, is significant for a STOL
> airplane.
>
> Dan


And angling an engine UP is a real BAD (tm) idea.


--

Richard

(remove the X to email)

[Morgans[_2_]]

"Alan Baker" > wrote

> Read this. Read it again and again until you get it:
>
> Don't take moments about anything other than the centre of mass.

Sorry if I don't take your word for it, so if you have some online sources,
lay 'em on me.
--
Jim in NC

[[email protected]]

On Thu, 13 Nov 2008 23:28:09 -0500, "Morgans"
> wrote:

>
> wrote
>
>> For those unfamiliar with the plane it is a highwing STOL 2 placer
>> roughly the same size as a Cessna 150 (150 sq ft wing,33 ft wingspan,
>
> Good to see you around, again. :-) Howz the project going?
>
>Did you have a web page or blog with your project on it? I lost everything
>with a total computer melt-down a while back.

Project is at www.pegazair.on-the-net.ca/ClareSquared

[[email protected]]

On Fri, 14 Nov 2008 11:08:57 -0800 (PST),
wrote:

>On Nov 14, 8:25 am, Alan Baker > wrote:
>
>> Did a quick little check:
>>
>> As an example, a Cessna 150 is about 25 feet long and from looking at
>> wikipedia's little jpeg, the centre of mass should be about 5 feet
>> behind the propellor disc.
>>
>> So if you raise the thrust line 4 inches, you need to angle the engine
>> up an additional 3.8 degrees; arctan(4/60).
>>
>
> Don't bother with center of mass. It's not really relevant.
>Angling the engine up 3.8 degrees would lead to trouble. That's a lot
>of angle. Most engines are aligned with the longitudinal axis or
>parallel to it (the waterline) or angled *down* a bit (Ercoupe has
>lots; Cherokee and its brethren have some, 172 has none at all) and
>some are angled to the side a bit as well to control P-factor.
>
> Thrust works against the center of DRAG, which is much harder
>to locate than CG. Lowering the thrust line would tend to raise the
>nose more on powering up, which would require more nose-down trim to
>control, which would lead to a bigger drop in attitude when the power
>is removed.
> But I don't think four inches lower is going to be a big
>deal. The loss of ground clearance, OTOH, is significant for a STOL
>airplane.
>
> Dan

Plane is designed for 72 inch prop. I will be running a 68" prop, so I
have 2 inches more to play with. Also running bigger wheels and tires,
which gives me another 1/2 inch at worst case,perhaps 1 1/2 with a
full load of air. Not sure if I will need to go down 4 inches - 2 will
likely do it but I needed a number to ask the question.

[[email protected]]

On Fri, 14 Nov 2008 13:24:47 -0600, cavelamb himself
> wrote:

wrote:
>> On Nov 14, 8:25 am, Alan Baker > wrote:
>>
>>
>>>Did a quick little check:
>>>
>>>As an example, a Cessna 150 is about 25 feet long and from looking at
>>>wikipedia's little jpeg, the centre of mass should be about 5 feet
>>>behind the propellor disc.
>>>
>>>So if you raise the thrust line 4 inches, you need to angle the engine
>>>up an additional 3.8 degrees; arctan(4/60).
>>>
>>
>>
>> Don't bother with center of mass. It's not really relevant.
>> Angling the engine up 3.8 degrees would lead to trouble. That's a lot
>> of angle. Most engines are aligned with the longitudinal axis or
>> parallel to it (the waterline) or angled *down* a bit (Ercoupe has
>> lots; Cherokee and its brethren have some, 172 has none at all) and
>> some are angled to the side a bit as well to control P-factor.
>>
>> Thrust works against the center of DRAG, which is much harder
>> to locate than CG. Lowering the thrust line would tend to raise the
>> nose more on powering up, which would require more nose-down trim to
>> control, which would lead to a bigger drop in attitude when the power
>> is removed.
>> But I don't think four inches lower is going to be a big
>> deal. The loss of ground clearance, OTOH, is significant for a STOL
>> airplane.
>>
>> Dan
>
>
>And angling an engine UP is a real BAD (tm) idea.


Lowering the thrust line to below the center of aerodynamic drag would
cause nose up - OK I get that. Now where is the center of drag on a
peg? and it will DEFINETLY change with flying attitude - ie with the
flaps on, or the slats extended.

I guess what it boils down to is it will not be a HUGE effect.
On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
roughly 1/2 degree. One 1/8" washer at the firewall and one at the
engine rubber on both sides will make 1/2 degree change if I need to
do a bit od "fine" tuning.

Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
offset - guess I'll put in about .875 and see what happens

[cavelamb himself[_4_]]

wrote:
>
>
> Lowering the thrust line to below the center of aerodynamic drag would
> cause nose up - OK I get that. Now where is the center of drag on a
> peg? and it will DEFINETLY change with flying attitude - ie with the
> flaps on, or the slats extended.
>
> I guess what it boils down to is it will not be a HUGE effect.
> On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
> roughly 1/2 degree. One 1/8" washer at the firewall and one at the
> engine rubber on both sides will make 1/2 degree change if I need to
> do a bit od "fine" tuning.
>
> Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
> offset - guess I'll put in about .875 and see what happens

I thought about this a bit last night.
And a couple of thoughts seemed worth relaying.

First, (and most obviously) a new mount will be needed.
So build it as close as you can guess to what you'll need.

Adjusting the mount at the firewall end strikes me as a bit "iffy".

More that a washer or two makes for a noticeable misalignment between top
and bottom bolts. When torqued down, something it GOING to give.

Either the mount gets twisted or the firewall support structure does.
Or both?

The engine end, if rubber cushioned would be a lot more compliant.
Might consider all that when designing the new mount.

The Corvair would use a bearer style mount, wouldn't it?

Rubber pads front and rear would give quite a bit of adjustment room.

I think Stealth got it right.

Same side alignment and a touch more down.

[Morgans[_2_]]

"Alan Baker" > wrote
>
> First: yes, any *fixed* point will do, which the centre of drag is not.
>
> Second, the math is easiest when you pick the point that is actually the
> one about which the body will rotate.

I think I get your point, about the plane rotating about the center of
mass, but I do not believe that a change in the amount of thrust and its
location in relation to the center of mass is what is relevant, in this
issue.

For a plane to not require a change in trim with a change in power, and not
to rotate, the thrust line must be in line with the center of drag in level
flight, no?

The downthrust or upthrust is added to partially negate the rotation caused
due to the distance the thrust is away from the center of drag.
--
Jim in NC

[Morgans[_2_]]

> wrote
>
> Project is at www.pegazair.on-the-net.ca/ClareSquared

Tanks! I'll go have a gander! Or a goose! <g>
--
Jim in NC

[Morgans[_2_]]

> wrote

> Don't bother with center of mass. It's not really relevant.

Indeed

> Angling the engine up 3.8 degrees would lead to trouble. >That's a lot
> of angle.

Absolutely

> Thrust works against the center of DRAG, which is much harder
> to locate than CG.

Thanks for a verification of my thinking.
--
Jim in NC

[Alan Baker]

In article >,
cavelamb himself > wrote:

> wrote:
> > On Nov 14, 8:25 am, Alan Baker > wrote:
> >
> >
> >>Did a quick little check:
> >>
> >>As an example, a Cessna 150 is about 25 feet long and from looking at
> >>wikipedia's little jpeg, the centre of mass should be about 5 feet
> >>behind the propellor disc.
> >>
> >>So if you raise the thrust line 4 inches, you need to angle the engine
> >>up an additional 3.8 degrees; arctan(4/60).
> >>
> >
> >
> > Don't bother with center of mass. It's not really relevant.
> > Angling the engine up 3.8 degrees would lead to trouble. That's a lot
> > of angle. Most engines are aligned with the longitudinal axis or
> > parallel to it (the waterline) or angled *down* a bit (Ercoupe has
> > lots; Cherokee and its brethren have some, 172 has none at all) and
> > some are angled to the side a bit as well to control P-factor.
> >
> > Thrust works against the center of DRAG, which is much harder
> > to locate than CG. Lowering the thrust line would tend to raise the
> > nose more on powering up, which would require more nose-down trim to
> > control, which would lead to a bigger drop in attitude when the power
> > is removed.
> > But I don't think four inches lower is going to be a big
> > deal. The loss of ground clearance, OTOH, is significant for a STOL
> > airplane.
> >
> > Dan
>
>
> And angling an engine UP is a real BAD (tm) idea.

Really?

So when you're flying with a large AOA, it's BAD(tm)?

Please.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
"Morgans" > wrote:

> > wrote
>
> > Don't bother with center of mass. It's not really relevant.
>
> Indeed
>
> > Angling the engine up 3.8 degrees would lead to trouble. >That's a lot
> > of angle.
>
> Absolutely
>
> > Thrust works against the center of DRAG, which is much harder
> > to locate than CG.
>
> Thanks for a verification of my thinking.

Sorry.

But all forces on a body act around it's centre of *mass* when
considering its rotation. You can take the moments about anywhere fixed,
but the CoM is what actually matters.

As for angling the engine 3.8 degrees up being a bad idea, how can that
possibly be? When you pull up to a high AOA, does the engine suddenly
explode? No.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >, "Morgans" >
wrote:

> "Alan Baker" > wrote
>
> > Read this. Read it again and again until you get it:
> >
> > Don't take moments about anything other than the centre of mass.
>
> Sorry if I don't take your word for it, so if you have some online sources,
> lay 'em on me.

Sorry, man, but it's just basic physics.

You can take them about any fixed point, but the essence of it is that a
rigid body rotates about its center of mass.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article
>,
wrote:

> On Nov 14, 8:25 am, Alan Baker > wrote:
>
> > Did a quick little check:
> >
> > As an example, a Cessna 150 is about 25 feet long and from looking at
> > wikipedia's little jpeg, the centre of mass should be about 5 feet
> > behind the propellor disc.
> >
> > So if you raise the thrust line 4 inches, you need to angle the engine
> > up an additional 3.8 degrees; arctan(4/60).
> >
>
> Don't bother with center of mass. It's not really relevant.
> Angling the engine up 3.8 degrees would lead to trouble. That's a lot
> of angle. Most engines are aligned with the longitudinal axis or
> parallel to it (the waterline) or angled *down* a bit (Ercoupe has
> lots; Cherokee and its brethren have some, 172 has none at all) and
> some are angled to the side a bit as well to control P-factor.

The longitudinal axis is purely notional. What is it defined by?

Go ahead: write a definition in words that will let anyone deduce what
any arbitrary aircraft's "longitudinal axis" is...

>
> Thrust works against the center of DRAG, which is much harder
> to locate than CG. Lowering the thrust line would tend to raise the
> nose more on powering up, which would require more nose-down trim to
> control, which would lead to a bigger drop in attitude when the power
> is removed.

Which is why you change the angle...

If the thrust line is changed to keep the torques the same, then no need
for increased trim, right?

> But I don't think four inches lower is going to be a big
> deal. The loss of ground clearance, OTOH, is significant for a STOL
> airplane.
>
> Dan

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
wrote:

> On Fri, 14 Nov 2008 13:24:47 -0600, cavelamb himself
> > wrote:
>
> wrote:
> >> On Nov 14, 8:25 am, Alan Baker > wrote:
> >>
> >>
> >>>Did a quick little check:
> >>>
> >>>As an example, a Cessna 150 is about 25 feet long and from looking at
> >>>wikipedia's little jpeg, the centre of mass should be about 5 feet
> >>>behind the propellor disc.
> >>>
> >>>So if you raise the thrust line 4 inches, you need to angle the engine
> >>>up an additional 3.8 degrees; arctan(4/60).
> >>>
> >>
> >>
> >> Don't bother with center of mass. It's not really relevant.
> >> Angling the engine up 3.8 degrees would lead to trouble. That's a lot
> >> of angle. Most engines are aligned with the longitudinal axis or
> >> parallel to it (the waterline) or angled *down* a bit (Ercoupe has
> >> lots; Cherokee and its brethren have some, 172 has none at all) and
> >> some are angled to the side a bit as well to control P-factor.
> >>
> >> Thrust works against the center of DRAG, which is much harder
> >> to locate than CG. Lowering the thrust line would tend to raise the
> >> nose more on powering up, which would require more nose-down trim to
> >> control, which would lead to a bigger drop in attitude when the power
> >> is removed.
> >> But I don't think four inches lower is going to be a big
> >> deal. The loss of ground clearance, OTOH, is significant for a STOL
> >> airplane.
> >>
> >> Dan
> >
> >
> >And angling an engine UP is a real BAD (tm) idea.
>
>
> Lowering the thrust line to below the center of aerodynamic drag would
> cause nose up - OK I get that. Now where is the center of drag on a
> peg? and it will DEFINETLY change with flying attitude - ie with the
> flaps on, or the slats extended.

Which is why one should take moments about a fixed point...

....say the centre of mass. :-)

>
> I guess what it boils down to is it will not be a HUGE effect.
> On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
> roughly 1/2 degree. One 1/8" washer at the firewall and one at the
> engine rubber on both sides will make 1/2 degree change if I need to
> do a bit od "fine" tuning.
>
> Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
> offset - guess I'll put in about .875 and see what happens

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
cavelamb himself > wrote:

> wrote:
> >
> >
> > Lowering the thrust line to below the center of aerodynamic drag would
> > cause nose up - OK I get that. Now where is the center of drag on a
> > peg? and it will DEFINETLY change with flying attitude - ie with the
> > flaps on, or the slats extended.
> >
> > I guess what it boils down to is it will not be a HUGE effect.
> > On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
> > roughly 1/2 degree. One 1/8" washer at the firewall and one at the
> > engine rubber on both sides will make 1/2 degree change if I need to
> > do a bit od "fine" tuning.
> >
> > Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
> > offset - guess I'll put in about .875 and see what happens
>
> I thought about this a bit last night.
> And a couple of thoughts seemed worth relaying.
>
> First, (and most obviously) a new mount will be needed.
> So build it as close as you can guess to what you'll need.
>
> Adjusting the mount at the firewall end strikes me as a bit "iffy".
>
> More that a washer or two makes for a noticeable misalignment between top
> and bottom bolts. When torqued down, something it GOING to give.
>
> Either the mount gets twisted or the firewall support structure does.
> Or both?

Not if you use wedge washers...

<http://www.instron.us/wa/acc_catalog/detail.aspx?aid=1706&ref=http://www
..google.com/search>

>
> The engine end, if rubber cushioned would be a lot more compliant.
> Might consider all that when designing the new mount.
>
> The Corvair would use a bearer style mount, wouldn't it?
>
> Rubber pads front and rear would give quite a bit of adjustment room.
>
> I think Stealth got it right.
>
> Same side alignment and a touch more down.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Jim Logajan]

Alan Baker > wrote:
> In article >,
> "Morgans" > wrote:
>
>> > wrote
>>
>> > Don't bother with center of mass. It's not really relevant.
>>
>> Indeed
>>
>> > Angling the engine up 3.8 degrees would lead to trouble. >That's a lot
>> > of angle.
>>
>> Absolutely
>>
>> > Thrust works against the center of DRAG, which is much harder
>> > to locate than CG.
>>
>> Thanks for a verification of my thinking.
>
> Sorry.
>
> But all forces on a body act around it's centre of *mass* when
> considering its rotation. You can take the moments about anywhere fixed,
> but the CoM is what actually matters.

I know you are trying to be helpful and you know that objects in free space
rotate about their center of mass, but I'm fairly confident that aspects
such as the center of pressure and affects on control surface authority
need to be taken into account when changing the thrust line.

> As for angling the engine 3.8 degrees up being a bad idea, how can that
> possibly be? When you pull up to a high AOA, does the engine suddenly
> explode? No.

A high angle of attack during cruise would presumably place the wings
closer to their stall angle. I presume that is what makes it dangerous.

[Morgans[_2_]]

"Alan Baker" > wrote

> Sorry, man, but it's just basic physics.
>
> You can take them about any fixed point, but the essence of it is that a
> rigid body rotates about its center of mass.

Yes, but I believe the problem is that you are not seeing the whole picture.

For example, what forces are involved that cause an airplane to not be
rotating around the center of mass while at cruise?

The answer is that thrust is cancel by drag. Where those two forces are
located is behind what amount of thrust differential is needed.
--
Jim in NC

[Alan Baker]

In article >,
"Morgans" > wrote:

> "Alan Baker" > wrote
> >
> > First: yes, any *fixed* point will do, which the centre of drag is not.
> >
> > Second, the math is easiest when you pick the point that is actually the
> > one about which the body will rotate.
>
> I think I get your point, about the plane rotating about the center of
> mass, but I do not believe that a change in the amount of thrust and its
> location in relation to the center of mass is what is relevant, in this
> issue.
>
> For a plane to not require a change in trim with a change in power, and not
> to rotate, the thrust line must be in line with the center of drag in level
> flight, no?

No.

Thrust, drag, lift weight are the way we normally coin the forces on an
aircraft, but the reality isn't quite that.

That formalism has both drag and thrust operating in the horizontal
axis, but we know that the engine's thrust will only actually be
horizontal at one particular angle of attack. Hence, only then will the
thrust line be aligned with the drag line.

That's why I keeps saying take all the moments about a fixed point to
really have a clear picture.

>
> The downthrust or upthrust is added to partially negate the rotation caused
> due to the distance the thrust is away from the center of drag.

Forget all about drag for a moment: you can boil down all the forces of
lift and drag -- the aerodynamic forces -- to a single force through the
aerodynamic centre of pressure. Now imagine an aircraft with the engine
off and the prop feathered and in a trimmed out glide.

Got that pictured in your mind? Good.

Now: when you start the engine and add its thrust into the equation,
what is the one direction in which you can apply that thrust and not
cause the aircraft to pitch.

Where must the axis of the thrust vector be?

Through the centre of mass. Period.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Morgans[_2_]]

"Alan Baker" > wrote

> Which is why you change the angle...
>
> If the thrust line is changed to keep the torques the same, then no need
> for increased trim, right?

Sorry Alan, but you should just take a pass on further arguing this one.

I don't know what you don't understand, but rest assured, you don't
understand it. ;-)

You are plain and simple wrong on this one. The notion that you think
changing the thrust angle of an engine by 3.8 degrees, for as small of a
change in the thrust as line 4 inches, illustrates how far off you are.

Case closed, for me.
--
Jim in NC

[Alan Baker]

In article >,
Jim Logajan > wrote:

> Alan Baker > wrote:
> > In article >,
> > "Morgans" > wrote:
> >
> >> > wrote
> >>
> >> > Don't bother with center of mass. It's not really relevant.
> >>
> >> Indeed
> >>
> >> > Angling the engine up 3.8 degrees would lead to trouble. >That's a lot
> >> > of angle.
> >>
> >> Absolutely
> >>
> >> > Thrust works against the center of DRAG, which is much harder
> >> > to locate than CG.
> >>
> >> Thanks for a verification of my thinking.
> >
> > Sorry.
> >
> > But all forces on a body act around it's centre of *mass* when
> > considering its rotation. You can take the moments about anywhere fixed,
> > but the CoM is what actually matters.
>
> I know you are trying to be helpful and you know that objects in free space
> rotate about their center of mass, but I'm fairly confident that aspects
> such as the center of pressure and affects on control surface authority
> need to be taken into account when changing the thrust line.

But by taking moments about the CoM, you can isolate the contribution of
the engine and work to keep it the same despite the change in its
location.

That's precisely why you don't do it with respect to loci that change.

Let's say the engine is mounted such that it is acting through the CoM,
OK? In that case, changes in thrust cannot *possibly* cause any net
torque, right?

OK, move the engine up or down, and if you reangle it to set the thrust
line through the CoM, then the same situation holds true.

>
> > As for angling the engine 3.8 degrees up being a bad idea, how can that
> > possibly be? When you pull up to a high AOA, does the engine suddenly
> > explode? No.
>
> A high angle of attack during cruise would presumably place the wings
> closer to their stall angle. I presume that is what makes it dangerous.

Not dangerous for the engine, though, right?

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
"Morgans" > wrote:

> "Alan Baker" > wrote
>
> > Sorry, man, but it's just basic physics.
> >
> > You can take them about any fixed point, but the essence of it is that a
> > rigid body rotates about its center of mass.
>
> Yes, but I believe the problem is that you are not seeing the whole picture.
>
> For example, what forces are involved that cause an airplane to not be
> rotating around the center of mass while at cruise?
>
> The answer is that thrust is cancel by drag. Where those two forces are
> located is behind what amount of thrust differential is needed.

In level flight, drag is horizontal. Engine thrust is not except at one
particular angle of attack (and it is technically possible that it is
never level for any angle of attack the aircraft can achieve). So drag
and thrust cannot *possibly* cancel each other except at the one angle
of attack.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
"Morgans" > wrote:

> "Alan Baker" > wrote
>
> > Which is why you change the angle...
> >
> > If the thrust line is changed to keep the torques the same, then no need
> > for increased trim, right?
>
> Sorry Alan, but you should just take a pass on further arguing this one.
>
> I don't know what you don't understand, but rest assured, you don't
> understand it. ;-)
>
> You are plain and simple wrong on this one. The notion that you think
> changing the thrust angle of an engine by 3.8 degrees, for as small of a
> change in the thrust as line 4 inches, illustrates how far off you are.
>
> Case closed, for me.

Sorry, but I can do the math in my head, and you apparently cannot.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
"Morgans" > wrote:

> "Alan Baker" > wrote
>
> > Which is why you change the angle...
> >
> > If the thrust line is changed to keep the torques the same, then no need
> > for increased trim, right?
>
> Sorry Alan, but you should just take a pass on further arguing this one.
>
> I don't know what you don't understand, but rest assured, you don't
> understand it. ;-)
>
> You are plain and simple wrong on this one. The notion that you think
> changing the thrust angle of an engine by 3.8 degrees, for as small of a
> change in the thrust as line 4 inches, illustrates how far off you are.
>
> Case closed, for me.

I don't really have time to draw up a free body diagram and show you the
math, so here:

<http://www.av8n.com/how/htm/4forces.html>

Note the diagrams correctly show that thrust and drag do NOT operate in
the same axis.

Also, a short quote:

"In reality, it isn¹t safe to assume that lift always matches weight, or
thrust exactly matches drag. Consider a bomb falling straight down
(figure 4.5) * it has no lift and no thrust; when it reaches terminal
velocity its weight is supported purely by drag. Another interesting
case is a moon lander hovering on its rocket plume (figure 4.6) ‹ it has
no lift and no drag; its weight is supported by its thrust."

Read that first sentence again:

"In reality, it isn¹t safe to assume that lift always matches weight, or
thrust exactly matches drag."

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Bob Kuykendall]

On Nov 14, 4:10*pm, Alan Baker > wrote:

> ...Where must the axis of the thrust vector be?
>
> Through the centre of mass. Period.

Ah, I see. All those self-launch gliders and amphibs with pylon-
mounted engines must be figments of imagination. Thanks for clarifying
that.

Bob K.

[Morgans[_2_]]

"Alan Baker" > wrote
>
> In level flight, drag is horizontal. Engine thrust is not except at one
> particular angle of attack (and it is technically possible that it is
> never level for any angle of attack the aircraft can achieve). So drag
> and thrust cannot *possibly* cancel each other except at the one angle
> of attack.

Now, you are starting to get close. You are splitting hairs, though.

Airplanes are said to be a loose flying formation of compromises. In the
question of thrust angle, also.

Parasite drag goes up for higher speeds, induced drag goes up with higher
loads. The angle of attack changes the center of lift, loading affects the
center of gravity of the airplane, and the tail balances it all out, with
help from the thrust angle.

Everything is designed to achieve a compromise of performance and safety,
speed and comfort, and many other factors. So goes it with questions of
thrust, trim and what goals you are trying to maximize. Same with this
whole question. A different engine will cause a different thrust line, and
changes would need to be made to keep the handling qualities approximately
the same. They can never be the exact same, but an attempt can be made to
keep it close.

A change of 1/2 degree would be close, but the best answer will be to try it
and see.

Drag still is the paramount factor in attempting to quantify the changes
that will need to be made. Once things start rotating, then they will
indeed rotate around the center of mass. If the change in the angle is made
successfully, there won't be any rotating going on. ;-)
--
Jim in NC

[Alan Baker]

In article
>,
Bob Kuykendall > wrote:

> On Nov 14, 4:10*pm, Alan Baker > wrote:
>
> > ...Where must the axis of the thrust vector be?
> >
> > Through the centre of mass. Period.
>
> Ah, I see. All those self-launch gliders and amphibs with pylon-
> mounted engines must be figments of imagination. Thanks for clarifying
> that.
>
> Bob K.

Bob,

Let's put back what you snipped:

> > Now: when you start the engine and add its thrust into the equation,
> > what is the one direction in which you can apply that thrust and not
> > cause the aircraft to pitch?

I'm not saying that you can't have an engine mounted with the thrust
line pointed off the CoM. You can, but when you change engine power, the
aircraft is going to experience a pitch change.

I'm saying if you don't want pitch changes with changing engine thrust,
you have to mount it with the thrust line through the CoM.

And I'm right.

But if you don't believe me, will a reference from Princeton suffice?

<http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>

"Pitching Moment due to Thrust
€ Thrust line above or below center of mass induces a pitching
moment"

Note: no mention of drag line...

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Morgans[_2_]]

> Let's say the engine is mounted such that it is acting through the CoM,
> OK? In that case, changes in thrust cannot *possibly* cause any net
> torque, right?

Yep. Drag is not at the center of mass

> OK, move the engine up or down, and if you reangle it to set the thrust
> line through the CoM, then the same situation holds true.

Nope. Drag is not at the CoM
--
Jim in NC

[Morgans[_2_]]

"Alan Baker" > wrote

> Sorry, but I can do the math in my head, and you apparently cannot.

The problem is that word problems have to be set up properly.

I have set it up in my head. Apparently, you can not.
--
Jim in NC

[Alan Baker]

In article >,
"Morgans" > wrote:

> "Alan Baker" > wrote
> >
> > In level flight, drag is horizontal. Engine thrust is not except at one
> > particular angle of attack (and it is technically possible that it is
> > never level for any angle of attack the aircraft can achieve). So drag
> > and thrust cannot *possibly* cancel each other except at the one angle
> > of attack.
>
> Now, you are starting to get close. You are splitting hairs, though.

No. I'm understanding the situation and so far, you've not shown that
you do.

>
> Airplanes are said to be a loose flying formation of compromises. In the
> question of thrust angle, also.
>
> Parasite drag goes up for higher speeds, induced drag goes up with higher
> loads. The angle of attack changes the center of lift, loading affects the
> center of gravity of the airplane, and the tail balances it all out, with
> help from the thrust angle.

If your claim that thrust and drag line always canceled each other out,
then that last statement would not be true, would it?

>
> Everything is designed to achieve a compromise of performance and safety,
> speed and comfort, and many other factors. So goes it with questions of
> thrust, trim and what goals you are trying to maximize. Same with this
> whole question. A different engine will cause a different thrust line, and
> changes would need to be made to keep the handling qualities approximately
> the same. They can never be the exact same, but an attempt can be made to
> keep it close.

They can be kept close by keeping the change in torque about the centre
of mass the same as they were in the original design...

>
> A change of 1/2 degree would be close, but the best answer will be to try it
> and see.

Where did you get that figure? Show your work if you're going to try and
be quantitative...

>
> Drag still is the paramount factor in attempting to quantify the changes
> that will need to be made. Once things start rotating, then they will
> indeed rotate around the center of mass. If the change in the angle is made
> successfully, there won't be any rotating going on. ;-)

Yup. And in order for that to happen, you need to keep the same
relationship with the centre of *mass*.

:-)

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
"Morgans" > wrote:

> > Let's say the engine is mounted such that it is acting through the CoM,
> > OK? In that case, changes in thrust cannot *possibly* cause any net
> > torque, right?
>
> Yep. Drag is not at the center of mass

What does that have to do with my question?

Take the air away for a moment and use a rocket. The only way to place
it such that it won't cause a pitching moment is to align its thrust
through the centre of mass, correct?

>
> > OK, move the engine up or down, and if you reangle it to set the thrust
> > line through the CoM, then the same situation holds true.
>
> Nope. Drag is not at the CoM

Show your work or a reference...

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Jim Logajan]

Alan Baker > wrote:
> Forget all about drag for a moment: you can boil down all the forces
> of lift and drag -- the aerodynamic forces -- to a single force
> through the aerodynamic centre of pressure. Now imagine an aircraft
> with the engine off and the prop feathered and in a trimmed out glide.
>
> Got that pictured in your mind? Good.

Okay, so far I have:

air flow ----> M
|
P--->

Where M = "center of mass," P = "center of pressure," and the arrow
represents the direction and magnitude of the resultant aerodynamic
force.

> Now: when you start the engine and add its thrust into the equation,
> what is the one direction in which you can apply that thrust and not
> cause the aircraft to pitch.
>
> Where must the axis of the thrust vector be?

Well the following seems to work, where T = "a point that thrust acts
through:"

air flow ----> M
|
<---TP--->

> Through the centre of mass. Period.

You mean this?:

air flow ----> <---TM
|
P--->

That's a couple whose only important property is its moment. Looks like
the aircraft will rotate. Sure, it will rotate about the center or mass,
but so what?

[Alan Baker]

In article >,
"Morgans" > wrote:

> "Alan Baker" > wrote
>
> > Sorry, but I can do the math in my head, and you apparently cannot.
>
> The problem is that word problems have to be set up properly.
>
> I have set it up in my head. Apparently, you can not.

Take it up with this guy...

<http://www.av8n.com/how/htm/4forces.html>

Drag -- by definition -- always acts through the centre of aerodynamic
pressure; which is pretty much somewhere in the main plane.

When in horizontal flight, drag is horizontal, right?

OK. A Cessna 150. That means the drag line is somewhere above the pilots
head. So unless they've moved the engine up three feet when we weren't
looking, a Cessna somehow manages to stay in the air with a thrust line
that is nowhere *near* the drag line.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Jim Logajan]

Alan Baker > wrote:
> <http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>
>
> "Pitching Moment due to Thrust
> Thrust line above or below center of mass induces a pitching
> moment"
>
> Note: no mention of drag line...

No mention of pitching moment due to elevator controls either. Does that
mean elevators don't affect pitch!? You are taking a reference out of
context.

[Alan Baker]

In article >,
Jim Logajan > wrote:

> Alan Baker > wrote:
> > <http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>
> >
> > "Pitching Moment due to Thrust
> > Thrust line above or below center of mass induces a pitching
> > moment"
> >
> > Note: no mention of drag line...
>
> No mention of pitching moment due to elevator controls either. Does that
> mean elevators don't affect pitch!? You are taking a reference out of
> context.

No, I'm not.

I'm showing what what you need to consider when changing the location of
an engine is keeping the thrust line in the same relationship with
respect to its moment arm with the CoM.

Just as if you wanted to change the size of the tail plane and keep the
same control authority: you'd consider how much you need to move it in
relation to the CoM.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Jim Logajan]

Alan Baker > wrote:
> In article >,
> Jim Logajan > wrote:
>
>> Alan Baker > wrote:
>> > <http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>
>> >
>> > "Pitching Moment due to Thrust
>> > Thrust line above or below center of mass induces a pitching
>> > moment"
>> >
>> > Note: no mention of drag line...
>>
>> No mention of pitching moment due to elevator controls either. Does
>> that mean elevators don't affect pitch!? You are taking a reference
>> out of context.
>
> No, I'm not.
>
> I'm showing what what you need to consider when changing the location
> of an engine is keeping the thrust line in the same relationship with
> respect to its moment arm with the CoM.

That's appears to be a different claim than in your post up-thread when you
claimed the one place you can apply thrust to avoid an engine pitching
force is through the center of mass. Now you're saying the thrust line need
not go through the center of mass, just that the new thrust line coincide
with the old one.

(Hopefully the original poster can find a decent book on the subject.)

> Just as if you wanted to change the size of the tail plane and keep
> the same control authority: you'd consider how much you need to move
> it in relation to the CoM.

Um, just what is your training in the physical sciences, if I may ask?

[Alan Baker]

In article >,
Jim Logajan > wrote:

> Alan Baker > wrote:
> > Forget all about drag for a moment: you can boil down all the forces
> > of lift and drag -- the aerodynamic forces -- to a single force
> > through the aerodynamic centre of pressure. Now imagine an aircraft
> > with the engine off and the prop feathered and in a trimmed out glide.
> >
> > Got that pictured in your mind? Good.
>
> Okay, so far I have:
>
> air flow ----> M
> |
> P--->
>
> Where M = "center of mass," P = "center of pressure," and the arrow
> represents the direction and magnitude of the resultant aerodynamic
> force.

Yup. Except you're neglecting to show that this is an aircraft in a
trimmed glide and thus the total aerodynamic force must be vertical and
acting directly through the centre of mass.

:-)

>
> > Now: when you start the engine and add its thrust into the equation,
> > what is the one direction in which you can apply that thrust and not
> > cause the aircraft to pitch.
> >
> > Where must the axis of the thrust vector be?
>
> Well the following seems to work, where T = "a point that thrust acts
> through:"
>
> air flow ----> M
> |
> <---TP--->

Yup. And if you add the thrust there to a system that is in balance,
what's going to happen? Pitch up, right?

>
> > Through the centre of mass. Period.
>
> You mean this?:
>
> air flow ----> <---TM
> |
> P--->
>
> That's a couple whose only important property is its moment. Looks like
> the aircraft will rotate. Sure, it will rotate about the center or mass,
> but so what?

No. Your ASCII drawings aren't fine enough to show the situation broken
into components, but I can show total forces...

In a glide in a low wing aircraft:

Total aerodynamic force (lift and drag!)
^
|
|
M (Centre of Mass)
|
C (Centre of Aerodynamic Pressure)
|
|
Weight (no down arrow head... ...sorry)

Now remember, the aircraft must be descending to make this work.

Now if you add thrust at the "drag line" (the line through the CoP
parallel to the aircraft's motion):

Total aerodynamic force
^
|
|
M (Centre of Mass)
|
(Thrust)<--C (Centre of Aerodynamic pressure)
|
|
Weight

You can align the engine any way you want and it will still create a
pitch up, right?

But:

Total aerodynamic force
^
|
|
(Thrust)<--M (Centre of Mass)
|
C (Centre of Aerodynamic Pressure)
|
|
Weight

Add the thrust at the centre of mass, and you get no pitching moment.

Back to our original discussion, if the aircraft happened to be designed
with the engine installed for zero pitching moment, then if have to
change the height of the prop with a different engine, what you need to
do is change the angle so that it once again points back through the CoM.

And similarly if you want the same pitch response with changing throttle
as an engine mounted such that it has moment arm with respect to the
centre of mass, then you want to keep *that* moment arm the same in
order to make the aircraft's flying characteristics stay the same.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
Jim Logajan > wrote:

> Alan Baker > wrote:
> > In article >,
> > Jim Logajan > wrote:
> >
> >> Alan Baker > wrote:
> >> > <http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>
> >> >
> >> > "Pitching Moment due to Thrust
> >> > Thrust line above or below center of mass induces a pitching
> >> > moment"
> >> >
> >> > Note: no mention of drag line...
> >>
> >> No mention of pitching moment due to elevator controls either. Does
> >> that mean elevators don't affect pitch!? You are taking a reference
> >> out of context.
> >
> > No, I'm not.
> >
> > I'm showing what what you need to consider when changing the location
> > of an engine is keeping the thrust line in the same relationship with
> > respect to its moment arm with the CoM.
>
> That's appears to be a different claim than in your post up-thread when you
> claimed the one place you can apply thrust to avoid an engine pitching
> force is through the center of mass. Now you're saying the thrust line need
> not go through the center of mass, just that the new thrust line coincide
> with the old one.

I was using the claim you make first to show a simplified case of the
general problem.

I don't know where this particular aircraft (the "Pegazair", IIRC) has
its thrust line with respect to its centre of mass, but for the sake of
demonstrating my argument I simplified to the idea of an engine mount
with no moment arm.

In the real situation, the thrust line is almost certainly *not* through
the CoM. There are simply infinitely more places for it to be. :-) Plus,
some of the reading I've done since this discussion began suggests one
can use a thrust line above the CoM to create a stabilizing effect where
a perturbation of the aircraft that increases its speed causes a
decrease in thrust and thus a pitch up.

<http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>

So let's say for the sake of argument that that's precisely the case
with this "Pegazair" then, and let's assume that the amount the thrust
line is above the CoM is precisely amount that the propellor will be
lower if he build his craft with a Corvair engine and positions it to
keep the CoM in the same location: 4 inches (the designer's figure).

OK. Doing that then removes that stabilizing effect. And note that
Professor Stengel makes no reference to the "drag line" when talking on
the subject,.

>
> (Hopefully the original poster can find a decent book on the subject.)

It looks like Professor Stengel might have one:

<http://www.princeton.edu/~stengel/FlightDynamics.html>

>
> > Just as if you wanted to change the size of the tail plane and keep
> > the same control authority: you'd consider how much you need to move
> > it in relation to the CoM.
>
> Um, just what is your training in the physical sciences, if I may ask?

Some university physics and a lot of study for interest. :-)

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[[email protected]]

On Fri, 14 Nov 2008 16:16:21 -0600, cavelamb himself
> wrote:

wrote:
>>
>>
>> Lowering the thrust line to below the center of aerodynamic drag would
>> cause nose up - OK I get that. Now where is the center of drag on a
>> peg? and it will DEFINETLY change with flying attitude - ie with the
>> flaps on, or the slats extended.
>>
>> I guess what it boils down to is it will not be a HUGE effect.
>> On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
>> roughly 1/2 degree. One 1/8" washer at the firewall and one at the
>> engine rubber on both sides will make 1/2 degree change if I need to
>> do a bit od "fine" tuning.
>>
>> Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
>> offset - guess I'll put in about .875 and see what happens
>
>I thought about this a bit last night.
>And a couple of thoughts seemed worth relaying.
>
>First, (and most obviously) a new mount will be needed.
>So build it as close as you can guess to what you'll need.
>
>Adjusting the mount at the firewall end strikes me as a bit "iffy".
>
>More that a washer or two makes for a noticeable misalignment between top
>and bottom bolts. When torqued down, something it GOING to give.
>
>Either the mount gets twisted or the firewall support structure does.
>Or both?
>
>The engine end, if rubber cushioned would be a lot more compliant.
>Might consider all that when designing the new mount.
>
>The Corvair would use a bearer style mount, wouldn't it?

Not on this plane. I'll get pics of the mount design on line soon.
I've put mounting tabs on the top and bottom rear so I'm mounting it
like a Conti O200, but using 1" diameter Licoming type homebuilder
mounts.The typical bed mount would interfere with my 180 degree header
system.
>
>Rubber pads front and rear would give quite a bit of adjustment room.
>
>I think Stealth got it right.
>
>Same side alignment and a touch more down.

[[email protected]]

On Fri, 14 Nov 2008 19:48:00 -0500, "Morgans"
> wrote:

>
>"Alan Baker" > wrote
>>
>> In level flight, drag is horizontal. Engine thrust is not except at one
>> particular angle of attack (and it is technically possible that it is
>> never level for any angle of attack the aircraft can achieve). So drag
>> and thrust cannot *possibly* cancel each other except at the one angle
>> of attack.
>
>Now, you are starting to get close. You are splitting hairs, though.
>
>Airplanes are said to be a loose flying formation of compromises. In the
>question of thrust angle, also.
>
>Parasite drag goes up for higher speeds, induced drag goes up with higher
>loads. The angle of attack changes the center of lift, loading affects the
>center of gravity of the airplane, and the tail balances it all out, with
>help from the thrust angle.
>
>Everything is designed to achieve a compromise of performance and safety,
>speed and comfort, and many other factors. So goes it with questions of
>thrust, trim and what goals you are trying to maximize. Same with this
>whole question. A different engine will cause a different thrust line, and
>changes would need to be made to keep the handling qualities approximately
>the same. They can never be the exact same, but an attempt can be made to
>keep it close.
>
>A change of 1/2 degree would be close, but the best answer will be to try it
>and see.
>
>Drag still is the paramount factor in attempting to quantify the changes
>that will need to be made. Once things start rotating, then they will
>indeed rotate around the center of mass. If the change in the angle is made
>successfully, there won't be any rotating going on. ;-)


U guess the way to intelligently assess this is to determine the
longitudinal center of balance of the finished plane, and project the
thrust line of the engine (from the plans) back to where the 2
intesect. THAT point is where I want the new thrust lines to intersect
as well. If I drop the engine 2 inches I change the angle down enough
for the thrust line to hit the same point and the flight
characteristics SHOULD, theoretically be the same.
No?
Any reason this should not work?

[[email protected]]

On Fri, 14 Nov 2008 16:52:24 -0800, Alan Baker >
wrote:

>In article
>,
> Bob Kuykendall > wrote:
>
>> On Nov 14, 4:10Â*pm, Alan Baker > wrote:
>>
>> > ...Where must the axis of the thrust vector be?
>> >
>> > Through the centre of mass. Period.
>>
>> Ah, I see. All those self-launch gliders and amphibs with pylon-
>> mounted engines must be figments of imagination. Thanks for clarifying
>> that.
>>
>> Bob K.
>
>Bob,
>
>Let's put back what you snipped:
>
>> > Now: when you start the engine and add its thrust into the equation,
>> > what is the one direction in which you can apply that thrust and not
>> > cause the aircraft to pitch?
>
>I'm not saying that you can't have an engine mounted with the thrust
>line pointed off the CoM. You can, but when you change engine power, the
>aircraft is going to experience a pitch change.
>
>I'm saying if you don't want pitch changes with changing engine thrust,
>you have to mount it with the thrust line through the CoM.
>
>And I'm right.
>
>But if you don't believe me, will a reference from Princeton suffice?
>
><http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>
>
>"Pitching Moment due to Thrust
>€ Thrust line above or below center of mass induces a pitching
>moment"
>
>Note: no mention of drag line...


Let's step back and take another look. The plane is trimmed for level
flight.All forces are ballanced. The aerodynamic drags (parasitic and
dynamic) and lifts (main wing and rear horizontal stab) balance out,
so the center of mass is what the plane will rotate around when
something other than aerodynamic trim causes the plane to pitch.
Moving the center of mass forward or backwards from the center of lift
will cause a pitching motion, as will adding thrust (or drag) which is
not applied through the center of mass.

[Alan Baker]

In article >,
wrote:

> On Fri, 14 Nov 2008 19:48:00 -0500, "Morgans"
> > wrote:
>
> >
> >"Alan Baker" > wrote
> >>
> >> In level flight, drag is horizontal. Engine thrust is not except at one
> >> particular angle of attack (and it is technically possible that it is
> >> never level for any angle of attack the aircraft can achieve). So drag
> >> and thrust cannot *possibly* cancel each other except at the one angle
> >> of attack.
> >
> >Now, you are starting to get close. You are splitting hairs, though.
> >
> >Airplanes are said to be a loose flying formation of compromises. In the
> >question of thrust angle, also.
> >
> >Parasite drag goes up for higher speeds, induced drag goes up with higher
> >loads. The angle of attack changes the center of lift, loading affects the
> >center of gravity of the airplane, and the tail balances it all out, with
> >help from the thrust angle.
> >
> >Everything is designed to achieve a compromise of performance and safety,
> >speed and comfort, and many other factors. So goes it with questions of
> >thrust, trim and what goals you are trying to maximize. Same with this
> >whole question. A different engine will cause a different thrust line, and
> >changes would need to be made to keep the handling qualities approximately
> >the same. They can never be the exact same, but an attempt can be made to
> >keep it close.
> >
> >A change of 1/2 degree would be close, but the best answer will be to try it
> >and see.
> >
> >Drag still is the paramount factor in attempting to quantify the changes
> >that will need to be made. Once things start rotating, then they will
> >indeed rotate around the center of mass. If the change in the angle is made
> >successfully, there won't be any rotating going on. ;-)
>
>
> U guess the way to intelligently assess this is to determine the
> longitudinal center of balance of the finished plane, and project the
> thrust line of the engine (from the plans) back to where the 2
> intesect. THAT point is where I want the new thrust lines to intersect
> as well. If I drop the engine 2 inches I change the angle down enough
> for the thrust line to hit the same point and the flight
> characteristics SHOULD, theoretically be the same.
> No?
> Any reason this should not work?

If I understand your language correctly, not that I can see.

The "longitudinal center of balance" is just the longitudinal position
of the centre of mass, and as such the "line" with which the thrust line
would intersect would be the vertical line through the CoM, which would
be very close to the moment line which would be the line perpendicular
to the TL that passes through the CoM.

What you want is for the aircraft to react to change in thrust in the
way its designers intended, and to do that, you need to keep the moment
arm the same, but keeping the vertical line segment the same should be
pretty close in any rational aircraft layout.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
wrote:

> On Fri, 14 Nov 2008 16:52:24 -0800, Alan Baker >
> wrote:
>
> >In article
> >,
> > Bob Kuykendall > wrote:
> >
> >> On Nov 14, 4:10Â*pm, Alan Baker > wrote:
> >>
> >> > ...Where must the axis of the thrust vector be?
> >> >
> >> > Through the centre of mass. Period.
> >>
> >> Ah, I see. All those self-launch gliders and amphibs with pylon-
> >> mounted engines must be figments of imagination. Thanks for clarifying
> >> that.
> >>
> >> Bob K.
> >
> >Bob,
> >
> >Let's put back what you snipped:
> >
> >> > Now: when you start the engine and add its thrust into the equation,
> >> > what is the one direction in which you can apply that thrust and not
> >> > cause the aircraft to pitch?
> >
> >I'm not saying that you can't have an engine mounted with the thrust
> >line pointed off the CoM. You can, but when you change engine power, the
> >aircraft is going to experience a pitch change.
> >
> >I'm saying if you don't want pitch changes with changing engine thrust,
> >you have to mount it with the thrust line through the CoM.
> >
> >And I'm right.
> >
> >But if you don't believe me, will a reference from Princeton suffice?
> >
> ><http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>
> >
> >"Pitching Moment due to Thrust
> >€ Thrust line above or below center of mass induces a pitching
> >moment"
> >
> >Note: no mention of drag line...
>
>
> Let's step back and take another look. The plane is trimmed for level
> flight.All forces are ballanced. The aerodynamic drags (parasitic and
> dynamic) and lifts (main wing and rear horizontal stab) balance out,
> so the center of mass is what the plane will rotate around when
> something other than aerodynamic trim causes the plane to pitch.
> Moving the center of mass forward or backwards from the center of lift
> will cause a pitching motion, as will adding thrust (or drag) which is
> not applied through the center of mass.

Correct. And given that there can be at most precisely *one* AoA at
which the thrust line acts through the CoM, changing thrust will
normally cause a pitching moment.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article
>,
Alan Baker > wrote:

> In article >,
> wrote:
>
> > On Fri, 14 Nov 2008 16:52:24 -0800, Alan Baker >
> > wrote:
> >
> > >In article
> > >,
> > > Bob Kuykendall > wrote:
> > >
> > >> On Nov 14, 4:10Â*pm, Alan Baker > wrote:
> > >>
> > >> > ...Where must the axis of the thrust vector be?
> > >> >
> > >> > Through the centre of mass. Period.
> > >>
> > >> Ah, I see. All those self-launch gliders and amphibs with pylon-
> > >> mounted engines must be figments of imagination. Thanks for clarifying
> > >> that.
> > >>
> > >> Bob K.
> > >
> > >Bob,
> > >
> > >Let's put back what you snipped:
> > >
> > >> > Now: when you start the engine and add its thrust into the equation,
> > >> > what is the one direction in which you can apply that thrust and not
> > >> > cause the aircraft to pitch?
> > >
> > >I'm not saying that you can't have an engine mounted with the thrust
> > >line pointed off the CoM. You can, but when you change engine power, the
> > >aircraft is going to experience a pitch change.
> > >
> > >I'm saying if you don't want pitch changes with changing engine thrust,
> > >you have to mount it with the thrust line through the CoM.
> > >
> > >And I'm right.
> > >
> > >But if you don't believe me, will a reference from Princeton suffice?
> > >
> > ><http://www.princeton.edu/~stengel/MAE331Lecture9.pdf>
> > >
> > >"Pitching Moment due to Thrust
> > >€ Thrust line above or below center of mass induces a pitching
> > >moment"
> > >
> > >Note: no mention of drag line...
> >
> >
> > Let's step back and take another look. The plane is trimmed for level
> > flight.All forces are ballanced. The aerodynamic drags (parasitic and
> > dynamic) and lifts (main wing and rear horizontal stab) balance out,
> > so the center of mass is what the plane will rotate around when
> > something other than aerodynamic trim causes the plane to pitch.
> > Moving the center of mass forward or backwards from the center of lift
> > will cause a pitching motion, as will adding thrust (or drag) which is
> > not applied through the center of mass.
>
> Correct. And given that there can be at most precisely *one* AoA at
> which the thrust line acts through the CoM, changing thrust will
> normally cause a pitching moment.

Note, you can simplify your statement by removing the division between
parasitic and dynamic drag and by removing mention of the aerodynamic
trim. A change in that just changes the vector (and technically, the
position by a very small amount) of total aerodynamic force.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[[email protected]]

On Fri, 14 Nov 2008 16:56:51 -0800, Alan Baker >
wrote:

>In article >,
> "Morgans" > wrote:
>
>> "Alan Baker" > wrote
>> >
>> > In level flight, drag is horizontal. Engine thrust is not except at one
>> > particular angle of attack (and it is technically possible that it is
>> > never level for any angle of attack the aircraft can achieve). So drag
>> > and thrust cannot *possibly* cancel each other except at the one angle
>> > of attack.
>>
>> Now, you are starting to get close. You are splitting hairs, though.
>
>No. I'm understanding the situation and so far, you've not shown that
>you do.
>
>>
>> Airplanes are said to be a loose flying formation of compromises. In the
>> question of thrust angle, also.
>>
>> Parasite drag goes up for higher speeds, induced drag goes up with higher
>> loads. The angle of attack changes the center of lift, loading affects the
>> center of gravity of the airplane, and the tail balances it all out, with
>> help from the thrust angle.
>
>If your claim that thrust and drag line always canceled each other out,
>then that last statement would not be true, would it?
>
>>
>> Everything is designed to achieve a compromise of performance and safety,
>> speed and comfort, and many other factors. So goes it with questions of
>> thrust, trim and what goals you are trying to maximize. Same with this
>> whole question. A different engine will cause a different thrust line, and
>> changes would need to be made to keep the handling qualities approximately
>> the same. They can never be the exact same, but an attempt can be made to
>> keep it close.
>
>They can be kept close by keeping the change in torque about the centre
>of mass the same as they were in the original design...
>
>>
>> A change of 1/2 degree would be close, but the best answer will be to try it
>> and see.
>
>Where did you get that figure? Show your work if you're going to try and
>be quantitative...
>
>>
>> Drag still is the paramount factor in attempting to quantify the changes
>> that will need to be made. Once things start rotating, then they will
>> indeed rotate around the center of mass. If the change in the angle is made
>> successfully, there won't be any rotating going on. ;-)
>
>Yup. And in order for that to happen, you need to keep the same
>relationship with the centre of *mass*.
>
>:-)
Using an engine offset calculator, and assuming the CL is over the CM
and it is 78" behind the prop center,1.5 degrees would have the prop
center 2.045" below the CM. If I lower the engine 2 inches, the angle
needs to change to 2.969

If theCM is farther back, the angle change will be less.
If I only drop the engine 1 inch instead of 2, the difference is half.
If I drop it 4 degrees, the angle WOULD be over 3.5 degrees - 4.432,
to be exact.

ASSUMING the CM is somewhere very close to the CL (which it MAY not
be)

[cavelamb himself[_4_]]

>>
>>And angling an engine UP is a real BAD (tm) idea.
>
>
> Really?
>
> So when you're flying with a large AOA, it's BAD(tm)?
>
> Please.
>


I don't know who you are, nor what your qualification really are.

But most of what you have espoused here I strongly disagree with.


You dance pretty well, though.

Richard

[cavelamb himself[_4_]]

Alan Baker wrote:

> Not if you use wedge washers...
>
> <http://www.instron.us/wa/acc_catalog/detail.aspx?aid=1706&ref=http://www
> .google.com/search>
>


The smallest of those are 1" in dimeter.

Do you think that's big enough???



--

Richard

(remove the X to email)

[cavelamb himself[_4_]]

wrote:

>>The Corvair would use a bearer style mount, wouldn't it?
>
>
> Not on this plane. I'll get pics of the mount design on line soon.
> I've put mounting tabs on the top and bottom rear so I'm mounting it
> like a Conti O200, but using 1" diameter Licoming type homebuilder
> mounts.The typical bed mount would interfere with my 180 degree header
> system.
>


How will the mount attach to the engine case?
I don't recall how the aft end of the engine is arranged.






--

Richard

(remove the X to email)

[Alan Baker]

In article >,
cavelamb himself > wrote:

> >>
> >>And angling an engine UP is a real BAD (tm) idea.
> >
> >
> > Really?
> >
> > So when you're flying with a large AOA, it's BAD(tm)?
> >
> > Please.
> >
>
>
> I don't know who you are, nor what your qualification really are.
>
> But most of what you have espoused here I strongly disagree with.
>
>
> You dance pretty well, though.
>
> Richard

Nope. I just don't like bull****.

Angling an engine "up" implies only the physical orientation of the
engine.

Now if you want to talk about what makes for a stable configuration with
respect to the *aircraft*, that's an entirely different story.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
cavelamb himself > wrote:

> Alan Baker wrote:
>
> > Not if you use wedge washers...
> >
> > <http://www.instron.us/wa/acc_catalog/detail.aspx?aid=1706&ref=http://www
> > .google.com/search>
> >
>
>
> The smallest of those are 1" in dimeter.
>
> Do you think that's big enough???

Why would it matter if the SMALLEST of something is BIG ENOUGH?

Surely even you are bright enough to realize that the there must
logically be larger ones than the SMALLEST of something...

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[cavelamb himself[_4_]]

Alan Baker wrote:
> In article >,
> cavelamb himself > wrote:
>
>
>>Alan Baker wrote:
>>
>>
>>>Not if you use wedge washers...
>>>
>>><http://www.instron.us/wa/acc_catalog/detail.aspx?aid=1706&ref=http://www
>>>.google.com/search>
>>>
>>
>>The smallest of those are 1" in dimeter.
>>
>>Do you think that's big enough???
>
>
> Why would it matter if the SMALLEST of something is BIG ENOUGH?
>
> Surely even you are bright enough to realize that the there must
> logically be larger ones than the SMALLEST of something...
>

BECAUSE WE GENERALLY USE 3/8" OR 1/2" DIAMETER BOLTS ON ENGINE MOUNTS.

--

Richard

(remove the X to email)

[Jim Logajan]

Alan Baker > wrote:
> In a glide in a low wing aircraft:
>
> Total aerodynamic force (lift and drag!)
> ^
> |
> |
> M (Centre of Mass)
> |
> C (Centre of Aerodynamic Pressure)
> |
> |
> Weight (no down arrow head... ...sorry)

It is late and I'm about to go to bed, but after seeing your force diagrams
like the one above I wrote a private e-mail to the original poster with the
advice that while you get many concepts correct you also appear to apply
them incorrectly on occasion, so your advice should be disregarded as
suspect. Nothing personal - but someone's time, money, and life are
involved. Getting things wrong is pretty common - we all do it - even Nobel
prize winners in physics have had their physical intuition fail them
(Feynman wrote of one such case in one of his books.)

Anyway, all the force diagrams you drew appear to have important errors.

And for what it is worth, I have a BSc in physics and was just reviewing a
textbook on basic mechanics a week ago (J. P. Den Hartog's "Mechanics" -
inexpensive and worth buying by the way) but even so I would have to think
the problem through carefully before attempting even a qualified answer.

[Alan Baker]

In article >,
cavelamb himself > wrote:

> Alan Baker wrote:
> > In article >,
> > cavelamb himself > wrote:
> >
> >
> >>Alan Baker wrote:
> >>
> >>
> >>>Not if you use wedge washers...
> >>>
> >>><http://www.instron.us/wa/acc_catalog/detail.aspx?aid=1706&ref=http://www
> >>>.google.com/search>
> >>>
> >>
> >>The smallest of those are 1" in dimeter.
> >>
> >>Do you think that's big enough???
> >
> >
> > Why would it matter if the SMALLEST of something is BIG ENOUGH?
> >
> > Surely even you are bright enough to realize that the there must
> > logically be larger ones than the SMALLEST of something...
> >
>
> BECAUSE WE GENERALLY USE 3/8" OR 1/2" DIAMETER BOLTS ON ENGINE MOUNTS.

1. Do you think this is the only supplier of wedge washers in the world?

2. Why did you ssy, "Do you think that's big enough???"?

3. You don't suppose the original poster could just design the engine
mounts to accomodate the different bolt angles do you? I mean, it's not
like he's building a kit or something...

....no, wait.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[jan olieslagers[_2_]]

Alan Baker schreef:

> ...no, wait.

No I won't.
Ploink.

[Alan Baker]

In article >,
Jim Logajan > wrote:

> Alan Baker > wrote:
> > In a glide in a low wing aircraft:
> >
> > Total aerodynamic force (lift and drag!)
> > ^
> > |
> > |
> > M (Centre of Mass)
> > |
> > C (Centre of Aerodynamic Pressure)
> > |
> > |
> > Weight (no down arrow head... ...sorry)
>
> It is late and I'm about to go to bed, but after seeing your force diagrams
> like the one above I wrote a private e-mail to the original poster with the
> advice that while you get many concepts correct you also appear to apply
> them incorrectly on occasion, so your advice should be disregarded as
> suspect. Nothing personal - but someone's time, money, and life are
> involved. Getting things wrong is pretty common - we all do it - even Nobel
> prize winners in physics have had their physical intuition fail them
> (Feynman wrote of one such case in one of his books.)
>
> Anyway, all the force diagrams you drew appear to have important errors.
>
> And for what it is worth, I have a BSc in physics and was just reviewing a
> textbook on basic mechanics a week ago (J. P. Den Hartog's "Mechanics" -
> inexpensive and worth buying by the way) but even so I would have to think
> the problem through carefully before attempting even a qualified answer.

I'm sorry, but the problem is simple.

You want the new engine configuration to behave like the old one. To
achieve that, it must have the same effect on pitch as the old one. To
achieve *that*, it must have the same moment arm with respect to the
center of mass.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
jan olieslagers > wrote:

> Alan Baker schreef:
>
> > ...no, wait.
>
> No I won't.
> Ploink.

Suit yourself, but running from factual arguments never appears to be a
good idea to me...

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article >,
wrote:

> On Fri, 14 Nov 2008 16:56:51 -0800, Alan Baker >
> wrote:
>
> >In article >,
> > "Morgans" > wrote:
> >
> >> "Alan Baker" > wrote
> >> >
> >> > In level flight, drag is horizontal. Engine thrust is not except at one
> >> > particular angle of attack (and it is technically possible that it is
> >> > never level for any angle of attack the aircraft can achieve). So drag
> >> > and thrust cannot *possibly* cancel each other except at the one angle
> >> > of attack.
> >>
> >> Now, you are starting to get close. You are splitting hairs, though.
> >
> >No. I'm understanding the situation and so far, you've not shown that
> >you do.
> >
> >>
> >> Airplanes are said to be a loose flying formation of compromises. In the
> >> question of thrust angle, also.
> >>
> >> Parasite drag goes up for higher speeds, induced drag goes up with higher
> >> loads. The angle of attack changes the center of lift, loading affects
> >> the
> >> center of gravity of the airplane, and the tail balances it all out, with
> >> help from the thrust angle.
> >
> >If your claim that thrust and drag line always canceled each other out,
> >then that last statement would not be true, would it?
> >
> >>
> >> Everything is designed to achieve a compromise of performance and safety,
> >> speed and comfort, and many other factors. So goes it with questions of
> >> thrust, trim and what goals you are trying to maximize. Same with this
> >> whole question. A different engine will cause a different thrust line,
> >> and
> >> changes would need to be made to keep the handling qualities approximately
> >> the same. They can never be the exact same, but an attempt can be made to
> >> keep it close.
> >
> >They can be kept close by keeping the change in torque about the centre
> >of mass the same as they were in the original design...
> >
> >>
> >> A change of 1/2 degree would be close, but the best answer will be to try
> >> it
> >> and see.
> >
> >Where did you get that figure? Show your work if you're going to try and
> >be quantitative...
> >
> >>
> >> Drag still is the paramount factor in attempting to quantify the changes
> >> that will need to be made. Once things start rotating, then they will
> >> indeed rotate around the center of mass. If the change in the angle is
> >> made
> >> successfully, there won't be any rotating going on. ;-)
> >
> >Yup. And in order for that to happen, you need to keep the same
> >relationship with the centre of *mass*.
> >
> >:-)
> Using an engine offset calculator, and assuming the CL is over the CM
> and it is 78" behind the prop center,1.5 degrees would have the prop
> center 2.045" below the CM. If I lower the engine 2 inches, the angle
> needs to change to 2.969
>
> If theCM is farther back, the angle change will be less.
> If I only drop the engine 1 inch instead of 2, the difference is half.
> If I drop it 4 degrees, the angle WOULD be over 3.5 degrees - 4.432,
> to be exact.
>
> ASSUMING the CM is somewhere very close to the CL (which it MAY not
> be)

Define: "CL".

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Stealth Pilot[_2_]]

On Fri, 14 Nov 2008 07:25:03 -0800, Alan Baker >
wrote:

>In article
>,
> Alan Baker > wrote:
>
>> In article >,
>> Stealth Pilot > wrote:
>>
>> > On Thu, 13 Nov 2008 22:20:25 -0500, wrote:
>> >
>> > >
>> > >
>> > >How does a person determine what the proper height of an engine should
>> > >be when building an airplane? If a particular engine design mandates
>> > >the prop is 4 inches, say, lower than where it would be with the
----------------------------------------^^^^
>> > >engine originally installed, what effect will it have on handling, and
>> > >what changes in downthrust might be advised?

>>
>> Draw few diagrams of the situation and you'll see what I mean. I'm not
>> even going to try to draw that situation here. Basically, if the thrust
>> line was already above the CoM and you move it up, then the change is
>> smaller than the arctangent of h/l and if it was below the CoM the
>> change is a little greater than the arctangent.
>
>Did a quick little check:
>
>As an example, a Cessna 150 is about 25 feet long and from looking at
>wikipedia's little jpeg, the centre of mass should be about 5 feet
>behind the propellor disc.
>
>So if you raise the thrust line 4 inches, you need to angle the engine
>up an additional 3.8 degrees; arctan(4/60).

alan his corvair engine has the thrustline lower than the original
O-200.
so that would be down an extra 3 degrees.

Stealth Pilot

[stol]

On Nov 14, 2:59*pm, wrote:
> On Fri, 14 Nov 2008 13:24:47 -0600, cavelamb himself
>
>
>
> Lowering the thrust line to below the center of aerodynamic drag would
cause nose up - OK I get that. Now where is the center of drag on a
peg? and it will DEFINETLY change with flying attitude - ie with the
flaps on, or the slats extended.

I guess what it boils down to is it will not be a HUGE effect.
On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
roughly 1/2 degree. One 1/8" washer at the firewall and one at the
engine rubber on both sides will make 1/2 degree change if I need to
do a bit od "fine" tuning.


Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
offset - guess I'll put in about .875 and see what happens


This is all good till you consider that cowling you spent days
trimming to get it to fit perfectly will now be junk.

[[email protected]]

On Fri, 14 Nov 2008 22:53:23 -0600, cavelamb himself
> wrote:

wrote:
>
>>>The Corvair would use a bearer style mount, wouldn't it?
>>
>>
>> Not on this plane. I'll get pics of the mount design on line soon.
>> I've put mounting tabs on the top and bottom rear so I'm mounting it
>> like a Conti O200, but using 1" diameter Licoming type homebuilder
>> mounts.The typical bed mount would interfere with my 180 degree header
>> system.
>>
>
>
>How will the mount attach to the engine case?
>I don't recall how the aft end of the engine is arranged.


I'll get pictures, but I used a chunk of auminum channel, cut away to
make a "U" shaped bracket that bolts to the top surface of the engine
case, with "ears" to which mounting blocks are fastened, immitating
the top ears of an O200 case. The bottom has an angle boted down each
side, like the typical bed mount but without rubber isolation, with
mount blocks fastened to them as well,.

Very similar to the way it is mounted on my engine test stand,
pictured on my website.

[[email protected]]

On Fri, 14 Nov 2008 21:37:00 -0800, Alan Baker >
wrote:

>In article >,
> cavelamb himself > wrote:
>
>> Alan Baker wrote:
>>
>> > Not if you use wedge washers...
>> >
>> > <http://www.instron.us/wa/acc_catalog/detail.aspx?aid=1706&ref=http://www
>> > .google.com/search>
>> >
>>
>>
>> The smallest of those are 1" in dimeter.
>>
>> Do you think that's big enough???
>
>Why would it matter if the SMALLEST of something is BIG ENOUGH?
>
>Surely even you are bright enough to realize that the there must
>logically be larger ones than the SMALLEST of something...


Um, look again - the smallest BOLT DIAMETER is 1 inch. We are using
3/8" bolts to fasten engine mounts to firewalls. They go up to 1.5"
BOLT diameter.

Not an option, Sorry Alan.

[[email protected]]

On Sat, 15 Nov 2008 03:22:14 -0800, Alan Baker >
wrote:

>In article >,
> wrote:
>
>> On Fri, 14 Nov 2008 16:56:51 -0800, Alan Baker >
>> wrote:
>>
>> >In article >,
>> > "Morgans" > wrote:
>> >
>> >> "Alan Baker" > wrote
>> >> >
>> >> > In level flight, drag is horizontal. Engine thrust is not except at one
>> >> > particular angle of attack (and it is technically possible that it is
>> >> > never level for any angle of attack the aircraft can achieve). So drag
>> >> > and thrust cannot *possibly* cancel each other except at the one angle
>> >> > of attack.
>> >>
>> >> Now, you are starting to get close. You are splitting hairs, though.
>> >
>> >No. I'm understanding the situation and so far, you've not shown that
>> >you do.
>> >
>> >>
>> >> Airplanes are said to be a loose flying formation of compromises. In the
>> >> question of thrust angle, also.
>> >>
>> >> Parasite drag goes up for higher speeds, induced drag goes up with higher
>> >> loads. The angle of attack changes the center of lift, loading affects
>> >> the
>> >> center of gravity of the airplane, and the tail balances it all out, with
>> >> help from the thrust angle.
>> >
>> >If your claim that thrust and drag line always canceled each other out,
>> >then that last statement would not be true, would it?
>> >
>> >>
>> >> Everything is designed to achieve a compromise of performance and safety,
>> >> speed and comfort, and many other factors. So goes it with questions of
>> >> thrust, trim and what goals you are trying to maximize. Same with this
>> >> whole question. A different engine will cause a different thrust line,
>> >> and
>> >> changes would need to be made to keep the handling qualities approximately
>> >> the same. They can never be the exact same, but an attempt can be made to
>> >> keep it close.
>> >
>> >They can be kept close by keeping the change in torque about the centre
>> >of mass the same as they were in the original design...
>> >
>> >>
>> >> A change of 1/2 degree would be close, but the best answer will be to try
>> >> it
>> >> and see.
>> >
>> >Where did you get that figure? Show your work if you're going to try and
>> >be quantitative...
>> >
>> >>
>> >> Drag still is the paramount factor in attempting to quantify the changes
>> >> that will need to be made. Once things start rotating, then they will
>> >> indeed rotate around the center of mass. If the change in the angle is
>> >> made
>> >> successfully, there won't be any rotating going on. ;-)
>> >
>> >Yup. And in order for that to happen, you need to keep the same
>> >relationship with the centre of *mass*.
>> >
>> >:-)
>> Using an engine offset calculator, and assuming the CL is over the CM
>> and it is 78" behind the prop center,1.5 degrees would have the prop
>> center 2.045" below the CM. If I lower the engine 2 inches, the angle
>> needs to change to 2.969
>>
>> If theCM is farther back, the angle change will be less.
>> If I only drop the engine 1 inch instead of 2, the difference is half.
>> If I drop it 4 degrees, the angle WOULD be over 3.5 degrees - 4.432,
>> to be exact.
>>
>> ASSUMING the CM is somewhere very close to the CL (which it MAY not
>> be)
>
>Define: "CL".
Center of Lift.

[[email protected]]

On Sat, 15 Nov 2008 06:44:21 -0800 (PST), stol >
wrote:

>On Nov 14, 2:59Â*pm, wrote:
>> On Fri, 14 Nov 2008 13:24:47 -0600, cavelamb himself
>>
>>
>>
>> Lowering the thrust line to below the center of aerodynamic drag would
>cause nose up - OK I get that. Now where is the center of drag on a
>peg? and it will DEFINETLY change with flying attitude - ie with the
>flaps on, or the slats extended.
>
>I guess what it boils down to is it will not be a HUGE effect.
>On a 28" long engine, 3 degrees is roughly 1.5" offset, so 1/4" is
>roughly 1/2 degree. One 1/8" washer at the firewall and one at the
>engine rubber on both sides will make 1/2 degree change if I need to
>do a bit od "fine" tuning.
>
>
>Spec for the O200 mount is 1.5 degrees down IIRC,amounting to .75"
>offset - guess I'll put in about .875 and see what happens
>
>
>This is all good till you consider that cowling you spent days
>trimming to get it to fit perfectly will now be junk.


Not a chance. The cowling has not even been designed yet, much less
built or trimmed.

This plane has not been completed - still a work in progress.
One of the other local builders is building with an O200 and has his
mount that I can compare to.

[[email protected]]

Remember the Four Forces? weight ahead of lift, thrust below drag.
Only weight need be at centre of mass.

See http://www.myaeromodelling.com/wp/wp-content/uploads/2008/01/aerodynamic-force2.jpg

Thrust below centre of mass will have an effect ONLY during
acceleration by the propeller, or decelleration if it has enough drag.
The rotational couple will be much smaller than that caused by the
thrust/drag or lift/weight offsets, and pitch changes are largely due
to the propwash over the stabilizer anyway.

There have been numerous airplanes built with low thrust lines.
Lemme See:

The deHavilland Dragon Rapide:
http://www.deltaaviation.co.uk/gifs/Gallery/Large/Dragon_Rapide/Pleasure_Flights_Dragon_Rapide_Airbourne.jpg

The deHavilland Cirrus Moth: http://www.apda61.dsl.pipex.com/Av12/G-EBLV.jpg

Curtiss R: http://www.aviationhalloffamewisconsin.com/images/maitland_curtissR6.jpg

The Lincoln Standard: http://cdn-www.airliners.net/aviation-photos/middle/1/6/7/0817761.jpg

Koohoven FK-41: http://www.henrikaper.nl/koolhoven-fk41/images/koolhoven-fk41-phagq-sunlight.jpg

I don't see any of those engines perceptibly angled up or down. They
fly just fine. The original Knight Twister used an upright inline
engine, with the resultant low thrust line, and also flew well.

Having a lower thrust line will pull the nose up more. Decreasing the
stab incidence a tiny bit will fix it.

Dan

[Alan Baker]

In article >,
wrote:

> On Fri, 14 Nov 2008 21:37:00 -0800, Alan Baker >
> wrote:
>
> >In article >,
> > cavelamb himself > wrote:
> >
> >> Alan Baker wrote:
> >>
> >> > Not if you use wedge washers...
> >> >
> >> > <http://www.instron.us/wa/acc_catalog/detail.aspx?aid=1706&ref=http://www
> >> > .google.com/search>
> >> >
> >>
> >>
> >> The smallest of those are 1" in dimeter.
> >>
> >> Do you think that's big enough???
> >
> >Why would it matter if the SMALLEST of something is BIG ENOUGH?
> >
> >Surely even you are bright enough to realize that the there must
> >logically be larger ones than the SMALLEST of something...
>
>
> Um, look again - the smallest BOLT DIAMETER is 1 inch. We are using
> 3/8" bolts to fasten engine mounts to firewalls. They go up to 1.5"
> BOLT diameter.
>
> Not an option, Sorry Alan.

<sigh>

I was using that as an *example*. Surely you don't believe that no one
in the world makes wedge washers for smaller bolts...

<https://www.acehardwareoutlet.com/(ashbal45ovewf4ivkfrwldeb)/RequestForQ
uote.aspx?SKU=2147007679>

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Jim Logajan]

Alan Baker > wrote:
> In a glide in a low wing aircraft:
>
> Total aerodynamic force (lift and drag!)
> ^
> |
> |
> M (Centre of Mass)
> |
> C (Centre of Aerodynamic Pressure)
> |
> |
> Weight (no down arrow head... ...sorry)
>
> Now remember, the aircraft must be descending to make this work.

The above diagram is simplified too soon in the analysis. You may as well
have dispensed with the weight and aerodynamic forces too, as they
contribute nothing to your subsequent argument since you never vary them.

> Now if you add thrust at the "drag line" (the line through the CoP
> parallel to the aircraft's motion):
>
> Total aerodynamic force
> ^
> |
> |
> M (Centre of Mass)
> |
> (Thrust)<--C (Centre of Aerodynamic pressure)
> |
> |
> Weight
>
> You can align the engine any way you want and it will still create a
> pitch up, right?

Sure - and the object will rotate about M until it reaches a rotation
speed in equilibrium with air drag (by definition, the only point where
we are allowed to add that drag component is at point C):

Total aerodynamic force
^
|
|
M (Centre of Mass)
|
(Thrust)<--C-->(air drag) (Centre of Aerodynamic pressure)
|
|
Weight


> But:
>
> Total aerodynamic force
> ^
> |
> |
> (Thrust)<--M (Centre of Mass)
> |
> C (Centre of Aerodynamic Pressure)
> |
> |
> Weight
>
> Add the thrust at the centre of mass, and you get no pitching moment.

The diagram above is of a system that isn't in equilibrium. Furthermore,
there is no vector we can anchor at C that brings it into equilibrium -
if we add a vector so that we get a pure couple, like so:

Total aerodynamic force
^
|
|
(Thrust)<--M (Centre of Mass)
|
C-->(air drag) (Centre of Aerodynamic Pressure)
|
|
Weight

....then the _couple_ rotates the aircraft around M in a counterclockwise
direction (i.e. pitch down!) Your force diagram is flawed because it
makes incorrect assumptions about the location of C at equilibrium and
the direction of the total aerodynamic forces.

Running the thrust line through M does _not_ guarantee you wont get any
couple.

In fact none of the diagrams you or I drew are complete and do not
accurately capture the reality. Center of mass changes with each flight
and even during flight, and center of pressure changes with aircraft
orientation.

[guynoir]

On Nov 13, 7:20*pm, wrote:
> How does a person determine what the proper height of an engine should
> be when building an airplane? If a particular engine design mandates
> the prop is 4 inches, say, lower than where it would be with the
> engine originally installed, what effect will it have on handling, and
> what changes in downthrust might be advised?
>
> We are building a Pegazair, and my Corvair engine would need to have
> the cowl higher than ideal to keep the crank centerline at the same
> hight as say, an O200. Weight wize, the engines are just about
> identical as equipped Have not determined the center of gravity of the
> engine yet, to determine the overall length of the mount.
>
> For those unfamiliar with the plane it is a highwing STOL 2 placer
> roughly the same size as a Cessna 150 *(150 sq ft wing,33 ft wingspan,
> )

I've posted a spreadsheet to calculate a new thrust angle based on
changing the waterline location of an engine. The data needed is
horizontal distance from center of propeller to CG, original vertical
distance from center of propeller to CG, original thrust angle, and
new vertical distance from propeller center to CG. The formula is not
sensitive to vertical CG location, an estimate will do. What matters
is the change in the engine location.

http://www.spiretech.com/~guynoir/sldwrks/downthrust.xls

[Alan Baker]

In article >,
Jim Logajan > wrote:

> Alan Baker > wrote:
> > In a glide in a low wing aircraft:
> >
> > Total aerodynamic force (lift and drag!)
> > ^
> > |
> > |
> > M (Centre of Mass)
> > |
> > C (Centre of Aerodynamic Pressure)
> > |
> > |
> > Weight (no down arrow head... ...sorry)
> >
> > Now remember, the aircraft must be descending to make this work.
>
> The above diagram is simplified too soon in the analysis. You may as well
> have dispensed with the weight and aerodynamic forces too, as they
> contribute nothing to your subsequent argument since you never vary them.

No, it's not.

It represents all the forces on an aircraft in a trimmed glide: total
aerodynamic force perfectly balancing weight.

>
> > Now if you add thrust at the "drag line" (the line through the CoP
> > parallel to the aircraft's motion):
> >
> > Total aerodynamic force
> > ^
> > |
> > |
> > M (Centre of Mass)
> > |
> > (Thrust)<--C (Centre of Aerodynamic pressure)
> > |
> > |
> > Weight
> >
> > You can align the engine any way you want and it will still create a
> > pitch up, right?
>
> Sure - and the object will rotate about M until it reaches a rotation
> speed in equilibrium with air drag (by definition, the only point where
> we are allowed to add that drag component is at point C):

It will never reach such an equilibrium. That's the problem. With the
increased thrust, the aircraft will both: pitch up and gain airspeed.
Remember: drag is notional. It is just the component of the total
aerodynamic force anti-parallel to the motion of the aircraft. In this
situation of a low wing aircraft, if you add thrust at the CoA, the
aircraft will pitch up, and that will rotate the craft and you'll have
to trim the aircraft. No waiting for drag to grow will do it.

>
> Total aerodynamic force
> ^
> |
> |
> M (Centre of Mass)
> |
> (Thrust)<--C-->(air drag) (Centre of Aerodynamic pressure)
> |
> |
> Weight
>
>
> > But:
> >
> > Total aerodynamic force
> > ^
> > |
> > |
> > (Thrust)<--M (Centre of Mass)
> > |
> > C (Centre of Aerodynamic Pressure)
> > |
> > |
> > Weight
> >
> > Add the thrust at the centre of mass, and you get no pitching moment.
>
> The diagram above is of a system that isn't in equilibrium. Furthermore,
> there is no vector we can anchor at C that brings it into equilibrium -
> if we add a vector so that we get a pure couple, like so:

So, what do you expect an aircraft in a stable glide to do when you add
thrust: accelerate. The natural consequence of a system that isn't in
equilibrium.

>
> Total aerodynamic force
> ^
> |
> |
> (Thrust)<--M (Centre of Mass)
> |
> C-->(air drag) (Centre of Aerodynamic Pressure)
> |
> |
> Weight
>
> ...then the _couple_ rotates the aircraft around M in a counterclockwise
> direction (i.e. pitch down!) Your force diagram is flawed because it
> makes incorrect assumptions about the location of C at equilibrium and
> the direction of the total aerodynamic forces.

Sorry, but no.

By definition, an aircraft in a stable glide has a *total* aerodynamic
force acting on it that must be precisely equal to the aircraft's weight
and *must* be acting through the centre of mass. You're suddenly adding
a new force as if it isn't accounted for in the previous diagram.

>
> Running the thrust line through M does _not_ guarantee you wont get any
> couple.

It guarantees you won't get a couple from the thrust. You say you have a
B.SC: from where?

>
> In fact none of the diagrams you or I drew are complete and do not
> accurately capture the reality. Center of mass changes with each flight
> and even during flight, and center of pressure changes with aircraft
> orientation.

So? The point I've been trying to make is that if you're trying to keep
the aircraft's flight characteristics, what you need to consider is
orientation of the thrust line with respect to the CoM. For the
purposes of argument, I've been using a thrust line through the centre
of mass to illustrate my point, but at no time have I argued that it is
the only place you can have the thrust line and have a stable aircraft.

But by using the zero point, I can illustrate it well. If you have an
airframe with an engine installation where the thrust line goes through
the centre of mass, then you're noting going to have a pitching moment
generated by thrust, period. So if you install a new engine and have to
adjust it's mounting point such that it maintains the CoM in the same
location, but moves the thrust line up or down, all of sudden you *will*
have a pitching moment generated by changes in thrust.

That is a change in the aircraft's flying characteristics, period.

To remove that change, simply reangle the engine to once again have the
thrust line pass through the CoM. Then once again, you will have no
thrust induced pitch changes.

Do the same reasoning for an aircraft with a thrust line above the CoM,
where a new engine lowers it to coincide with the CoM. You'll once again
change the flying characteristics from one where increased thrust causes
a pitch up, to one where thrust does not. Reangle the engine and you'll
restore the original flying characteristics.

Period.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[[email protected]]

OK - got some more info.

The center of mass is something like 34 inches behind the firewall
and roughly 7 inches above the top engine mount point on the firewall.
so roughly speeking 13 inches above the prop centerline.
The prop flange with the O200 is 29.75 inches from the firewall.
This means it is 63.75 inches from the prop flange to the CM.(center
of mass)

This means there is NO WAY the thrust line is aligned anywhere close
to the center of mass.
This would require a downward displacement of almost 15 degrees.

THAT is not going to fly - PERIOD. We are hitting about 5.5 inches
BELOW the center of mass

If we aim for the middle of the rear stabilizer, about 183 inches from
the prop flange, 1.5 degrees down is 5.5 inches above the prop center,
which is about the middle of the rear of the fuselage and roughly 10
inches below the center of the rear horizontal stabilizer .

If I want to hit the same spot with the engine down 1.5 inches, i need
to change the angle to 1.875 degrees.
2 inches goes to 2 degrees.
2.5 inches would be 2.15 degrees, +/-
3 inches would be 2.31 degrees
4 inches would be 2.58 degrees.

Does this make any sense??
It sounds right to me.

[cavelamb himself[_4_]]

wrote:
> OK - got some more info.
>
> The center of mass is something like 34 inches behind the firewall
> and roughly 7 inches above the top engine mount point on the firewall.
> so roughly speeking 13 inches above the prop centerline.
> The prop flange with the O200 is 29.75 inches from the firewall.
> This means it is 63.75 inches from the prop flange to the CM.(center
> of mass)
>
> This means there is NO WAY the thrust line is aligned anywhere close
> to the center of mass.
> This would require a downward displacement of almost 15 degrees.
>
> THAT is not going to fly - PERIOD. We are hitting about 5.5 inches
> BELOW the center of mass
>
> If we aim for the middle of the rear stabilizer, about 183 inches from
> the prop flange, 1.5 degrees down is 5.5 inches above the prop center,
> which is about the middle of the rear of the fuselage and roughly 10
> inches below the center of the rear horizontal stabilizer .
>
> If I want to hit the same spot with the engine down 1.5 inches, i need
> to change the angle to 1.875 degrees.
> 2 inches goes to 2 degrees.
> 2.5 inches would be 2.15 degrees, +/-
> 3 inches would be 2.31 degrees
> 4 inches would be 2.58 degrees.
>
> Does this make any sense??
> It sounds right to me.
>


I'd fly it.

--

Richard

(remove the X to email)

[[email protected]]

On Sat, 15 Nov 2008 20:32:00 -0800 (PST), guynoir >
wrote:

>On Nov 13, 7:20Â*pm, wrote:
>> How does a person determine what the proper height of an engine should
>> be when building an airplane? If a particular engine design mandates
>> the prop is 4 inches, say, lower than where it would be with the
>> engine originally installed, what effect will it have on handling, and
>> what changes in downthrust might be advised?
>>
>> We are building a Pegazair, and my Corvair engine would need to have
>> the cowl higher than ideal to keep the crank centerline at the same
>> hight as say, an O200. Weight wize, the engines are just about
>> identical as equipped Have not determined the center of gravity of the
>> engine yet, to determine the overall length of the mount.
>>
>> For those unfamiliar with the plane it is a highwing STOL 2 placer
>> roughly the same size as a Cessna 150 Â*(150 sq ft wing,33 ft wingspan,
>> )
>
>I've posted a spreadsheet to calculate a new thrust angle based on
>changing the waterline location of an engine. The data needed is
>horizontal distance from center of propeller to CG, original vertical
>distance from center of propeller to CG, original thrust angle, and
>new vertical distance from propeller center to CG. The formula is not
>sensitive to vertical CG location, an estimate will do. What matters
>is the change in the engine location.
>
>http://www.spiretech.com/~guynoir/sldwrks/downthrust.xls


Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
inches.
One inch change in prop height according to your spreadsheat makes a
change of 2.14 degrees.
I cannot buy that. Particularly since it would go from 1.5 down to .64
up.

[Dudley Henriques[_2_]]

On Sat, 15 Nov 2008 23:11:33 -0600, cavelamb himself wrote:

> wrote:
>> OK - got some more info.
>>
>> The center of mass is something like 34 inches behind the firewall
>> and roughly 7 inches above the top engine mount point on the firewall.
>> so roughly speeking 13 inches above the prop centerline.
>> The prop flange with the O200 is 29.75 inches from the firewall.
>> This means it is 63.75 inches from the prop flange to the CM.(center
>> of mass)
>>
>> This means there is NO WAY the thrust line is aligned anywhere close
>> to the center of mass.
>> This would require a downward displacement of almost 15 degrees.
>>
>> THAT is not going to fly - PERIOD. We are hitting about 5.5 inches
>> BELOW the center of mass
>>
>> If we aim for the middle of the rear stabilizer, about 183 inches from
>> the prop flange, 1.5 degrees down is 5.5 inches above the prop center,
>> which is about the middle of the rear of the fuselage and roughly 10
>> inches below the center of the rear horizontal stabilizer .
>>
>> If I want to hit the same spot with the engine down 1.5 inches, i need
>> to change the angle to 1.875 degrees.
>> 2 inches goes to 2 degrees.
>> 2.5 inches would be 2.15 degrees, +/-
>> 3 inches would be 2.31 degrees
>> 4 inches would be 2.58 degrees.
>>
>> Does this make any sense??
>> It sounds right to me.
>>
>
> I'd fly it.

You would. But you have NEVER been known for your intelligence. :)))))

[cavelamb himself[_4_]]

Dudley Henriques wrote:
> On Sat, 15 Nov 2008 23:11:33 -0600, cavelamb himself wrote:
>
>
wrote:
>>
>>>OK - got some more info.
>>>
>>>The center of mass is something like 34 inches behind the firewall
>>>and roughly 7 inches above the top engine mount point on the firewall.
>>>so roughly speeking 13 inches above the prop centerline.
>>>The prop flange with the O200 is 29.75 inches from the firewall.
>>>This means it is 63.75 inches from the prop flange to the CM.(center
>>>of mass)
>>>
>>>This means there is NO WAY the thrust line is aligned anywhere close
>>>to the center of mass.
>>>This would require a downward displacement of almost 15 degrees.
>>>
>>>THAT is not going to fly - PERIOD. We are hitting about 5.5 inches
>>>BELOW the center of mass
>>>
>>>If we aim for the middle of the rear stabilizer, about 183 inches from
>>>the prop flange, 1.5 degrees down is 5.5 inches above the prop center,
>>>which is about the middle of the rear of the fuselage and roughly 10
>>>inches below the center of the rear horizontal stabilizer .
>>>
>>>If I want to hit the same spot with the engine down 1.5 inches, i need
>>>to change the angle to 1.875 degrees.
>>>2 inches goes to 2 degrees.
>>>2.5 inches would be 2.15 degrees, +/-
>>>3 inches would be 2.31 degrees
>>>4 inches would be 2.58 degrees.
>>>
>>>Does this make any sense??
>>> It sounds right to me.
>>>
>>
>>I'd fly it.
>
>
> You would. But you have NEVER been known for your intelligence. :)))))

:)




--

Richard

(remove the X to email)

[cavelamb himself[_4_]]

There is not one right answer here.

What we are looking for is predictible and managable responses.

That's why the Cooper-Harper index exists and is written the way it is.


http://en.wikipedia.org/wiki/Cooper-Harper_rating_scale

[John Kimmel]

wrote:

> Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
> inches.
> One inch change in prop height according to your spreadsheat makes a
> change of 2.14 degrees.
> I cannot buy that. Particularly since it would go from 1.5 down to .64
> up.
Here is REV A, with a couple math errors fixed:
http://www.spiretech.com/~guynoir/sldwrks/downthrustreva.xls

--
John Kimmel


I think it will be quiet around here now. So long.

[[email protected]]

On Sun, 16 Nov 2008 00:58:06 -0800, John Kimmel
> wrote:

wrote:
>
>> Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
>> inches.
>> One inch change in prop height according to your spreadsheat makes a
>> change of 2.14 degrees.
>> I cannot buy that. Particularly since it would go from 1.5 down to .64
>> up.
>Here is REV A, with a couple math errors fixed:
>http://www.spiretech.com/~guynoir/sldwrks/downthrustreva.xls


That looks a lot closer.

What is it based on?

[Jim Logajan]

Alan Baker > wrote:
> Jim Logajan > wrote:
>> Running the thrust line through M does _not_ guarantee you wont get
>> any couple.
>
> It guarantees you won't get a couple from the thrust.

I think I see one of your problems. How many forces are needed for a
couple? Can one of those forces pass through the center of mass?

[Alan Baker]

In article >,
Jim Logajan > wrote:

> Alan Baker > wrote:
> > Jim Logajan > wrote:
> >> Running the thrust line through M does _not_ guarantee you wont get
> >> any couple.
> >
> > It guarantees you won't get a couple from the thrust.
>
> I think I see one of your problems. How many forces are needed for a
> couple? Can one of those forces pass through the center of mass?

One. There only has to be a one force on a body in order for it to
experience to angular acceleration. That force just has to act on a line
that is not through the centre of mass.

Questions for you:

Do you believe that in the absence of any other forces, if the
aerodynamic forces act on a line that is not through the centre of mass,
will the aircraft experience angular acceleration?

If you add thrust acting through the centre of mass to the situation,
will it change the angular acceleration?

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Alan Baker]

In article
>,
Alan Baker > wrote:

> In article >,
> Jim Logajan > wrote:
>
> > Alan Baker > wrote:
> > > Jim Logajan > wrote:
> > >> Running the thrust line through M does _not_ guarantee you wont get
> > >> any couple.
> > >
> > > It guarantees you won't get a couple from the thrust.
> >
> > I think I see one of your problems. How many forces are needed for a
> > couple? Can one of those forces pass through the center of mass?
>
> One. There only has to be a one force on a body in order for it to
> experience to angular acceleration. That force just has to act on a line
> that is not through the centre of mass.

I apologize, I was using the term "couple" incorrectly.

But that doesn't matter to my argument. So ignore the first bit and
answer my questions below:

>
> Questions for you:
>
> Do you believe that in the absence of any other forces, if the
> aerodynamic forces act on a line that is not through the centre of mass,
> will the aircraft experience angular acceleration?
>
> If you add thrust acting through the centre of mass to the situation,
> will it change the angular acceleration?

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[Jim Logajan]

Alan Baker > wrote:
> Alan Baker > wrote:
>> Jim Logajan > wrote:
>> > Alan Baker > wrote:
>> > > Jim Logajan > wrote:
>> > >> Running the thrust line through M does _not_ guarantee you wont
>> > >> get any couple.
>> > >
>> > > It guarantees you won't get a couple from the thrust.
>> >
>> > I think I see one of your problems. How many forces are needed for
>> > a couple? Can one of those forces pass through the center of mass?
>>
>> One. There only has to be a one force on a body in order for it to
>> experience to angular acceleration. That force just has to act on a
>> line that is not through the centre of mass.
>
> I apologize, I was using the term "couple" incorrectly.

Not surprising, given that most freshman level college physics textbooks
don't even mention the concept (a pity). Most introductory mechanics
(statics and dynamics) textbooks, on the other hand, introduce the couple
concept in the first or second chapters.

> But that doesn't matter to my argument.

It does matter in the sense that it is an important (perhaps essential)
concept needed to properly set up and solve these sorts of problems.
Another concept needed is the fictitious inertia force (m*A) that is
anchored at the center of mass and always points opposite the net
external force. That means there is always at least a second force around
to create a couple.

> So ignore the first bit and answer my questions below:
>
>>
>> Questions for you:
>>
>> Do you believe that in the absence of any other forces, if the
>> aerodynamic forces act on a line that is not through the centre of
>> mass, will the aircraft experience angular acceleration?

It will undergo both angular and translational acceleration. The motion
of the system over time is generally not obvious and holds a few
surprises. For example:

There is a related problem that often goes by the label "center of
percussion" (try Googling it) that shows that at the instantaneous point
in time that the system is placed at rest with respect a reference frame,
the instantaneous center of rotation in that reference frame is not at
the center of mass.

>> If you add thrust acting through the centre of mass to the situation,
>> will it change the angular acceleration?

When moving in a vacuum? No. When moving in a fluid? Yes it could. It is
not hard to imagine scenarios where that can happen.

[Alan Baker]

In article >,
Jim Logajan > wrote:

> Alan Baker > wrote:
> > Alan Baker > wrote:
> >> Jim Logajan > wrote:
> >> > Alan Baker > wrote:
> >> > > Jim Logajan > wrote:
> >> > >> Running the thrust line through M does _not_ guarantee you wont
> >> > >> get any couple.
> >> > >
> >> > > It guarantees you won't get a couple from the thrust.
> >> >
> >> > I think I see one of your problems. How many forces are needed for
> >> > a couple? Can one of those forces pass through the center of mass?
> >>
> >> One. There only has to be a one force on a body in order for it to
> >> experience to angular acceleration. That force just has to act on a
> >> line that is not through the centre of mass.
> >
> > I apologize, I was using the term "couple" incorrectly.
>
> Not surprising, given that most freshman level college physics textbooks
> don't even mention the concept (a pity). Most introductory mechanics
> (statics and dynamics) textbooks, on the other hand, introduce the couple
> concept in the first or second chapters.
>
> > But that doesn't matter to my argument.
>
> It does matter in the sense that it is an important (perhaps essential)
> concept needed to properly set up and solve these sorts of problems.
> Another concept needed is the fictitious inertia force (m*A) that is
> anchored at the center of mass and always points opposite the net
> external force. That means there is always at least a second force around
> to create a couple.

No. It is a *useful* concept, but it is in no way essential. Any problem
you can solve by taking couples and pure moments, you can solve without
reference to couples.

And the fictitious force you suggest would, if used, make the concept
conflict with its own definition. The very point of a couple is that it
can be represented by a pure vector moment, which by definition, cannot
accelerate a body linearly.

So if you say there is a couple when you talk about a system with a
single external force that isn't aligned with the centre of mass of the
body it is acting on, then your solution would suggest that the mass
doesn't undergo linear acceleration.

>
> > So ignore the first bit and answer my questions below:
> >
> >>
> >> Questions for you:
> >>
> >> Do you believe that in the absence of any other forces, if the
> >> aerodynamic forces act on a line that is not through the centre of
> >> mass, will the aircraft experience angular acceleration?
>
> It will undergo both angular and translational acceleration. The motion
> of the system over time is generally not obvious and holds a few
> surprises. For example:

A yes or no would have sufficed.

>
> There is a related problem that often goes by the label "center of
> percussion" (try Googling it) that shows that at the instantaneous point
> in time that the system is placed at rest with respect a reference frame,
> the instantaneous center of rotation in that reference frame is not at
> the center of mass.
>
> >> If you add thrust acting through the centre of mass to the situation,
> >> will it change the angular acceleration?
>
> When moving in a vacuum? No. When moving in a fluid? Yes it could. It is
> not hard to imagine scenarios where that can happen.

No, it couldn't. The other effects that result in the thrust *might*
cause a change in the angular acceleration (increased/changed airflow
over the aerodynamic surfaces), that that isn't the thrust doing it.

--
Alan Baker
Vancouver, British Columbia
<http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg>

[John Kimmel]

wrote:
> On Sun, 16 Nov 2008 00:58:06 -0800, John Kimmel
> > wrote:
>
>
wrote:
>>
>>
>>>Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
>>>inches.
>>>One inch change in prop height according to your spreadsheat makes a
>>>change of 2.14 degrees.
>>>I cannot buy that. Particularly since it would go from 1.5 down to .64
>>>up.
>>
>>Here is REV A, with a couple math errors fixed:
>>http://www.spiretech.com/~guynoir/sldwrks/downthrustreva.xls
>
>
>
> That looks a lot closer.
>
> What is it based on?

"Vector Mechanics For Engineers: Statics and Dynamics", by Beer and Johnston, 5th edition, Chapter 4: "Equilibrium of Rigid Bodies",
page 126, equation 4.1. Equation 4.1 can be easily found on the internet, just type "statics" into Google and go to the first hit,
which should be Wikipedia.

All my spreadsheet does is keep the distance from the thrust line to the CG the same with different engine locations, so that the sum
of moments about the CG remains the same (as in: Zero. See equation 4.1).


--
John Kimmel


I think it will be quiet around here now. So long.

[cavelamb himself[_4_]]

John Kimmel wrote:

> wrote:
>
>> On Sun, 16 Nov 2008 00:58:06 -0800, John Kimmel
>> > wrote:
>>
>>
>>> wrote:
>>>
>>>
>>>> Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
>>>> inches.
>>>> One inch change in prop height according to your spreadsheat makes a
>>>> change of 2.14 degrees.
>>>> I cannot buy that. Particularly since it would go from 1.5 down to .64
>>>> up.
>>>
>>>
>>> Here is REV A, with a couple math errors fixed:
>>> http://www.spiretech.com/~guynoir/sldwrks/downthrustreva.xls
>>
>>
>>
>>
>> That looks a lot closer.
>>
>> What is it based on?
>
>
> "Vector Mechanics For Engineers: Statics and Dynamics", by Beer and
> Johnston, 5th edition, Chapter 4: "Equilibrium of Rigid Bodies", page
> 126, equation 4.1. Equation 4.1 can be easily found on the internet,
> just type "statics" into Google and go to the first hit, which should be
> Wikipedia.
>
> All my spreadsheet does is keep the distance from the thrust line to the
> CG the same with different engine locations, so that the sum
> of moments about the CG remains the same (as in: Zero. See equation 4.1).
>
>

I tried the first sheet.
Set the offset to 4 inches and got 8 something degrees angle!

That may have been a right answer for the problem you've set up.

But no way it would work on the airplane.



--

Richard

(remove the X to email)

[John Kimmel]

cavelamb himself wrote:

>
> I tried the first sheet.
> Set the offset to 4 inches and got 8 something degrees angle!
>
> That may have been a right answer for the problem you've set up.
>
> But no way it would work on the airplane.
>
>
>
I checked the math on my rev a spread sheet more carefully than on the original, and included a check feature within the spreadsheet
itself. I also checked the spreadsheet graphically using a cad program. The spread sheet as currently posted will work for all
aircraft, but if you don't have a good understanding of the vector mechanics underlying it in the references I included in my previous
post, you should not use it.

If you think there is an error in my spread sheet, you are welcome to identify it, correct it, and re-post it, or you can show me the
problem graphically with a free body diagram and I'll make the correction, but your post above doesn't give me enough information.

--
John Kimmel


I think it will be quiet around here now. So long.

[[email protected]]

On Mon, 17 Nov 2008 20:41:30 -0800, John Kimmel
> wrote:

wrote:
>> On Sun, 16 Nov 2008 00:58:06 -0800, John Kimmel
>> > wrote:
>>
>>
wrote:
>>>
>>>
>>>>Spreadsheet is not quite right.Prop center is BELOW the CG by about 13
>>>>inches.
>>>>One inch change in prop height according to your spreadsheat makes a
>>>>change of 2.14 degrees.
>>>>I cannot buy that. Particularly since it would go from 1.5 down to .64
>>>>up.
>>>
>>>Here is REV A, with a couple math errors fixed:
>>>http://www.spiretech.com/~guynoir/sldwrks/downthrustreva.xls
>>
>>
>>
>> That looks a lot closer.
>>
>> What is it based on?
>
>"Vector Mechanics For Engineers: Statics and Dynamics", by Beer and Johnston, 5th edition, Chapter 4: "Equilibrium of Rigid Bodies",
>page 126, equation 4.1. Equation 4.1 can be easily found on the internet, just type "statics" into Google and go to the first hit,
>which should be Wikipedia.
>
>All my spreadsheet does is keep the distance from the thrust line to the CG the same with different engine locations, so that the sum
>of moments about the CG remains the same (as in: Zero. See equation 4.1).


Only problem I see is you still have it set up with the thrust above
the CG. The center line of the prop is significantly lower than the
CG. Do I just use negative numbers? Or should the spreadsheet still be
corrected?

[Jim Logajan]

wrote:
> OK - got some more info.
>
> The center of mass is something like 34 inches behind the firewall
> and roughly 7 inches above the top engine mount point on the firewall.
> so roughly speeking 13 inches above the prop centerline.

What is that the center of mass of, exactly? The empty airframe only (i.e.
no engine)? Or is it the center of mass of a complete airplane (sans fuel,
pilot, passenger, and baggage) with the engine mounted at the original
point?

I'm also curious as to how you determined the location in two dimensions. I
can think of a few ways of determining it, but they all involve a bunch of
effort.

[[email protected]]

On Tue, 18 Nov 2008 12:05:40 -0600, Jim Logajan >
wrote:

wrote:
>> OK - got some more info.
>>
>> The center of mass is something like 34 inches behind the firewall
>> and roughly 7 inches above the top engine mount point on the firewall.
>> so roughly speeking 13 inches above the prop centerline.
>
>What is that the center of mass of, exactly? The empty airframe only (i.e.
>no engine)? Or is it the center of mass of a complete airplane (sans fuel,
>pilot, passenger, and baggage) with the engine mounted at the original
>point?
>
>I'm also curious as to how you determined the location in two dimensions. I
>can think of a few ways of determining it, but they all involve a bunch of
>effort.


Center of mass for the completed plane when built to specs. Provided
by the designer.

[John Kimmel]

wrote:

>
> Only problem I see is you still have it set up with the thrust above
> the CG. The center line of the prop is significantly lower than the
> CG. Do I just use negative numbers? Or should the spreadsheet still be
> corrected?

If you downloaded the corrected version, the numbers you see when you open the spreadsheet are:
60 -13 -1.5 -14 -2.408899631

CG is 60" aft of propeller center. Propeller center is 13" below CG. Thrust angle is 1.5 degrees down.
If you move propeller center down 1" to 14" below CG, the new thrust angle will be 2.4 down.
--
John Kimmel


I think it will be quiet around here now. So long.

[[email protected]]

On Tue, 18 Nov 2008 21:20:43 -0800, John Kimmel
> wrote:

wrote:
>
>>
>> Only problem I see is you still have it set up with the thrust above
>> the CG. The center line of the prop is significantly lower than the
>> CG. Do I just use negative numbers? Or should the spreadsheet still be
>> corrected?
>
>If you downloaded the corrected version, the numbers you see when you open the spreadsheet are:
>60 -13 -1.5 -14 -2.408899631
>
>CG is 60" aft of propeller center. Propeller center is 13" below CG. Thrust angle is 1.5 degrees down.
>If you move propeller center down 1" to 14" below CG, the new thrust angle will be 2.4 down.


That's what I suspected - and it seams to work. The resulting
reduction in the angle is counterintuitive however.

[Jim Logajan]

wrote:
> Jim Logajan > wrote:
wrote:
>>> OK - got some more info.
>>>
>>> The center of mass is something like 34 inches behind the firewall
>>> and roughly 7 inches above the top engine mount point on the
>>> firewall. so roughly speeking 13 inches above the prop centerline.
>>
>>What is that the center of mass of, exactly? The empty airframe only
>>(i.e. no engine)? Or is it the center of mass of a complete airplane
>>(sans fuel, pilot, passenger, and baggage) with the engine mounted at
>>the original point?
>>
>>I'm also curious as to how you determined the location in two
>>dimensions. I can think of a few ways of determining it, but they all
>>involve a bunch of effort.
>
> Center of mass for the completed plane when built to specs. Provided
> by the designer.

I don't suppose the designer was or is willing to provide general advice
or guidance on mounting your engine? Sure beats asking total strangers on
the net. :-)

If I were in your position these are a few of the things I'd keep in
mind:

* The center of mass (CoM) of an empty airplane and full airplane are in
different locations in three dimensions. In the two dimensions of
interest, depending on how fuel, passengers, and baggage are loaded, it
moves around in those two dimensions such that it is geometrically
impossible to move engine "down" and find a single angle that yields the
original moment arms for all load configurations. The one configuration I
don't think I would use to compute engine mounting angle would be the
empty configuration.

* Moving the engine down relative to the original design specs also moves
the empty airplane CoM down. Assuming the Corvair and O-200 are both the
same weight of about 200 lbs, and assuming the empty weight of the plane
is 815 lbs, then if the engine is moved D inches down, the CoM moves down
D*200/815 inches. If D = 4, CoM moves down about 1 inch.

* I don't know where the CoM of passengers and baggage would be, but they
probably move CoM down (and presumably fuel moves it up). As an example,
if all that usable load is about 600 lbs, then if that load's CoM is L
inches below the empty airplane's CoM, the CoM of the whole airplane
moves down L*600/(815+600). So if, for example, L = 5, then CoM moves
down about 2 inches. So between lowering the engine and throwing in
stuff, the CoM of the airplane in this configuration differs by about 3
inches from that used in the original empty airplane spec. But the load
probably moved the CoM forward or backward too, affecting another
variable in the equations. Fun, huh?

* How much is the horizontal tail in the propwash? That and a different
angle of incidence of the propwash on the fuselage might give me more
down pitch than I calculated by assuming the only factor affected by the
engine move and rotation was the thrust moment arm. I might also risk
losing thrust efficiency because of increasing fuselage interference in
the propwash.

* At some point I'd realize that the CoM is variable enough that for the
small amount I'd be moving the engine, it would probably be best to mount
the engine so that the engine thrust line is aligned with the most
natural fuselage longitudinal. Then make trim adjustments during the test
flights.

[[email protected]]

On Wed, 19 Nov 2008 13:15:02 -0600, Jim Logajan >
wrote:

wrote:
>> Jim Logajan > wrote:
wrote:
>>>> OK - got some more info.
>>>>
>>>> The center of mass is something like 34 inches behind the firewall
>>>> and roughly 7 inches above the top engine mount point on the
>>>> firewall. so roughly speeking 13 inches above the prop centerline.
>>>
>>>What is that the center of mass of, exactly? The empty airframe only
>>>(i.e. no engine)? Or is it the center of mass of a complete airplane
>>>(sans fuel, pilot, passenger, and baggage) with the engine mounted at
>>>the original point?
>>>
>>>I'm also curious as to how you determined the location in two
>>>dimensions. I can think of a few ways of determining it, but they all
>>>involve a bunch of effort.
>>
>> Center of mass for the completed plane when built to specs. Provided
>> by the designer.
>
>I don't suppose the designer was or is willing to provide general advice
>or guidance on mounting your engine? Sure beats asking total strangers on
>the net. :-)
>
>If I were in your position these are a few of the things I'd keep in
>mind:
>
>* The center of mass (CoM) of an empty airplane and full airplane are in
>different locations in three dimensions. In the two dimensions of
>interest, depending on how fuel, passengers, and baggage are loaded, it
>moves around in those two dimensions such that it is geometrically
>impossible to move engine "down" and find a single angle that yields the
>original moment arms for all load configurations. The one configuration I
>don't think I would use to compute engine mounting angle would be the
>empty configuration.
>
>* Moving the engine down relative to the original design specs also moves
>the empty airplane CoM down. Assuming the Corvair and O-200 are both the
>same weight of about 200 lbs, and assuming the empty weight of the plane
>is 815 lbs, then if the engine is moved D inches down, the CoM moves down
>D*200/815 inches. If D = 4, CoM moves down about 1 inch.
>
>* I don't know where the CoM of passengers and baggage would be, but they
>probably move CoM down (and presumably fuel moves it up). As an example,
>if all that usable load is about 600 lbs, then if that load's CoM is L
>inches below the empty airplane's CoM, the CoM of the whole airplane
>moves down L*600/(815+600). So if, for example, L = 5, then CoM moves
>down about 2 inches. So between lowering the engine and throwing in
>stuff, the CoM of the airplane in this configuration differs by about 3
>inches from that used in the original empty airplane spec. But the load
>probably moved the CoM forward or backward too, affecting another
>variable in the equations. Fun, huh?
>
>* How much is the horizontal tail in the propwash? That and a different
>angle of incidence of the propwash on the fuselage might give me more
>down pitch than I calculated by assuming the only factor affected by the
>engine move and rotation was the thrust moment arm. I might also risk
>losing thrust efficiency because of increasing fuselage interference in
>the propwash.
>
>* At some point I'd realize that the CoM is variable enough that for the
>small amount I'd be moving the engine, it would probably be best to mount
>the engine so that the engine thrust line is aligned with the most
>natural fuselage longitudinal. Then make trim adjustments during the test
>flights.


Thanks Jim.
I just talted to a P.Eng who has a LOT of aeronautic experience and he
said I likely would not notice any change, but he would cut the
recommended 1.5 degrees roughly in half for my install... I don't
think there will be any detrimental effect as far as propwash is
concerned - might even improve very marginally.

[[email protected]]

On Sat, 15 Nov 2008 11:46:10 -0500, wrote:

>On Fri, 14 Nov 2008 22:53:23 -0600, cavelamb himself
> wrote:
>
wrote:
>>
>>>>The Corvair would use a bearer style mount, wouldn't it?
>>>
>>>
>>> Not on this plane. I'll get pics of the mount design on line soon.
>>> I've put mounting tabs on the top and bottom rear so I'm mounting it
>>> like a Conti O200, but using 1" diameter Licoming type homebuilder
>>> mounts.The typical bed mount would interfere with my 180 degree header
>>> system.
>>>
>>
>>
>>How will the mount attach to the engine case?
>>I don't recall how the aft end of the engine is arranged.
>
>
>I'll get pictures, but I used a chunk of auminum channel, cut away to
>make a "U" shaped bracket that bolts to the top surface of the engine
>case, with "ears" to which mounting blocks are fastened, immitating
>the top ears of an O200 case. The bottom has an angle boted down each
>side, like the typical bed mount but without rubber isolation, with
>mount blocks fastened to them as well,.
>
>Very similar to the way it is mounted on my engine test stand,
>pictured on my website.


OK guys, the pictures of the mount are up on my website at:
http://www.pegazair.on-the-net.ca/ClareSquared/Mount.htm

Drop by and take a gander. You can get to the rest of the project from
there as well.

[Stealth Pilot[_2_]]

On Sun, 23 Nov 2008 17:51:18 -0500, wrote:


>OK guys, the pictures of the mount are up on my website at:
>http://www.pegazair.on-the-net.ca/ClareSquared/Mount.htm
>
>Drop by and take a gander. You can get to the rest of the project from
>there as well.

that is either a bloody big aeroplane or the wife is the cutsiest
little thing.
her washing line is half the height of the rudder!!


......yeah I know. it was taken from the second floor....

nice workmanship. thank god you're not a simmer!
Stealth Pilot