effect of changed thrust line.

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,
)

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" 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

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

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

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)

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

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)

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

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