effect of changed thrust line.

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.

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...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.

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.

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.

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...mic-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/..._Airbourne.jpg

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

Curtiss R: http://www.aviationhalloffamewiscons..._curtissR6.jpg

The Lincoln Standard: http://cdn-www.airliners.net/aviatio.../7/0817761.jpg

Koohoven FK-41: http://www.henrikaper.nl/koolhoven-f...q-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

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...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/(a...b)/RequestForQ
uote.aspx?SKU=2147007679

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

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.

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/sl...downthrust.xls

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

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.