effect of changed thrust line.

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

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

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?

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

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.

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

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?

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

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

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.