effect of changed thrust line.

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?

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.

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

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

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

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)

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

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???



--

Richard

(remove the X to email)

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)

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