effect of changed thrust line.

"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

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

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

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

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 he

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

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.

"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

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

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

"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