effect of changed thrust line.

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.

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.

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

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

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