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)