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?