tommyann wrote:

what problems are you creating buy not aligning engine properly
and what is needed to be done to correct whatever problem you are creating


Ever tried to push a large piece of furniture from the side rather than
the middle. Remember how it gets all squirrly and doesn't really want
to go where you want it to. That's the sort of problem your looking at,
except that you can't just stop and go to the other side.

Airplanes have all sorts of centers. Everyone's well versed in CG,
cause that's the one we get taught about in flight training. We don't
hear much about the rest, because as pilots we can't change them much.
Story's different when we put on the builder's hat.

Not only do we have a CG, but there is a center of lift. Those
published numbers from the kit manufacturer or plans publisher don't
mean much if you change the position of the wing, thus moving the center
of lift. The CG must be just forward of CL or you risk not being able
to pull the nose up on takeoff (if it is to far forward), or
inadvertently entering a deep stall (if it is even a little behind).

Then there is the center of drag, CD. This one is fun because it is in
two dimensions. Make your wings assymetrical and the plane will pull to
one side or the other. This is because the drag is offset to one side.
This is the laterall element. Then there's the vertical element.
Suppose you put those big draggy wings on top, like on a Cessna. Short
final, deer on the runway, you firewall the throttle. The nose wants to
pull up. On a Cherokee, you have to be careful, or you might find
yourself nosing over into the runway. The engine is providing the
center of thrust, CT. You can create all sorts of havoc, or very docile
behaviour, depending on how you arrange CD and CT.

That propeller spinning moves the air around the plane in a circle, till
it hits the tail fin and tries to turn you to the side. Especially
notable on takeoff where you need to add a lot of rudder to keep going
straight. One solution is to angle the tail fin to the side a degree or
so. Another is to cock the engine to the side a bit. How much angle is
optimized for a certain flight condition by the designer.

So the CG is forward of CL. This is by design. The plane stalls, then
falls. You want it to fall nose first, so that it will stop falling
very soon thereafter. The downside is that the tail has to provide down
force to keep the nose up. Lots of extra drag there. The drag can be
reduced if the engine is pointed up just a bit, though. Drag the
airplane through the air by pulling it's nose up.

On a pusher, all of these things will be reversed, or maybe not. If you
decide to play with these factors, find out what you're doing first.
The designer should have done the math and flight test to optimize the
parameters for reasonable flight regimes. If you start fiddling with
them willy-nilly, the best that you can hope for will be a very draggy
bird that is constantly fighting with itself to get through the air,
just like a car with a bad alignment job. The likely outcome is that
you will have a bird with some strange and possibly unpredictable ways.