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Re: Physics of Flight



OK to the n-th power. Very well explained. Dihedral and roll control while
flying upside down was to be my next question. Thanx.

poj

John Denker wrote:

At 04:28 PM 8/17/99 -0500, paul o johnson wrote:
how can an airplane remain stable while
flying upside down?

Good question.

1) The key stability concept is this: The airplane wants to fly at a
definite angle of attack. Even a dime-store balsa-wood glider is trimmed
for a definite angle of attack. This is discussed in
http://www.monmouth.com/~jsd/how/htm/aoastab.html

Suppose you take said glider, hold it upside down, and throw it northward.
It will immediately do a half-loop downwards and fly away right-side-up,
soutbound.

2) If you don't want that behavior -- if you want it to fly upside down
northward, you need to retrim it. Aerobatic aircraft are built with
super-duper trim controls (lots of authority, fast action).

You can re-trim the balsa glider. Typically the tail slides into a slot in
the body; cut a new slot using a razor saw. You may need to glue on an
extra bit of wood to make a place for the new slot.

You need to change the tail's angle of incidence in the direction that
corresponds to a "push" on the yoke. That is, the leading edge needs to be
higher and the trailing edge needs to be lower (where higher and lower
refer to the airplane in the normal upright orientation).

3) Side issue: In addition to angle-of-attack stability, there are other
things such as roll-axis stability. When you turn an airplane upside down,
some of the contributions to roll-axis stability change sign. Typical
aerobatic aircraft have practically zero roll-axis stability, so they don't
lose anything when they fly inverted. Negating zero doesn't have much
effect. For more on this, refer to
http://www.monmouth.com/~jsd/how/htm/roll.html#sec_dihedral

OTOH alas many dime-store gliders have tons of dihedral, to make up for the
fact that there's no pilot on board to keep an eye on the roll axis. So
you may find that you've got roll-axis stability problems. Shop for a
model that's got zero or minimal dihedral to minimize this problem.

OK? --- jsd