Does this mean that airplanes cannot fly upside down, or does the shape of
the wing change when the airplane rolls over?

Colin

Good question. In most aircraft, the airfoil shape does not change.
However, assuming that an airplane maintains level flight at +5 degrees
AOA relative to the horizon, and then rolls over upside down, what is
its new AOA? That would neg. 5 degrees. Gravity aside, the plane would
fly in a downward direction because the air flowing over the "upper"
surface of the airfoil would still be traveling at a higher rate
relative to the "lower" surface.

Of course, if a pilot rolls her plane over and then pushes forward on
the stick, that would produce a positive AOA relative to the horizon.
How much would be enough to increase the flow over the "lower" surface
of the wing (now sunny-side up) than on the "upper" surface? I dunno.
Push the stick forward till we quit flying down toward the dirt.

Some planes, and some helicopters, have semetrical airfoil wings. They
achieve a differential airspeed/pressure by flying at a positive AOA.

So why is AOA so important? If the leading edge is up, and the trailing
edge is down, doesn't that mean that the wing is still forcing air down
as it travels through the air? Maybe somewhat, but that's not where the
magic is. Its all about the relative difference in airspeed and
therefore air pressue. Since AOA directly effects airflow over the
wing, is it not reasonable to think that it alone could produce enough
of a difference in speed/pressure to sustain flight?


Old Regallo wings could/would change shape. Google "luff-dive"
sometime. Things get ugly in a hurry when your airfoil reverses its
loveley curved shape and slams you into the ground with over 300 lbs of
force. Been there. Done that. Lived.

By the way, I'm not professing to know much more about aerodynamics
than what was discovered in the Wright wind tunnel. But I'm pretty well
convinced by those results that most airplanes do not stay in the aloft
by forcing air downward.

Now here's a simple 19th Century way to prove the point. Attach a
length of yarn to the "lower" surface of a slow-flying plane. Watch the
yarn and see what direction it takes in flight. Is it straight back? Or
is it down? If air is indeed being forced downward by the wing,
shouldn't we be able to see the results in the yarn?

Harry