what the heck is lift?

buttman wrote:


My instructor, which is a very knowledgable guy tried telling me that
lift has nothing to do with airspeed. He said that lift is directly and
soley related to AOA and AOA only.

Uh-huh. So if you use a crane to lift up the nose of a 747 sitting on
the ramp, your instructor believes it will be generating the same
amount of lift as it would at that AOA and 400 knots. Detach the crane
and the 747 will just stay there in a nose up attitude without any
visible means of support. Or maybe your instructor has a poor
understanding of lift.

Lift is actually a mathematical formula: L = 1/2 air density * velocity
squared * area of the wing * coefficient of lift for that wing. Your
instructor should know that; it is on both the commercial and flight
instructor written exams.

You generally can't do much about the air density, but you usually can
change your velocity and the coefficient of lift. The coefficient of
lift for most wings increases with AOA, peaking at the critical AOA and
dropping off sharply at higher AOA after that. Some flaps and other
devices (variable geometry wngs come to mind) can change the area of
the wing and/or its coefficient of lift. Also, "the area of the wing"
is not quite right; it really is a reference area which might have
little to do with the actual wing size. A helicopter, for example, uses
a reference area equal to the entire disk, not just the blades. The
same rule applies to propellers. The reference area on a fixed wing
plane includes the area through the fuselage, as if the wing was all
one piece.

You can use either sq. feet or sq. meters (or, heck, sq. rods if you
want to) for the reference area; it all works out as long as you use
the same type of units all through the calculation, including air
density.