Lift is a
force generated by turning a moving fluid.
Lift theory in the FAA text book (AC 61-23C) (as known as
"Longer Path" or "Equal Transit" theory) is
obsolete. The airflow over the top of a lifting airfoil
travels faster than the flow beneath the airfoil. However, the
actual velocity over the top of an airfoil is much faster than
that predicted by the "Longer Path" theory and
particles moving over the top arrive at the trailing edge
before particles moving under the airfoil. Two molecules
near each other at the leading edge will not end up next to
each other at the trailing edge. This part of the
"Longer Path" theory attempts to provide us with a
value for the velocity over the top of the airfoil based on a
non-physical assumption (the molecules meet at the aft end). We
can calculate a velocity based on this assumption, and use
Bernoulli's equation to compute the pressure, and perform the
pressure-area calculation and the answer we get does not agree
with the lift that we measure for a given airfoil. The lift
predicted by the "Longer Path" theory is much less
than the observed lift, because the velocity is too low.
According to the modern lift theory, a wing is mathematically
equivalent to a rotating
cylinder. The shape of an airfoil is obtained by applying
Transform to a rotating cylinder. This page
also has detailed descriptions on the modern lift theory.
NOTE: I do not recommend that you explain the modern lift
theory at your oral examination since you are not expected to
know this much detail. It is better to follow the FAA book to
pass the examination.
Last modified: Wed Jul 26 12:18:57 PDT 2006