Aerodynamics Information

Lift Theory

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 Lift theory in the FAA text book (AC 61-23C) (as known as "Longer Path" or "Equal Transit" theory) is obsolete.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 Joukowski 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.



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Last modified: Wed Jul 26 12:18:57 PDT 2006