The Longer Path Explanation

What is it?
The Longer Path explanation holds that the top surface of a wing is more curved than the bottom surface. Air particles that approach the leading edge of the wing must travel either over or under the wing. Let's assume that two nearby particles split up at the leading edge, and then come back together at the trailing edge of the wing. Since the particle traveling over the top goes a longer distance in the same amount of time, it must be traveling faster.

Bernoulli's equation, a fundamental of fluid dynamics, states that as the speed of a fluid flow increases, its pressure decreases. The Longer Path explanation deduces that this faster moving air develops a lower pressure on the top surface, while the slower moving air maintains a higher pressure on the bottom surface. This pressure difference essentially "sucks" the wing upward (or pushes the wing upward, depending on your point of view).

Why is it not entirely correct?
There are several flaws in this theory, although this is a very common explanation found in high school textbooks and even encyclopedias:

1. The assumption that the two air particles described above rejoin each other at the trailing edge of the wing is groundless. In fact, these two air particles have no "knowledge" of each other's presence at all, and there is no logical reason why these particles should end up at the rear of the wing at the same moment in time.

2. For many types of wings, the top surface is longer than the bottom. However, many wings are symmetric (shaped identically on the top and bottom surfaces). This explanation also predicts that planes should not be able to fly upside down, although we know that many planes have this ability.

Why is it not entirely wrong?
The Longer Path explanation is correct in more than one way. First, the air on the top surface of the wing actually does move faster than the air on the bottom -- in fact, it is moving faster than the speed required for the top and bottom air particles to reunite, as many people suggest. Second, the overall pressure on the top of a lift-producing wing is lower than that on the bottom of the wing, and it is this net pressure difference that creates the lifting force.