Why is it that the air traveling over the top of the airfoil has to reach the end of it at the same time that the air traveling underneath the airfoil reaches it.
The idea that the air traveling over the top of the airfoil must reach the end of the wing at the same time as the air traveling along the bottom of the airfoil is called the 'principle of equal transit times.' This follows from Bernoulli's principle, which says that when fluid velocity increases, pressure decreases. As the air over the top of the airfoil has higher velocity than that of air at the bottom of the airfoil, there is a decrease in pressure at the top of the airfoil, which causes lift. This explanation of lift is actually inaccurate. The reason this explanation has been around for so long is that it is the easiest to understand.
The Bernoulli principle is true, as is the fact that the air at the top of the airfoil goes faster than that at the bottom. However, it is untrue that the seperated air must reach the end of the airfoil at the same time. In reality, the air at the bottom of the airfoil is slowed down.
In order to reach the correct explanation, we need to look at Newton's
laws. The third law says that for every action there is an equal and
opposite reaction. In order for an airfoil to produce lift, it must
obtain an equal and opposite reaction. So, an airfoil changes the
direction of the air: it sends a stream of air downward, in order to
send the wing upward. (See picture.) The lift of an airfoil is
proportional to the amount of air diverted downward times the downward
velocity of the air. The more air the airfoil sends downward (by
changing the shape or angle of the airfoil), the more lift will be
obtained. For a more detailed description, and more information on
flight, you might try the ALLSTAR Network.
Submitted by Bob (Maryland, USA)
(August 2, 1999)