Lift and the Physics of Flight

Since the beginning of recorded history, humans have always had a fascination with flight. Now that we live in a world where boarding an airplane and flying across the country – or even the world - is simply a part of everyday life, the wonder of flight has diminished for many. Despite this, physics students from all around continue to delight in the many physical forces that play a part in keeping these huge objects (like jumbo jets) from falling out of the sky!

The common explanation given to those curious about how an airplane wing produces lift uses the Bernoulli Principle. This is the concept that because of the airfoil shape of a wing, the air traveling over the top of the wing must travel faster than the air going under the wing because it has to travel a farther distance. The resulting difference in pressure between the two (higher pressure under the wing) creates lift, keeping the plane in the air.

This explanation is unsatisfactory in a number of ways. It does not consider or explain the important role that the angle of attack plays in flight, nor does not explain how planes can fly upside down (where according to the Bernoulli Principle, the pressure would actually be higher on the top of the wing, pushing the plane down to the ground!). For some interesting arguments and calculations refuting the Bernoulli Principle as a sound explanation for lift in an airplane, check out Gail Craig’s book “Stop Abusing Bernoulli! How Airplanes Really Fly” available from Regenerative Press (see bibliography).

Although the Bernoulli Principle is used to describe many physical phenomena, it does not explain lift. Luckily for us there is a much more sound explanation for how an airplane flies! There are four main forc...

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...er angle of attack helps divert more air downwards, thus creating more lift. If one imagines the air particles as bullets hitting the wing of the airplane, an increased angle of attack increases the number of air particles that will hit the bottom of the wing, thus increasing the amount of air being “scooped” and diverted downwards.

However, there is a limit to this great thing we call angle of attack. Generally any angle greater than 15 degrees will cause the plane to stall. The stall happens because, as discussed earlier, the viscous property of air “wants” to follow a curve, but is limited to its level of “stickiness” to the surface of the plane. As the angle of attack increases, the air has a harder time “sticking” to the surface and eventually simply passes right over the wing without following the surface, resulting in the loss of lift (a.k.a. stall).