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 forces that act on an airplane wing during flight: lift, weight, thrust, and drag. To further explain the physical phenomena that lift a plane during flight, I will instead use Newton’s Laws.
Let’s say a plane is flying at a constant velocity and is not changing in altitude. Newton’s first law states: “Any object at rest will remain at rest or an object in motion will remain in motion unless a force acts on it.” Newton’s second law tells us that the sum of the forces (F) acting on a body is the mass (m) times the acceleration (a) of the body. In the case of our airplane, there is no acceleration because the velocity does not change up or down, forwards or backwards, so the sum of the forces must be zero. (F = ma = 0) In the case of the up and down forces, we know that the weight of the plane is acting down, so the counteracting lift force must be of magnitude equal and opposite the weight (which can be very large depending on the size of the airplane in question)!
First, we must clarify that air does not fly over an airfoil as smoothly and as straight as is commonly depicted. Let’s take a little detour to look at air as a fluid to explain how the shape of an airfoil diverts air down. When a moving fluid such...