Page 30 - Understanding Flight

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How Airplanes Fly 17

Fig. 2.1. Simulated smoke flowing around an airfoil.

transit times, the popular description of lift loses its explanation for
acceleration of the air over the top of the wing.
In explaining the physics of the lift on the wing, we will reveal other
problems with this description. But first we would like to introduce
two other descriptions of lift. These are the “mathematical description
of lift” and the “physical description of lift” The latter is the
description followed in this book.

The Mathematical Description of Lift

Aeronautical engineers use the mathematical description of lift, a name
coined for the sake of this discussion. It uses complex mathematics
and computer simulations, and is a powerful design tool. Typically, the
velocities of the air around the wing are generated with a computer
program. Then, using Bernoulli’s equation, the pressures and lift forces
are accurately calculated. Since this description often calculates the lift
of a wing from the acceleration of the air, this description has quite a
bit in common with the popular description of lift.
The aeronautical engineers know that the principle of equal transit
times is not true. They often use a mathematical concept called
circulation to calculate the acceleration of the air over the wing.
Circulation is a measure of the apparent rotation of the air around the
wing. In the mathematical description of lift circulation is used to