Textbook

Forces

Aerodynamics is the study of the motions and forces of gases on an object. It aids in the design of airplanes, sailboats, trains, cars, and other objects moving quickly through the air as well as buildings, bridges, and windmills affected by the wind moving past them.

The fact that air can create a force strong enough to support an airplane in flight is sometimes difficult to imagine. When we walk through air, we don't really feel anything, unless there is a wind! Then we feel the force that the air makes on us. Yet, even when there is no wind and we can not feel the air, it does exert a force on objects.

The flight of an airplane, a tennis ball, a bird, or any other object involves forces that may be measured and compared. For an airplane those forces are: lift, drag, thrust, and weight. As can be seen in the figure below, these four forces are distributed with the:

lift force pointing upward;
weight pushing downward;
thrust pointing forward in the direction of flight;
and the drag force opposing the thrust.

In order for the plane to fly, the lift force must be greater than or equal to the weight. The thrust force must be greater than or equal to the drag force. You'll learn that for a sports ball and a vehicle like a glider that there is no thrust force during its flight.

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