The Four Forces of Flight

Inventors and scientists struggled for centuries to understand the basic principles of flight. Experts still debate the details of aerodynamics, but pilots need to understand a few fundamental concepts, starting with the four forces that affect flight: lift, weight, thrust, and drag.

These four forces act in pairs. Lift (actually, the sum of all upward forces) opposes weight (actually, the sum of all downward forces), and thrust opposes drag. The opposing forces balance one another in steady-state flight. Steady-state flight includes straight-and-level flight and constant-rate climbs or descents at a steady airspeed. You can assume that the four forces act through a single point called the center of gravity (CG).

Lift

Lift is the force that makes an airplane fly. Most of an airplane's lift comes from its wings. You control the amount of lift a wing creates by adjusting airspeed and angle of attack (AOA)—the angle at which the wing meets the oncoming air. In general, as an aircraft's airspeed or angle of attack increases, so does the amount of lift created by the wings.

As an airplane's speed increases, you must reduce the angle of attack—lower the nose slightly—to maintain a constant altitude. As the airplane slows down, you must increase the angle of attack—raise the nose slightly—to generate more lift and maintain altitude.

Remember that even in a climb or descent, lift essentially equals weight. An aircraft's rate of climb or descent is primarily related to the amount of thrust generated by its engines, not by the amount of lift created by its wings.

Weight

Weight opposes lift. As a practical matter, you can assume that weight always acts along a line from the airplane's center of gravity to the center of the earth.

At first you might assume that weight changes only as fuel is consumed. In fact, as an airplane maneuvers, it experiences variations in load factor, or G forces, which change the load supported by the wings. For example, an airplane making a level turn in a 60-degree bank experiences a load factor of 2. If that airplane weighs 2,000 lbs (907 kg) at rest on the ground, its effective weight becomes 4,000 lbs (1,814 kg) during that turn.

To maintain the balance between lift and weight during maneuvers, you must adjust the angle of attack. During a steeply banked turn, for example, you must raise the nose slightly (increase the angle of attack) to produce more lift and thus balance the increased weight.

Thrust

Thrust provided by an aircraft's powerplant propels it through the air. Thrust is opposed by drag, and in steady-state flight thrust and drag are equal. If you increase thrust and maintain altitude, thrust momentarily exceeds drag, and the airplane accelerates. Drag increases, too, however, and soon drag once again balances thrust. The airplane stops accelerating and resumes steady-state flight at a higher, but constant airspeed.

Thrust is also the most important factor in determining your airplane's ability to climb. In fact, an airplane's maximum rate of climb is related not to the amount of lift its wings create, but to the amount of power available beyond that required to maintain level flight.

Drag

Two kinds of drag affect an airplane. Parasite drag is friction between the air and an aircraft's structure—landing gear, struts, antennas, and so forth. Parasite drag increases as the square of an aircraft's velocity. If you double airspeed, parasite drag quadruples.

Induced drag is a byproduct of lift. It is caused by air moving from the high-pressure area below a wing into the low-pressure area above the wing. This effect is most pronounced at slow airspeeds where a high angle of attack is necessary to produce enough lift to balance weight. In fact, induced drag varies inversely as the square of the airspeed. If you reduce airspeed by half, induced drag increases four times.

A Balancing Act -