Lift, Weight, Thrust And Drag

One of the first things that is likely to be noticed during a visit to the local airport is thewide variety of airplane styles and designs. No matter what each looks like like they alldepend on the the same four factors which are lift, weight, thrust, and drag

Lift is the upward force created by the effect of airflow as it passes over and under the wings. It supports the airplane in flight. Weight opposes lift. It is caused by the downward pull of gravity. Thrust is the forward force which propels the airplane through the air. It varies with the amount of engine power being used. Opposing thrust is drag, which is a backward, or retarding, force that limits the speed of the airplane.

Lift is the key aerodynamic force. It is the force that opposes weight. In straight-and-level, unaccelerated flight, when weight and lift are equal, an airplane is in a state of equilibrium. If the other aerodynamic factors remain constant, that airplane neither gains nor loses altitude. When an airplane is stationary on the ramp, it is also in equilibrium, but the aerodynamic forces are not a factor. In calm wind conditions, the atmosphere exerts equal pressure on the upper and lower surfaces of the wing. Movement of air about the airplane, particularly the wing, is necessary before the aerodynamic force of lift becomes effective. During flight, however, pressures on the upper and lower surfaces of the wing are not the same. Although several factors contribute to this difference, the shape of the wing is the principal one. The wing is designed to divide the airflow into areas of high pressure below the wing and areas of comparatively lower pressure above the wing. This pressure differential, which is created by movement of air about the wing, is the primary source of lift.

The weight of the airplane is not a constant. It varies with the equipment installed, passengers, cargo, and fuel load. During the course of a flight, the total weight of the airplane decreases as fuel is consumed. Additional weight reduction may also occur during some specialized flight activities, such as crop dusting, fire fighting, or sky diving flights.

The direction in which the force of weight acts is constant. It always acts straight down toward the center of the earth.

Thrust is the forward-acting force which opposes drag and propels the airplane. In most airplanes, this force is provided when the engine turns the propeller.