FORCES ACTING ON THE AEROPLANE

The aeroplane in straight-and-level, unaccelerated flight is acted on by four forces-lift, the upward acting force; weight, or gravity, the downward acting force; thrust, theforward acting force; and drag, the backward acting, or retarding force of wind resistance. Lift opposes weight and thrust opposes drag. These four forces act on an aeroplane in any attitude of flight.

Drag and weight are forces inherent in anything lifted from the earth and moved through the air. Thrust and lift are artificially created forces used to overcome the forces of nature and enable an aeroplane to fly. The engine-propeller combination is designed to produce lift to overcome the weight (or gravity).

In straight-and-level, unaccelerated flight, lift equa1s weight and thrust equals drag, though lift and weight will not equal thrust and drag. Any inequality between lift and weight will result in the aeroplane entering a climb or descent. Any inequality between thrust and drag while maintaining straight-and-level flight will result in acceleration or deceleration until the two forces become balanced.

An airfoil is a device that gets a useful reaction from air moving over its surface. Wings, horizontal tail surfaces, vertical tail surfaces and propellers are all examples of airfoils.

For convenience we will use a cross-sectional view of a wing in discussion. The forward part of an airfoil is rounded and is called the leading edge. The aft part is narrow and tapered and is called the trailing edge. An imaginary straight line joining the extremities of the leading and trailing edges is the chord.

The angle of incidence is the angle formed by the longitudinal axis of the aeroplane and the chord of the wing.

The relative wind is the direction of the air flow with respect to the wing. The flight path and relative wind are parallel but travel in opposite directions.

An aeroplane on the take off roll is subject to the relative wind created by its motion along the ground and also by the moving mass of air (the wind). For this reason, take off should be made into the wind.

The angle of attack is the angle between the wing chord line and the direction of the relative wind (or between the chord line and the flight path).

The airfoil is designed to increase the velocity of the airflow above its surface, thereby decreasing pressure above the airfoil. Simultaneously, the impact of the air on the lower surface of the airfoil increases the pressure below. This combination of decrease above and increase below produces lift.