Aeromodelling

Forces Acting on an Airplane

During the aircraft in straight-and-level unaccelerated flight, there are fore forces acted on the aircraft. These four main forces acting on an airplane are lift, drag, gravity, and thrust. Maintaining a steady flight of the aircraft requires a balance of the four forces, often described as equilibrium.

To maintain equilibrium the following conditions must be met:

L =W and T = D

Lift opposes Weight and Thrust opposes Drag

In general view, the vectors of the four forces are always assumed act on a single point. The presentation of the four forces as illustrated in figure above. However, in reality as shown below figure, Weight is acted on the Center of Gravity. Lift is acted at the Center of Pressure. Thrust and Drag are paired to reduce the pitching moment which is created by the lift.

Weight and Drag forces are inherent in any objects that lifted from the ground and moved through the air. Thrust and lift are artificially created forces used to overcome the forces of nature and enable an airplane to fly. The engine and propeller combination is designed to produce Thrust to overcome Drag. The wing is designed to produce Lift to overcome the Weight (gravity).

When the four forces are in equilibrium, the aircraft in motion will tend to keep moving along the same flight path with a constant speed, whether it is flying straight and level, descending or climbing.

If the pilot sets the engine throttle fully, and maintains straight-and-level flight. The aircraft accelerates in a direction; the thrust force is initially greater than drag. That mean the forces are not in equilibrium state. However the aircraft will soon reach the full throttle speed. And the aircraft speed remains constant, where the forces are again balanced.