There are four forces that act on an aircraft in flight: lift, weight, thrust, and drag. Aircraft’s motion in air is dependent on the relative magnitude and direction of these forces. Fig -1 below shows the direction of these forces. The weight of an airplane is always directed towards the center of the earth. The thrust is normally directed forward along the center-line of the aircraft. Lift and drag are aerodynamic forces on the airplane.
Drag acts in a direction opposite to the motion of the aircraft and hence is sometimes referred to as the aerodynamic friction, while lift force acts perpendicular to the motion. An aircraft is in a state of equilibrium when the thrust and drag are equal and opposite. It will continue to move forward at the same uniform speed. If thrust or drag becomes greater than the opposite force, the aircraft loses its state of equilibrium. If thrust is greater than drag, the aircraft will accelerate.
If drag is greater than thrust, the aircraft will lose speed and eventually descend. When lift and weight are equal and opposite, the airplane is in a state of equilibrium. If lift is greater than weight, the aircraft will climb. If weight is greater than lift, the airplane will descend. Drag is the aerodynamic force encountered as an airplane pushes through the air, which tends to slow the airplane down. Drag is generated by the contact of a solid body with a fluid, in this case due to the interaction between the plane body and air.
Drag force, which is a mechanical force, is generated by every part of the airplane including the engines. It is a vector quantity i. e. has both magnitude and direction. Drag must be overcome by thrust in order to achieve forward motion. Drag is generated by nine conditions associated with the motion of air particles over the aircraft. Although prediction of drag and wind tunnel drag measurements of models yield good results, final drag evaluation must be obtained by flight tests