The four forces of flight

The four forces of flight are lift, drag, thrust, and weight.

Lift is the upward force created by the wings moving through the air that sustains the airplane in flight. Lift operates to overcome weight. It must be equal to or greater than the weight of the object in flight and acting in the opposite direction. Lift can be increased by increasing the forward speed of the aircraft or by increasing the angle of attack

Drag is the resistance of the airplane to forward motion. It is directly opposed to thrust and is caused by the resistance of air.

Thrust is the force exerted by the engine and its propeller(s). It pushes the air backward with the object of causing movement of the airplane in the forward direction.

Weight is the downward force due to the weight of the airplane and its load. It is directly opposed to lift

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.