Lift
Lift is an artificial force manipulated by pilot; it is generated through the wings, acts perpendicular to the relative wind and wingspan. The theoretical concept that summarizes the direction and force of lift is the centre of pressure. Lift opposes weight—during level cruise, lift equals weight; during climb, lift is greater than weight; and during descent, weight is greater that lift.
Weight
Weight is a natural (uncontrollable) force generated by gravity (g force) that acts perpendicular to earth’s surface; theoretically, weight is exerted through the centre of gravity, and opposes lift.
Thrust
Thrust is an artificial force manipulated by pilot and generated through engine(s) that acts horizontally, parallel to flight path; thrust opposes drag—when airspeed constant, thrust equals drag; when airspeed accelerating, thrust is greater than drag; and when decelerating, drag is greater than thrust.
Drag
Drag is the natural resistance of an aeroplane while it is moving through air; it is partially controlled by pilot. Drag is a horizontal force acting parallel to flight path, and is opposed to thrust.
