The following are the forces acting on an aircraft in flight: -
(a) Lift is a positive force caused by the difference in air pressure under and above a wing. The higher air pressure beneath a wing creates lift and is affected by the shape of the wing. Changing a wing's angle of attack affects the speed of the air flowing over the wing and the amount of lift that the wing creates.
(b) Thrust is the force that propels an object forward. An engine spinning a propeller or a jet engine expelling hot air out the tailpipe are examples of thrust. In bats, thrust is created by muscles making the wings flap.
(c) Drag is the resistance of the air to anything moving through it. Different wing shapes greatly affect drag. Air divides smoothly around a wing's rounded leading edge, and flows neatly off its tapered trailing edge this is called streamlining.
(d) Weight is the force that causes objects to fall downwards. In-flight, the force of the weight is countered by the forces of lift and thrust.
OR
a) Total reaction: It is one single force representing all the pressures (force per unit area) over the surface of the Aerofoil. It acts through the center of pressure which is situated on the chord line.
b) Angle of Attack: It is the angle between the chord line and the relative airflow undisturbed by the presence of Aerofoil.
c) Aerofoil: It is a body designed to produce more lift than drag. A typical Aerofoil section is cambered on a top surface and is more or less straight at the bottom
