What are the four principal forces acting on an airplane in flight?

The four principal forces acting on an airplane in flight are lift, drag, thrust, and gravity.

Lift is the force that acts perpendicular to the relative wind and supports the weight of the aircraft, allowing it to rise. It is generated mainly by the wings as they interact with the airflow.

Drag is the resistance force that opposes an aircraft's forward motion through the air. It is caused by the friction of air molecules against the surface of the aircraft and increases with speed and surface area.

Thrust is the forward force produced by the aircraft's engines, propelling the aircraft through the air. It must overcome drag for the airplane to accelerate.

Gravity, or weight, is the force that pulls the aircraft downward towards the earth. It is based on the mass of the airplane and is always acting downwards, opposing lift.

These four forces are critical in understanding flight dynamics, as they interact continuously and determine the aircraft's behavior in the air, including its ability to climb, cruise, and descend.