Understanding the four forces of flight
Published: Nov 8, 2025 |The four forces of flight are the cornerstone of aerodynamics and a fundamental concept for any pilot. These forces are Lift, Weight, Thrust, and Drag. In steady, unaccelerated flight, these forces are in equilibrium: Lift equals Weight, and Thrust equals Drag.
1. Lift
Lift is the upward force created by the movement of air over the wings (airfoils). It is generated primarily due to Bernoulli's Principle and Newton's Third Law. The curved upper surface of the wing causes air to travel faster, creating a lower pressure area, while the flatter bottom surface has a higher pressure area. This pressure differential "pushes" the wing upwards.
The amount of lift generated depends on several factors, including the shape of the airfoil, the angle of attack (the angle between the wing and the oncoming air), the air density, and the square of the aircraft's velocity.
2. Weight
Weight is the force of gravity acting on the aircraft, pulling it towards the center of the Earth. It acts vertically downwards through the aircraft's center of gravity (CG). Weight is a constant force, though it decreases slightly during flight as fuel is consumed.
For an aircraft to climb, lift must be greater than weight. To descend, lift must be less than weight. In level flight, lift and weight are equal.
3. Thrust
Thrust is the forward-acting force generated by the aircraft's engine(s), whether it's a propeller or a jet engine. It opposes the force of drag. Thrust is what propels the aircraft through the air, creating the airflow over the wings necessary to generate lift.
To accelerate, thrust must be greater than drag. To decelerate, thrust must be less than drag. In steady-state flight at a constant speed, thrust and drag are equal.
4. Drag
Drag is the aerodynamic force that opposes the aircraft's motion through the air. It is a rearward-acting force and is parallel to the relative wind. Drag is the price we pay for lift and comes in two main forms: parasite drag and induced drag.
- Parasite Drag: This is composed of form drag (due to the aircraft's shape), skin friction drag (due to the smoothness of the surfaces), and interference drag (where different airflows meet, e.g., at the wing root). Parasite drag increases with the square of the airspeed.
- Induced Drag: This is the byproduct of generating lift. It is created by the wingtip vortices and is greatest at slow airspeeds, high angles of attack, and high weights.
Total drag is the sum of parasite and induced drag. The point where these two drag curves intersect is the minimum drag speed, which is the most efficient speed for the aircraft.