The "law of flight" isn't a single, codified law, but rather a description of how an aircraft achieves and maintains flight based on fundamental physics. The principles of flight are governed by the interplay of four primary forces.
The Four Forces of Flight
The principle of flight is made up of four fundamental forces: lift, weight, drag, and thrust. These forces work together in a delicate balance to determine an aircraft's trajectory, with lift and weight opposing each other and thrust and drag doing the same.
- Lift: The force that opposes weight, pushing the aircraft upward. It's primarily generated by the wings as air flows over them.
- Weight: The force of gravity pulling the aircraft downward.
- Thrust: The force that propels the aircraft forward, overcoming drag. This is typically generated by engines (e.g., jet engines or propellers).
- Drag: The force that opposes thrust, resisting the aircraft's movement through the air.
How the Forces Interact
An aircraft achieves flight when lift is greater than or equal to weight, and thrust is greater than or equal to drag. The interaction of these forces determines whether an aircraft ascends, descends, maintains altitude, accelerates, decelerates, or maintains a constant speed.
| Force | Description | Direction | Opposing Force |
|---|---|---|---|
| Lift | Upward force generated by the wings | Upward | Weight |
| Weight | Downward force due to gravity | Downward | Lift |
| Thrust | Forward force generated by the engine(s) | Forward | Drag |
| Drag | Resistance to motion through the air | Backward | Thrust |
Achieving and Maintaining Flight
For an aircraft to take off, lift must exceed weight. Once airborne, maintaining level flight requires lift and weight to be equal. To accelerate, thrust must exceed drag, and to decelerate, drag must exceed thrust. These dynamic relationships are continuously adjusted by the pilot through the aircraft's controls to achieve the desired flight path.
