Theory of Flight, Forces and Stability

An aircraft in steady flight balances four forces: lift upward against weight downward, and thrust forward against drag backward. Increase thrust and the aircraft accelerates; increase lift and it climbs. When the forces are unbalanced the aircraft accelerates in the direction of the surplus force.

Lift is produced by the wing deflecting air downward and by the pressure difference created as air accelerates over the curved upper surface. Lift rises with angle of attack up to the critical angle, where airflow separates and the wing stalls with a sharp loss of lift. Recovery is to reduce the angle of attack and add power.

Stability describes how the aircraft returns to a trimmed condition: longitudinal stability in pitch through the horizontal tail, lateral stability in roll through dihedral and the fin, and directional stability in yaw through the vertical fin. Trim surfaces and tabs relieve the control forces needed to hold an attitude. Weight and balance sets the centre of gravity, and the published envelope limits are hard limits, not guidance.