Aerodynamics and Aircraft Control

Four forces of flight

An aircraft in steady, straight-and-level flight is acted upon by four forces: lift, weight, thrust and drag. Lift acts perpendicular to the relative wind; weight acts toward the centre of the Earth; thrust acts along the propeller or engine axis; drag opposes the direction of flight.

When lift equals weight and thrust equals drag, the aircraft maintains altitude and airspeed. Increasing angle of attack increases lift coefficient but also increases induced drag.

Angle of attack and stalls

Angle of attack (AOA) is the angle between the chord line and the relative wind. A wing stalls when the critical angle of attack is exceeded, regardless of airspeed. Recovery requires decreasing AOA (reduce back pressure), adding power if appropriate, and rolling wings level with coordinated rudder.

• Stall speed increases in turns because load factor increases required lift.
• Power-on stalls can produce a stronger left-turning tendency from P-factor and spiraling slipstream.
• Spin entry requires stall plus yaw; recovery is PARE — Power, Ailerons neutral, Rudder opposite, Elevator forward.

Load factor

Load factor is the ratio of total lift to weight. In a level 60° bank, load factor is 2 G and stall speed increases by about 41%. Structural limits (normal category typically +3.8 G / −1.52 G flaps up) must be respected during manoeuvring flight.

Stability

Static stability is the initial tendency to return to equilibrium after a disturbance. Longitudinal stability depends on CG position relative to the centre of lift — aft CG reduces stability and can make recovery from stalls more difficult. Dihedral contributes to lateral stability; sweepback and keel effect aid directional stability.