Principles of Flight - Helicopter Aerodynamics
This domain tests lift, drag, rotor-disc behaviour, translational lift, autorotation, vortex-ring state and retreating-blade stall.
Core aerodynamics
Relative airflow and angle of attack produce lift; induced drag increases with lift, profile drag increases with speed. The rotor disc has advancing and retreating sides in forward flight. Dissymmetry of lift is equalised by flapping. Coning increases with lift; Coriolis changes blade speed as radius changes. Effective translational lift occurs as the rotor moves into undisturbed air. Ground effect reduces induced velocity near the surface. Torque requires anti-torque pedal input; translating tendency is corrected with cyclic.
Worked example
Main rotor turns at 300 RPM; radius 5 m; forward speed 30 m/s. Angular velocity = 2π × 300/60 = 31.4 rad/s. Tip speed at radius 5 m = 31.4 × 5 = 157 m/s. At radius 4 m, rotational speed = 31.4 × 4 = 125.7 m/s. Advancing blade speed = 125.7 + 30 = 155.7 m/s; retreating = 125.7 − 30 = 95.7 m/s. The retreating blade is closer to stall.
Traps
Confusing translational lift with ground effect; thinking flapping is a malfunction; and ignoring that retreating-blade stall worsens with high mass, low RPM and high speed.