Principles of Flight – Helicopter
Lift and the rotor disc
Understand how rotor thrust is generated and controlled through collective and cyclic pitch. Know induced flow, induced drag, and the power curve (profile, induced, parasite power) with its minimum at best endurance speed. Best range speed is higher than best endurance speed.
Asymmetry of lift and retreating blade stall
Dissymmetry of lift arises from advancing versus retreating blade airspeed; flapping and cyclic feathering equalise lift. Retreating blade stall is triggered by high weight, high altitude, high speed, low rotor rpm, and turbulence. Trap: questions that claim retreating blade stall produces a left roll in a counter-clockwise rotor - it produces a roll toward the retreating side.
Autorotation and vortex ring
Know the three autorotative regions: driven, driving, and driven region reversal, plus the effect of forward speed on rate of descent. Vortex ring state occurs with low forward speed, high rate of descent, and high power; recovery is to lower collective and gain forward speed.
Worked example
A helicopter at high gross weight and high density altitude begins to stall the retreating blade at a lower speed than usual. Rule of thumb: retreating blade stall speed falls as weight and altitude rise - the Vne you use must be the chart value, not the placard value.