Principles of Flight - Helicopter
Lift and the rotor
A helicopter rotor produces lift by the same aerodynamic principles as a wing, but with rotating blades. Understand angle of attack, relative airflow, induced drag, profile drag and parasite drag. Know that total drag is the sum of these, and that induced drag is greatest at low speed and high blade loading. Understand translational lift - as airspeed increases, the rotor becomes more efficient and less power is needed.
Asymmetry and control
Know dissymmetry of lift (advancing blade has higher relative airspeed than retreating blade) and how flapping and cyclic pitch compensate. Understand retreating blade stall, its causes (high speed, high weight, low RPM, high altitude, turbulence) and its symptoms (vibration, pitch up, roll). Know ground effect, autorotation, and the vortex ring state (settling with power) - descending vertically with power, which is dangerous and is recovered by forward cyclic.
Traps
Questions confuse dissymmetry of lift with retreating blade stall. Another trap is the cause of vortex ring state: low airspeed, high power, high rate of descent. Watch for items on torque effect and translating tendency.
Worked example
If a helicopter at high weight and high density altitude approaches VNE, retreating blade stall risk rises; reduce speed and weight, and check RPM.