Principles of Flight - Helicopters (Aerodynamics)
Lift, thrust and rotor aerodynamics
Understand how the rotor produces lift through angle of attack and airflow, and how induced drag, profile drag and parasite drag combine. Know the power required curve: induced power dominates at low speed, parasite power at high speed, and the total curve has a minimum at the best endurance speed. Understand translational lift, dissymmetry of lift, retreating blade stall, and blade tip vortices.
Stability and control
Know how the helicopter achieves control about each axis: cyclic for pitch and roll, pedals for yaw, collective for lift. Understand the concept of pendular stability and why helicopters are less stable than fixed-wing aircraft. Learn autorotation aerodynamics: the driving region, driven region and stall region of the blade.
Traps in questions
Questions often confuse dissymmetry of lift (a flapping/feathering solution) with retreating blade stall (a high-speed or high-load limit). Another trap is asking what happens to power required as speed increases: it decreases then increases, with a minimum at best endurance. Watch for questions about ground effect, which reduces induced velocity and thus power required.
Worked example
Rule of thumb: best rate of climb speed (VY) is higher than best angle (VX), and best endurance speed is lower than best range speed. If a question asks for the speed to stay airborne longest, choose best endurance, not best range.