Helicopter Aerodynamics, Structures and Systems
What the module tests
Helicopter aerodynamics, structures and systems covers rotor theory, hovering, translational lift, autorotation, retreating-blade stall, tail-rotor function, transmissions, vibration and special handling hazards. Expect relationship questions and simple tip-speed calculations.
Core rules
- Rotor tip speed = π × diameter × RPM / 60, in m/s.
- Dissymmetry of lift arises from advancing and retreating blades; flapping and blade pitch changes equalise lift.
- Translational lift improves efficiency as the rotor leaves its own downwash, typically above about 10-20 kt.
- Autorotation uses upward airflow through the rotor; rotor RPM is controlled with collective pitch and airspeed.
- Retreating-blade stall occurs at high speed and high disc loading; it causes vibration and roll.
- Loss of tail-rotor effectiveness and dynamic rollover are handling hazards, not merely power limits.
Worked example
A main rotor of 10 m diameter turns at 400 RPM. Find the tip speed. Circumference = π × 10 = 31.42 m. Distance per minute = 31.42 × 400 = 12,568 m. Tip speed = 12,568/60 = 209.5 m/s. This is a subsonic tip speed; if RPM or diameter rises, the advancing blade approaches its critical Mach number sooner.
Traps
Using radius instead of diameter; mixing RPM and revolutions per second; treating translational lift as a fixed altitude; confusing retreating-blade stall with engine failure; assuming tail-rotor thrust is unaffected by wind direction; ignoring density altitude in hover performance.
Exam approach
For rotor sums, convert diameter to circumference, then RPM to seconds. For handling questions, identify the airflow or thrust change first, then the control response.