Helicopter Aerodynamics, Structures and Systems
Core concepts
Helicopter aerodynamics differs fundamentally from fixed wing. Cover rotor systems (fully articulated, semi-rigid, rigid), the controls (collective, cyclic, pedals), and the aerodynamic phenomena unique to rotors: dissymmetry of lift, blade flapping, lead-lag, retreating blade stall, translational lift, autorotation, ground effect, vortex ring state, and tail rotor drift and roll. Structures include the main rotor head, tail boom, transmission, and skid or wheeled landing gear.
Traps to watch
- Dissymmetry of lift: advancing blade has higher airspeed than retreating; flapping and cyclic pitch equalise lift.
- Retreating blade stall: occurs at high speed, high weight, high altitude, or high load factor; the retreating blade stalls first.
- Translational lift: effective translational lift occurs as the rotor escapes its own downwash, improving efficiency.
- Autorotation: rotor is driven by upward airflow; you must lower collective to maintain rotor RPM.
- Tail rotor drift: the tail rotor thrust causes lateral drift corrected by a small bank or cyclic input; tail rotor roll is a related effect.
Worked example
In forward flight, the advancing blade sees airspeed V + ωr and the retreating blade sees V − ωr. At high forward speed the retreating blade's effective angle of attack rises until it stalls - this sets the helicopter's forward speed limit. Reducing speed, weight or altitude restores the margin.
Rule of thumb
When a helicopter question mentions high speed, high weight, high altitude or turbulence, think "retreating blade stall". When it mentions descent with low airspeed, think "vortex ring state".