Helicopter Aerodynamics, Structures and Systems
What you must understand
Know how a rotor generates lift: relative airflow, angle of attack, blade pitch, and the difference between rotational and induced velocity. Understand rotor systems (articulated, semi-rigid, rigid) and the function of the swashplate in collective and cyclic control. Study dissymmetry of lift and how flapping and blade lead-lag compensate for it, plus retreating blade stall, blade tip stall and compressibility effects at high forward speed. Understand autorotation: the energy state, the driving region, and the pilot's actions. Know translating tendency and tail rotor drift, ground resonance, dynamic rollover, and vortex ring state. Study vibration sources and tracking, plus the structure: mast, gearbox, tail boom and skids, and the systems: hydraulics, electrical and fuel.
Traps in questions
- Dissymmetry of lift: advancing blade sees higher relative airflow and produces more lift; flapping equalises it.
- Retreating blade stall: occurs at high forward speed and high disc loading; it limits maximum speed.
- Vortex ring state: descending too fast with low airspeed into your own downwash; recovery is to lower collective and gain forward speed.
- Ground resonance: can destroy the aircraft quickly; correct action is to lift off or shut down, not to sit still.
- Autorotation: lowering collective is essential to preserve rotor energy.
Worked example
In forward flight the advancing blade tip sees rotational speed plus forward speed, while the retreating tip sees rotational speed minus forward speed. The lift difference is equalised by blade flapping, which changes the local angle of attack and tilts the disc.
Rule of thumb
For any helicopter handling question, identify the energy source: engine, rotor inertia, or height. The correct action usually protects the rotor RPM.