Helicopter General Knowledge - Airframe, Systems, Rotor Dynamics and Powerplant
This domain tests how the helicopter is built and how its systems interact: rotor head type, controls, transmission, engine, hydraulics, electrical supply and the dynamic behaviour that limits the machine.
Airframe and rotor systems
A fully articulated head has flapping hinges, lead-lag hinges and feathering bearings; a semi-rigid head teeters as a unit; a rigid head relies on blade flexibility. The swashplate converts control inputs into blade pitch: cyclic tilts the disc, collective changes pitch collectively, and pedals control tail-rotor thrust. The transmission includes clutch, freewheel unit, main gearbox, drive shaft, intermediate gearbox and tail gearbox. The freewheel unit lets the rotor turn the transmission during autorotation without driving the engine.
Powerplant and systems
Piston or turbine engines have limits on RPM, torque, temperature and oil pressure. Turbine start checks include hot-start and hung-start recognition. Hydraulic and electrical systems may be duplicated; know which controls are affected by a failure.
Rotor dynamics
Coning occurs as lift bends blades upward; flapping equalises lift across the disc; lead-lag absorbs acceleration and deceleration; Coriolis effects change chordwise speed. Dissymmetry of lift, retreating-blade stall and ground resonance are dynamic hazards, not engine faults.
Worked example
Main rotor turns at 300 RPM and the engine at 3,000 RPM. Main gearbox ratio = 3,000/300 = 10:1. If the tail rotor runs at five times main-rotor speed, tail RPM = 300 × 5 = 1,500 RPM. Check tail-rotor limits, not just main-rotor limits.
Traps
Confusing cyclic with collective; thinking the freewheel unit drives the rotor in autorotation; and mixing up ground resonance with retreating-blade stall.