Rotorcraft Airframe Systems and Structures
Rotorcraft airframe questions test whether you can connect the rotor head, control system, transmission and structure into one load path. The exam favours system understanding over part naming: what moves, what limits the movement, and what failure mode follows.
Rotor head and controls
Articulated heads use separate flapping, lead-lag and feathering hinges, with lag dampers controlling in-plane oscillation. Semi-rigid two-blade heads teeter as a unit: no lag hinges, and mast bumping is possible when the disc is unloaded. Rigid heads use flexures or elastomeric bearings to absorb flap and lag. The swashplate has a stationary ring moved by collective and cyclic inputs and a rotating ring that drives pitch links. Collective changes pitch on all blades equally; cyclic tilts the tip-path plane.
Transmission and structure
Main gearbox, drive shafts, intermediate and tail gearboxes transmit power within torque limits; chip detectors and oil analysis are early warnings. Ground resonance occurs when lag damping is degraded and the landing gear oscillates in phase with blade motion. Tracking and balancing reduce vibration and prevent fatigue.
Worked example
A two-blade teetering helicopter reports a low-G pushover and a sudden mast 'knock'. Reasoning: with no flapping hinge, unloading the disc lets blades reach the teeter stops, producing a large mast bending moment. The correct response is to inspect the mast and hub for cracks and check control rigging against the manual, not simply to increase damper pressure.
Traps
- Confusing collective (all-blade pitch) with cyclic (tip-path tilt).
- Assuming every articulated head has the same damper arrangement.
- Signing off tracking without a vibration check.