Helicopter General Knowledge: Airframe, Systems and Powerplant
Core framework
This domain covers the airframe, rotor systems, transmission, engine and associated systems. You must understand the main rotor head (articulated, semi-rigid/teetering, rigid), the tail rotor or alternative anti-torque systems (Fenestron, NOTAR), and the transmission that links engine to rotors. Know that a freewheeling unit (sprag clutch) disconnects the engine from the rotor system so the rotor can autorotate after power loss.
Systems to know
Fuel system (tanks, boost pumps, shut-off valve), lubrication, ignition, engine governing (governor maintains rotor RPM), hydraulic systems for flight controls and, where fitted, stability augmentation. Understand the difference between a piston and turbine powerplant, and the meaning of limits: torque, TOT/ITT, N1/N2 and rotor RPM (NR).
Traps
- Confusing Nr (rotor RPM) with engine RPM (N1/N2) - they are linked through gearing but governed separately.
- Assuming the freewheeling unit drives the rotor from the engine in both directions; it only transmits engine-to-rotor torque.
- Forgetting that hydraulic failure usually means higher control forces, not total loss of control.
- Mixing mast bumping (semi-rigid heads) with blade sailing.
Rule of thumb
"Torque = power demanded; TOT = heat produced." If torque is high but TOT is low, the engine is working but not overtemping; if TOT rises with modest torque, suspect a cooling or fuel-scheduling issue.
Worked example
After an engine failure, the question asks what allows autorotation. Answer: the freewheeling unit releases the engine, so the rotor turns freely, driven by airflow through the disc.