Helicopter Power Transmission
Helicopter Power Transmission covers the path from engine output to main and tail rotors: shafts, couplings, gearboxes, bearings and the mounts that carry torque and thrust. The exam tests power, torque and gear ratio relationships, plus alignment and inspection.
Core rules
- The main transmission gearbox reduces engine RPM to rotor RPM and drives the tail take-off.
- Tail drive includes driveshafts, hanger bearings, intermediate gearbox and tail gearbox.
- Couplings accommodate misalignment and flex; hanger bearings support long shafts and must be aligned.
- Torque = power / angular velocity; reducing speed increases torque proportionally.
- Lubrication and cooling of the main gearbox are as critical as engine oil.
- Mounts carry rotor thrust and torque reactions; degraded mounts change alignment and vibration.
Worked example
An engine delivers 500 kW at 6000 rpm to a rotor turning at 300 rpm. Engine shaft torque = 500,000 / (2π × 100) ≈ 796 N·m. Rotor shaft torque at 300 rpm = 500,000 / (2π × 5) ≈ 15,915 N·m. The gear ratio is 20:1; ignoring losses, torque rises by 20. With 97% transmission efficiency, rotor torque is about 16,400 N·m. This is why the main gearbox is massive and why torque limits are not just engine limits.
Traps
- Confusing power in kW with torque in N·m.
- Forgetting losses in gear ratio calculations.
- Misaligning hanger bearings and preloading the shaft.
- Reusing coupling bolts or locking devices.
- Ignoring tail drive vibration because the main rotor is smooth.