User:Noleander/sandbox

The transmission is a mechanical system that transmits power from the engine(s) to the rotors. The transmission is a system of gears, clutches, shafts, and other mechanisms. The transmission performs several functions (1) Translates the alignment of the drive shaft to match the alignment of the rotor shafts; (2) Reduces the RPM of the drive shaft to the lower RPMs of the rotors; and (3) Supports situations where the engine engages or disengages from the rotors (such as engine start-up, or autorotation after engine failure). For helicopters with tail rotors, the transmission gear chain forks into two paths: one leading to the main rotor, and one leading to the tail rotor.

The transmission must translate the alignment of the drive shaft to match the shafts of the rotors. The drive shafts of helicopter engines are typically not in the same alignment as the rotor shafts. Many engine drive shafts are aligned horizontally, yet the main rotor shaft is usually vertical, and the tail rotor shaft is often perpendicular to the engine's drive shaft. The transmission contains a series of gears, usually bevel gears, that translate the alignment of the drive shaft to the alignment of the rotor shafts.

The transmission also reduces the RPMs of the engine to the lower RPMs required by the rotors. The drive shaft of the engine, before any gearing is applied, is typically between 3,000 and 50,000 RPM (turbine engines typically have higher RPM than piston engines). The main rotor typically rotates between 300 to 600 RPM. The tail rotor, if present, usually rotates between 1,000 to 5,000 RPM. (The RPMs of the engine and rotors is essentially fixed for a given model of helicopter; the ranges cited above span most models). The transmission contains a series of reduction gears to reduce the engine RPM to the rotor RPMs. Several types of reduction gears may be used, including bevel gears, planetary gears, helical gears, and spur gears. Most transmissions contain several reduction gears: the engine itself may contain reduction gears (often spur gears) at the output end, where the drive shaft emerges; the main rotor may have a reduction gear at its base (typically a planetary gear); and there may be reduction gears at the tail rotor, and on the shaft leading to the tail rotor.

The transmission often includes a clutch, which permits the rotors to engage or disengage from the engine. A clutch is required so the engine can start up and gain speed before taking the load of the rotors. The clutch is also required in the case of engine failure: in that situation, the rotors must disengage from the engine so that the rotors can continue spinning and support autorotation. Helicopter clutches are usually freewheel clutches relying on centrifugal forces - most commonly, a sprag clutch is used.