Helicopter Engines

Most modern helicopters use turboshaft engines with electronic controllers.  Some older or smaller helicopters use piston engines.  In this article, we describe how these engines behave in a typical helicopter with a main and tail rotor configuration.

Turboshaft Engine

At a high level, a turboshaft engine performs three functions.  First, it sucks in outside air and compresses it to high pressure.  Second, it mixes the high pressure air with gas and ignites it.  Finally, it allows the this expanding air to blow through a turbine and turn the output “power shaft.”  This last step is like wind applying torque to a wind turbine.  The engine controller keeps the shaft turning at a constant speed, say 6500 RPM, by adjusting fuel flow and other engine parameters. 

Turboshaft engine diagram

Power Transmission To Rotors

The engine power shaft is geared to the main and tail rotors and keeps them turning at a constant, but different, speed.  A typical main rotor gear ratio is around \(\frac{1}{20}\), meaning 20 turns of the power shaft result in one turn of the main rotor.  The tail rotor typically spins about 5 times faster than the main rotor, with a gear ratio of about \(\frac{1}{4}\) to the engine.  The tail rotor also rotates slower than the power shaft. 

The power shaft cannot slow the rotors, it can only maintain or increase rotor speed.  If the power shaft slows, e.g. during an engine failure, it declutches from the rotors.  The rotors can maintain speed due to inertia and / or aerodynamic forces.  This is what happens in autorotation - the helicopter descends at high speed so the upflow of air powers the main rotor.  Think of it like a wind turbine.  Since the power shaft declutches, it and the engine do not "drag down" the rotor speed.  However, the main rotor shaft always remains clutched to the tail rotor shaft.  In autorotation, the main rotor effectively powers the tail rotor to provide yaw control. 

Throttle Control

Although it’s not considered a primary flight control like the collective, cyclic and pedal; the pilot does have a throttle control for the engine.  It’s typically a twist grip on the collective stick.  There will be two such grips for a dual engine helicopter.  A pilot can turn or "roll the throttle" to override the engine electronic controller.  This may be done if an engine controller malfunctions or if the pilot wants to practice autorotation.  Otherwise this is almost never used in flight.  The usual behavior is for the throttle to stay in a designated "fly" position where the engine controller automatically maintains rotor speed and optimizes engine efficiency.

When manually controlling the engine via throttle, a pilot must maintain proper rotor speed without damaging the engine.  For a dual engine helicopter, the pilot would only take control of the failed engine - the other engine controller would continue to maintain rotor speed (at least within the engines limitations).  Normally the pilot would control the failed engine’s throttle to provide slightly less power than the controlled engine, mimicking the action of the electronic controller (minus a small margin).