Helicopter Engines

Most modern helicopters use turboshaft engines with electronic controllers.  Some older or smaller helicopters use piston engines.  In this article we provide a concise description of 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:  (1) sucks in outside air and compresses it to high pressure, (2) mixes the high pressure air with gas and ignites it and (3) allows the resulting/expanding air to blow through and turn a turbine (like wind applying torque to a wind turbine) which turns the output “power shaft.”  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 main rotor may have a gear ratio of around \(\frac{1}{20}\), meaning 20 full turns of the power shaft result in one full turn of the main rotor.  The tail rotor typically spins about 5 times faster than the main rotor (still slower than the engine) and hence a gear ratio of about \(\frac{1}{4}\) to the engine.

The power shaft declutches from the rotors if it slows below the rotor speed (divided by the gear ratio).  E.g. if an engine fails the power shaft may slow down while the rotor maintains speed due to inertia or aerodynamic forces.  This is what happens in autorotation - the helicopter descends at high speed so the upflow of air powers the main rotor like a wind turbine while the power shaft is declutched.  The main rotor shaft always remains clutched to the tail rotor shaft, effectively powering the tail rotor during autorotation.

Throttle Control

Although it’s not considered a primary flight control like the collective, cyclic and pedal; the pilot does have throttle control of an engine.  It’s typically a twist grip on the collective stick (there will be 2 such grips for a dual engine helicopter) that the pilot can turn/roll to override the engine electronic controller, e.g. if an engine controller malfunctions.  A pilot will rarely if ever use this in flight - the default 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).