What is the best power source for a helicopter? The two choices are a turboshaft or a reciprocating engine. A turboshaft engine has the same basic structure as a turbojet; however, the energy produced by the expanding gases is used to drive a turbine instead of producing thrust. The turbine is connected to a gearbox that drives the helicopter’s main rotor transmission. Likewise, the reciprocating engine’s output drives the main rotor transmission; however, these engines have traditionally been viewed as less reliable.
To understand where that reputation came from we need to look at early helicopter designs. Helicopter manufactures took piston engines used in airplanes and installed them in their helicopters. However, these engines didn’t quite have enough horsepower for hovering. So to increase the power, manufactures ran the engines at a higher rpm, and as a result reliability suffered. So much so that Lycoming reduced the TBO on the O-360 from 2,000 hours to 1,600 hours for engines installed in helicopters. This fueled the unreliable reputation of the piston engine.
In 1979 Frank Robinson introduced the two-seat R22. His idea was to reduce the helicopter’s weight to reduce the power required. For example, the T-bar cyclic system is simple and weighs less than the conventional dual control system. He then took the reliable Lycoming O-320 engine and reduced the rpm from 2,700 to 2,652 and de-rated the maximum horsepower from 160 to 124. Lycoming then approved the same 2,000-hour TBO it had for fixed-wing installations. He did the same thing with the R44’s Lycoming O-540 engine. The engine’s reliability proved so good that Lycoming increased the TBO to 2,200 hours for both airframes, giving these helicopter installations a higher TBO than the same engine installed in a fixed wing. NTSB accident data supports the higher reliability achieved by derating a reciprocating engine.
Even with the vast improvement in reliability, reciprocating engines suffer from a low power to weight ratio. So for helicopters above about 2,500 lbs gross weight, a turbine engine makes sense. It is compact, light weight, and has a simple design that gives it excellent reliability. However, perhaps the most important feature is its high power-to-weight ratio. This makes turboshaft engines the only choice for large single and all twin-engine helicopters. However, the downside to these engines is the high cost to acquire, maintain, and operate them.
Tags: Tim McAdams