Helicopter Piston Engine Maintenance
For the most part, piston-powered helicopters are a relatively rare breed. Turbine-powered helicopters are more popular because of their increased utility and also for what many consider to be better reliability.
Yet, there continues to be a strong market for piston-powered helicopters for the definite advantages that they offer and namely low cost, light weight, and high maneuverability.
Although piston-powered helicopters are used primarily for personal transportation, they are also employed in law enforcement (surveillance), power line patrol, sightseeing, electronic news gathering operations, and helicopter flight training schools.
By far, most of these helicopters, to include the Robinson, Enstrom, and Schweizer lines, are powered by Lycoming engines. There are some exceptions, but they are rare.
If there is one difference between maintaining piston-powered helicopters versus fixed-wing, it is that helicopter engines require quite different treatment from installation to installation. Don't ever make the mistake of assuming that because the helicopter contains a theoretically similar engine to its fixed-wing counterpart, it should be maintained the same. You really have to take the airframe, operating environment, and the instructions of the helicopter manufacturer into consideration in your approach to maintaining these engines. Following are two examples of helicopter installations that illustrate the radical differences in the maintenance approach — Robinson and Enstrom.
Patrick Cox, customer support for Robinson, explains that the engines it uses in its newest helicopters are not Lycoming's H-Model engines that are typically used in helicopter installations. Neither is the engine on the Robinson a V-series or an I-series engine. It is, instead, a slightly modified Lycoming 360 (used on the R-22) or 540 (used on the R-44) Lycoming mounted horizontally, as in a fixed-wing application, except that it is pointed backward. Most of the older R-22s use a 320 Lycoming.
The difference in these engines from their fixed-wing counterparts, according to Cox, is that the cylinders are manufactured to be thinner in order to reduce the weight by 1-1/2 lbs. per cylinder.
"As a consequence, we de-rate these engines for our installation. For example, an O-360 will be rated at 145 horsepower on the data plate and even though it is theoretically capable of 180 hp, as on fixed-wing installations," he says.
"Lycoming makes these cylinders up for us to our specifications. It was critical that we reduce the weight of the engine for the application, so Lycoming came up with a lighter engine design. The engines that we have done this to are the O-360 and the O-540-F1B5. Other than the cylinders, there are no other changes to the basic engine and the block is still the same and the fuel control system is the same," says Cox.
Further, even though the engine is rated at 145 hp on the data plate, it's de-rated further in operation by only allowing 131 hp for takeoff. The thinner cylinder walls dictate that the horsepower rating on the engine be reduced. The advantage to de-rating the engine, however, is that there is significantly reduced wear and tear on the remainder of the engine.
According to Cox, the thinner cylinders have no adverse affect on the life of the engine. In fact, Cox says that Robinson is trying to extend the TBO to 2,200 hours from 2,000. This would work well for Robinson because their airframe TBO is 2,200 hours and making it convenient for maintenance purposes.
Cox explains that further contributing to the low wear and tear on its engines is the fact that the engine is not subjected to excessive gyroscopic loading from a fast-spinning propeller. The engine, on this helicopter is isolated from unusual loading through the gearbox.