Helicopter Piston Engine Maintenance

Helicopter Piston Engine Maintenance By Greg Napert April 2000 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...

A critical part of properly maintaining Enstrom's engines is to understand their turbocharging system. Burdue says it's important to understand that with a turbocharger, you always develop deck or manifold pressure whether the turbocharger is working. On a normal non-turbo engine this would normally be around 27.5 inches of manifold pressure, yet with a turbocharger, you develop at least 30 inches of pressure, even if it isn't working. This is important to know for troubleshooting purposes. Just because you have 30 inches of pressure, it doesn't mean the turbocharger is working correctly. Turbocharger TBO is around 2,000 hours and but that depends on how it is used and if it's abused.

"Another problem area we see in terms of troubleshooting is the engine operating at high density altitude due to elevated intake temperatures," says Burdue. "This can be attributed to where the air filter intake assembly mounts to the intake pipe. If it is not seated correctly and there is a gap behind the air inlet the engine air, which is hot inside the compartment, will blow out and leak across the gasket and get sucked back into the intake. This will require that the engine make more power to do the same job. By ensuring this area is sealed, you can gain up two to three more inches of manifold pressure."

Burdue explains, "We use a dual Bendix magneto on our turbocharged engines. Again, the high heat in this engine compartment can contribute to premature wear on the magneto points, and this requires more frequent inspection. We recently have developed a magneto cooling kit to blow outside air into the magneto area to help cool it down. And we strongly recommend this kit in southern climates where outside air temperatures are high to begin with — and especially if you are doing a lot of prolonged hovering, such as in training exercises. This is a fairly inexpensive modification that is well worth it. Other operators who have experienced this problem simply schedule a point change every 100 hours (at the 100-hour inspection). Many helicopters don't need it, however and particularly if you operate the helicopter in a cool climate."

Another consideration with these magnetos is the critical nature of the magneto timing. Newer engines are timed at 20 degrees, and the old "A" model is timed at 25 degrees. Unfortunately, you're forced to look through a small hole in the fan to align a small dot in the magneto, and this can be difficult to do correctly. If you time the magneto only two degrees out of time, it will have a negative impact on power.

"We stress getting the engine at top dead center (using one of several methods) on compression and then precisely lining up the marks on the magneto and then double checking to achieve proper timing. We also stress how to tell by instrument readings that the engine is out of time. For instance, if it is slightly advanced, you will have the engine run better (lots of power) with the EGTs being on the cool side. This may seem better, but it is actually detrimental to the engine as it may result in carbon fouling or other related problems," says Burdue.

Another item that is unique to Enstrom engines is the "correlated" throttle system. The correlated throttle is a system that connects throttle linkage to collective lever movement. The result is that you set the throttle at one place, and then, as the collective pitch is increased, the throttle automatically increases to provide the power you need to keep the engine rpm constant. "This gives the recip helicopter the feel of a turbine helicopter," says Burdue.

Correlation is accomplished through proper rigging of the throttle cables. It's an all mechanical system that uses cables, pulleys, and cams.

Burdue continues, "Unfortunately, rigging the correlator can be a bit confusing as you are balancing one part of the system against another and you have to keep going back and forth in order to achieve the desired adjustment. We like to impress always starting from step one of the post-flight rigging procedure and then following the procedure until you find what is out of rig — then making the necessary adjustments. Don't start with the pre-flight rigging procedures (preflight rigging is set at the factory and seldom needs adjustment) and don't just take a guess at what needs adjusting as you'll become even more confused and then make it difficult to get it back into rig.

"One of the tools we like to encourage technicians to use on our engines is the instruments in the cockpit and particularly the exhaust gas temperature on all of our helicopters and the graphic engine monitor that comes standard with some of our models and is optional on others," he says.

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