Governing a piston engine
One issue that had always plagued the piston engine was how to effectively govern it. Some of the original governors were slightly more than crutches that barely kept the RPM’s at a steady rate. Robinson set out to develop a governing system to install on its helicopters that would be able to govern as good as the turbine ones were able to.
Initial R-22’s used a correlator to govern the engine. It consisted of a mechanical linkage in the throttle system that tied it to the collective input. It was a good system, and worked better than other correlators on piston helicopters, but it only performed a rough governing job. It was not able to handle fine adjustments in RPM. The turbine engine governors still reigned supreme.
Robinson continued experimenting with governing systems in order to obtain a more precise control of RPM. They ended up installing an electronic system that governed the engine directly. This system applies throttle through a friction clutch – increasing or decreasing the throttle. The system works by first taking a signal from a set of points that are put in the spark system. An electronic circuit converts this signal into a usable signal that controls permanent magnet DC motors. These motors control the throttle by having a direct input into the throttle linkage, using a friction clutch. After numerous flight tests and tweaking, they were finally able to get the combination of both damping and static stability that worked the best.
When it comes to the actual governing duties, the correlator and electronic governor each provide different input into the process. The fine RPM adjustments are taken care of by the electronic governor while the large changes, such as during take off or going into a descent, are still controlled by the mechanical correlator. The new electronic governor added the fine tuning that Robinson had been striving to achieve. The governing system actually holds the RPM as close as the turbine engine governors do.
There are two safeguards to the governing system that provide protection in the event of governor malfunction. One is the way the throttle is set up. It is designed so that the governor can be overriden by applying firm input into the throttle. The other is a switch on the end of the collective that can directly switch the governor off.
This new governing system was incorporated in the mid 1990’s. It has since been retrofitted on all R-22’s.
One problem inherent to carburetors is that of icing. In the R-22, this problem is solved by the introduction of carburetor heating assist. The heating assist used is a linkage utilizing a friction clutch arrangement so that carburetor heat can be added or subtracted manually just like in other systems. But with the automatic carburetor heat assist, as long the carburetor heat is left on "locked," then when the collective is raised, the carburetor heat is reduced. Likewise, when the collective is lowered, carburetor heat is added. So when the aircraft enters a descent (where carburetor icing is most likely to occur), the system will automatically increase carburetor heat. Additonally, at the bottom of a descent, when the aircraft is going into a hover, the carburetor heat assist is automatically turned off. This allows for an additional inch and a half of manifold pressure, allowing for maximum power at this important stage.
It appears that by designing a helicopter that operates at lower RPM’s, Robinson has been able to produce a product with a respected reliability track record and increased TBO time. The added systems of governing and carburetor heat assist help to make the aircraft a viable choice for many helicopter operators.
One issue inherent to helicopters is that of high engine operating temperatures. Helicopters that are hovering or not moving forward fast do not have a lot of cooling air available. The problem is increased when student training is involved due to the amount of hovering practice. It is also worse in hotter environments like Florida or Arizona. These higher operating temperatures could lead to coking formation in the valve guide area that might eventually stick a valve.
Textron Lycoming has issued service bulletin 388B that addresses the issue of valve guide clearance. This inspection helps prevent engine failure due to sticking valves which can be the result of excessive carbon build up between the valve guide and valve stem. It also helps to detect excessive wear (bell-mouthing) of the exhaust valve guide that could lead to broken exhaust valves. Textron Lycoming recommends that helicopter engines have this inspection performed every 300 hours.