What is the purpose of exercising a McCauley prop?
Answers:
While it is a good idea to insure the prop is working properly, more importantly excercising the prop exchanges the oil that has been sitting in the hub over night (or even longer).
McCauley governors control PRM by hydraulically adjusting the blade angles with oil via the aircrafts oil system. Oil that sits in the prop hub becomes very thick. By excercising the props, it is exchanged with warmerer, more free flowing oil that has passed through the engine. This is especially important to do in cold weather conditions. A quick cycle won't do the trick either. Too quick a cycle, and the oil will not be exchanged. The prop control must drop back to the tach reading specified by the manufacturer for a good exchange.
If the oil in the hub remains cool and thick, this can cause the prop not to hold it's preset RPM ("hunting") or have poor response to prop control changes. Extended ops at high, cold altitudes can cause this issue as well. Periodically changing the RPM (within MP limits) can help limit that problem.
McCauley constant speed props are used on lots of aircraft, not just the Cessna 421.
The purpose of exercising the prop during the pre-flight run-up is to insure they're working properly before attempting to launch the aircraft. If they're not working properly, it could be an indication of low engine oil pressure, or other impending mechanical failure. don't attempt to take off.
You see the prop gets it's oil pressure from the crank via a collar that is inside the engine. The collar transfers oil into the prop. The way this was explained to me was, the collar is only oiled when the prop is actuating. In flight the prop is always searching, (searching for ideal blade angle) but on the ground it's not. so to compensate for this you must. "exercises the prop"
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