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2 Answers

Constant Speed VS Full-Feathering Propellers

Asked by: 6772 views , , , ,
Aircraft Systems, Commercial Pilot

I found constant speed and full-feathering props are not acting in the same way. There must be reasons since making both systems in the same way would save manufacturing cost but I couldn't find why. My questions are 1. Why Constant speed and full-feathering props have reverse engine oil flow and piston position? I understand full-feathering could make coarse pitch if oil pressure is lost but how about constant speed? once oil pressure is lost, constant speed prop would make fine pitch due to spring pushing the piston and twisting centrifugal force. 2. Why Full-feathering props have counterweights while constant speed don't? Both full-feathering and constant speed props have twisting centrifugal force as I understand but why only full-feathering props have counterweights? Thank you all for enlightening me in advance and fly fix my understanding if there is. I read McCauley propeller system technology guide and it is a great material if you haven't read it yet. http://www.mccauley.textron.com/tech_guide.pdf

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2 Answers



  1. Mark Kolber on Nov 16, 2015

    I suspect a number of folks seeing your question didn’t quite understand it. That probably includes me but here is a shot.

    My non-expert opinion is that, like most else in aviation, there is a compromise between desired performance on the one hand and weight and cost on the other. I figure the parts and work that go into full-feathering setup are simply more expensive and/or the assembly heavier.

    That makes sense for a twin where the effects of yaw in the case of the loss of the critical engine make the removal of as much drag as possible a very high priority. OTOH, the slight gain in glide distance that might be gained by feathering in the typical single is probably close to negligible.

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  2. Brian on Nov 17, 2015

    Both systems are constant speed system. Each of them are absolutely mechanically identical in their constant speed operation except that the flow of oil is reversed. The reasoning here is that in a single engine you want the engine to go to a flat pitch so as to allow the possibility for a restart. Where as in a twin you\’d rather it feather.

    In both systems the pilot sets the speeder spring with the blue handle to command an rpm. The system then maintains that rpm with flyweights that move a pilot valve up or down during over/under speeding. The movement of the pilot valve guides oil into or out of the system to manipulate blade pitch and arrest the over/under speed condition keeping the rpm constant.

    The only other difference in the system is the feathering rod and, in some aircraft, a backup system, usually some form of highly pressurized gas, to take the engine out of feather for a restart.

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