Propeller speed reduction unit
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A Propeller Speed Reduction Unit is a gearbox used in some airplanes so to allow fast engines to turn propellers within an efficient speed range.
Experimental homebuilt aircraft are often built with automotive engines that develop peak power at high rpm (unlike aircraft engines, which are usually designed to develop peak power at optimum propeller speeds). A propeller that spins too fast will have its tips go supersonic and lose efficiency, so a PSRU is used to gear down the speed of the automotive engine to a range better suited for aircraft propellers, usually between 2,500 and 3,000 rpm (compared to 6,000+ rpm revs where these engines provide the most power).
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