Propeller speed reduction unit
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A Propeller Speed Reduction Unit (PSRU) is a gearbox used in some airplanes to allow small, fast-turning engines to turn propellers within an efficient speed range.
Experimental homebuilt aircraft are often built with automotive engines that develop peak power at high revolutions per minute (rpm), unlike aircraft engines, which are usually designed to develop peak power at optimum propeller speeds. The tips of a propeller that spins too fast will go supersonic and lose efficiency. A PSRU is, therefore, used to reduce the speed of the automotive engine output shaft to a range better suited for aircraft propellers, usually between 2 500 and 3 000 rpm, compared its maximum-power speed of 6 000+ rpm.
