Propeller speed reduction unit
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A Propeller Speed Reduction Unit (PSRU) is a gearbox or a belt and pulley device used to reduce the output revolutions per minute (rpm) when compared to the higher input rpm of the powerplant. These arrangements allow the use of small displacement internal combustion automotive engines to turn propellers within an efficient speed range.
The use of a PSRU is common in the construction of experimental homebuilt aircraft when automotive engines may be used. These engines, in addition to their lower cost, are customarily smaller in cubic inch displacement and develop peak torque at high revolutions per minute (rpm), typically near 4,500 rpm. True aircraft engines, where the propeller most commonly is fastened directly to the engine crankshaft, develop peak power near the peak safe and efficient speed for the propeller--2,500 to 3,000 rpm. This speed is considered the typical maximum rpm for a single engine aircraft propeller.
Note that there are examples of factory certified aircraft that have used a PSRU. The Cessna 170 used a a geared unit during its production run the 1970s.
