Propeller speed reduction unit
![The propeller speed reduction unit of a Rolls-Royce R engine](/Images/godic/202502/04/RollsRoyceRpropreductiongear2627.JPG")
![](/Images/godic/202502/04/Gears_animation2627.gif")
A propeller speed reduction unit is a gearbox or a belt and pulley device used to reduce the output revolutions per minute (rpm) from the higher input rpm of the powerplant. This allows the use of small displacement internal combustion automotive engines to turn aircraft propellers within an efficient speed range.