Propeller Speed Reduction Unit

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.

The Wright brothers recognised the need for propeller reduction gearing in 1903, but it was not generally used on aircraft engines until larger engines were designed in the 1920s. Large engines with high crankshaft speeds and power outputs demanded propeller reduction, pilots noted the increase in performance of similar aircraft fitted with reduction gearing.

Types of propeller speed reduction units include:

The Rolls-Royce Falcon engine of 1915 featured epicyclic propeller reduction gearing which contained a clutch designed to limit the maximum torque, thus protecting the reduction gears. The later Merlin engine from the same company used opposite rotation reduction gears to provide counter-rotating propellers for twin-engined aircraft, a much cheaper method than designing and building the engines to run in opposite directions.

The challenge with coupling gearboxes to internal combustion engines is primarily the torsional resonance that can develop at certain speeds. The load of a propeller and reduction unit can alter the resonant frequencies of the crankshaft, allowing torsional vibration to increase rapidly to high levels at certain rotational speeds. Measures taken by the designer to mitigate torsional resonances in the original design of the engine can be rendered ineffective if the resonant frequency is altered by the use of a reduction unit. Clutches and/or flexible couplings are sometimes used to prevent torsional resonance from reaching damaging levels.

The Continental Tiara series engines were designed to drive their propellers directly from the camshaft (running at half engine speed).

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