Propeller speed reduction unit
| Propeller speed reduction unit | |
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| The propeller reduction unit of a Rolls-Royce R engine |
A Propeller Speed Reduction Unit (PSRU) 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.[1] This allows the use of small displacement internal combustion automotive engines to turn aircraft propellers within an efficient speed range.
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[edit] History and operation
The Wright brothers recognised the need for propeller reduction gearing in 1903, it was not generally used on aircraft engines until larger engines were designed in the 1920s.[1] 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.[1]
[edit] Types
Types of propeller speed reduction units include:[1]
- Chain drive
- Single reduction or spur gear.
- Internal spur gear.
- Farman or bevel planetary type.
- Fixed sun gear.
- Fixed internal gear
- Belt drive
- Epicyclic gearing
[edit] Design variations
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.[2]. 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.[3]
The Continental Tiara series engines were designed to drive their propellers directly from the camshaft (running at half engine speed).[4]
[edit] Applications
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 175 used a geared unit on the Continental GO-300 engine.
[edit] See also
[edit] References
[edit] Notes
[edit] Bibliography
- Flight No. 1935, Volume XLIX, 24 January 1946.
- Gunston, Bill. Development of Piston Aero Engines. Cambridge, England. Patrick Stephens Limited, 2006. ISBN 0-7509-4478-1
- Guttery, T.E. The Shuttleworth Collection. London: Wm. Carling & Co, 1969. ISBN 901319-01-5
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