Propeller Speed Reduction Unit
Encyclopedia
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. This allows the use of small displacement
internal combustion automotive engines to turn aircraft propeller
s within an efficient speed range.
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. 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.
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 Continental Tiara series
engines were designed to drive their propellers directly from the camshaft
(running at half engine speed).
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.
Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...
or a belt and pulley device used to reduce the output revolutions per minute
Revolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
(rpm) from the higher input rpm of the powerplant. This allows the use of small displacement
Engine displacement
Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...
internal combustion automotive engines to turn aircraft propeller
Propeller (aircraft)
Aircraft propellers or airscrews convert rotary motion from piston engines or turboprops to provide propulsive force. They may be fixed or variable pitch. Early aircraft propellers were carved by hand from solid or laminated wood with later propellers being constructed from metal...
s within an efficient speed range.
History and operation
The Wright brothersWright brothers
The Wright brothers, Orville and Wilbur , were two Americans credited with inventing and building the world's first successful airplane and making the first controlled, powered and sustained heavier-than-air human flight, on December 17, 1903...
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. 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
Types of propeller speed reduction units include:- Chain drive
- Single reduction or spur gear.
- Internal spur gear.
- FarmanFarmanFarman Aviation Works was an aeronautic enterprise founded and run by the brothers; Richard, Henri, and Maurice Farman. They designed and constructed aircraft and engines from 1908 until 1936; during the French nationalization and rationalization of its aerospace industry, Farman's assets were...
or bevelBevel gearBevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped.Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well...
planetarySun and planet gearThe sun and planet gear was a method of converting reciprocal motion to rotary motion and was utilised in a reciprocating steam engine....
type. - Fixed sun gear.
- Fixed internal gear
- Belt driveBelt (mechanical)A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the...
- Epicyclic gearingEpicyclic gearingEpicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear...
Design variations
The Rolls-Royce Falcon engine of 1915 featured epicyclicEpicyclic gearing
Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear...
propeller reduction gearing which contained a clutch
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
designed to limit the maximum torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
, thus protecting the reduction gears.. The later Merlin
Rolls-Royce Merlin
The Rolls-Royce Merlin is a British liquid-cooled, V-12, piston aero engine, of 27-litre capacity. Rolls-Royce Limited designed and built the engine which was initially known as the PV-12: the PV-12 became known as the Merlin following the company convention of naming its piston aero engines after...
engine from the same company used opposite rotation reduction gears to provide counter-rotating propellers
Counter-rotating propellers
Counter-rotating propellers, found on twin- and multi-engine propeller-driven aircraft, spin in directions opposite one another.The propellers on both engines of most conventional twin-engined aircraft spin clockwise . Counter-rotating propellers generally spin clockwise on the left engine and...
for twin-engined aircraft, a much cheaper method than designing and building the engines to run in opposite directions.
The Continental Tiara series
Continental Tiara series
|-See also:-External links:* * * *...
engines were designed to drive their propellers directly from the camshaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
(running at half engine speed).
Applications
The use of a PSRU is common in the construction of experimental homebuilt aircraftHomebuilt aircraft
Also known as amateur-built aircraft or kit planes, homebuilt aircraft are constructed by persons for whom this is not a professional activity. These aircraft may be constructed from "scratch," from plans, or from assembly kits.-Overview:...
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
Revolutions per minute
Revolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
(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
Cessna 175
thumb|right|Cessna 175C Skylark The Cessna 175 Skylark is a four-seat, single-engine, high-wing airplane produced between 1958 and 1962.-Production history:The 175 was designed to fill a niche between the Cessna 172 and the faster Cessna 182...
used a geared unit on the Continental GO-300 engine.