Propeller speed reduction unit

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The propeller speed reduction unit of a Rolls-Royce R engine RollsRoyceRpropreductiongear.JPG
The propeller speed reduction unit of a Rolls-Royce R engine

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. [1] This allows the use of small displacement internal combustion automotive engines to turn aircraft propellers within an efficient speed range.

Transmission (mechanics) machine in a power transmission system for controlled application of the power;gearbox,uses gears/gear trains to provide speed,torque conversions from a rotating power source to another device;reduces the higher engine speed to the slower wheel speed

A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device.

Revolutions per minute is the number of turns in one minute. It is a unit of rotational speed or the frequency of rotation around a fixed axis.

Engine displacement

Engine displacement is the swept volume of all the pistons inside the cylinders of a reciprocating engine in a single movement from top dead centre (TDC) to bottom dead centre (BDC). It is commonly specified in cubic centimetres, litres (L), or cubic inches (CID). Engine displacement does not include the total volume of the combustion chamber.


History and operation

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. [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]

Wright brothers American aviation pioneers, inventors of the airplane

The Wright brothers, Orville and Wilbur, were two American aviation pioneers generally credited with inventing, building, and flying the world's first successful airplane. They made the first controlled, sustained flight of a powered, heavier-than-air aircraft with the Wright Flyer on December 17, 1903, four miles south of Kitty Hawk, North Carolina. In 1904–05, the brothers developed their flying machine into the first practical fixed-wing aircraft, the Wright Flyer III. Although not the first to build experimental aircraft, the Wright brothers were the first to invent aircraft controls that made fixed-wing powered flight possible.


Simple spur gear reduction Gears animation.gif
Simple spur gear reduction

Types of propeller speed reduction units include: [1]

Bevel gear

Bevel 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. The pitch surface of bevel gears is a cone.

Epicyclic gearing consists of two gears mounted so that the center of one gear revolves around the center of the other

An epicyclic gear train consists of two gears mounted so that the centre of one gear revolves around the centre of the other. A carrier connects the centres of the two gears and rotates to carry one gear, called the planet gear, around the other, called the sun gear. The planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve. In this simplified case, the sun gear is fixed and the planetary gear(s) roll around the sun gear.

Belt (mechanical) loop of flexible material used to mechanically link rotating shafts

A belt is a loop of flexible material used to link two or more rotating shafts mechanically, most often parallel. Belts may be used as a source of motion, to transmit power efficiently or to track relative movement. Belts are looped over pulleys and may have a twist between the pulleys, and the shafts need not be parallel.

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]

Rolls-Royce Falcon V-12 piston aircraft engine

The Rolls-Royce Falcon is an aero engine developed in 1915. It was a smaller version of the Rolls-Royce Eagle, a liquid-cooled V-12 of 867 cu in capacity. Fitted to many British World War I-era aircraft, production ceased in 1927. The Falcon was designed by R.W. Harvey-Bailey.

Clutch machine element for rigid, elastic, movable or releasable connection of two shafts

A clutch is a mechanical device which engages and disengages power transmission especially from driving shaft to driven shaft.

Torque tendency of a force to rotate an object

Torque, moment, or moment of force is the rotational equivalent of linear force. The concept originated with the studies of Archimedes on the usage of levers. Just as a linear force is a push or a pull, a torque can be thought of as a twist to an object. The symbol for torque is typically , the lowercase Greek letter tau. When being referred to as moment of force, it is commonly denoted by M.

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.

Resonance phenomenon in which a vibrating system or external force drives another system to oscillate with greater amplitude at specific frequencies

In mechanical systems, resonance is a phenomenon that only occurs when the frequency at which a force is periodically applied is equal or nearly equal to one of the natural frequencies of the system on which it acts. This causes the system to oscillate with larger amplitude than when the force is applied at other frequencies.

Torsional vibration is angular vibration of an object—commonly a shaft along its axis of rotation. Torsional vibration is often a concern in power transmission systems using rotating shafts or couplings where it can cause failures if not controlled. A second effect of torsional vibrations applies to passenger cars. Torsional vibrations can lead to seat vibrations or noise at certain speeds. Both reduce the comfort.

The Continental Tiara series engines used a single gearset to drive both the propeller and the camshaft, this allowed the propeller to run at half the engine speed. [4]


The use of propeller reduction gearing was very common during the height of piston engine use in aviation (the 1930s through the 1940s), with essentially all of the most powerful piston engines ever built for use in aircraft being designed to make use of reduction gearing.

The use of a reduction unit is common in the construction of experimental homebuilt aircraft when automotive engines may be used. These engines, in addition to their lower cost, typically have less displacement than purpose-built light aircraft engines and develop peak power at high revolutions per minute (rpm), typically above 4,000 rpm. Traditional aircraft engines, where the propeller is most commonly 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 due to the need to keep the propeller tip speed below the speed of sound.

Factory-certified aircraft engines have also used reduction units integral to their design. The Cessna 175 used a geared unit which is part of the Continental GO-300 engine while the Helio Courier and several Beechcraft twins, among others, used the geared Lycoming GO-435 and GO-480. Many light sport aircraft use engines from Rotax such as the Rotax 912 which incorporates a geared reducer.

See also

Related Research Articles

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Reduction drive Device, containing a different gears.

A reduction drive is a mechanical device to shift rotational speed. A planetary reduction drive is a small scale version using ball bearings in an epicyclic arrangement instead of toothed gears.

Harmonic damper

A harmonic damper is a device fitted to the free end of the crankshaft of an internal combustion engine to counter torsional and resonance vibrations from the crankshaft. This device must be interference fit to the crankshaft in order to operate in an effective manner. An interference fit ensures the device moves in perfect step with crankshaft. It is essential on engines with long crankshafts and V8 engines with cross plane cranks. Harmonics and torsional vibrations can greatly reduce crankshaft life, or cause instantaneous failure if the crankshaft runs at or through an amplified resonance. Dampers are designed with a specific weight (mass) which is dependent on the damping material/method used and the freedom it affords the mass move allowing to reduce mechanical Q factor, or damp, crankshaft resonances. A harmonic balancer is the same thing as a harmonic damper except that the balancer includes a counterweight to externally balance the rotating assembly. The harmonic balancer often serves as a pulley for the accessory drive belts turning the alternator, water pump and other crankshaft driven devices.

Rotax 185

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Fiat AS.6

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Pratt & Whitney R-1860 Hornet B 575 hp radial aircraft engine

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Packard X-2775 aircraft piston engine - X configuration

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Aircraft engine starting

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A cam engine is a reciprocating engine where, instead of the conventional crankshaft, the pistons deliver their force to a cam that is then caused to rotate. The output work of the engine is driven by this cam.

The Rotax 915 iS is an Austrian aircraft engine, produced by Rotax of Gunskirchen for use in ultralight aircraft, homebuilt aircraft, light-sport aircraft, small helicopters and gyroplanes. The engine was type certified in 2017.



  1. 1 2 3 4 Gunston 2006, p. 82.
  2. Guttery 1969, p.27.
  3. Flight 1946, pp. 92–94.
  4. Gunston 2006, p. 191.


  • 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   0-901319-01-5