In aeronautics, a variable-pitch propeller is a type of propeller (airscrew) with blades that can be rotated around their long axis to change the

blade pitch Blade pitch or simply pitch refers to the angle of a blade in a fluid. The term has applications in aeronautics, shipping, and other fields. Aeronautics In aeronautics, blade pitch refers to the angle of the blades of an aircraft propeller o ...

. A controllable-pitch propeller is one where the pitch is controlled manually by the pilot. Alternatively, a constant-speed propeller is one where the pilot sets the desired

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ...

speed (

RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

), and the blade pitch is controlled automatically without the pilot's intervention so that the rotational speed remains constant. The device which controls the propeller pitch and thus speed is called a propeller governor or constant speed unit. Reversible propellers are those where the pitch can be set to negative values. This creates

reverse thrust Thrust reversal, also called reverse thrust, is the temporary diversion of an aircraft engine's thrust for it to act against the forward travel of the aircraft, providing deceleration. Thrust reverser systems are featured on many jet aircraft to ...

for braking or going backwards without the need to change the direction of shaft revolution. Some aircraft have

ground-adjustable propeller A ground-adjustable propeller is a simple type of aircraft variable-pitch propeller where the blade angle can be adjusted between pre-set limits of fine and coarse pitch.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 86. Av ...

s, however these are not considered variable-pitch. These are typically found only on

light aircraft A light aircraft is an aircraft that has a maximum gross takeoff weight of or less.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 308. Aviation Supplies & Academics, 1997. Light aircraft are used as utility aircraft co ...

and

microlights Ultralight aviation (called microlight aviation in some countries) is the flying of lightweight, 1- or 2-seat fixed-wing aircraft. Some countries differentiate between weight-shift control and conventional three-axis control aircraft with aile ...

Purpose

When an aircraft is stationary with the propeller spinning (in calm air), the

relative wind In aeronautics, the relative wind is the direction of movement of the atmosphere relative to an aircraft or an airfoil. It is opposite to the direction of movement of the aircraft or airfoil relative to the atmosphere. Close to any point on the su ...

vector for each propeller blade is from the side. However, as the aircraft starts to move forward, the relative wind vector comes increasingly from the front. The propeller blade pitch must be increased to maintain optimum angle of attack to the relative wind. The first propellers were fixed-pitch, but these propellers are not efficient over a range of conditions. If the propeller blade angle is set to give good takeoff and climb performance, the propeller will be inefficient in cruising

flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...

because the blade will be at too low an angle of attack. In contrast, a propeller set for good cruise performance may stall at low speeds, because the angle of attack is too high. A propeller with adjustable blade angle is more efficient over a range of conditions. A propeller with variable pitch can have a nearly constant efficiency over a range of airspeeds. A shallower angle of attack requires the least torque, but the highest

, because the propeller is not moving very much air with each revolution. This is similar to a car operating in

low gear A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission ...

. When the motorist reaches cruising speed, they will slow down the engine by shifting into a higher gear, while still producing enough power to keep the vehicle moving. This is accomplished in an airplane by increasing the angle of attack of the propeller. This means that the propeller moves more air per revolution and allows the engine to spin slower while moving an equivalent volume of air, thus maintaining velocity. Another use of variable-pitch propellers is to feather the blades of the propeller, in order to reduce drag. This means to rotate the blades so that their leading edges face directly forwards. In a multi-engine aircraft, if one engine fails, it can be feathered to reduce drag so that the aircraft can continue flying using the other engine(s). In a single-engine aircraft, if the engine fails, feathering the propeller will reduce drag and increase glide distance, providing the pilot with more options for the location of a

forced landing A forced landing is a landing by an aircraft made under factors outside the pilot's control, such as the failure of engines, systems, components, or weather which makes continued flight impossible. For a full description of these, see article on ' ...

Mechanisms

Three methods are used to vary the pitch: oil pressure, centrifugal weights, or electro-mechanical control. Engine

oil pressure {{unreferenced, date=May 2016 Oil pressure is an important factor in the longevity of most internal combustion engines. With a forced lubrication system (invented by Frederick Lanchester), oil is picked up by a positive displacement oil pump and f ...

is the usual mechanism used in commercial propeller aircraft and the Continental and Lycoming engines fitted to light aircraft. In aircraft without a constant speed unit (CSU), the pilot controls the propeller blade pitch manually, using oil pressure. Alternatively, or additionally, centrifugal weights may be attached directly to the propeller as in the

Yakovlev Yak-52 The Yakovlev Yak-52 (russian: Яковлев Як-52) is a Soviet primary trainer aircraft which first flew in 1976. It was produced in Romania from 1977 to 1998 by Aerostar, as ''Iak-52'', which gained manufacturing rights under agreement wit ...

. The first attempts at constant-speed propellers were called counterweight propellers, which were driven by mechanisms that operated on

centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is parall ...

. Their operation is identical to the

centrifugal governor A centrifugal governor is a specific type of governor with a feedback system that controls the speed of an engine by regulating the flow of fuel or working fluid, so as to maintain a near-constant speed. It uses the principle of proportional con ...

used by James Watt to limit the speed of steam engines. Eccentric weights were set up near or in the spinner, held in by a spring. When the propeller reached a certain RPM, centrifugal force would cause the weights to swing outwards, which would drive a mechanism that twisted the propeller into a steeper pitch. When the propeller slowed, the RPM would decrease enough for the spring to push the weights back in, realigning the propeller to the shallower pitch. Small, modern engines with a constant speed unit (CSU), such as the Rotax 912, may use either the conventional hydraulic method or an electrical pitch control mechanism. Hydraulic operation can be too expensive and bulky for

. Instead, these may use propellers that are activated mechanically or electrically.

Constant-speed propellers

A constant-speed propeller is a variable-pitch propeller that automatically changes its

in order to maintain a chosen rotational speed, regardless of the operational conditions of the aircraft. This is achieved by use of a constant-speed unit (CSU) or propeller governor, which automatically changes the propeller's

. Most engines produce their maximum power in a narrow speed band. The CSU allows the engine to operate in its most economical range of

rotational speed Rotational frequency (also known as rotational speed or rate of rotation) of an object rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second (cps), etc. The symbol fo ...

s, regardless of whether the aircraft is taking off or cruising. The CSU can be said to be to an aircraft what the continuously variable transmission is to the motorcar: the engine can be kept running at its optimum speed, regardless of the speed at which the aircraft is flying through the air. The CSU also allows aircraft engine designers to keep the ignition system simple: the automatic spark advance seen in motor vehicle engines is simplified, because aircraft engines run at a roughly constant RPM. Virtually all high-performance propeller-driven aircraft have constant-speed propellers, as they greatly improve fuel efficiency and performance, especially at high altitude. The first attempts at constant-speed propellers were called counterweight propellers, which were driven by mechanisms that operated on

. Their operation is identical to the

used by James Watt to control the speed of steam engines. Eccentric weights were set up near or in the spinner, held in by a spring. When the propeller reached a certain RPM, centrifugal force would cause the weights to swing outwards, which would drive a mechanism that twisted the propeller into a steeper pitch. When the propeller slowed, the RPM would decrease enough for the spring to push the weights back in, realigning the propeller to the shallower pitch. Most CSUs use oil pressure to control propeller pitch. Typically, constant-speed units on a single-engine aircraft use oil pressure to increase the pitch. If the CSU fails, the propeller will automatically return to fine pitch, allowing the aircraft to be operated at lower speeds. By contrast, on a multi-engine aircraft, the CSU will typically use oil pressure to decrease the pitch. That way, if the CSU fails, that propeller will automatically feather, reducing drag, while the aircraft continues to be flown on the good engine. An "unfeathering accumulator" will enable such a propeller to return to fine pitch for an in-flight engine restart. Operation in a single-engine aircraft is as follows: Oil is pumped through the propeller shaft to push on a piston that drives the mechanism to change pitch. The flow of oil and the pitch are controlled by a governor, consisting of a speeder spring, flyweights, and a pilot valve. The tension of the spring is set by the propeller control lever, which sets the RPM. The governor will maintain that RPM setting until an engine overspeed or underspeed condition exists. When an overspeed condition occurs, the propeller begins to rotate faster than the desired RPM setting. This would occur as the plane descends and airspeed increases. The flyweights begin to pull outward due to centrifugal force which further compresses the speeder spring. As that happens, the piston moves forward, allowing the pilot valve to open and oil to flow from the oil

sump A sump is a low space that collects often undesirable liquids such as water or chemicals. A sump can also be an infiltration basin used to manage surface runoff water and recharge underground aquifers. Sump can also refer to an area in a cave ...

into the hub. This increase in oil pressure will increase the pitch of the propeller angle causing it to slow back down to the desired RPM setting. When an underspeed condition occurs, such as a climb with a loss of airspeed, the opposite takes place. The airspeed decreases, causing the propeller to slow down. This will cause the flyweights to move inward due to a lack in centrifugal force, and tension will be released from the speeder spring. As this happens, the piston will move in the opposite direction causing the pilot valve to allow oil to flow from the hub back to the oil sump. The propeller blade angle will now decrease to a lower pitch allowing the propeller to speed back up to the desired RPM setting. This process usually takes place frequently throughout flight. A pilot requires some additional training and, in most jurisdictions, a formal sign-off before being allowed to fly aircraft fitted with a CSU. CSUs are not allowed to be fitted to aircraft certified under light-sport aircraft regulations in the United States.

History

A number of early aviation pioneers, including A. V. Roe and Louis Breguet, used propellers that could only be adjusted while the aircraft was on the ground. This was also the case in late

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

with one testbed example, "R.30/16", of the low-production (56 examples in 1917 and 1918) Zeppelin-Staaken R.VI German "giant" four-engined heavy bomber. In 1919 L. E. Baynes patented the first automatic variable-pitch airscrew.

Wallace Rupert Turnbull Wallace Rupert Turnbull (October 16, 1870 – November 24, 1954) was a Canadian engineer and inventor. The Saint John Airport was briefly named after him. He was inducted in Canada's Aviation Hall of Fame in 1977. Biography Born in Saint John, ...

of Saint John, New Brunswick, Canada is credited in Canada for creating the first variable pitch propeller in 1918. The French aircraft firm Levasseur displayed a variable-pitch propeller at the 1921

Paris Air Show The Paris Air Show (french: Salon international de l'aéronautique et de l'espace de Paris-Le Bourget, Salon du Bourget) is a trade fair and air show held in odd years at Paris–Le Bourget Airport in north Paris, France. Organized by the French ...

. The firm claimed that the French government had tested the device in a ten-hour run and that it could change pitch at any engine RPM. Dr Henry Selby Hele-Shaw and T.E. Beacham patented a hydraulically-operated variable-pitch propeller (based on a variable-stroke pump) in 1924 and presented a paper on the subject before the

Royal Aeronautical Society The Royal Aeronautical Society, also known as the RAeS, is a British multi-disciplinary professional institution dedicated to the global aerospace community. Founded in 1866, it is the oldest aeronautical society in the world. Members, Fellows, ...

in 1928; it met with scepticism as to its utility. The propeller had been developed with

Gloster Aircraft Company The Gloster Aircraft Company was a British aircraft manufacturer from 1917 to 1963. Founded as the Gloucestershire Aircraft Company Limited during the First World War, with the aircraft construction activities of H H Martyn & Co Ltd of Chelte ...

as the Gloster Hele-Shaw Beacham Variable Pitch propeller and was demonstrated on a

Gloster Grebe The Gloster Grebe was developed by the Gloster Aircraft Company from the Gloster Grouse (an experimental aircraft later developed as a trainer), and was the Royal Air Force's first post- First World War fighter aircraft, entering service in 1 ...

, where it was used to maintain a near-constant RPM. The first practical controllable-pitch propeller for aircraft was introduced in 1932. French firm

Ratier Ratier-Figeac is an aircraft components manufacturer in Figeac, France. From 1926 until 1930 it also built a car with a 746 cc overhead camshaft engine. From 1959 until 1962 Ratier made motorcycles, having taken over the motorcycle business of ...

pioneered variable-pitch propellers of various designs from 1928 onwards, relying on a special ball-bearing

helicoid The helicoid, also known as helical surface, after the plane and the catenoid, is the third minimal surface to be known. Description It was described by Euler in 1774 and by Jean Baptiste Meusnier in 1776. Its name derives from its similarity ...

al ramp at the root of the blades for easy operation. Walter S Hoover'
patent
for a variable pitch propeller was filed in the U.S. Patent Office in 1934. Several designs were tried, including a small bladder of pressurized air in the propeller hub providing the necessary force to resist a spring that would drive the blades from fine pitch (take-off) to coarse pitch (level cruising). At a suitable airspeed a disk on the front of the

spinner Technology *Spinner (aeronautics), the aerodynamic cone at the hub of an aircraft propeller * Spinner (cell culture), laboratory equipment for cultivating plant or mammalian cells * Spinner (computing), a graphical widget in a GUI * Spinner (MIT Med ...

would press sufficiently on the bladder's air-release valve to relieve the pressure and allow the spring to drive the propeller to coarse pitch. These "pneumatic" propellers were fitted on the

de Havilland DH.88 Comet The de Havilland DH.88 Comet is a British two-seat, twin-engined aircraft built by the de Havilland Aircraft Company. It was developed specifically to participate in the 1934 England-Australia MacRobertson Air Race from the United Kingdom to ...

aircraft, winner of the famed long-distance 1934

MacRobertson Air Race The MacRobertson Trophy Air Race (also known as the London to Melbourne Air Race) took place in October 1934 as part of the Melbourne Centenary celebrations. The race was devised by the Lord Mayor of Melbourne, Sir Harold Gengoult Smith, and th ...

and in the Caudron C.460 winner of the 1936 National Air Races, flown by . Use of these pneumatic propellers required presetting the propeller to fine pitch prior to take-off. This was done by pressurizing the bladder with a bicycle pump, hence the whimsical nickname ''Gonfleurs d'hélices'' (prop-inflater boys) given to the aircraft ground-mechanics in France up to this day. A Gloster Hele-Shaw hydraulic propeller was shown at the 1929 International Aero Exhibition at Olympia. American Tom Hamilton of the Hamilton Aero Manufacturing Company saw it and, on returning home, patented it there. As the

Hamilton Standard Hamilton Standard was an American aircraft propeller parts supplier. It was formed in 1929 when United Aircraft and Transport Corporation consolidated Hamilton Aero Manufacturing and Standard Steel Propeller into the Hamilton Standard Propeller C ...

Division of the United Aircraft Company, engineer Frank W. Caldwell developed a hydraulic design, which led to the award of the Collier Trophy of 1933. de Havilland subsequently bought up the rights to produce Hamilton propellers in the UK, while

Rolls-Royce Rolls-Royce (always hyphenated) may refer to: * Rolls-Royce Limited, a British manufacturer of cars and later aero engines, founded in 1906, now defunct Automobiles * Rolls-Royce Motor Cars, the current car manufacturing company incorporated in ...

and Bristol Engines formed the British company

Rotol Dowty Propellers is a British engineering company based in Brockworth, Gloucestershire that specialises in the manufacture, repair and overhaul of propellers and propeller components for customers around the world. It is owned by General Elect ...

in 1937 to produce their own designs. The French company of Pierre Levasseur and Smith Engineering Co. in the United States also developed controllable-pitch propellers.

Wiley Post Wiley Hardeman Post (November 22, 1898 – August 15, 1935) was a famed American aviator during the interwar period and the first pilot to fly solo around the world. Also known for his work in high-altitude flying, Post helped develop on ...

(1898-1935) used Smith propellers on some of his flights. Another electrically-operated mechanism was originally developed by Wallace Turnbull and refined by the

Curtiss-Wright Corporation The Curtiss-Wright Corporation is a manufacturer and services provider headquartered in Davidson, North Carolina, with factories and operations in and outside the United States. Created in 1929 from the consolidation of Curtiss, Wright, and v ...

. This was first tested in on June 6, 1927 at Camp Borden, Ontario, Canada, and received a patent in 1929 (). Some pilots in World War II (1939-1945) favoured it, because even when the engine was no longer running the propeller could be feathered. On hydraulically-operated propellers the feathering had to happen before the loss of hydraulic pressure in the engine.

References

aircraft engine development
Kimble D. McCutcheon * Gunston, Bill. ''Development of Piston Aero Engines''. Cambridge, England. Patrick Stephens Limited, 2006. * For more technical information on the first constant speed propeller governor mechanism invented by Elmer E. Woodward, see

External links