A controllable pitch propeller
(CPP) or variable pitch propeller
is a type of propeller
A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...
with blades that can be rotated around their long axis to change their pitch
Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller or helicopter rotor into or out of the wind to control the production or absorption of power. Wind turbines use this to adjust the rotation speed and the generated power...
. If the pitch can be set to negative values, the reversible propeller
can also create reverse thrust for braking or going backwards without the need of changing the direction of shaft revolutions.
The earliest known attempt towards creating a variable-pitch propeller of any type for an aircraft, though only adjustable on the ground before or after flight, was on the IdFlieg
The Idflieg was the bureau of the German War Office that oversaw German military aviation prior to and during World War I....
-numbered "R.30/16" example of the Zeppelin-Staaken R.VI four-engined World War I-era Riesenflugzeug
heavy bomber in 1918.
The French aircraft firm Levasseur displayed a variable pitch propeller at the 1921 Paris Airshow which it claimed had been tested by the French government in a ten-hour run and could change pitch at any engine rpm.
Dr Henry Selby Hele-Shaw
Henry Selby Hele-Shaw FRS was an English mechanical and automobile engineer. He was the inventor of the variable-pitch propeller, which contributed to British success in the Battle of Britain in 1940, and he experimented with flows through thin cells. Flows through such configurations are named in...
and T E Beacham patented an 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 in 1928, though it was received with scepticism as to its utility. The propeller had been developed with Gloster - as the "Gloster Hele-Shaw Beachem" - and was demonstrated on a Gloster Grebe
-See also:-References:NotesBibliography* James, Derek N. Gloster Aircraft since 1917. London: Putnam and Company Ltd., 1987. ISBN 0-85177-807-0.* Thetford, Owen. Aircraft of the Royal Air Force 1918-57. London:Putnam, First edition 1957....
where it was used to maintain a near-constant rpm.
The first practical controllable pitch propeller for aircraft was introduced in 1932.
Such propellers are used in propeller
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...
-driven aircraft to adapt the propeller to different thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
levels and air speeds so that the propeller blades don't stall, hence degrading the propulsion system's efficiency. Especially for cruising, the engine can operate in its most economical range of rotational speed
Rotational speed tells how many complete rotations there are per time unit. It is therefore a cyclic frequency, measured in hertz in the SI System...
s. With the exception of going into reverse for braking after touch-down, the pitch is usually controlled automatically without the pilot's intervention. A propeller with a controller that adjusts the blades' pitch so that the rotational speed always stays the same is called a constant speed propeller
A constant speed propeller is a type of propeller that can change its blade pitch to take better advantage of the power supplied by an engine in much the same way that a transmission in a car takes better advantage of its power source...
. A propeller with controllable pitch can have a nearly constant efficiency over a range of airspeeds.
The most common type of controllable pitch propeller is hydraulically actuated; it was originally developed by Frank W. Caldwell of the Hamilton Standard
Hamilton Standard, an aircraft propeller parts supplier, was formed in 1929 when United Aircraft and Transport Corporation consolidated Hamilton Aero Manufacturing and Standard Steel Propeller into the Hamilton Standard Propeller Corporation. Other members of the corporation included Boeing,...
Division of the United Aircraft Company. This design led to the award of the Collier Trophy
The Collier Trophy is an annual aviation award administered by the U.S. National Aeronautics Association , presented to those who have made "the greatest achievement in aeronautics or astronautics in America, with respect to improving the performance, efficiency, and safety of air or space...
of 1933.http://www.time.com/time/magazine/article/0,9171,754215,00.html?promoid=googlep de Havilland
The de Havilland Aircraft Company was a British aviation manufacturer founded in 1920 when Airco, of which Geoffrey de Havilland had been chief designer, was sold to BSA by the owner George Holt Thomas. De Havilland then set up a company under his name in September of that year at Stag Lane...
subsequently bought up the rights to produce Hamilton propellers in the UK, while the British company Rotol was formed to produce its own designs. The French company of Pierre Levasseur and the US Smith Engineering Co. also developed controllable pitch propellors. Smith propellers were used by Wiley Post
Wiley Hardeman Post was a famed American aviator, the first pilot to fly solo around the world. Also known for his work in high altitude flying, Post helped develop one of the first pressure suits. His Lockheed Vega aircraft, the Winnie Mae, was on display at the National Air and Space Museum's...
on some of his flights.
As experimental aircraft and microlights have become more sophisticated, it has become more common for such light aeroplanes to fit variable-pitch propellers, both ground-adjustable propeller
A ground-adjustable propeller is a simple type of variable-pitch aircraft propeller, , whose blades can be rotated around their long axis to change their pitch, but in this instance any adjustment can be performed only on the ground...
s and in-flight-variable propellers. Hydraulic operation is too expensive and bulky, and instead light aircraft use propellers that are activated mechanically or electrically. Some are manually operated, some are controlled by electronics; and one, the "Silence V-Prop", is fully self-powering and self-adjusting.
Controllable pitch propellers (CPP) for marine propulsion systems have been designed to give the highest propulsive efficiency over a broad range of speeds and load conditions. When the vessel is fully loaded with cargo the propulsion required at a given ship speed is much higher than when the vessel is empty. By adjusting the blade pitch, the optimum efficiency can be obtained and fuel can be saved. Also, the controllable pitch propeller has a "vane"-stance, which is useful with combined sailing / motor vessels as this stance gives the least water resistance when not using the propeller (e.g. when the sails are used instead).
A fixed pitch propeller (FPP) is more efficient than a controllable pitch propeller under a specific
rotational speed and load condition. At that particular rotational speed and load, a FPP can transmit power more efficiently than a CPP. At any other rotational speed, or any other vessel loading, the FPP will not be more efficient, either being over pitched or under pitched. A correctly sized controllable pitch propeller can be efficient for a wide range of rotational speeds, since pitch can be adjusted to absorb all the power that the engine is capable of producing at nearly any rotational speed.
The CPP also improves maneuverability of a vessel. When maneuvering the vessel the advantage of the CPP is the fast change of propulsion direction. The direction of thrust can be changed without slowing down the propeller and depending on the size of the CPP can be changed in approximately 15 to 40 seconds. The increased maneuverability can eliminate the need for docking tug
Tuğ is a village in the Khojavend Rayon of Azerbaijan....
s while berthing.
A reversing gear or a reversible engine is not necessary for ships utilizing CPP, saving money to install and service these components. Depending on the main engine rotational speed and the size of the CPP, a reduction gear may still be required. A CPP does require a hydraulic system to control the position of the blades. A CPP does not produce more or less wear or stress on the propeller shaft or propulsion engine than an FPP. Therefore this will not be an argument to choose between an FPP or a CPP.
Large vessels that make long trips at a constant service speed, for example crude oil tankers or the largest container ships, do not utilize a CPP, since the amount of power generated exceeds the current CPP design capabilities. A CPP is usually found on harbor or ocean-going tugs, dredgers, cruise ship
A cruise ship or cruise liner is a passenger ship used for pleasure voyages, where the voyage itself and the ship's amenities are part of the experience, as well as the different destinations along the way...
s, ferries, cargo vessels and larger fishing vessels that sail to ports with limited or no tug assistance. Prior to the development of CPPs, some vessels would alternate between "speed wheel" and "power wheel" propellers depending on the task.
Current CPP designs can tolerate a maximum output of 44000 kW (60,000 hp).
Bruntons, an engineering firm in Essex, has patented the "AutoProp", a marine propeller where the blades swivel freely and automatically set to the correct angle. The Autoprop is suitable for small to medium yachts and boats, and is particularly beneficial for motor-sailers.