Variable pitch propeller
A controllable pitch propeller (CPP) or variable pitch propeller is a type of propeller with blades that can be rotated around their long axis to change their pitch. 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 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 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 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 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-driven aircraft to adapt the propeller to different thrust 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 speeds. 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 propeller with controllable pitch can have a nearly constant efficiency over a range of airspeeds.
A common type of controllable pitch propeller is hydraulically actuated; it was originally developed by Frank W. Caldwell of the Hamilton Standard Division of the United Aircraft Company. This design 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 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 on some of his flights.
Another common type was originally developed by Wallace R. Turnbull and refined by the Curtiss-Wright Corporation. This electrically operated mechanism was first tested in on June 6, 1927 at Camp Borden, Ontario, Canada and patented in 1929 (United States No. 1,828,348). It was favoured by some pilots in WWII because even when the engine was no longer running the propellor could be feathered. On hydraulically operated propellors the feathering had to happen before the loss of hydraulic pressure in the engine.
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 propellers 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. The Silence Twister prototype kitplane was fitted with the V-Prop, an automatic self-energising and electronically self-adjusting VP propeller.
A fixed pitch propeller (FPP) is often the optimal propeller for most vessels, not least because it is both cheaper and more robust than a variable pitch propeller (VPP). Also, a FPP is typically more efficient than a VPP for a single specific rotational speed and load condition. Accordingly, large bulk carriers, tankers and container ships, that normally operate at a standard speed will have a FPP optimized for that speed. At the other extreme, an English canal narrowboat will have a FPP for two reasons: speed is limited to 4 mph (to protect the canal bank), and the propeller needs to be robust (when encountering underwater obstacles).
At this given rotational speed and load, an FPP can typically transmit power slightly more efficiently than a VPP. At any other rotational speed, or at any other vessel loading, the FPP will be inefficient, either being over-pitched or under-pitched. By contrast, a VPP can be efficient for the full range of rotational speeds and load conditions, since its pitch will be varied to absorb the maximum power that the engine is capable of producing. When fully loaded, a vessel obviously needs more propulsion power than when empty. By varying the propeller blades to the optimal pitch, higher efficiency can be obtained, thus saving fuel.
A vessel with a VPP can accelerate faster from a standstill, and can decelerate much more effectively, making stopping quicker and safer. A VPP can improve vessel maneuverability by directing a stronger flow of water onto the rudder.
Most vessels use a reduction gear to reduce the engine output speed to an optimal propeller speed. While a FPP-equipped vessel needs either a reversing gear or a reversible engine to reverse, a VPP vessel may not. On a large ship the VPP requires a hydraulic system to control the position of the blades. Compared to a FPP, a VPP is more efficient in reverse as the blades’ leading edges remain as such in reverse also, so that the hydrodynamic cross-sectional shape is optimal for both forward and reverse.
In the mid-1970s, Uljanik shipyard in Yugoslavia produced four VLCCs with VPPs - a tanker and three ore/oil carriers - each powered by two 20,000 bhp B & W diesel engines directly driving Kamewa variable-pitch propellers. Due to the high construction cost none of these vessels ever returned a profit over their lifetimes. For these vessels, fixed pitch propellers would have been more appropriate.
Variable pitch propellers are usually found on harbour or ocean-going tugs, dredgers, cruise ships, ferries, cargo vessels and larger fishing vessels. Prior to the development of VPPs, some vessels would alternate between "speed wheel" and "power wheel" propellers depending on the task. Current VPP designs can tolerate a maximum output of 44000 kW (60,000 hp).
Just like an aeronautical propeller, a marine VPP may be “feathered”. This is useful for motorsailers as this mode gives the least water resistance when sailing without using power. Also, when motorsailing, (i.e. voyaging under both power and sail) the VPP can be coarsened to incoprporate the wind component.
Bruntons, a UK engineering firm in Essex, manufactures their patented "AutoProp", a marine propeller where the blades swivel freely and automatically set to the optimum angle. The Autoprop is suitable for small to medium yachts and working boats. Unlike a fixed blade propeller, the Autoprop is exactly as efficient in reverse as in forwards, which is important for stopping and manoeuvering. The AutoProp is particularly beneficial for motorsailers, since if the sails are set but the wind strength is insufficient to make good progress, the engine can be run and the Autoprop will then automatically coarsen to acknowledge the vessel's wind-driven speed component. When solely under sail, a motorsailer's Autoprop will automatically feather to give the minimum drag resistance.
- Aeroplane propellersFlight 9 January 1909
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- Lutze (05-May-2011). "Level flight performance". The Department of Aerospace and Ocean Engineering, Virginia Tech. p. 8. Retrieved 2011-01-06.
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- 1941 Cutaway Drawing of Hydromatic Variable Pitch Propeller Operation on B-24 bomber
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- CONTROLLABLE-PITCH AIRSCREWS explained in Flight 2 May 1935 and 9 May 1935