Engineering:Variable-pitch propeller

Short description: Propeller with blades that can be rotated to control their pitch while in use

A variable-pitch propeller or controllable-pitch propeller (CPP) is a type of propeller with blades that can be rotated around their long axis to change the blade pitch. Reversible propellers—those where the pitch can be set to negative values—can also create reverse thrust for braking or going backwards without the need to change the direction of shaft revolution.

Aircraft

One of a C-130J Super Hercules' 6 bladed Dowty Rotol R391 composite controllable- and reversible-pitch propellers.

A Hamilton Standard variable-pitch propeller on a 1943 model Stinson V77 Reliant

Propellers whose blade pitch could be adjusted while the aircraft was on the ground were used by a number of early aviation pioneers, including A. V. Roe and Louis Breguet - this also happened late in World War I with one testbed example, "R.30/16", of the low-production (56 examples) Zeppelin-Staaken R.VI German "giant" four-engined heavy bomber. In 1919 L. E. Baynes AFRAeS patented the first automatic variable-pitch airscrew.

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 Aircraft Company — as the Gloster Hele-Shaw Beacham Variable Pitch Propellor — 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. French firm Ratier pioneered variable-pitch propellers of various designs from 1928 onwards, relying on a special ball bearing helicoidal ramp at the root of the blades for easy operation.

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 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 DH88 Comet aircraft, winner of the famed long distance 1934 Mac Robertson race and in the Caudron C.460 winner of the 1936 National Air Races, flown by Michel Detroyat. 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 boy) given to the aircraft ground mechanics in France up to this day.

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 blade 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 Smith Engineering Co. in the United States also developed controllable-pitch propellers. 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 (U.S. Patent 1,828,348). It was favoured by some pilots in World War II, 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.

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. The Silence Twister prototype kitplane was fitted with the V-Prop, an automatic self-energising and electronically self-adjusting VP propeller.

Ships

A ship's variable-pitch propeller

A controllable pitch propeller (CPP) 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 CPP can also improve vessel maneuverability by directing a stronger flow of water onto the rudder.

However, a fixed variable-pitch propeller (FVPP) is both cheaper and more robust than a CPP. Also, an FVPP is typically more efficient than a CPP for a single specific rotational speed and load condition. Accordingly, vessels that normally operate at a standard speed (such as large bulk carriers, tankers and container ships) will have an FVPP optimized for that speed. At the other extreme, a canal narrowboat will have a CPP 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).

Vessels with medium or high speed diesel or gasoline engines use a reduction gear to reduce the engine output speed to an optimal propeller speed—although the large low speed diesels, whose cruising RPM is in the 80 to 120 range, are usually direct drive with direct-reversing engines. While an FVPP-equipped vessel needs either a reversing gear or a reversible engine to reverse, a CPP vessel may not. On a large ship the CPP requires a hydraulic system to control the position of the blades. Compared to an FPP, a CPP 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 forward propulsion and satisfactory for reverse operations.

In the mid-1970s, Uljanik shipyard in Yugoslavia produced four VLCCs with CPPs – 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 variable-pitch propellers would have been more appropriate.

Controllable-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 CPPs, 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).

Sail boats

A sailboat or motorsailer when voyaging on sail alone will benefit from reduced drag, and just like an aeronautical propeller, a marine VPP may be “feathered” to give the least water resistance when sailing without using power. A VPP is particularly useful when motorsailing (i.e. voyaging under both power and sail), as the VPP can be coarsened to incorporate the wind component. If the prop remained at the "normal" setting, it would be too fine and the engine would provide little useful contribution; but by coarsening the prop, the engine provides useful thrust, resulting in a higher speed yet reduced fuel consumption because of the sailing component. For sailboats and motorsailers, a proven VPP design needing no external inputs is the Bruntons "AutoProp", a marine propeller where the blades swivel freely and automatically set to the optimum angle. The Autoprop works as efficiently in astern as in ahead, aiding stopping and manoeuvering. The Autoprop is beneficial for motorsailers sailing in light winds, as the engine can be run and the propeller 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. A simpler and cheaper design common on sailboats is the feathering (or "folding") prop, which automatically feathers when not in use, reducing drag and allowing a higher speed.

See also

• Constant-speed propeller

External links

• 1941 Cutaway Drawing of Hydromatic Variable Pitch Propeller Operation on B-24 bomber
• Contemporary discussion of merits of variable-pitch propellers in Flight magazine 1935
• CONTROLLABLE-PITCH AIRSCREWS explained in Flight 2 May 1935 and 9 May 1935