Controllable-pitch propellers

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 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.

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. 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 helicoïdal 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.

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Wikipedia