Variable-pitch Propellers

Fixed-pitch props are quite suitable for low-speed light aircraft with low horsepower. However, as engines with greater horsepower were developed producing greater cruise speeds, the fixed-pitch props suffered in performance. Thus problem was resolved when the next important step in propeller history occurred in 1916 when a variable-pitch (VP) propeller with forward and reverse thrust was first used on an airship to help with manoeuvring. The idea of a VP prop was first mooted by the Frenchman Croce-Spinelli as early as 1871 without any progress in that area. The first variable-pitch propeller mounted on an aircraft was flight-tested on 23 October 1922 by Sandy Fairchild; this was a VP propeller made by the American Propeller Company, with two positions for forward and reverse flight. Nevertheless, the major part of the prop’s development was performed in 1926 in the United Kingdom at the Royal Aircraft Factory in Farnborough (now known as the Royal Aircraft Establishment or simply as Farnborough). A Royal Aircraft Factory RE-8 airplane was used to flight-test the four-blade, wooden VP propeller. The flight-testing of VP props was also progressing well in Germany, with the German R-36 bomber being the first multi-engine aircraft to fly with twin VP props. Variable-pitch props were used on many aircraft types between 1928 and 1940. The propeller manufacturer Standard Steel in the USA made the ground adjustable VP propeller for Charles Lindbergh’s Ryan NY-P ‘Sprit of St. Louis’ in which he achieved the first solo transatlantic flight on 20-21 May 1927.

The US Army aircraft Engineering Division at Wright Field, Dayton, Ohio, further developed VP props under the direction of Frank Caldwell. He later joined Hamilton Standard Division of the United Aircraft Company as their Chief Engineer, where he perfected controllable pitch propellers in 1933 for which he received the Collier Trophy.

Reverse thrust props were not operational until 1943 when they became popular for manoeuvring flying boats on the water. Two years later reverse thrust found its way onto conventional, wheeled aircraft to assist landing roll braking. One of the first aircraft to be so equipped was the Boeing B17 Flying Fortress bomber. Just about all modern turboprop aircraft are now equipped with reverse pitch propellers. It is rare to find a reverse pitch prop on a single-engine aircraft but not altogether unknown. Although VP props have remained in limited production over the year’s right up to the present day, its place has been taken over by the introduction of the constant-speed propeller for high-performance aircraft.