Like many Sikorsky helicopters, the S-55 started out as a military helicopter for the US armed forces that was later adapted for commercial service.
Sikorsky developed the S-55 in response to a US military requirement for a large general purpose helicopter. The US Defense Department awarded Sikorsky a contract to develop such a helicopter in 1948, and the first prototype flew for the first time on November 10 1949. The S-55 saw widespread US military service as H-19 in the Air Force, as H-19 Chickasaw in the Army, as HO4S in the Navy and Coast Guard, and as HRS in the Marine Corps. Civil certification for the commercial S-55 series was first awarded on March 25 1952.
The initial civil variant was the S-55, powered by a 450kW (600hp) Pratt & Whitney Wasp radial piston engine. Later civil variants include the S-55A which introduced a 520kW (700hp) Wright R-1300 radial piston engine, while the S-55C had a P&W R-1340 engine and the repositioned tail boom of the S-55A.
Westland in the UK licence built 400 S-55s as the WS-55 Whirlwind for mainly military but also commercial use. Early Whirlwinds were similar to the S-55 save for their Alvis Leonides radial engine, later developments were powered by a 785kW (1050shp) Rolls-Royce Bristol Gnome H.1000 turboshaft. S-55 licence manufacture was also undertaken in Japan by Mitsubishi and in France by Sud-Est.
In January 1971 Aviation Specialties was awarded a type certificate for its turbine powered conversion of the S-55, dubbed the S-55T. Aviation Specialties formed the Helitec Corp to market and convert aircraft, and approximately 40 were fitted with a Garrett AiResearch TSE331 turboshaft. The conversion reduced the S-55's empty weight by approximately 410kg (900lb).
In 1993 VAT (Vertical Aviation Technologies Inc) started the development of a super quiet conversion of the S-55, the S-55QT Whisper Jet. Full certification was awarded in December 1999. The 3-bladed main rotor of the standard S-55 was replaced by a 5-bladed one, and the powerplant is an Allied Signal TSE 331-10 turbine engine. It is certified at a MTOW of 3492kg (7700lb). As the S-55QT was specially developed for sightseeing, an additional left side cabin door, center floor window, and larger cabin windows were fitted, and sightseeing-optimized systems and equipment were added. The first customer for the Whisper Jet was Papillon Grand Canyon Helicopters.
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