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Sikorsky S-75 ACAP

Like the Bell D292, the Sikorsky Model S-75 helicopter was developed as part of the Army's Advanced Composite Airframe Programme (ACAP), the goal of which is the development of an all-composite helicopter fuselage which is considerably lighter and less costly to build than the predominantly metal airframes now in general use. Both Sikorsky and Bell were awarded contracts in February 1981 for the design, construction and initial testing of two ground test airframes and one flying prototype, all three to be built entirely of such composite materials as glass-reinforced plastic (GRP), graphite and Kevlar. Sikorsky's S-75 ACAP aircraft flew for the first time in July 1984.

The S-75 is a hybrid machine that uses the twin turboshaft engines, transmission and main and tail rotors of Sikorsky's successful S-76 civil transport helicopter mated to an entirely new composite airframe. Most of the aircraft's basic load-bearing structure is built of graphite or a graphite/epoxy blend, while the floors, roof and most exterior surfaces are of more ballistically-resistant Kevlar. In keeping with the Army's requirement that the ACAP aircraft meet or exceed all existing military crashworthiness standards, the S-75 is equipped with specially designed impact-resistant crew and passenger seats and high strength pneumatic shock absorbers on its non-retracting tricycle landing gear. The machine is operated by two crew members, and can carry up to six passengers in its one hundred cubic-foot rear cargo cabin.

Evaluation of the S-75 was still under way as this book went to press, but in tests already completed the machine was found to have exceeded the weight- and cost-saving criteria set by the Army in the original ACAP specification.

S.Harding "U.S.Army Aircraft since 1947", 1990

Sikorsky S-75 ACAP

Technical data for Sikorsky S-75 ACAP

Crew + passengers: 2+6, engine: 2 x Allison 250-C30S turboshaft, rated at 485kW, main rotor diameter: 13.41m, fuselage length: 13.31m, height: 4.01m, take-off weight: 3820kg, empty weight: 2895kg, max speed: 296km/h, cruising speed: 256km/h, ceiling: 6090m, range: 640km

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