Sikorsky S-75 ACAP
The Sikorsky Model S-75 helicopter was developed as part of the Army's Advanced Composite Airframe Programme (ACAP), to develop an all-composite helicopter fuselage which is lighter and less costly to build than metal airframes. 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 composite materials. 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 S-76 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.
In tests the machine was found to have exceeded the weight- and cost-saving criteria set by the Army in the original ACAP specification.