Design
The S-76 is of conventional configuration, with a four-bladed fully articulated main rotor and a four-bladed anti-torque rotor on the port side of the tailboom. Two turboshaft engines are located above the passenger cabin. In the prototypes and initial production aircraft, these engines were Allison 250-C30s, a new version of the popular Allison 250 engine developed specially for the S-76, with a single-stage centrifugal compressor instead of the multi-stage axial/centrifugal compressor of earlier models of the engine, rated at 650 shp (480 kW) for take-off. These engines are connected to the main rotor by the main gearbox, a three-stage unit with a bull gear as its final stage rather than the planetary gear used by previous generations of Sikorsky helicopters. This arrangement gave 30% fewer parts and lower costs than a more conventional design.
The main rotor hub has a single piece aluminum hub with elastomeric bearings designed not to require lubrication or any other kind of maintenance throughout its design life. The main rotor blades have titanium spars and incorporate a 10 degree twist to give an even loading when hovering, while the they use a non-symmetrical airfoil section with a drooped leading edge. The rotor tips are tapered and swept back. Flight controls are servo-assisted, with a Stability Augmentation System fitted. A retractable nosewheel undercarriage is fitted, which gave the S-76A a 6 knots (6.9 mph; 11 km/h) increase in cruising speed and emergency flotation gear can be fitted, with flotation bags that can be filled with helium in the event of a forced landing on water.
The fuselage is of mixed metal and composites construction, with a fiberglass nose and a light alloy honeycomb cabin structure. The semi-monocoque tailboom is also constructed of light alloy. Two pilots (or a pilot and a passenger) sit side in the cockpit, situated ahead of the cabin, which can accommodate a further 12 passengers in three rows of four, or four to eight passengers in more luxurious executive seating.
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