Configuration Features

The S-76 helicopter was designed to serve the off-shore oil market and meet its requirements to carry 12 passengers and a crew of 2 on a 400 nautical mile radius mission with flotation equipment and operate IFR Category A. In order to achieve the design goals, the S-76 employed technologies developed for the UH-60 BLACK HAWK helicopter. Among the features are: titanium and composite main rotor blades, a bearingless composite tail rotor, bifilar vibration absorbers, and a simplified main rotor transmission.

The main rotor blades incorporate composite materials including a hollow titanium spar that is plasma arc welded and hot formed to its oval shape, a fiberglass cover, a redundant graphite root end, a Kevlar tip cap and Nomex honeycomb in the trailing edge. The main rotor blades use an SC1095 airfoil and have a non-linear twist to load the blades evenly in a hover. Tip caps are swept to reduce noise and vibration.

Ride quality in the S-76 was enhanced by a pitch-cone coupling to a maximum of 17 degrees. The cyclical feathering motion, usually known by the mathematical shorthand “Delta-3”, functions to wash out blade pitch during blade flapping. In other words, when the blade flaps up as it would in turbulence, Delta-3 mechanically washers out some of the pitch angle to prevent tip stall and soften the ride. Delta-3 was incorporated in the S-76 by a phase shifter incorporated in a complex mixer unit.

The composite bearingless tail rotor principle structural element is the cross beam spar. The cross beam tail rotor is designed to take advantage of the special capabilities of unidirectional graphite composites. The tail rotor consists two assemblies each of which provide two blades for the four bladed tail rotor. The spar which connects opposite blades is made of unidirectional graphite and is laid up in a rectangular cross sectional shape. The resultant beam is flexible in the flapwise and torsional direction but very stiff in the edgewise direction. An airfoil shape is bolted to each end of the cross beam to form one half of the tail rotor. Two of these assemblies are clamped at right angles to form the tail rotor. The resultant tail rotor is free of instabilities and requires no lubrication or maintenance.

The Main Gearbox has as its final reduction a bull gear with two spur gear inputs. This is considerably simpler than the planetary gear system used in previous Sikorsky helicopter models Each engine has a separate power train all the way up to the bull gear through a single spur and a single bevel mesh. The tail takeoff is from the left engine power train. In case of a left engine failure the tail takeoff still drives through the right engine to the bull gear back through the bevel set to the tail take off. The freewheeling units are inside the first spur gear forward of each engine. Using the bull gearing system reduces the number of bearings and gears by a significant number over a conventional planetary gearing approach. This design results in a lower total cost and increased reliability.

The main rotor hub is a single piece aluminum forging with elastomeric bearings and Bifilar vibration absorbers tuned to different frequencies to dampen vibrations at the source to reduce cabin noise and protect the airframe from vibration damage.

The S-76 fuselage is designed for low drag to improve efficiency. Flush rivets are used, a first for any Sikorsky helicopter. The S-76 was the first Sikorsky commercial helicopter to make extensive use of advanced composites. Kevlar is used for engine cowlings and fairings which is half the weight and twice the strength of fiberglass.