Sikorsky S-76 – Igor I Sikorsky Historical Archives (2024)

Background

The S-76 was launched in the 1975 timeframe as the first Sikorsky model to be designed for commercial rather than military missions in order to increase the commercial portion of the company’s business. Its intended missions included the transport of workers to and from offshore oil rigs in a 12 passenger version and transport of business executives in a 6 passenger VIP version. Funding for its design, development and certification was provided from company sources. The technology basis for the S-76 was the Utility Tactical Transport Aircraft System (UTTAS) that had been awarded to Sikorsky in 1972. In particular, the rotor blade aerodynamic and structural features including blade airfoil, twist, swept tip and titanium spar were all embodied in the S-76. The UTTAS elastomeric main rotor head design as well as the cross beam bearingless tail rotor design was also applied to the S-76.

The Sikorsky S-76® is an intermediate class twin engine commercial helicopter powered by two turboshaft engines, which drive both the main and tail rotors, each with four blades. The S-76 landing gear is retractable.

The S-76 was announced at The HAA (now HAI) Convention in Las Vegas, Nevada in February 1975.

The sleek lines of the S-76 combined with significant advancements in rotor technologies and with improved engine fuel consumption, resulted in a helicopter having excellent efficiency.

The executive transport version was designed with a quiet cabin which would provide a work environment during flight and allow conversations without the need for headsets. The S-76 was designed to replace small business jets for short to medium business trips.

Sikorsky’s development of the UH-60 Black Hawk served as the basis for the S-74, which was later re-designated the S-76 Spirit in honor of the U.S. Bicentennial. The design accommodates 2 pilots and 12 passengers, to accommodate an oil rig crew change, and was initially powered by two Allison 250 C30 turboshaft engines of 650 shp (Takeoff Power) each. The prototype first flew on March 13, 1977. The name Spirit was officially dropped by the company on October 9, 1980 due to unanticipated complications when Spirit was translated into certain foreign languages.

The first production variant was the S-76A. In 1982 this model set 12 world records for speed and time to climb in its weight class. A closed course 500 km (311 mi) speed record was set at 214.8 mph.

Several airlines operate the S-76A on scheduled services including Helijet International of Vancouver, British Columbia, Canada. Helijet International was recognized in February 2012 as the operator of the world’s highest flight time Sikorsky S-76® airframe an S-76A™ helicopter (Serial Number 760074) with 37,025 flight hours.

The Executive S-76 has become the “First Choice” for Fortunes 500 companies. The Queen’s Helicopter Flight which transports Queen Elizabeth II and the Royal Family has flown a Sikorsky S-76C+ since 1998. In 2009 a Sikorsky S-76C++ took over the duties

Although not in the original target market, Air Ambulance (EMS) has developed as an S-76 market based on its high dash speed, long range, and room for 2 patients plus equipment which are not available in the EMS competition.

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.

General Arrangement Drawing

General Characteristics and Performance

Production History

Related Models: Military

AUH-76 Armed utility transport version, developed from the S-76 Mk. II. Four were sold to the Philippine Air Force

H-76 Eagle Military version of the S-76B. Designed to provide a lower cost alternative to the AH-64 Apache. One built.

H-76N Planned naval version proposed to New Zealand

Related Models: Experimental

SHADOW (Sikorsky Helicopter Advance Demonstrator of Operator Workload) An S-76A with an extended pilot station added to the nose to develop a fly-by wire flight control system for the RAH-66 Comanche program. A safety pilot flew in the normal pilot position to take control of the helicopter if required.

SHADOW was used to test three and four axis side arm controllers. It also tested unique flight control software concepts such as velocity hold which allowed the pilot to release the stick and the helicopter would continue to fly at the set speed. This and other concepts tested were designed to minimize the pilot’s flying workload so he could concentrate on the mission requirements.

Fantail S-76B with a fan-in-fin tail rotor to test the configuration for the RAH-66 Comanche which required the ability to do a 180 degree turn in 6 seconds with a 45 knot crosswind under hot and high conditions. This led to a very powerful fan and the ability to fly sidewards at 70 mph.

• byVinny Devine