Graham Warwick/Stratford

So little has been said about the S-92 Helibus programme since its 1995 launch that the sight of two near-complete prototypes at Sikorsky's Stratford, Connecticut, factory comes as something of a surprise. The wraps are coming off now - nine months before the first flight - in a bid to confound competitive disinformation about the project's status, and to prepare for the long-awaited campaign to sell the private-venture helicopter.

The S-92 was conceived as a companion to Sikorsky's S-70 family of medium-weight military helicopters - the UH-60 Black Hawk and SH-60 Seahawk. The aircraft is aimed equally at commercial and military medium-helicopter markets, particularly those requiring a larger cabin than the H-60 series provides. The programme uses S-70 technology where possible, upgrading it where necessary, always with an eye to retrofitting the result to the H-60 series.

Programme vice-president Kenneth Kelly describes the S-92 as a "latter-day S-61". The company sold 1,431 S-61s into commercial, as well as military, markets and this success forms the basis for the S-92 programme. Sikorsky has sold some 2,370 S-70s so far, and the aircraft will remain in production into the next century, but it has not tapped the civil and other markets so successfully served by the S-61.

Sikorsky forecasts the demand for replacement of S-61-class civil helicopters at 340 units over 20 years. "This is not a big market," Kelly admits. The S-92 is also being aimed at the "largely government" international-utility market, which the company forecasts will account for sales of 1,390 aircraft over 20 years. "The Black Hawk does not satisfy the market totally," Kelly believes. "Some customers want a larger cabin."


Sikorsky estimated that it could sell 240 civil and 520 military S-92s, but based its business case on a "conservative" 600-700 sales over 20 years. "That would be a good programme," Kelly acknowledges. The company has subsequently updated its assessment, and the sales target has stayed relatively unchanged. "The helicopter market is going up worldwide, and the medium segment has the biggest potential," he believes.

To meet the needs of civil and international-military customers, the S-92 will be available in two versions: a 19-passenger civil transport with airline interior and commercial avionics and a utility transport with 22 side-facing seats in the cabin and a 1553 databus for military avionics. The civil version has 1.82m stand-up headroom and three-abreast seating: "The cabin is about the same size as the S-61's," Kelly says.

Performance for both versions is the same: a 155kt (260km/h) cruise and 910km (490nm) range at a gross weight of 11,430kg. The utility S-92 has a 12,020kg gross weight and 4,540kg cargo-hook capability. The likely price for the civil version is about $13-14 million.


Lowest cost has been the driving factor in designing the S-92 - both in terms of purchase price and operating cost. "Targets have been broken back to the design teams tied to selling price," Kelly says, "and, for the first time, we have taken in operating cost and broken that back to the individual product-development teams, so they have two types of cost target. Major parts have their own targets."

Design requirements include the latest US and European FAR/JAR 29 certification regulations, which include more-stringent rules for damage-tolerant dynamics and single-engine performance. "We shot for the best cost and performance in the medium class," Kelly says.

The fuselage, for example, is a mix of metal and composite construction. "All the parts that face the airstream are composite, "he says. These include the nose, rotor pylon, sponsons and fin leading-edge. The cabin, tailboom and fin are semi-monocoque aluminium. "We use high-speed machining to get the same weight benefit as composites," Kelly explains. "It's the best way to keep cost down."

Cost has also been reduced by involving risk-sharing partners around the world and linking them electronically to create a "virtual corporation". About 28% of the helicopter's value is provided by the partners, Kelly says. Sikorsky is responsible for the dynamic system, integration and management. All partners use the CATIA three-dimensional design system, exchanging drawings, documents and solid models via an encrypted wide-area network.

Partners were selected, Kelly says, based on their ability to use the CATIA, to invest in the programme and to open doors to strategic markets. Companies involved are:

Taiwan's Aero Industrial Development - cockpit. This arrives "fully stuffed", with all harnesses and ducting installed;

Brazil's Embraer - sponsons. These are delivered complete with fuel cells supplied by Aerazur and gauging provided by Intertech-nique, both of France. Embraer's equipment division, EDE, supplies the landing gear;

Spain's Gamesa - main rotor pylon, aft fuselage and tail boom, and interior panels;

Japan's Mitsubishi Heavy Industries - cabin. This includes the aft sections of the sponsons, to which the main gear is mounted;

China's Jingdezhen Helicopter Group - vertical tail.

Each partner is responsible for detailed design of its part of the aircraft, and for production of five prototype shipsets. Kelly says that the design effort has been a success. "The quality is great and the fit is great," he says. The partners supplied solid models for an electronic mock-up at Sikorsky. "This allowed us to head off 1,300 interferences before they happened."

Sikorsky's philosophy is "one design, two models". The fuselages are similar, with differences in interior and mission equipment only. The fuel system, landing gear and the seats are the same. "The passenger seats are crashworthy for the first time," Kelly says. Early differences have been eliminated. Both versions have the same cabin windows and rear loading-ramp, and are powered by the General Electric CT7-8 engine, rated at 1,790kW (2,400shp) for take-off.

"We started out with two [CT7] versions, but customers say the higher-powered -8 is needed, "Kelly says. Sikorsky originally thought that the 1,300kW CT7-6 would appeal for North Sea operations in cool, sea-level, conditions, but says that civil customers are increasingly operating in hot and high regions of the world and require the higher power.

The dynamic system is 20% more powerful than the latest UH-60L's. The unlimited-life main-rotor head is a new design, but, like other improvements to the S-92 dynamic system, is designed for retrofit to the H-60 series. The rotor head consists of a titanium hub with titanium yokes which enclose elastomeric bearings and attach to composite cuffs on the blade roots. The design is intended to be easily inspected visually, and flaw-tolerant requirements result in beefier parts than those of the H-60, Kelly says.


Sitting atop the hub is a new design of vibration absorber. This consists of a metal drum enclosing five composite springs which allow the absorber to move in opposition to in-plane forces so as to remove vibration at the rotor.

The main-rotor blade is a new, all-composite, design with a 50mm-wider chord and 400mm-longer radius than the H-60 blade. The tip is swept back and down, and tapered, to reduce noise and improve performance.

Main-transmission capacity has been increased to 3,100kW while retaining compatibility with the H-60 drive system by using a system of 12 planetary gears, compared with five in the Black Hawk. The resulting gearbox is only 50mm higher. Anew corrosion-resistant magnesium alloy is used for the housing.

The tail-rotor driveshaft is supported by elastromeric bearings and is driven at a higher speed than in the H-60. This reduces the number of connections and is more durable, Kelly says. Intermediate and tail gearboxes are similar to those in the H-60, but the tail rotor is new. The blades are attached to a bearingless composite-flexbeam hub and are individually removable. The tail rotor is "birdstrike-tolerant" for the first time, Kelly says.

Both versions share the same cockpit, with liquid-crystal displays (LCDs) supplied by Lockheed Martin Sanders. Four 150 x 200mm displays are standard, with an optional fifth, central, LCD. Kelly says that the "unique" symbology includes a display of engine ratings. A collective-lever symbol fills like a thermometer as the 2min single-engine power rating is used up.

The core avionics suite common to both versions is based on commercial ARINC 429-standard equipment. The utility S-92 also has a 1553 databus, allowing sensors and other mission equipment to be added. Use of both the ARINC and 1553 databuses provides an "open architecture" for installation of a mix of commercial and military systems, Kelly says. Avionics are mounted in racks behind the cockpit, and wiring runs over frames and along troughs in the cabin floor for easy access.



Hamilton Standard supplies the dual digital automatic flight-control system (AFCS). This provides stabilisation and autopilot functions, including automatic approach to a hover. Solid-state rate sensors are used which have a mean time between failures of 50,000h, Kelly says. HamStan also provides the aircraft-system computer, which drives the displays, and the active noise- and vibration-control systems. These units, and the AFCS, use the same family of HamStan computers, he says.

Sikorsky plans to install active vibration-control as standard, but active noise-control will be an option, possibly only for VIP aircraft. An option Kelly hopes that most customers will pick is the health- and usage-monitoring system (HUMS). This equipment is under development by BFGoodrich for the S-92, and the smaller S-76, to monitor operational and structural usage so that the HUMS can tell operators when to remove components, rather than replacing them at hard time limits.


The S-92's basic design reflects a combination of requirements from the two market segments. These include Category A vertical-take-off performance - the ability to continue with a take-off after the loss of an engine - and crashworthy design of the fuel system, landing gear and seats. Designing the aircraft to meet the safety requirements of North Sea operators led to the installation of four large cabin exits, plus pop-out windows.

Flotation gear provided by the UK's FPT Industries will keep the aircraft upright for 10min and consists of floats either side of the nose and under the ramp, which deploy automatically on immersion. Externally mounted RFD Aviation liferafts deploy from the front of each sponson, and work either way up.

Kelly says that the S-92 programme is still on the schedule laid out at the launch. The first of five prototypes, the ground-test vehicle (GTV), is expected to be delivered to Sikorsky's West Palm Beach, Florida, flight-test centre in March. The second prototype will be the first flight-test aircraft, with the first flight planned for September, at West Palm Beach.

The GTV and first flight-test aircraft will be powered by CT7-6Ds, as GE is still developing an improved compressor for the CT7-8. The first S-92 to be powered by the -8 will be the second flying prototype. The CT7-8 is the first member of the T700 turboshaft family to have dual-channel full-authority digital engine-control , which is supplied by Hamilton Standard.

A 1,600h flight-test programme is planned with four aircraft, all in the core "North Sea oil-market" civil configuration. This is expected to lead to basic US and European type-certification in the third quarter of 2000.

Sikorsky and its partners have not yet taken the decision to launch production of the S-92. That is due after the first flight, but could be brought forward if Sikorsky bows to increasing pressure from potential customers to commit to production. A decision early in 1998 would allow S-92 deliveries to begin in 2001, says Kelly.



Source: Flight International