Helicopter technology - Setting the standards

Designing a helicopter with equal appeal for civil and military users poses a significant challenge. Sikorsky's answer is the S-92

Paul Lewis/STRATFORD, CONNECTICUT

When Sikorsky set out in the early 1990s to design the S-92, it intended to develop a derivative of the highly successful S-70/H-60 military machine that would appeal to the commercial market and rival the Eurocopter Super Puma and provide a replacement for the S-61. The helicopter that eventually emerged from Sikorsky is essentially an all-new design incorporating many important advances in safety, reliability and maintainability, much of which will find its way on to future enhanced S-70s.

The brief for Sikorsky's functional design team was a twin-turbine helicopter with a cabin large enough to seat 19 passengers in relative comfort and fly 740km (400nm). "Nineteen passengers provides a break in regulations and 400nm matches a lot of realistic missions patterned on that of the North Sea," explains Bob Blauch, S-92 system requirements manager.

In this respect Sikorsky has more than met its initial goals. During recent demonstrations to the Portuguese military, the S-92 was flown at an alternate take-off weight of 14,074kg (31,000lb), well in excess of its 12,835kg maximum take-off weight (MTOW). Furthermore, the helicopter has shown that with additional auxiliary fuel tanks, a search-and-rescue (SAR) version can reach as far as 1,480km.

The adoption of Federal Aviation Requirement and Joint Aviation Requirement Pt 29.571 stipulated that the helicopter be designed to new flaw tolerant criteria, effectively ruling out a S-70 derivative approach. Sikorsky claims that the S-92 is the first to be designed to the amended requirements, ensuring there is sufficient structural strength and system redundancy to avoid failure from fatigue or damage.

Acceptable damage

This includes "barely visible damage" up to 5/1000th of an inch without compromising any component's lifespan and then accepting larger damage up to 1mm deep without any crack propagation between and beyond the S-92's scheduled 1,250h inspection cycle. This dictated a new rotorhead incorporating dual-arm titanium yokes for load path redundancy and 4,000-5,000h minimal wear elastomeric bearings.

"We started with a S-70 spindle-based rotorhead, but in order to meet the new requirements we would have had to have grown it by such huge amounts that we switched to a new concept to optimise the design," says James Williams, S-92 business development. The new rotorhead, along with other new dynamic components, is now a candidate for growth versions of the Black Hawk such as the US Army's UH-60X.

The S-92's composite main rotor blade was previously tested on a UH-60. The main difference is a 400mm (16in) extended radius, wider chord, new cuff rotorhead mounting and a swept, tapered anhedral blade tip. The latter reduces blade vortex interaction and in the process lowers drag and noise. Sikorsky says that it also improves hover efficiency equivalent to a 360-400kg load, or three passengers.

Other blade wake remedial measures include increasing the spacing between the main and tail rotor to 350mm - four times that of the S-70. New airfoil technology, first tested on the S-76, has also been incorporated into the S-92 to optimise the shaping of the bearingless composite flexbeam tail rotor blade to lower noise and RPM to 1,190RPM at 100% power and 1,250RPM at 105%.

The S-92's main transmission features a new 3,130kW (4,170hp) dual planetary gearbox, designed for 6,000h of flight between overhaul. Interconnecting shafts drive the aft intermediate and tail rotor gearboxes. Sikorsky uses titanium diaphragm-type couplings in place of a flexible, stacked disc coupling to simplify maintenance, eliminate shimming and eliminate old-school inspection requirements.

Sikorsky initially selected the General Electric CT7-6D turboshaft to power the first S-92 prototype, before switching to the uprated 1,865kW CT7-8. More than 10,000 CT7 and T700 military variants have been built amassing 28 million hours of operation. "The engine has a very good service record and GE has put in all the improvements from all the experience it has gathered over the years. They're offering a very good fixed-price-per-hour engine maintenance programme," says Williams.

GE's on-condition philosophy is a critical component of Sikorsky's aim of reducing direct maintenance costs to $800 per flight hour - one third lower than the earlier S-70. Sikorsky has launched a failure reporting and corrective action system to fix problems during testing to "ensure our customers don't become the lab," says Williams.

One shortcoming found in flight test was an increased nose-up attitude. Sikorsky corrected this, in the process refining the S-92's airframe by stretching the fuselage aft of the cockpit by 400mm, lowering the tail by 1.04m (3.4ft) and repositioning the horizontal stabiliser to the starboard side. The result is a better centre of gravity, flatter hover and a more compact tail fold for shipboard storage. The "soon-to-fly" third and fourth flying prototypes have been modified accordingly.

The CT7-8 features a modified compressor and the addition of a dual channel solid state full authority digital engine control (FADEC) from Hamilton Sundstrand. The powerplant was certified by the US FAA last October and given 30s and 2min one engine inoperative (OEI) ratings of 1,914kW and 1,852kW respectively, permitting Category A operations over a wide range of ambient conditions.

An aft centre-mounted Honeywell 36-150 series auxiliary power unit (APU) provides engine start, ground power, heating and inflight supplemental/ emergency power.

Safety in mind

FADEC features include an OEI pilot training mode, which causes the helicopter's powerplant and instrumentation to simulate an engine failure at MTOW. "It's important the pilot reads these indications. For safety, if the RPM falls too low the system drops out of training mode," explains Nick Lappos, S-92 director of programmes.

The baseline S-92 has two fuel tanks in the side sponsons with combined capacity of 2,650litres (700 USgal). There is provision for two 701litre tie-down auxiliary tanks inside the cabin and a pair of external side-mounted 870litre tanks. Sikorsky is also proposing adding even larger 1,820litre external drop tanks.

The S-92 fuel system uses a suction feed system and self-sealing breakaway valves with the sponson designed to detach in the event of a crash. Another safety feature is a tricycle landing gear capable of absorbing up to a 7.9m/s (1,550ft/min) vertical crash, or a 67kt landing at MTOW. The tube inside of the gear's oleo is designed to fragment once the strut passes the maximum compression point and "in doing so, absorbs a tremendous amount of energy," says Williams.

A recurring safety design theme on the S-92 is system redundancy. The helicopter has three 4,000psi (276bar) transmission-driven hydraulic pumps. Systems one and two drive the machine's three-axis stability augmentation systems (SAS) and dual redundant main and tail rotor servos. System three acts as a back-up, as well as powering the hydraulic hoist, landing gear, rear loading ramp and APU accumulator.

The electrical system comprises two interchangeable 75kV AC generators running off the main gearbox accessory module, supplemented by a smaller 35kV generator hooked up to the APU. These upgraded generators ensure a greater margin of power for optional items such as electrical rescue hoists or anti-submarine warfare (ASW) equipment, without having to resort to the APU.

Its tail rotor cable quadrant ensures full control even in the event that one of the two cables fails, and has been credited already with saving several crews. "Typically when you lose a cable the other flaps loose," explains Lappos. "We've a redundant mechanism, which when you cut one cable, throws over a spring and puts the remaining cable into tension."

Standard avionics equipment on the S-92 includes dual digital Hamilton Sundstrand Automatic Flight Control Systems (AFCS), with fully coupled flight directors. The flight control computers each contain dual independent SAS and airspeed, attitude and heading hold functions. "There are a total of four, of which any one will stabilise the helicopter," adds Lappos.

For the S-92's avionics, Sikorsky originally selected Lockheed Martin's former Sanders division to supply the electronic flight instrumentation system. Then early last year, the company decided to switch to a Rockwell Collins system, which is being installed in the fourth and final flight test machine.

While Sanders' decision to exit the display business left Sikorsky with little option, it did feel that the Rockwell Collins system better lent itself to both the military and civil community. Similar ProLine-based avionics solutions have also been selected by Turkey for a planned new fleet of Sikorsky CH-53E Stallion heavylift helicopters, and for upgrading its UH-60 Black Hawk and SH-60 Seahawk machines.

Rockwell Collins' S-92 suite is being developed by the military side of the avionics house with commercial input. This approach is critical for the future success of the S-92 as a multi-market machine. It must be compatible with civil Arinc-429 and military 1553 systems.

"Collins is developing this with an eye towards the civil and military. You can take a system which the FAA is comfortable with and be able to drop in MIL-STD 1553 capability and go directly in the military utility category." says Steven Brainard, S-92 senior avionics engineer.

Optional fifth screen

The cockpit is designed around four 150x200mm LCDs, with the option to add a fifth screen. Each multi-function LCD can be tailored to provide primary flight, navigation, engine instrument/caution advisory and built-in test displays. The displays use Power PC 603-based processors and have the ability to interface with 1553 systems, such as forward looking infrared (FLIR)imager.

Rockwell Collins is also supplying its ProLine IV communications/navigation suite. This comprises two VHF-422 radios, dual VIR-432 VOR/ILS navigation receivers, DME-442 distance measuring equipment transceiver, ADF-462 automatic direction finder receiver and TDR-94D Mode-S transponder.

Other systems include a Litef dual attitude heading reference system, a maintenance data computer, a BFGoodrich health and usage monitoring system and BAE Systems' cockpit voice and data recorders. All these interface with the helicopter's LCDs through two Rockwell Collins data concentrator units.

Sikorsky test pilots have teamed with Rockwell to better configure the helicopter's smart displays for enhanced situational awareness. This includes OEI available power and time prompts for CAT A operations, actual/required power indications for hover-out-of-ground-effect landings and a schematic layout of the helicopter's systems, along with geographically distributed advisory and caution messages.

Obstacle warnings

This could be superimposed over the Enhanced Ground Proximity Warning System display (see P42), being offered as an S-92 option along with a universal flight management system and 12-channel UNS-1C global positioning system The Honeywell/Sikorsky system uses a terrain database to flag obstacles ahead, as well as modes including warning of excessive sink rates, terrain closure rates, and glide path deviation.

Sikorsky is compiling a comprehensive range of mission equipment options for the S-92. The civil version includes either an executive or 19-seat passenger configuration, complete with 16g seats, emergency push-out windows and an environmentally controlled cabin with an 80db interior noise level. It offers quick change for cargo operations and includes a stowable 4,540kg external load hook.

The military version will accommodate up to 22 troops using side-mounted crashworthy seats. Options include a belly-mounted surveillance radar such as the Telephonics APS-143, an ASW suite including dipping sonar and sonobuoy launcher, or airborne mine countermeasures systems. Sikorsky also plans to equip the S-92 with two wing stubs mounted above the sponsons to accommodate a range of external stores.

A further benefit from stretching the fuselage is the option of an enlarged 1.27m sliding side door in order to compete for the Nordic Standard Helicopter Programme. This represents perhaps the fullest test yet of the S-92's mission flexibility, with Denmark, Finland, Norway and Sweden having teamed together to find a select a single machine capable of fulfilling their differing needs for troop transport, SAR as well as shore and ship-based ASW.

Sikorsky S-92 chief system engineer Thomas Toner observes: "What they're asking for is what we've been planning to do with the aircraft all along, which is to have a basic helicopter and through different configurations be able to address differing requirements. It couldn't mirror our design philosophy any better."

Source: Flight International