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Composites confer agility and speed

02 April 2012

Composite materials manufactured at a site in Duxford, Combridge will provide ultra high strength and lightweight structures for Sikorsky's next generation military helicopter, the S-97 Raider, which is due to make its maiden flight in 2014. Les Hunt reports.

Sikorsky Aircraft’s next-generation S-97 Raider light tactical helicopter is at the advanced design stage and is expected to make its maiden flight in 2014
Sikorsky Aircraft’s next-generation S-97 Raider light tactical helicopter is at the advanced design stage and is expected to make its maiden flight in 2014

Earlier this year, Sikorsky Aircraft announced its preferred suppliers for the revolutionary S-97 Raider helicopter project - a select group of 35 companies, including composites specialist Hexcel. This next-generation helicopter is being developed for flight evaluation by the US military, and Sikorsky and its team of suppliers are self-funding the design and manufacture of two prototype aircraft for this evaluation. The programme is currently in the advanced design stage, and Sikorsky expects to achieve first flight in 2014.

The Raider aircraft programme follows Sikorsky’s successful X2 Technology demonstrator aircraft, which in September 2010 achieved more than 250 knots flight speed, or twice the average cruise speed of a conventional helicopter. As an X2 aircraft design, the S-97 RAIDER helicopter will feature coaxial counter-rotating main rotors and a pusher propeller to provide dramatic improvements in manoeuvrability, hover efficiency, high/hot climate performance, and speed.

Defined as a 'light tactical helicopter', the single-engine Raider will weigh in at less than 5,000kg maximum gross weight - an achievement largely attributed to the materials used for its structures and blades. Those materials include Hexcel’s HexPly prepregs and HexWeb honeycomb core composite technologies, which will give the aircraft’s structure and blades high strength and light weight properties.

These materials will be manufactured at Hexcel's Duxford, Cambridge site, which was founded in 1935 by Cambridge University don, Dr Norman De Bruyne, a pioneer in structural adhesives and composite materials. Today, the Duxford plant has six manufacturing plants and is also Hexcel's European R&D centre. An extensive range of prepregs - epoxy, phenolic or BMI resin systems that are reinforced with carbon, glass or aramid fibers – are manufactured at Duxford, along with hotmelt prepreg resin films, a range of adhesives and honeycomb.

The aerospace industry is the greatest consumer of Hexcel prepregs, which find applications across the board in civil aircraft, military jets, helicopters, engines and space satellite launchers. Hexcel’s range of resin formulations for aerospace prepregs includes a wide range of epoxies for highly loaded parts and supreme toughness; BMI systems for high temperature performance; phenolics for fire, smoke and toxicity performance in aircraft interiors; and cyanate esters for space structures and satellite applications. HexPly prepregs are available with HexForce woven and multi-axial reinforcements, or as unidirectional tapes in various forms.

The materials
HexPly M91 is Hexcel’s latest toughened epoxy prepreg, which is claimed to offer superior performance for primary aircraft structures and engines. Outperforming the company’s HexPly M21 prepreg, with which it is fully compatible, HexPly M91 is a toughened, controlled flow, epoxy resin system supplied with unidirectional fibres, best suited to autoclave cure to ensure optimum mechanical performance.

Hexcel provides what is possibly the largest variety of structural honeycomb to the aerospace industry, producing over 700 varieties and constantly developing new versions to meet customer demands. Honeycomb is an outstanding core material for sandwich structures and an efficient energy absorber. It is also used for airflow control, sound attenuation and dielectric applications. A variety of production processes, including the expansion method and the corrugated process, result in a range of cell configurations to meet different requirements for formability, flexibility, energy absorption and strength.

The HexWeb honeycombs chosen for the S-97 Raider project provide exceptional stiffness and strength with little added weight. The honeycomb yields a structure made from a wide variety of web materials, including thermoplastic, fibreglass, carbon, aluminium and aramid mechanical papers. Among recent developments, the HexWeb HRP-C fibreglass core is suitable for aircraft nacelle applications, while the HexWeb HDC-F high density fibreglass can be used as a potting replacement in honeycomb sandwich panels.

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