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X2 triplex super-chopper in final flight

Prototype retires: Military 'Raider' version forthcoming

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The (ahem) revolutionary X2 prototype copter, which can fly much faster than a normal helicopter but still make vertical landings and takeoffs, has made its last flight. Manufacturer Sikorsky now plans to produce an S-97 military version of the speedy chopper for sale to the US Army.

The X2 demonstrator airborne for the first time above New York State

“This flight is the culmination of a five-year, internally funded program to expand the operational envelope of helicopters,” said Sikorsky chief Jeffrey Pino in tinned quotes accompanying the flight. “A small empowered team was able to meet a specific number of very challenging goals within a small budget and a compressed schedule.

“The X2 program also has helped to develop the next generation of Sikorsky engineers, including some who are now in key positions on the S-97 Raider™ program."

With twin, contra-rotating main rotors, the X2 had a blade moving forward on both sides of the craft at once and thus did not suffer from the issue of "retreating blade stall" as normal helicopters do when they try to go really fast: the blades on one side of them, going backwards past the fast-moving aircraft, become stationary with respect to the air they cleave and lose lift, tending to make the copter flip over.

The other issues of high-speed choppers were tackled in the X2 by permitting it to slow down the spin of its blades as speed increased (normal craft spin their rotors at a fixed rate), so preventing the tips from going supersonic as forward speed climbed. High vibration - always a problem, especially at speed - was quelled by the use of active damping machinery and super-stiff rotors.

In order to actually get the X2 to go fast it had a tail-mounted pusher prop to increase speed beyond what was possible by tilting the main rotor discs forward as a regular copter does. The tail pusher ran off the same single engine as the main discs, avoiding the cost and complexity of extra engines as seen on previous efforts along these lines.

All these goodies allowed the X2 to reach a top speed of no less than 253 knots in level flight, according to Pino, and the forthcoming Raider is expected to cruise at 220 knots. Most ordinary helicopters struggle to beat 150 knots (though a specially pimped Westland Lynx, G-LYNX, once reached 216 knots). The only vertical-takeoff craft which can exceed this performance is the famous V-22 Osprey tiltrotor*, which is faster still at 270 knots flat out: but this is a larger and much more complex aircraft and X2-type designs, which can be single-engined, would probably be better for many tasks.

Sikorsky's Raider project "will produce two prototype assault/attack aircraft with six-passenger cabins and the ability to carry armament," says Sikorsky R&D veep Mark Miller. “In addition to the superior speed and maneuverability of X2 technology, these aircraft are designed to be capable of 10,000-foot hover out of ground effect on a 95-degree day. The future awaits this technology."

Such hot-and-high performance would be very attractive to US and allied forces serving now in the baking highlands of Afghanistan. Whether they'll actually get it is another matter. ®

*Jump jets like the Harrier and F-35B are faster still, but they aren't normally able to lift off vertically with a combat load - they generally make a short roll to take off and land vertically after having burned fuel and expended munitions.

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