Original URL: http://www.theregister.co.uk/2009/02/03/x2_tail_prop_ground_runs/

X2 triplex supercopter gets tail-drive hooked up

3 eggbeaters = 1 worldbeater?

By Lewis Page

Posted in Science, 3rd February 2009 13:10 GMT

US helicopter firm Sikorsky has announced successful ground testing of its "X2" twin-rotor, tail-prop high speed helicopter prototype with the tail propulsor fitted and working. The firm says the X2 is on track to achieve 250-knot speeds this year - beating normal helicopters by two thirds again.

Sikorsky art of the X2 demonstrator

Faster than a good hoverer: hovers better than anything fast. On the drawing board, anyway.

"The high-speed pusher prop produced the expected amount of thrust and the testing went according to plan," said X2 chief Steven Cizewski. "In addition, the pusher prop's drive system and oil distribution system checked out well. In all, we are pleased with this milestone and look forward to finishing the tests."

The X2 has already made its first flight, but that was without any use of the tail propellor - which will be necessary for the new machine to achieve its full potential. The idea of the X2 design is to achieve much faster cruising speed than a normal helicopter can manage, while retaining good performance in the hover and - according to Sikorsky - avoiding the complexity and high maintenance burden attendant on tiltrotor designs auch as the V-22 Osprey and BA609.

The normal problems suffered by helicopters attempting to beat 150 knots or so are avoided by the X2's design. The issue of "retreating blade stall" - where the backward-spinning blade briefly becomes almost stationary in the air owing to the speed of the aircraft, so losing lift - isn't a problem as there are forwardgoing blades on both sides at any moment. The problem of the forward-moving blade tips going supersonic is sidestepped by the use of variable speed rotors: most helicopters spin their blades at a steady rate, but the X2 can slow them down as speed increases, keeping the tips safely subsonic.

All this, Sikorsky engineers believe, will allow the tail prop to safely push the triplex copter up to cruise at 250 knots: beating the Osprey, and potentially doing so in an aircraft which would also work better in the hover than the controversial US military tiltrotor. (The Osprey's blades, in order to offer decent performance as propellors in the cruise, are necessarily shorter than one would choose to have them for use as vertical-lift rotors. No such constraint applies to the X2.)

Previous efforts along X2 lines have been bedevilled by vibration issues and excessive weight, but Sikorsky believes that modern superstiff rotors, variable rotor speed and active vibration damping tech will squelch the shudder. Use of a pusher prop driven by the main engine, rather than extra strap-on jet engines as with the ill-fated XH-59A demonstrator, is expected to offer a much lighter design. Sikorsky also like to emphasise that smaller X2 craft of the future could be single engined, something which is definitely not an option for tiltrotors.

So great is the firm's faith in its technology, indeed, that it is proceeding with development at its own expense rather than the more normal route of expecting funds by one or another channel from the US taxpayer. ®