Original URL: http://www.theregister.co.uk/2010/09/29/eurocopter_x3/

New Euro multicopter aims bitchslap at American X2, V-22

Targeting low end of supertwirlycraft market

By Lewis Page

Posted in Science, 29th September 2010 10:00 GMT

Franco-Germano-Spanish helicopter agglomorocorp Eurocopter has announced its aspirations to leave a glowing handprint upon the bitchcheek of US whirlycraft titan Sikorsky. In an apparent response to the Sikorsky X2 triplex speedchopper project, Eurocopter has now pulled the wraps off its own supercopter design - which it has chosen to dub the "X3".

The Eurocopter X3 prototype in flight tests. Credit: Eurocopter

Hah, it is not merely ze Americains who can make ze flying eggbeaters

Unlike the X2, which is single-engined and uses twin contra-rotating main rotors supplemented by a pusher tail propeller, the twin-engined X3 Euro bird has a normal main rotor and stub wings tipped by puller propellers.

“Innovation is at the core of Eurocopter’s strategy,” said Lutz Bertling, Eurocopter chief. “The teams at Eurocopter took this hybrid helicopter from concept to first flight in less than three years."

Eurocopter's engineers believe that the X3, now entering flight tests, will offer "sustained cruise speeds in excess of 220 kts". It should be nearly as capable in the hover as a normal twin-engined helicopter, as it carries very little extra equipment. Rather than a single counter-torque sideways-acting tail boom prop as on the normal copter, the X3 has two at its stub wingtips, driven from the main transmission just like the ordinary tailprop.

Transitioning to forward flight, as the X3 speeds up it will develop lift and thrust from the stub wings and their props to supplement that from the rotor disc, which should help the aircraft deal with the issues faced by a conventional helicopter trying to go fast.

Apart from the need for extra shove to push a rotor disc through the air sideways, there are the twin issues of forwardgoing rotor tips becoming supersonic as speed increases, and backwardgoing tips becoming almost stationary with respect to the air they cleave - resulting in loss of lift at varying points in the speed regime.

The Sikorsky X2 deals with these by slowing its rotors down as it speeds up, keeping the tips safely subsonic. As the X2's stacked rotors provide blades going forward on both sides at any given moment, the issue of retreating-blade stall doesn't arise for the American machine.

The upstart Eurocopter offering doesn't have stacked rotors or variable speed. On the other hand it does have wings: it may be that the X3 simply flies mostly as an aeroplane at higher speeds, pitching its rotor blades flat to the air and letting them spin freely. Even so, Eurocopter's 220 knot aspirations aren't as ambitious as Sikorsky's: the American firm hopes to achieve a blistering 250 knots.

It would appear that Eurocopter intend to compete not on performance but on simplicity of design, which in helicopters generally translates to much lower running costs. The X3 is being touted more for civilian missions - rescue, border patrol and intercity shuttle services in particular - than for military operations. Civil customers might not be able to afford the relatively complex and hi-tech Sikorsky superchopper or the famous V-22 Osprey tiltrotor now in service with the US Marines and special-forces units. Another selling point vis-a-vis the Osprey would be the avoidance of the terrifically powerful and destructive downblast produced by the tiltrotor, whose small thrust discs must blow air harder and whose engine exhausts point straight down while in the hover.

Military users may be able to afford the radical technologies of the X2 and the Osprey, and put up with the violent, possibly even pad-melting downblast of the latter: but civilian buyers may prefer the potentially cheaper and less destructive X3, which in engineering terms is little more than a normal chopper with an extra tail boom on it.

For now, Eurocopter says that it aims to get to 180 knots this year, then resume testing in March 2011 after a break. At that stage the company will be aiming for the full 220 knots.

The Sikorsky X2 prototype has already reached 180 knots and is being readied for its final flight tests in which it will try for 250. The V-22 Osprey, following software upgrades, is now thought to be capable of 270 knots. ®