The prototype X2 aircraft has an estimated cruising speed of 250 knots, or 464 km/h, and has been in development since June 2005, United Tech's Sikorsky unit said.

The fastest helicopters on the market today top out at about 180 knots, or 333 km/h, according to Sikorsky officials.

Textron's V-22 Osprey tilt- rotor aircraft, which takes off and lands like a helicopter but flies like an airplane, cruises at about 275 knots, or 509 km/h.

In its first 30-minute test flight in Horseheads, New York, about 320 km north-west of New York City, the pilot did not attempt to bring the aircraft to its top speed, keeping it to 20 knots, or 37 km/h.

It is common industry practice to test new aircraft at slower speeds first, said Mr Jim Kagdis, the programme manager over the X2 at Sikorsky.

He said the company planned to test the aircraft's top speed by next year.

Mr Kagdis estimated it could be another five to eight years before the aircraft is commercially available.

The X2 helicopter features two main rotors atop the cabin, which spin in opposite directions. That both neutralises the spinning force applied to a traditional single-main-rotor helicopter and provides a speed boost.

Using the two opposing rotors also eliminates the need for the sideways tail rotor used to stabilise traditional helicopters. Instead of that sideways tail rotor, Sikorsky has added a third, backwards-facing tail rotor that provides additional speed.

The world's largest helicopter maker is Eurocopter, a unit of EADS NV. Other helicopter makers include Italy's AgustaWestland, a unit of defence company Finmeccanica, Boeing and Textron's Bell unit. -- REUTERS