Those like us on the Vulture flying-car desk, who chafe at the limitations of present-day hovering aircraft, like to keep an eye on Sikorsky's X2 prototype. The helicopter firm announced today that the X2 made its first ground runs with blades fitted last week, and everything went swimmingly.

"We continue to move our experimental program closer and closer to first flight," said Sikorsky chief James Kagdis. "We anticipate that reaching this particular milestone, as with those yet to come, will continue to pique the interest of our military, commercial and international customers."

X2. Even more revolutionary than an Osprey.

The X2 is Sikorsky's rival concept to the well-known V-22 Osprey tiltrotor from Boeing, now coming into service with the US Marines and special-ops forces. Both aircraft are intended to deal with two of the most pressing downsides of ordinary helicopters: limited speed, and limited range.

Helicopters' speed is capped by two main factors. As their blades spin backwards with respect to the forward motion of the copter, a point can be reached where the blade is briefly stationary with respect to the air it cleaves, which causes it to stop generating lift - obviously a bad thing.

Secondly, on the other side of the fuselage the blade tip which is going forward will start going supersonic well before the actual helicopter does, and this too causes serious issues. If you speed up the rotors to avoid the retreating-blade stall, you bring on the sound barrier even quicker.

The tiltrotor deals with these by tipping its twin rotors forward to become propellors, and flying like an aeroplane. But this simple-sounding idea is actually technical hell to implement, as the V-22 programme has found over past couple of decades. In particular, dealing with engine failures in helicopter mode is especially hard. A normal twin-rotor chopper (eg, the Chinook) has its rotor assemblies linked, meaning that remaining power can be shared between them. In the case of a total power loss, the bird can still "autorotate" down to land under control, dumping the energy which would normally smash it to bits on impact into spinning its rotors faster instead.

The V-22 can do this, but it wasn't easy to make it happen. The complex, expensive Osprey may well be vulnerable to competing approaches.

Sikorsky think so, anyway. They reckon to deal with the retreating-blade stall issue by using coaxial contrarotating rotor discs - thus far seen mainly in Russian designs - so that there are blades going forward on both sides of the craft at any given moment. Then, as the advancing blade tips start to bump up against the sound barrier, the rotors will begin to spin more slowly - letting the X2 carry on accelerating.

Normally this wouldn't be easy, as a regular copter gets its forward poke as well as its lift from the rotor disc. But the X2, having twin main rotors, needs no sideways tail prop to keep it pointing in one direction in the hover. Instead, it uses the tail for a pusher prop which can drive it forward.

Sikorsky reckon the X2 should be able to cruise easily at 250 knots, well in excess of a regular whirlybird's 150 or so. The Osprey can do this already, but Sikorsky reckon their bird could beat the V-22 in other areas such as range, size, cost and maintenance effort. One also notes that variable-rotor-revs tech like that of the X2 is quite rare, most whirlycraft spinning their blades at a fixed rate. Boeing's unmanned A160T uses variable-spin, and is said to be "four times quieter" in the hover than a regular copter - not to mention having advantages in fuel economy.

Whisper mode would, of course, be attractive to those who don't care what colour their chopper is so long as it's black. However, more prosaically, noise during takeoff and landing is one of the biggest obstacles to using helicopters in urban environments.

Noise was a major factor in killing off another excellent fast-but-hoverable aircraft concept tried out in the 1950s, the "Rotodyne". In this, a rotor disc is spun using "tip jets" rather than a normal engine and transmission. Away from the pad, such a craft transitions to forward flight using thrust from ordinary turbofans or props, gradually taking more and more lift from stub wings and allowing the rotors to spin unpowered. The Rotodyne is currently being looked at again under the name "Heliplane", but is still struggling to deal with the horrendous racket generated by tip jets.

The X2 would seem to offer some prospect of being quieter in the hover than a normal chopper, not to mention an Osprey with a larger physical footprint - and it would surely make less row than a Rotodyne/Heliplane. So Sikorsky's new toy just might be the design which brings aerial buses into the city centre at long last, even if it doesn't look much like a flying car. ®