Russia looks set to join Eurocopter and Sikorsky in the race to commercialise a high-speed helicopter, with the allocation of Rb3.6billion ($1.3 billion) in government cash to support development of concepts by Russian Helicopters' Kamov and Mil design bureaux.

Ultimately, only one of the two designs will proceed. Deputy minister for industry and trade Denis Manturov described the Rb400 million allocated for 2011 - to be followed by Rb700 million in 2012 and Rb2.5 billion in 2013 - as "a moderate sum", and added: "Work is only picking up and at this stage the industry simply could not do more than its capacities allow."

Kamov's Ka-92 concept echoes Sikorsky's X2, with counter-rotating main rotors and a single rear-mounted pusher prop. With X2, Sikorsky has surpassed 250kt (460km/h) in testing and aims to demonstrate good low-speed handling and efficient hovering.

Kamov KA-92 
 © Vladimir Karnozov

Mil's Mi-X1 takes a different tack, with a single main rotor and pusher prop with steering vane. This design offers an interesting blend of the X2 or Ka-92 approach and Eurocopter's X3 hybrid concept which features a single main rotor and twin pushers mounted laterally on short wings that provide some lift in forward flight.

 Mil Mi-X1
 © Vladimir Karnozov

Eurocopter, which has since September 2010 been flying an X3 built around off-the-shelf components including a Dauphin 365 airframe, promises less speed than Sikorsky - but isn’t far off, having just achieved 232kts in sustained, level flight - but insisted its design will be more cost-effective. Critically, said Eurocopter, main rotors are high-drag and counter-rotating designs are thus inefficient as well as mechanically complex. But with X3, the lateral wings provide some lift so the main rotor - which needs provide no forward thrust because the aircraft flies level - can be slowed in cruise mode, reducing drag.

Rotor drag is just one reason why conventional helicopters cannot fly faster than about 180kts by simply applying more engine power to turn their blades more quickly. The combination of high rotor speeds and high forward air speed can make the blade tips go supersonic, particularly during the forward part of their sweep.

However, the critical problem is so-called retreating blade stall. In forward flight, a rotor blade's relative air speed is higher when sweeping forward than when sweeping rearward. Thus, each blade's angle of attack must be flatter on the way forward and steeper when retreating, so that blades on either side of centre provide equal lift. As helicopter air speed rises, this differential is exacerbated until such point as the retreating blades reach a stall angle of attack - and the helicopter becomes unstable.

Hence the attraction of counter-rotating blades; on each rotor, one blade is always moving forward on each side, so the angle of attack of retreating blades need not be raised to balance the lift. As a counter-rotating design, X2 enjoys this inherent advantage, but reducing blade tip speed has also been a significant achievement by Sikorsky engineers.

If the Mi-X1 can fly level, its single main rotor would, also, not have to provide any forward thrust and thus run slower than would be the case for a conventional helicopter in forward flight. However, without the added lift advantage enjoyed by Eurocopter thanks to X3's short fixed wings, Mil's designers face an interesting development challenge to keep rotor speed low enough.

Source: Flight International