Despite the astonishing versatility of the helicopter, conferred by its ability to take off and land vertically, many believe that today's rotorcraft are effectively limited to niche roles by their restricted performance envelope.
Although a heavily modified Westland Lynx set a helicopter world speed record of 249.1mph (216.4kt/400km/h) on 11 August 1986, aircraft such as the AgustaWestland AW139, Eurocopter EC155 and Sikorsky S-76 are "sub-175kt" helicopters, cruising more than 85kt slower than a prop-driven fixed-wing aircraft such as the Beechcraft King Air, and nearly 260kt slower than a typical business jet, such as the Hawker 800.
At ranges of much above 175nm (320km), it is often quicker to drive to an out-of-town airport and fly on a conventional fixed-wing aircraft than it is to use the "city centre to city centre" capability of the helicopter. This has naturally curtailed the appeal of the rotorcraft in the corporate and VIP passenger roles, an attraction that has been further eroded by the noise and vibration encountered in many traditional helicopter cabins.
Noise and vibration have only recently been reduced to the levels experienced in equivalent fixed-wing aircraft, and helicopters remain restricted to altitudes of typically less than 20,000ft (6,100m), often making it impossible to fly "above the weather", and making it harder to use some controlled airspace.
There are few helicopter roles in which increased speed would not be both useful and commercially beneficial, even where vertical take-off and landing capability is essential - such as for offshore oil and gas support and airborne emergency medical services, where speed is life, after all.
As a result, increasing the speed capability of the helicopter (or designing a faster alternative with similar VTOL capabilities) has long been a preoccupation of rotorcraft designers. The Fairey Rotodyne, which first flew on 6 November 1957, demonstrated speeds of almost 175kt (50kt or more faster than contemporary helicopters) and could carry 48 passengers. To achieve this, the aircraft combined a large, four-bladed main rotor, driven by tip jets, augmented by a fixed stub wing on which were mounted a pair of Napier Eland turboprop engines.
The Rotodyne could take off vertically with all lift provided by the tip-jet driven main rotor, but would then "translate" into forward flight, with all power from the engines being transferred to the propellers, and with the main rotor autorotating like that of an autogyro, with reduced collective pitch and much reduced drag. In the end, funding problems killed the Rotodyne, but not before it had set a number of speed records for rotorcraft, and not before it had wowed the crowds at several Farnborough air shows.
The Fairey Rotodyne, which first flew on 6 November 1957, demonstrated speeds of almost 175kt, 50kt or more faster than contemporary helicopters and could carry 48 passengers
The Rotodyne was just one of a generation of experimental rotorcraft used to try to find a faster alternative to the conventional helicopter. Another approach was pioneered by Bell, which chose to tilt the rotors of a broadly conventional aircraft to provide lift in vertical flight, or to provide thrust in forward flight.
The initial XV-3 had a conventional fixed wing, with swivelling wingtip mounted rotors, and the XV-15 that followed it was similar, with wingtip mounted engine pods that swivelled through 90° to tilt the massive three-bladed rotors. Canadair's CL-84 and LTV's XC-142A took a slightly different approach, swivelling the whole wing, with engines attached. Although none of these aircraft resulted in a successful production aircraft, Bell and Boeing borrowed heavily from the XV-15 when designing the V-22 Osprey.
For take-off and landing, a tiltrotor such as the V-22 operates with its wingtip engine nacelles in the vertical with the propellers acting as side-by-side main rotors. Once airborne, the nacelles are rotated forward for horizontal flight, with the massive propellers providing thrust and converting the V-22 from unconventional helicopter to wingborne turboprop aeroplane, with corresponding improvements in performance and fuel efficiency compared with an equivalent helicopter.
The V-22 is now in frontline military service with the US Air Force and US Marine Corps and has proved a success in operations in both Iraq and Afghanistan, after a long and sometimes challenging development programme. The V-22 can carry up to 32 troops, in addition to its four-man crew, and has a maximum speed of 250kt and a 25,000ft service ceiling.
A civil tiltrotor, the Bell/Agusta BA609, is already flying in prototype form, and its manufacturers claim that it is "one of the most useful and versatile aircraft in aviation history" making possible mission profiles that could not be flown by conventional airplanes or helicopters and representing a "totally new approach" to corporate transport, search and rescue, and law enforcement missions.
The BA609 will carry nine passengers, cruising at 275kt ("twice the speed of helicopters of comparable capacity") and providing the quickest possible way to travel from one downtown area to another over distances between 80km and 800km, or flying typical offshore missions in about half the time that they take today.
But it remains unclear as to whether the tiltrotor will ever be more than a niche solution to particular requirements, whether it offers a sufficient performance differential to outweigh the potential disadvantages of high cost, high noise and rotor downwash, low payload, and potential problems with steep, slow approaches.
It remains to be seen whether the tiltrotor approach, of trying to "make a conventional aircraft hover" offers the best way ahead for tomorrow's rotorcraft, or whether it will be more cost effective and efficient to undertake a more evolutionary approach with the aim of making "conventional helicopters fly faster".
With conventional main rotors, speed is limited by retreating-blade stall. This can be delayed by off-loading the rotor by adding a wing to generate lift, or by providing an alternative means of forward propulsion, or indeed by doing both.
For its X-49A SpeedHawk technology demonstrator, intended primarily to develop and evaluate a vectored-thrust ducted propeller system, Piasecki Aircraft has modified a Sikorsky YSH-60F to serve as an experimental high-speed compound helicopter testbed. The aircraft is fitted with a "ring tail" ducted pusher propeller (similar to that used in the company's Model 16H-1 Pathfinder testbeds from the early 1960s).
The X-49A is also fitted with a conventional wing to "offload" the main rotor by generating lift in forward flight, and this wing is fitted with flaperons for control. The X-49A made its maiden flight on 29 June 2007 and has been flown to around 180kt, with further work planned to allow it to fly at speeds in excess of 200kt.
Sikorsky began serious work on increasing helicopter speeds by delaying retreating blade stall on its XH-59 experimental helicopter, which first flew on 26 July 1973. The XH-59 used contra-rotating rigid coaxial main rotors. The rotor configuration meant that the retreating blades were offloaded, with most of the load supported by the advancing blades of the other rotor and any penalty imposed by the stall of the retreating blade was thus "cancelled out".
© Piasecki Aircraft
This allowed the aircraft to reach 160kt. The subsequent addition of turbojet engines increased speed to nearly 240kt, although the aircraft proved problematic, with excessive noise and vibration and with poor fuel efficiency.
Sikorsky revisited the basic XH-59 configuration with its X2 technology demonstrator, which first flew on 27 August 2008, and which exploits advances in materials, rotor technology and digital flight controls to overcome the problems encountered by the older XH-59. The X2 has a coaxial contra-rotating rotor system with rigid, high lift-to-drag blades in low drag hub fairings, and has a six-bladed tail-mounted propulsor.
The aircraft has an advanced integrated fly-by-wire control system that gives full control of rotor RPM throughout the flight envelope. The aircraft achieved 181kt in level flight in late May this year, and Sikorsky expects the X2 to eventually exceed 250kt during the forthcoming fourth phase of flight-testing, and to form the basis of a new generation of high-speed helicopters.
Others challenge the need to fixate on increasing speed performance. At this year's Royal Aeronautical Society Sopwith Lecture in London, Dr Sergei Mikheyev, designer general at Russia's Kamov design bureau since 1975, said that "speed is not the main thing. It is the need to improve the aerodynamic performance of the helicopter. It is that which will represent the big breakthrough in the helicopter industry."
Despite these words, Kamov is working on its own high-speed helicopter project, the Ka-92, which combines an X2-type coaxial contra-rotating main rotor with a pusher propeller tail rotor like that fitted to Piasecki's "Speed Hawk". Russian rival Mil is working on a similar configuration, the Mi-X1.