Tilting the balance in the air
Aircraft manufacturers are blurring the line between helicopters and planes in a bid to break rotary-wing performance barriers. Jon Pratty assesses the chances of them taking off.
Residents of Yeovil in Somerset are used to seeing helicopters circling overhead, making test flights from the GKN Westland factory on the outskirts of town. But within two years the Somerset skies could well be graced by a very different machine: the Westland compound helicopter.
The compound, now at concept stage, is one of a new breed of advanced, hybrid aircraft that bear little resemblance to the familiar chopper. It will combine the lifting surfaces and propulsion systems of a fixed-wing plane with the vertical take-off abilities of a high-tech rotor system.
US manufacturers are exploring concepts even more advanced than this British effort. Some are years from being practical and safe alternatives to current designs, but they may offer solutions to problems that have beset designers since the first days of rotary- wing flight in the 1930s.
Helicopter development has been held up by the fact that the rotor has to do several jobs at once. ‘The rotor lifts, propels and controls the machine. It gets less efficient the faster you get,’ says David Humpherson, head of advanced projects at Westland.
The lift-to-drag ratio of a helicopter is typically around five. For a Boeing 747, the ratio is about 20:1. A top-notch sailplane will have a lift-to-drag ratio exceeding 60:1. ‘Helicopters are fund-amentally inefficient — and that is our problem,’ says Humpherson.
Westland holds the world speed record for a helicopter in level flight — 249 knots — achieved in a modified Lynx in the 1980s. At such speeds, the tip of the rotor blades approach the speed of sound. The resulting vibration in the rotor makes flight uncomfortable and dangerous. The extra power and fuel needed increases exponentially as speeds of 230–250 knots are approached.
One attempt to sidestep the limited efficiency of the traditional helicopter is the Boeing Bell V-22 Osprey tiltrotor, now in small-scale service with the US Marines. In level flight it looks similar to a conventional prop-driven aircraft, but with enormously oversized prop-ellors. For take-off and landing, the wing-mounted engines tilt, allowing the propellers to act as helicopter rotors.
Once airborne, the pilot tilts the engines back to a horizontal position. Though this sounds like a simple idea, in reality the aircraft has turned into a very complex and expensive machine.
If one engine failed when hovering the Osprey would plummet to the ground. As a consequence, interlinked drive shafts distribute power from each engine to each prop. If one engine stops, the remaining power plant will drive both props, solving the problem at the expense of extra weight and reduced performance.
In addition, according to one industry insider, the complex arrangement of power shafts within the machine causes so much vibration that front-line pilots will find the V-22 tiring to fly.
Two recent fatal crashes have led to further scrutiny of the Osprey project. In one incident, a V-22 with 18 US Marines on board crashed while landing. It seems that as it descended, the propellers entered their own turbulent air, known as vortex rings, losing all lift.
Former Bell Helicopter engineer Jay Carter does not think that the tiltrotor is the way to go. The NASA-funded CarterCopter — a cross between a fixed-wing plane and a gyrocopter — is twice as efficient, he claims. In the CarterCopter, the rotor is spun up on the ground and the machine takes off just like a helicopter. Once airborne the rotor slows down to 70–80rpm, in theory reducing drag. Small, stub wings support the CarterCopter which then flies like an ordinary fixed-wing plane — and with the same aerodynamic efficiency.
For landing, the rotor spins up to speed and the craft descends like a helicopter. It can only hover for a limited period, until the inertia stored in the rotor runs down. The aircraft is now being tested. So far it has only been flown at relatively modest speeds, but its designers think that production models could exceed 400knots, reach heights of more than 50,000ft and have almost unlimited endurance.
CarterCopter has big plans for the future — design studies are now complete on the CarterCopter Heliplane Transport, or CCH-T. The same size as a Lockheed C-130 Hercules, the CCH-T would be by some margin the largest rotorcraft ever built. It is planned to be able to fly twice as high as an equivalent sized tiltrotor, using the same amount of fuel and costing roughly the same as a Hercules.
Some in the aviation community have yet to be convinced about the CarterCopter, though: ‘I think their claims for low drag at high speeds are somewhat fanciful,’ says Westland chief scientist Geoff Byham. ‘They are claiming even lower drag than conventional planes.’
Looking further ahead, several US concepts make even the ideas of Jay Carter seem conservative. Boeing has come up with the Canard Rotor Wing, or CRW, a small, agile and simple unmanned reconaissance vehicle. The single two-bladed rotor mounted on top of the machine can be slowed to a stop. As the blades slow, they transform into fixed wings, and aeroengines drive the craft forwards.
Again, not everyone is convinced. ‘The idea of stopping the rotor in flight sounds very fraught to me,’ says John Fay, former Westland chief test pilot.
‘Presumably the craft is descending at a rate of knots as it makes the transition into rotary-wing flight. If anything goes wrong in the changeover you would be in real trouble.’
Also in the US, the Frontier Systems A-160 is on the drawing board. This machine also slows its rotor like the CRW and is intended to be an unmanned reconnaissance drone. Looking at first sight like a conventional helicopter, the A-160 relies on ultra-stiff, extra-large rotor blades, which, in theory, should lead to a dramatic improvement in aerodynamic efficiency at low speeds and weights. The high-efficiency rotor has been successfully tested on a conventional helicopter and a full-size prototype is scheduled to fly within two years.
So where does this leave Westland? In comparison with some of the ideas coming out of the US Westland believes its compound helicopter is much more practical. ‘We were starting to think about what would take the design of helicopters forward,’ says Humpherson. ‘If you take one of the jobs away from the rotor, such as propulsion, you free some of the lost potential.’
Westland thinks its self-funded concept is the safest, cheapest and most practical way to get through the barriers that limit performance. It uses systems and concepts available now, off-the-shelf.
It involves modifying an existing design, such as a Lynx or EH-101, adding small wings and vectored thrust nozzles to the jetpipes of the engines. Moving control surfaces on the tail, engine nozzles and rotor head would all be actively controlled using an advanced, digital, fly-by-wire system. The aircraft could be flown using the normal helicopter controls.
‘In both the CarterCopter and the Westland compound you are duplicating the safety factors, compared to the tiltrotor, and you’ve got the control power of both systems, wings and rotors,’ says Byham.
Designers think the compound approach could bring lift to drag ratio improvements of 25–50%, and productivity and payload improvements of 20% plus: ‘We’re starting to make fairly large inroads into the shortcomings of the chopper,’ says Humpherson.Because the compound helicopter uses existing technology, it is likely that it would cost little more than a conventional helicopter, with considerable performance gains.
Fay says: ‘Evolutionary technology such as this is more likely to succeed in the market. The compound project sounds a good idea — give it some wings, a bit of thrust from somewhere else. The wings will unload the rotor in fast flight and you won’t get so many blade stall problem.’
An airborne early warning compound EH-101 could capitalise on longer in-service time, lower fuel consumption and higher efficiency. ‘Our calculations suggest the compound could be a really attractive combat machine,’ says Humpherson. ‘The range of roles it could perform is larger than for a conventional helicopter,’ says Byham. ‘It would be stealth-capable and highly manoeuvrable.’
The big question now is: will the Ministry of Defence want such an aircraft? Although the compound helicopter does not exactly match any official MoD requirement, Westland is seeking funding to build a prototype. The company has put a proposal in to the MoD and is awaiting a response.
Whether the project gets any further depends on whether the MoD can be convinced.
Sidebar: Rotary wings not just Fairey tales
Radical rotary-wings are nothing new. One of the most unusual-looking aircraft of all time was the Fairey Rotodyne of 1959. Conceived as a medium-sized commercial aircraft with vertical take-off and landing capability, it was intended to herald a new era of city-centre heliports and city-to-city rapid air travel.
All the technology worked well, including a system for driving the giant main rotor using jet nozzles in the rotor tips. Two gas turbine engines powered propellers to pull the machine through the air. When the time came to land, the Rotordyne would change into vertical flight from fixed-wing flight.
‘It was very, very noisy,’ says former Westland helicopter test pilot John Fay. ‘It made a noise like a couple of trains going through a tunnel — but it looked very impressive. It worked, but the expense of the whole project brought it down in the end.’
The Rotodyne very nearly became a commercial success: a New York-based airline ordered a small number of production versions of the craft, but sufficient funding from the UK government was never on hand to establish a production line or to iron out the remaining bugs in the system. Fairey’s helicopter business was taken over by Westland, and the Rotodyne was allowed to die quietly, a good idea, but well before its time.