The best of both worlds: Bridging the gap between helicopters and fixed-wing aircraft
The advent of unmanned aerial vehicles (UAVs) is having an interesting impact on the way engineers think about aircraft. In many cases the rulebook has been thrown out with nothing off limits and ingenuity key. Removing people has removed much of the risk associated with failure in the air allowing much more radical aerodynamic concepts to be trialled.
One example is ornithopters, the idea of mimicking nature by flapping wings to produce lift and thrust. These are increasingly seeing use in small and micro UAVs designed for everything from covert military use to children's toys. A relatively recent example comes from Israel Aerospace Industries that has unveiled a prototype of its 'Butterfly', a micro-UAV for covert reconnaissance missions.
Another example is the DEMON demonstrator aircraft that has been developed by BAE Systems with a consortium of UK universities. DEMON does away with using conventional flaps and instead the aircraft uses air blown over the trailing edges of its wings to manoeuvre. The aircraft, which weighs 90kg and has a wingspan of 2.5m, undertook the first 'flapless' flight ever in the UK in 2010. As it is designed to fly without conventional flight controls such as elevators and ailerons, it requires far fewer moving parts. The aim is to make the aircraft much more reliable as well as easier to maintain and repair.
UAVs are getting increasing performance demands placed on them in terms of very short – if not vertical – takeoff and landing, the ability to be highly manoeuvrable, need for low maintenance while having good range and speed. This means that many traditional helicopter and aeroplane layouts and architectures are being rethought and redeveloped.
It was this train of thought that originally set the ball rolling for D-Dalus, a cyclogyro rotorcraft that is being developed by Austrian aerospace firm, IAT 21. The company was set up after inventor Meinhard Schwaiger became intrigued with the shortcomings of helicopters.
"I was on a business trip when a television programme came on about helicopter crashes," he says. "It was clear why they crashed, they were unstable and the pilot was not able to control them. I wondered, why didn't they build them it in a different way and make them more stable?"
Schwaiger's background is in mechanical engineering and plastic processing and although he has a fascination with flying, he did not have experience in aeronautics. Despite this, he came up with a truly intriguing concept that perhaps borrows from pump design and a more classical mechanical viewpoint.
"I came up with the idea almost straight away, but thought as it was late in the evening it was stupid and told myself to forget it," he says. "The next day I thought actually it was not such a bad idea so I began to look in to it, did some research and quite quickly came up with a concept and wrote a patent."
Schwaiger carried out some initial work to establish the kind of performance and lift that could be expected against power. Once he concluded that this type of propulsion system could be used for an aircraft, he founded IAT 21 in 2006.
At the heart of D-Dalus are four cyclogyro rotor assemblies that are contra-rotating. The rotors are akin to those used on old paddle steamer riverboats. However, the individual blades can have their angle of attack altered via a mechanical linkage.
Though the aerodynamic body structure of D-Dalus is no doubt impressive, it is the propulsion system that really sets it apart, offering some unique characteristics over more conventional aircraft and helicopters in terms of its manoeuvrability and efficiency.
"If the centre point is exactly on the rotation axle you create no lift as the rotor blades are in a neutral position, so there is zero net thrust," says Schwaiger. "However, if you move out of that centre position you can vector the thrust around 360°."
D-Dalus can take off and land vertically as well as hover in the same way as a helicopter but it can also roll, flip or rotate very quickly around the same point. However, the real advantage comes in forward flight. It is the hover and vertical take offs and landings that is its most inefficient phase of flight – approximately 20% worse than a typical helicopter. However, this is usually only a small part of an overall mission.
"At a certain speed all the lift is generated from the wing and a 100% of thrust is used for the forward speed," says Schwaiger. "That is a big difference over helicopters and we are a lot more economical in forward flight. So we can fly from A to B faster, higher and more economically than a helicopter. We can also touch against solid objects like buildings, trees or mountainsides as we have nothing rotating outside the structure."
Perhaps the biggest rival to this kind of propulsion system, particularly when it comes to UAVs, is quadcopters. These have been getting increasingly put to use as small UAVs in both the military and civil arena.
"Quadcopters weigh 10kg and can carry a 20kg payload while D-Dalus weighs 20kg and can take a 10kg payload," says Vice President for defence and leader of the D-Dalus project, Brigadier David Wills. "They can carry more but we can fly further, faster and more efficiently so that is the trade off."
Another key design feature for the D-Dalus team was the desire to make the propulsion systems as low maintenance as possible. The aircraft already does away with traditional flaps relying completely on vectored thrust to manoeuvre.
"Our chef engineer was a trained BMW motorcycle mechanic," says Brigadier Wills. "If there is something we ask him to do that he can't, we have to redesign it. The aim is that it will need a one-hour service every year."
The main structure and much of the rotating components of the D-Dalus are made from a lightweight carbon fibre, allowing various powerplant options to be considered. There are several possible methods of powering the four cyclogyros: it could potentially be achieved using electric batteries or an internal combustion engine. In the case of a diesel engine, it is placed in the middle of the aircraft and a gearbox is used to transmit the power to each rotor. An electric version uses batteries and each cyclogyro would have its own electric motor. Last year IAT 21 conducted a successful test flight using an engine from a racing ski-doo with around 145-150hp.
Numerous innovative concepts have been developed and applied along the development process so far. As the rotors run at over 2000rpm the forces generated at the circumference can be more than 1000g. This means that any mechanical linkage to the rotating blades to alter the angle of attack needs to operate at this extreme. The team struggled to find a bearing that would do the job needed so they decided to develop their own patented ultra-low friction swivel bearings. These allow microsecond changes giving D-Dalus its unique combination of manoeuvrability, performance and efficiency.
IAT 21 are also examining other novel concepts and technologies to further increase these attributes. Amongst them is a revolutionary way to improve the aerodynamic performance through its rotors. The team is also examining a technique to electrically generate a plasma boundary layer that influences the airflow over the rotor blades using a Plasma Enhanced Cycloidal Thuster (PECyT).
"It is an electromagnetic plasma and is not hot," says Schwaiger. "The current is induced through the centre and there is a wire through the composite air blade. It is a pulsing electromagnetic field that is only a few microns across the surface thickness but it immediately influences the friction of the airflow and you can direct it precisely. So that allows almost pure laminar flow over the wing."
Furthermore, due to its manoeuvrable nature, the D-Dalus is also highly suited to swarming. At moment with air traffic the further apart aircraft are, the safer they are considered.
However air traffic is going to triple in the next 20 to 30 years with countless more unmanned aircraft likely to be utilised for all sorts of applications.
"The only way of sensibly doing that is to swarm them," says Brigadier Wills. "So then the air traffic can control the swarm and something like the D-Dalus can go and join a swarm at higher altitude for intercontinental travel and come away from it as needed. That way you can pack a much higher traffic density into the sky."
The idea is establish the technology as a UAV and then explore the possibility of using it as a manned aircraft. That development, of course, will take a lot more time and money and IAT 21 hopes to partner with a bigger aerospace company at some point in the future. In the nearer term, D-Dalus is likely to continue flight trials next year for both military and civilian applications. "It opens the doors of possibility," concludes Schwaiger. "We know there is big interest from the European community and they want to push us."
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