A UK engineering team is using the Coanda effect to create a novel UAV. Stuart Nathan reports.
The industrial estates of Great Britain hold many secrets. In draughty breezeblock buildings on the edges of towns all over the country, you might find the most advanced of technologies rubbing shoulders with the most basic. Head to a small estate just outside Peterborough, for example, and among the aquarium superstore, the party goods stockist and the brakepad installer, you’ll find a company that makes flying saucers.
Aesir, the company in question, is working on a range of UAVs, for which ‘flying saucer’ is a perfectly accurate description. Circular or octagonal, with smoothly bulging walls and topped by a doughnut- shaped housing enclosing a rotor fan, the craft look like scaled-up novelties, the sort found in plastic bags stapled to the front of comics.
In flight, the craft is far from a novelty. It lifts straight up from the ground, tilts and zooms around in any direction, rises and falls, slows and hovers. It can touch a wall, tree, or other obstacle and move away, unlike any other type of UAV. The fact that it can do this with barely any external moving parts gives it a slightly unreal air and although UAV-testing regulations in the UK mean that, currently, it can only be tested at low altitude under remote control, its capabilities are noticeably different from more conventional fixed-wing or helicopter-type UAVs.
Aesir has only existed since the start of this year, being set up to fully commercialise the work of the craft’s inventor, former hovercraft engineer Geoff Hatton. ‘Hatton was very interested in the possibilities of hovercraft, but he was frustrated by their reliance on the wing-in-ground effect, which means that they can only work close to the ground. They can’t go over large obstacles,’ explained Mark Broughton, marketing director. ‘So he tinkered with different variations on the fan system and the body shape, until he found a way to use a phenomenon called the Coanda effect to make something that could actually fly.’
The Coanda effect is named after Romanian aeronautical engineer and inventor Henri Coanda, who discovered it in the 1930s. It’s the tendency for a stream of a fluid to become attached to a curved surface placed near to it. Whenever you’ve tried to pour a cup of tea from a pot and the stream has dribbled down the body of the teapot and onto the table rather than arcing out into the cup, that’s the Coanda effect in action.
It’s also what generates lift on fixed wings, said Chris newline, chief executive and technical director. ‘As an aerofoil passes through [air], the air stream tends to stick to the top of the curved wing surface. That reduces the air pressure and forces the aerofoil upwards.’
In the Aesir craft, however, the Coanda effect is harnessed in a different way. The fan at the centre of the hollow fuselage canopy pulls air in from above the craft and blows it out radially, over the top of the curved body. Because of the Coanda effect, the airstream remains ‘stuck’ to the canopy and follows the curved surface, leaving the body at its base. This, along with the downwards thrust of the fan, pulls the aircraft upwards.
A rotating fan generates a torque, which gives the craft a tendency to spin. Helicopters counter this with their tail rotor; the Aesir craft has a different method. The original craft, as developed by Hatton, featured short vanes on the surface of the fuselage that act as stators, keeping the air flowing downwards over the curved body and counteracting the torque. Some of these stators can tilt, however, pivoting around their centres to create angled stators. These allow a certain amount of spin, which steers the craft.
Directional control is provided by two sets of flaps on the lower edges of four alternate sides of the octagonal body. These work in opposite pairs, and interrupt the airstream around the canopy, varying the amount of lift. Moving the two sets of flaps opposite each other makes the craft tilt along that axis, which then sets it moving in the direction of tilt.
‘You can think of what we’re doing with the craft as making a very stable hole in the air, which we direct the craft into,’ said newline. ‘It means we can go in any direction we want, at a range of speeds, or we can stop dead. We can take off and land, and we can hover. Because of the shape of the canopy, with its angled sides, our craft balances on a broad-based cone of air, which is much, much more stable.’
When Aesir acquired Hatton’s assets, he had built a flying demonstrator model of his aircraft. Newland’s team then set about defining possible markets for it and devised a family of products, in varying sizes, that could fulfil a variety of different applications. The initial target market, as with most UAVs, is the defence sector, with surveillance and reconnaissance at the top of the applications list.
The workhorse of the range is a metre-wide craft called Odin, which will probably be powered by a rotary internal combustion engine. ‘The aerodynamics are all handled by the upper surface and the area directly below the fan, so anything else under the canopy can be used to carry equipment,’ Broughton said. ‘We could install cameras, infra-red systems, or communications equipment.’
Odin would be small and light, as the main body of the craft is made entirely from carbon fibre. This would allow it to be carried on a troop carrier, Broughton explained. ‘The communications network for operations has several layers, from satellites to high-altitude reconnaissance aircraft down to standard, drone UAVs, but they all have set patrols, and if a unit wants to find out what’s directly ahead of them, they have no way of doing that. But if they had an Odin unit, they’d be able to send it up, send it out for about an hour to scout out the territory, and then bring it back — other UAVs wouldn’t be capable of that. Unlike fixed wing UAVs, we don’t have to keep moving; we can land and take off whenever we need to.’ If equipped with infra-red imaging, Odin would also be able to spot disturbed earth, Broughton added — a characteristic clue to the presence of an improvised roadside bomb.
The craft’s stable hovering characteristics provide very steady images. ‘If you want to put a camera on a helicopter, you need a complicated stabilised mount and you might also use image stabilisation software on the pictures, or they’d shake so much you couldn’t make out what was there,’ newline said. ‘This system is so steady that you don’t need any of that. It instantly produces what looks like a stabilised helicopter image without any special mounts or processing.’ As well as providing images, Odin could also act as a communications relay, hovering over a strategic location and transmitting the signals from troops on the ground. It could also fulfil a logistics role, for example, by bringing small packages to forward operations posts. ‘You could send a simple UAV in and out again, without endangering anyone, and it would be easy to put a self-destruct on it in case it crashed or got captured.’
Aesir is developing a heavy-lift craft called Hoder to fulfil that role. Equipped with multiple engines, the large craft will be capable of carrying a tonne of equipment to resupply troops on the front line.
The smaller member of the family, dubbed Vidar, is 30cm across and powered by lithium batteries. Designed to be carried by a soldier in or on a backpack, it will be able to operate in narrow alleys or indoors, with a 15-minute flight time. It will carry a lightweight video camera to provide quick images of the soldier’s surroundings.
Aesir’s staff are working on a version of the control system, replacing the stator vanes on the outside of the canopy with a circular cassette which surrounds the outlet of the fan. The cassette consists of a ring of vertical vanes supported by a ring above and below. This means that the air passes through the stators before it reaches the canopy, reducing the number of components.
Each of the vertical vanes is tipped by a moving fishtail section, also made by rapid prototyping, which can move from side to side, directing the airflow and adjusting the lift distribution. ‘We’re calling this a form of vectored thrust — it performs the same function as the moving stator vanes and the flaps on the previous model by altering where the airflow goes. It simplifies the assembly and means that we can have a circular canopy rather than an octagonal one,’ newline said.
The next stage in the development is to install an automated control system, so the craft no longer has to be piloted by remote control. ‘We know that an off-the-shelf system will be able to do the job for us,’ newline added. This will also allow the craft to be flown in gusty winds.
The craft will then have to be certified to fly in the UK’s designated UAV testing air spaces, at higher altitudes than is currently possible. ‘We could have units on the market as early as the end of next year,’ Broughton said.