Flapless technology that manipulates airflow could represent a new way to control aircraft.
Flying is the art of controlling the air. If you watch any flying object – insect, bird or aircraft – to see how it flies, you can’t see the most crucial aspect, which is how the air flows around and past the body. It’s this that supplies lift, which keeps the object in the air. The air flow is what allows the object to change direction, to swoop and turn. Thrust alone will get you into the air, but it won’t keep you there. If you want to fly, you have to control the air.
How to do it is a complex matter. Gliders aside, living aeronauts do it by combining their control surfaces with their thrust generators – they flap their wings, move them and change their shape in complex ways that we’re only just beginning to understand and can barely emulate. For man-made flying vehicles, the main way to control the flow of air, and therefore enable true flight, is to change the shape of the wings, with flaps and rudders, to deflect the air as it passes over them.
There might, however, be other ways. A major project led by BAE Systems and involving 10 universities has developed an unmanned-aerial-vehicle test bed for a different way to control flight. Embodied in a stubby diamond-shaped aircraft that had its first flight late last year, the FLAVIIR (Flapless Air Vehicle Integrated Industrial Research) project works by manipulating the air that flows immediately next to its skin, rather than changing its shape.
The goal of Flavir is to develop an aircraft controlled without flaps that matches the performance of a conventional aircraft of the same power, and brings in a variety of technologies, including aerodynamics, control systems, electromagnetics, manufacturing, materials and structures
The conventional flap approach to control surfaces is, of course, tried and tested, but the more moving components a machine has, the more likely it is to break down and the more maintenance it needs.
FLAVIIR is a five-year project whose £6.5m budget comes from BAE Systems and the EPSRC. The academic partners include Cranfield, which co-leads the project with BAE, along with Imperial College London, the universities of Leicester, Liverpool, Manchester, Nottingham, Southampton, Warwick and York, and the University of Wales in Swansea. The goal, to develop an aircraft controlled without flaps that matches the performance of a conventional aircraft of the same power, brings in a variety of technologies, including aerodynamics, control systems, electromagnetics, manufacturing, materials and structures, along with the numerical and computational systems needed to model and understand all the systems. ’What makes FLAVIIR unique is that research is aimed at producing an entire working system, rather than just looking at individual technologies,’ said Phil Woods, project manager at BAE Systems’ Advanced Technology Centre.
All these technologies are being brought to bear on the layer of air immediately next to the skin of the aircraft. The boundary layer, as it’s called, is responsible for the pressure on the body of the aircraft and it’s this that dictates how it flies. Aircraft wings generate lift because their shape – more curved on top than underneath – forces the air to take a longer path over the top than the bottom and, therefore, pushes the wing upwards. Similarly, flaps and rudders disrupt the flow of air so that one side of the aircraft is pushed upwards more than the other, forcing it to turn – it’s this effect that causes aircraft to bank.
But there are ways of achieving that other than changing the shape of the wing. The airflow can be altered by using more jets of air from the trailing edge of the wing, which ’entrain’ the boundary layer and force it to move away from or toward the wing surface.
The FLAVIIR project is exploring the use of these jets, known as synthetic jets or zero-mass jet actuators, to manipulate the boundary layer. About 5mm in diameter – half the size of the boundary layer – the jets are positioned at the back of the wing, with many hundreds needed to control a full-scale aircraft. These are combined with arrays of micro-sensors and actuators to measure airflow and ensure that the right synthetic jets are firing to move the aircraft in the right direction. Imperial and Leicester universities are working on this part of the project, with Imperial taking the lead on the jets and Leicester on the sensors and actuators.
A similar effect can also be used on the jet thrust from the engine. This is a version of thrust vectoring – the same system that’s used to vary the direction of thrust in vertical take-off jets such as the Harrier and the STOVL version of the new F-35 Lightning strike aircraft. But rather than physically moving the engine nozzle, the FLAVIIR vehicle uses secondary jets positioned around the engine nozzle to again entrain the thrust and move it around, using a phenomenon called the coanda effect. This exploits the tendency for a stream of fluid to stick to a curved surface (try holding the handle of a wooden spoon near the water from a tap for a demonstrat-ion). The inner edges of the jet nozzle are rounded, forcing the control jet, which is at a higher pressure than main jet, to follow the curve as it leaves the aircraft. This bends the main jet exhaust in the same direction, causing a control force to act in the opposite direction. A control jet at the top of the nozzle, for example, bends the jet exhaust upwards, which forces the aircraft downwards.
Another thread of the research, based at Cranfield, is focused on manufacturing techniques that are based, in particular, around low-cost reinforcement fabrics and laminates, while Warwick is looking at reducing the cost of tooling for modifying the aircraft. The thinking behind this is that such UAVs are likely to be produced in low volumes and will probably be heavily modified for each specific mission – tooling, therefore, would be a major cost on such a project.
The outcome of FLAVIIR is an experimental UAV called Demon, which was designed at Cranfield and built by the university’s Composite Manufacturing Centre and apprentices at BAE Systems. Weighing 80kg and with a 2.7m wingspan, Demon will evaluate a number of systems for manipulating the boundary layer via blown air, with test flights taking place over the course of this year. ’These are the engineers of the future, working on the products of the future,’ said Matt Pearson, Demon delivery manager.
Systems aimed at reducing wing area were in use in the 1960sAs with many engineering innovations, this isn’t a new discovery. Systems known as ’blown flaps’ were used in the 1960s, when aircraft designers boosted the speed of aircraft by reducing their wing area. This cut drag, increasing the top speed, but also reduced lift, so the aircraft couldn’t take off unless they were going extremely fast. The blown flaps sent compressed air, bled off from the compressor stage of the aircraft engine, out of slots on the flaps behind the wings to increase lift at low speeds during take-off and landing. The first production aircraft to use them was the Vietnam War-era Lockheed Starfighter, whose wings were barely stubs; other notable US blown-flap aircraft were the Phantom and the Vigilante supersonic bomber. In the UK, the carrier-launched Blackburn Buccaneer and the TSR-2 fighter bomber also used this system. It fell out of favour, however, because, in its early incarnation, it was expensive and difficult to maintain, as the air slots tended to become blocked with dirt.
More advanced versions are still used, notably on the Airbus A380, in conjunction with moving flaps. The FLAVIIR project represents the first attempt to use the system as the sole source of manoeuvring control, rather than to boost lift.