Clean machine: Cambridge's hybrid aircraft uses up to 30% less fuel than a petrol-only plane You might expect that those taking the lead in aerospace research and pushing forward innovation would be industry giants such as Boeing and Airbus. However, instead, our own academics and university engineering students have been hitting the headlines by developing technology and using advanced manufacturing techniques that promise to take the aerospace industry to new heights.
One set of resesarch engineers and graduates involved in such projects is the University of Sheffield’s Advanced Manufacturing Research Centre’s (AMRC) design and prototyping group, who have developed an unmanned aerial vehicle (UAV) that pushes the boundaries of design and rapid manufacturing (RM) technology. And, in an advance for clean technology, a team at the University of Cambridge have built and tested an aircraft with a parallel hybrid-electric engine – the first to be able to recharge its batteries in flight.
At Sheffield, Dr Garth Nicholson, principal design engineer at the AMRC, headed a team that developed a ‘flying wing’ created using fused deposition modelling (FDM). “We wanted to explore the design for RM,” he says. “We chose a blended-wing-body airframe because it is an aerodynamically efficient design, which lends itself to RM because of the flowing nature of its geometry. It can be made in a small number of pieces, which is ideal for this kind of advanced manufacturing.” FDM works on an additive principle, laying down material in layers. A plastic filament or metal wire is unwound from a coil and supplies material to produce a part.
The team decided to build the first prototype as a glider, to check the aerodynamics and ensure it could fly before adding power. “As a rule, if an aircraft flies as a glider, it will fly under power,” he says.
Since then, the centre’s design and prototyping group (DPG) have taken another step forward and incorporated electric-powered, ducted fan engines on the glider. But developing the original flying wing was no easy task, says Mark Cocking, lead additive manufacturing engineer. “With the longitudinal balance point on this type of aircraft, there will always be a degree of trial and error during testing.”
To minimise this uncertainty, computational fluid dynamics was used to optimise the glider and to assess the lift, drag, pitching moment and other characteristics over a range of angles of incidence. Development engineer Sam Bull, who conducted these analyses, says: “The final configuration comprised two aerofoil sections blending from a thick, reflexed section in the body to a thinner, conventional section on the outer wing. The trailing edge was extended aft near the centre, where the reflexed aerofoil aids the longitudinal stability of the tail-less design.”
Creating shapes: Sheffield used computational fluid dynamics to optimise its glider
The CAD design was tailored for the additive manufacturing process – an aspect that Cocking says is regularly overlooked. “There is the impression that just because you have the CAD data you can input this straight onto the additive manufacturing machine and it will produce the shape. Part of the problem is that people are not embracing the human capability in designing for the process,” he says.
By feeding the FDM machines’ parameters into the CAD software, the group came up with a design that suited the process down to a tee, says Cocking. All nine parts of the airframe were built using the FDM process: two wings, two elevons, two spars, two wing end fences and a central spine. Working in this way also allowed the design to incorporate swept wings with straight leading edges, suited to the low Mach number flight regime in which the UAV would operate.
The aircraft was designed to split into halves about the central spine. This configuration allowed a larger wingspan to be created within the FDM machines’ build envelope, and made transport easier, says Nicholson. A pair of short spars – front and rear – clip into sockets formed within each wing half, giving a rapid set-up time for flight.
Tailoring the UAV for FDM saved a huge amount of time. An original design would have taken around 120 hours to produce on the machine, but the optimised design for the complete nine-part airframe was printed in just 22.5 hours. Designing for the RM process also led to a reduction in materials and cost savings. The team used a Fortus 900mc FDM printer, with two ‘heads’ that alternate between printing a layer of thermoplastic with a layer of support material, to create the chosen shape.
Ordinarily, an FDM-built aircraft would require significant amounts of support material around its component parts, to prevent the airframe structures from deforming during the build process, but the team’s design for the glider eliminated the need for this support. “Every layer that we can reduce the support material it has to lay down cuts the time and cost,” says Cocking. The design of the glider is such that each layer is supported by the model layer beneath it, thereby eliminating the need for support material. “You can find time saving in most components if you apply additive manufacturing knowledge to the design,” he says.
The team has put a case study on its website to demonstrate how small and medium-sized manufacturers can design a simple component for the additive manufacturing process to reduce the amount of support material needed.
When it came to the powered version, the team did have to retain some support material. “The concept of the glider was that we would build the whole airframe using RM, which is what we did,” says Nicholson. “For the powered version, we were unable to completely eliminate the support material – although it was reduced as much as possible.”
However, this was not too much of a problem, he says. “The elimination of the support material wasn’t a priority for the powered prototype as there were other RM techniques we wanted to explore.” These techniques included creating carbon-fibre wing skins using 3D printed moulds rather than conventionally made metal moulds. FDM was further used to create component jigs, fixtures and moulds, as well as parts of the UAV's airframe. In addition, the team used laser-cut carbon wing ribs to support the internal structure of the wing.
The DPG succeeded in making the central body of the electric-powered UAV, complete with twin engine ducts and complex internal features, as a single printed part. The advantage of printing a one-piece central fuselage, says Nicholson, is that the brackets and mounting points for everything from the engine to the electronics were printed in, saving weight and time on assembly. The ducts themselves could be printed in, so they didn’t have to be milled or formed using another process that could cause steps or joins, making them much more efficient.
Sometimes, however, buying off-the-shelf parts is easier and quicker, he and Cocking agree. And materials for advanced manufacturing, such as acrylonitrile butadiene styrene, can be brittle as they are built up in layers rather than being a solid block – although attempts have been made, with some success, using development material Nylon 12 to build the airframe. This gave it a tougher structure, at the expense of surface quality.
Despite these minor manufacturing drawbacks, the University of Sheffield team have successfully transformed their 2kg glider into a 3.5kg powered UAV, which can cruise at around 20 metres per second. Their next challenge is to replace the electric ducted fans with miniature gas turbine engines and to double the UAV’s wingspan to three metres. Other potential improvements include developing batteries made from carbon composites that could act as part of the UAV’s structure.
The team hopes the project will make the advanced manufacturing process more mainstream, says Nicholson. “We’ve demonstrated we can build an aircraft that flies entirely out of FDM. Once you demonstrate that, people will come up with their own possibilities.”
Meanwhile, at the University of Cambridge, Dr Paul Robertson and his team of researchers have successfully built and tested an aircraft with a parallel hybrid-electric engine that can recharge its batteries in flight. The demonstrator, based on a commercially available single-seat aircraft, uses up to 30% less fuel than a comparable plane with a petrol-only engine.
Robertson is an electrical engineer and a qualified pilot. Around nine years ago, when he was learning to fly, he began to question the potential of applying electricity to the pursuit. “At the time, the batteries weren’t good enough to make a viable electric aircraft,” he says. Since then, several purely electric aircraft have been developed, some of them even commercially available. However, they all suffer from limited flight duration, he says. “It is similar to electric cars, where you have range anxiety.”
To overcome this concern, Robertson had the idea of creating a hybrid plane to combine the high efficiency of an electric motor with the high energy density of liquid fuel. The project sparked industry interest, and Boeing came onboard to fund it.
Over the past few years, Robertson and his team at Cambridge have adopted a two-pronged approach. The first goal was to create a computer model of the aircraft and propulsion system. The second was to build the aircraft and collect real data to validate against the computer model. “From there, we are able to extend the concept to look at smaller and bigger aircraft,” he says.
The one-seat microlight uses a combination of a four-stroke piston engine and an electric motor/generator, coupled through the same drive pulley to spin the propeller. During take-off and climb, when maximum power is required, the engine and motor work together to power the plane. But once cruising height is reached, the electric motor can be switched into generator mode to recharge the batteries or used in motor-assist mode to minimise fuel consumption. The principle is the same as that at work in a hybrid car.
“We have essentially combined the best electric motor we could get our hands on with a suitable-sized petrol engine based on a commercial Honda engine. We have bought a tuned version of it – commonly used in karting. We have married these two to give a power plant that is a bit more powerful than the standard plant found in that particular aircraft,” says Robertson.
On the wing: Cambridge's hybrid engine wing can recharge batteries in flight
To provide electric power, the plane is fitted with 16 batteries. The team opted for KoKam lithium polymer cells, which are commercially available and used in cars and other electric aircraft. “They are a similar technology to what you find in laptops and mobile phones. However, the individual cells are much larger – each is about the size of an A4 page and 1cm thick,” he says.
The team had to get a special, bare-bones version of the plane’s airframe made by Czech manufacturer Gramex, so that it could fit the batteries in the wing roots, which would usually hold the fuel tanks. Eight cells are located on each side, while the fuel tank sits behind the power plant. “We also added a power controller that handles the energy from the batteries to the motor, or if we wish to use the motor as a generator we can send power back the other way,” says Robertson.
The pilot controls this system using two throttles: one for electric and one for the petrol engine. A mode switch indicates whether the electric generator or a motor is being used. When the pilot wants to take off, they set it in motor mode and push both throttles to maximum to achieve full power, he says. “This is the whole point of the hybrid. You’ve got plenty of power for taking off and climbing.”
When the plane is up in the air, the electric generator becomes less necessary. “Once you are up and level at your desired height, you throttle it back,” he says. The pilot can then throttle the electric engine back to zero if desired and run solely on petrol. “It isn’t like a large engine which you are running at 25% power; you are running it at around 80% power, so it is at its most efficient. The aircraft will cruise in that setting,” says Robertson.
The team also built in their own lightweight avionics – or instrument panel – designed by a final-year master’s degree student at Cambridge. The electronic digital devices record all the flight data and log it on an SD card, so it can be fed back later to validate or improve the computer model.
The purpose of designing the avionics in-house, other than for convenience, was the tight weight restrictions for the class of single-seat microlight aircraft that the team were using, says Robertson. “In this class of aircraft, the maximum take-off weight is 235kg, and that includes fuel and the pilot. When you extract the weight of the pilot with all their flying gear – which could be around 95kg – the whole aeroplane doesn’t weigh a great deal more than the pilot. We have been up against it from the weight point of view from the beginning.”
Flying colours: The hybrid has been flown for several hours in tests and reached an altitude of 500m
Test flights for the project took place last year at the Sywell Aerodrome near Northampton. The tests involved a series of ‘hops’ at a height of 6 metres along the 1km long tarmac strip. “If there are any issues, the pilot can put it down quickly and the altitude is not particularly hazardous,” says Robertson. “We have done around a dozen of those, and the pilot is happy to take it up for longer flights.”
The plane has reached a maximum altitude of around 500 metres and has notched up hours of flight time without any problems. The only modification the team have made to the design so far is tweaking the propeller to operate more efficiently.
The test flight data has been fed back into the team’s computer model, which uses desktop packages that link together. The aircraft’s aerodynamics are modelled in X-Plane, a flight simulator package that acts like a desktop wind tunnel and allows the team to work out the lift-to-drag ratio for the airframe. The ‘calculation brains’ to validate the thrust from the hybrid power-plant comes from the software package Matlab Simulink, while the team developed its own computer models for the engine and motor. Finally, JavaProp was used to model propeller behaviour.
“We get all that data and put it into a package which can then simulate the entire aircraft,” says Robertson. “We also have an autopilot model that we have written, where we can vary the distance, altitude and speed of a virtual aeroplane flight, allowing us to calculate what the fuel and battery energy consumption will be.”
With the current battery capacity onboard, the plane can achieve an hour’s worth of flight – around the typical time of a pleasure flight. Limited battery capacity is one of the main factors holding back the project, he says, and the team will have to wait for battery technology to improve before they can introduce the hybrid-electric system in larger aircraft.
Another obstacle is aircraft legislation that prohibits testing of hybrid engines in anything other than single-seat microlights – which were recently deregulated. “At the moment, legislation doesn’t allow us to move up to testing in a two-seater. I’d love to gradually move up the scale as the batteries get better,” says Robertson.
In the meantime, the University of Cambridge team will continue to gather data from further flight tests to improve the system and refine their computer models, so that they can begin virtually testing other classes of aircraft. Other areas of interest may include testing alternative fuels. While it may be decades before we see this technology used in commercial airliners, gains can already be made with current battery densities at the small end of aviation, Robertson believes.
While these two projects from the universities of Sheffield and Cambridge demonstrate the kind of advances in aviation technology that are being made by our academics and engineering students, they barely scratch the surface of the range of inventive aviation projects running across the country. When it comes to university-led innovation for the aerospace industry, the sky really is the limit.
Undergraduates compete to create humanitarian drones of the future
Some of Britain’s best young engineers will design and build life-saving unmanned aircraft systems (UAS) in a competition run by the Institution of Mechanical Engineers. Fourteen undergraduate teams from across the UK are competing and have put forward their preliminary design review proposals.
The demonstration flights of this design-and-build competition – to produce a UAS for a humanitarian aid mission – will take place on 1-3 July at Elvington Airfield and the Yorkshire Air Museum.
The UAS Challenge, for which Northrop Grumman is the lead sponsor, will see
teams of young engineers develop the UASs. They will then take on other teams in a ‘fly-off’, requiring them to transport aid and deliver a package as accurately and rapidly as possible.
“The event has three aims: recruitment opportunities for industry, employment opportunities for soon-to-be graduates, and a proving ground for innovation,” says John Turton, chairman of the UAS Challenge.
“The challenge will assist students to learn practical aerospace engineering skills for industry, prepare aerospace engineering students for industry employment, and encourage links between industry and universities,” adds Turton.