Solar powered aircraft take off

The drive for increasing efficiency is something that features regularly on the pages of Eureka. Getting the biggest bang for your buck, extending range and limiting emissions are all fairly mainstream targets for most major sectors.

Driving these innovations are factors such as the cost of fuel and the need to reduce environmental impact. So it is interesting to follow the developments of aircraft that have zero fuel costs and that produce absolutely no emissions at all during flight.

There have been two main European solar powered aircraft in recent years that have attracted much media attention. The two have taken different approaches in that one is piloted, while the other is not. However, despite the other obvious difference of scale, both have shown impressive flight capabilities.

Solar Impulse is a Swiss solar-powered aircraft led by aeronaut Bertrand Piccard, who co-piloted the first balloon to circle the world non-stop, and Swiss businessman André Borschberg. Though neither expects solar aircraft to scale up to larger airliners, it is the principle of sustainable travel that they want to demonstrate.

Inherent to this principle is the desire to push back the frontiers of knowledge in materials science, energy management and the man-machine interface that has inspired the aeroplane's creation. The ultimate goal is to continually fly around the world in 2015 in the Solar Impulse HB-SIB.

However, its current development aircraft – the Solar Impulse HB-SIA – has allowed concepts to be proven, lessons to be learned and systems to be trialled. The aircraft itself is actually more substantial than you may imagine. Its 63.4m wingspan is covered in 11,628 photovoltaic cells over an area of 200m2. This produces enough electricity to power its four brushless electric motors, each with a set of polymer lithium batteries, and a management system controlling charge and temperature thresholds.

Each motor has a maximum power output of 10HP and a gearbox limits the rotation of each 3.5m diameter, twin-bladed propeller to 400rpm. Over a 24-hour period the average power used is 8hp, or 6kW, roughly the power used by the Wright brothers' pioneering Flyer in 1903.

Yet the structure itself is significantly heavier than the Flyer. With a loaded weight of 1600kg and maximum takeoff weight of 2000kg, many cynics said it would never get off the ground. Yet it's had numerous successful flights, including a non-stop 19-hour trip from Spain to Morocco as well as numerous other day and night flights.

The opportunity to push technological boundaries was what inspired Bayer Material Science to join the project. "That is a major aspect of our involvement," says Martin Kreuter, project leader at Bayer Material Science for Solar Impulse. "Our business is very streamlined, there are many cost restrictions and other limitations. That's something that can be overcome in this project. It allows the people to think outside the box, to be creative and look for innovation without the pressure of daily business."

On the first aeroplane, the HB-SIA, Bayer was using off-the-shelf products. Polyurethane foam was used for the wing tips and in the motor gondolas. A polycarbonate film was also used in the cabin window as well as various commercially available adhesives and coatings.

However, it is currently optimising materials for the second aircraft (HB-SIB), in which it has system responsibility for the cockpit module. The windows were previously made from a polycarbonate film, but will now use a thermally formed polycarbonate sheet to allow for a more aerodynamic design.

This ability to apply innovation in a fairly open environment not only acts as inspiration for the engineers, it also has potential technology transfer benefits. One example of this is the work done to optimise the density of polyurethane ridged foam, which provides better insulation properties. This can in turn reduce wall thickness.

"We can also see that foam being used in electric vehicles," says Kreuter. "Insulation of lightweight vehicles is a major issue. You can't use thick foams of the sort used in building insulation, and that is where these solutions could really have benefit. And if you look at carbon fibre reinforced plastics (CFRP), we tried polyurethane as a resin material. The main resin material in CFRP is normally epoxy which has a long curing time. This makes it unfeasible for mass production. But polyurethane could act as an alternative as it has a short cycle times and is capable of high output. It is important to Bayer to have these innovation impulses. Not everything we investigate will make it to the aircraft as a solution. Many things will stay on a conceptual level and might well be used on other projects in the future."

While inspiration and a sense of adventure play a big part in the development of the Solar Impulse project, it was practical application that was a key driver for UK defence and aerospace company Qinetiq. The team were faced with finding a better way of operating military satellites to provide surveillance.

The result was Zephyr, an ultra-lightweight unmanned solar aircraft that can fly for months at a time and can carry a similar payload as a satellite in space. The advantage is that Zephyr is a fraction of the cost of space-bound satellites, can be quickly launched from anywhere and can land for routine maintenance at anytime. Qinetiq hope it will act as an alternative to surveillance, communication and remote sensing satellites in a variety of civil and military applications.

Chris Kelleher, the chief designer of high-altitude endurance unmanned aerial vehicles at Qinetiq, led the project team and began exploring whether aircraft could be left in the stratosphere permanently. The problem, however, was that the technology simply did not exist.

"We investigated all possible routes to create an aircraft that would fly permanently in the sky unmanned," says Kelleher. "We recognised the operational cost savings that this would create for defence teams that would use these aircraft for communication purposes."
The team began by evolving a bike-driven aeroplane originally designed in 1923. From this concept, the team was able to explore a series of developments and test flights that would evolve into Zephyr.

"We were building something very light and strong while at the same time exploring aerodynamics that no other aircraft had ever operated," says Kelleher. "The reason no one had done this before was because you needed a threshold performance level to fly continuously overnight as well as the ability to store energy and maintain altitude."

The team began developing various sections of the aircraft and an improved version was built and continuously flown for three and a half days. The aircraft has gone through many developments and iterations with the current Zephyr 7 having a wingspan of 22m and weighing just 40kg. Its wings are designed to maximise the use of thermal air currents to reach high altitudes and to launch Zephyr a team runs gently into the wind until it lifts out of their hands.

The structure is made up almost entirely of carbon fibre. Again it is the 3kWh lithium sulphur batteries that take up most weight. During the day, Zephyr uses amorphous silicon array solar cells no thicker than a sheet of paper spread across its wings to feed electricity to recharge its batteries and power two brushless motors that drive the propellers. At night, the energy stored in the batteries is sufficient to keep Zephyr in the sky, although it is likely to lose about 20,000ft.

"Zephyr 7 truly tested the mechanics of its structure and its ability to store power," says Kelleher. "One of the biggest breakthroughs we had was getting extremely low drag through the aircraft along with a very light structural weight, while at the same time not being too fragile to handle. By using very high modular carbon fibres, we were able to take this to the extreme."

In 2010 the aircraft flew for a record time of 14 days and 22 minutes reaching altitudes in excess of 70,000ft. Aiming for a target of three months, Qinetiq's engineers will tailor Zephyr for operational roles such as fire detection, piracy or surveying large areas of land and ocean.

Solar Impulse and Zephyr are impressive examples of innovation. While it is unlikely that we will all one day board a solar-powered airliner, it is not so farfetched to think that solar energy may well be harnessed to supply some onboard power. It is also likely that space satellites could be replaced, at least on some scale, by unmanned aircraft cruising high in the stratosphere.

The true benefit of these projects, however, is the transferable developments in materials, sensors and solar technology it has driven, as well as the creativity and innovation it has inspired among the engineers that have worked on them.

Author
Justin Cunningham

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