It is easy to see the appeal of a solar-powered aircraft. Next time you are aboard a plane, try to get a window seat. As you taxi out for takeoff, look at the amount of light hitting the plane’s wings. In perfect flying conditions, the polished metal surface will brilliantly reflect the sun.
Then, as you are pushed back into your seat when the aircraft accelerates along the runway, perhaps the sun will catch a wing edge, suddenly illuminating the cabin.
Once the aircraft climbs through the clouds, the seatbelt sign pings off and the light can rapidly become piercing. The wings and fuselage are often bathed in constant sunlight at these higher altitudes during the daytime, prompting passengers to shield their eyes and pull down their window blinds.
In these kinds of conditions, flying by the sun could seem a relatively easy proposition – cover the plane in solar panels and you will have an aircraft that taps this free, limitless and clean energy source that abounds at altitude.
But then think about other flights you have taken. Foul weather or low cloud can cut out almost all sunlight. A winter sun can mean that the light barely glances off the plane as it soars into the sky. A night flight in the wrong direction can end up never seeing sunlight at all.
These kinds of considerations give some insight into the massive hurdles that designers of solar-powered aircraft face. But that has not stopped people trying.
“The promise is almost unlimited duration – as long as the sun is out they can fly,” says Bob van der Linden, the curator of special purpose aircraft at the Smithsonian National Air and Space museum in Washington DC.
For example, his collection contains Pathfinder Plus, an unmanned aircraft which set an altitude record of over 80,000ft back in 1998.
“It’s a bizarre-looking thing with a really, really long wingspan and very short length. If you look at it in flight, the wings are extremely flexible, and under load the wing takes the shape of a U with 8 motors on it,” says van der Linden.
Other high-profile craft include the fragile Nasa Helios, which disintegrated above the Pacific Ocean near Hawaii after hitting unexpected turbulence. Prior to the crash, it had set a new altitude record of 96,836ft (29515.6m).
Although embarrassing, accidents like this have not stopped the development of solar-powered craft, particularly by the military.
One solar-powered high-altitude long-endurance (Hale) craft under development is called Zephyr. The unmanned aerial vehicle (UAV), built by UK defence firm Qinetiq, has shown that it can stay aloft for more than two weeks, a valuable asset for the armed forces.
“You have a machine with solar cells and a battery that can fly, at high or low altitudes, over a troubled area and send back data as a substitute for a satellite,” says van der Linden.
“If the point is to circle over a couple of spots and take pictures, it’s perfect.”
And these have other uses too. They could become slow-flying perpetual aircraft, never having to land, never having to refuel. They could perform long-term science missions, sampling the atmosphere and supplementing the types of data we can get from satellites in much higher orbits. They could also be used to broadcast or relay TV and radio signals, providing a cheap way to extend coverage to areas where it is needed, after natural disasters or during major sporting events like the Olympics.
But it is the idea of a manned aircraft that captures people’s attention.