Could Solar Powered Flight Become a Reality?

According to Greek mythology, to escape imprisonment on Crete, the craftsman and inventor Daedalus made wings from wax and feathers for himself and his son Icarus. The legend then goes on to tell how Icarus flew too close to the sun, the wax melted, and he plunged into the sea and was drowned.

It seems that when it comes to human attempts at flight, the sun has not always been a help, but that may all be about to change, as a number of projects are poised to launch a range of solar-powered aircraft into the skies and, if all goes to plan, will soon see a two-man crew circumnavigate the globe.

Drone Wars

Unsurprisingly, much of the interest in solar-powered planes has come from the military. Recent conflicts around the world have highlighted the benefits of remotely operated, pilot-less 'drones' - and not just those that are equipped with rockets and bombs. A tireless 'eye-in-the-sky' can be a real boon for a whole host of missions, from intelligence gathering to humanitarian operations such as search-and-rescue, particularly in relatively inaccessible regions of the planet.

While shifting the pilot out of the cockpit and onto a computer console hundreds if not thousands of miles away from the action gets around the problem of human fatigue, for a conventionally powered aircraft, no matter how good its fuel economy, sooner or later it has to leave its station and refuel. That problem goes away with photovoltaics (PV) powering your plane.

Satellite Replacement

It's not, however, just about replacing those small pilot-less planes (or UAVs - unmanned aerial vehicles - to give them their proper title). Plans are afoot to put some truly massive solar-powered UAVs into the air.

A whole new class of unmanned aircraft are being developed that could revolutionise surveillance and communications forever, and provide a cheaper, more flexible - and vastly less energy-hungry - alternative to just about anything that you currently need a satellite to do. Instead of blasting cameras, sensors and sensitive scientific instruments into orbit on the backs of loud, flame-spewing rockets, the future could see the quieter and much less dramatic launches taking place of solar pseudo satellites - huge, long-endurance UAVs that can fly non-stop for years at a time.

Sounds a bit sci-fi? Not really; a number of aerospace companies around the world are already working on aircraft capable of doing just that - and two of the best candidates use engines designed at Newcastle University.

HAPS and HALE

Originally devised by QinetiQ, and now Airbus-owned as part of its High Altitude Pseudo-Satellite (HAPS) programme, Zephyr effectively broke the mould for solar-powered flight in 2010 when it smashed the then existing endurance record for UAV flight, flying to a height of 21,562 metres and for two-and-a-half weeks.

With a wingspan of nearly 75ft (22.5m), two 450W permanent-magnet synchronous motors, and with a lithium-sulphur battery to power the plane at night, Zephyr can cruise at 30 knots and at altitudes of up to 70,000ft or more.

Boeing's proposed Solar Eagle is set to be orders of magnitude bigger still, with a staggering 400ft (120m) wingspan - almost double a Jumbo jet's - and no fewer than 20 of the same engines that power the Zephyr, and it will have a performance to match. Officially dubbed a High Altitude, Long Endurance (HALE) aircraft, the Solar Eagle will ultimately be capable of flying for five years or more without needing to land.

Once that becomes a reality, PV pseudo satellites become a genuine alternative to real satellites, for everything from military reconnaissance to weather monitoring - and the savings in rocket fuel could be immense.

Round the World

Flying UAVs - even these really big ones - by solar power is one thing, and it may even be the biggest potential use of PV in aviation for some time to come, but it is manned flight that really fires the imagination like nothing else.

Two Swiss pioneers - Bertrand Piccard, veteran of the first non-stop, round the world, balloon trip and André Borschberg - clearly agree. Having already successfully crossed the US from coast to coast in their solar powered Solar Impulse, together they plan to circumnavigate the globe.

Their 21,000 mile pioneering flight is scheduled to begin in March next year, heading east from the Persian Gulf, over India, China and the US before finally heading for home over either Europe or Africa.

Test of Endurance

Although it is not intended to be a non-stop trip, the total 25-day flight time being spread over the course of about five months, it will still be a major feat of endurance for the two pilots. Confined in their unpressurised cabin and travelling at up to 27,000ft, although they will be able to take 20-minute cat naps on longer legs of the flight, when the aircraft is safely away from inhabited areas, on flights of less than 24 hours, they will have to go without sleep. Oxygen masks are, however, supplied!

Borschberg has described the plane as "of unlimited endurance." No doubt everyone involved in the project hopes the same will hold true for two men in its cockpit.

The Solar Impulse is a remarkable machine, with a wingspan the size of a 747, but weighing about the same as the average car, and equipped with over 17,000 solar cells, its four twin-bladed propellers are driven with roughly the same amount of power that drove the Wright Brothers' first flights in the early 1900s. Had solar technology been around then, they probably would have used it themselves.

PV certainly isn't going to be powering your holiday flights anytime soon, but it certainly does show just how far clean technology has come - and how far it could yet go.

Like so many themes from the past, it seems the Icarus story just got a reboot!