(Photo : UHI)

It has no engine, it doesn't need fuel but the best thing about it is that it can stay up in the air, for days, weeks, or months even; and yes it is an aircraft that's using a new type of propulsion system.

"It's a proper airplane," says Professor Andrew Rae of the University of the Highlands and Islands (UHI). Prof Rae is the project's chief engineer of Pheonix, unmanned aerial vehicle (UAV) designed to stay in the air indefinitely.

Besides UHI, it also involves Bristol, Newcastle, Sheffield and Southampton universities, four commercial companies and UK government's innovation agency Innovate UK.

It looks like an oversize airship, 15m (50ft) long and weighs at 120kg (19 stone), except that it has wings. It doesn't need fuel since its wings and tail carry solar panels. Albeit not having an engine, its revolutionary propulsion system called variable-buoyancy allows it to travel long distances and stay aloft for long periods. This is the first large-scale aircraft to be powered by this kind of system.

This propulsion technique is already used underwater. The Scottish Association for Marine Science (also part of UHI) has a small fleet of remotely operated vehicles - they call them gliders - that gather data in the North Atlantic, but air is much less dense than water and this has made the principle a trickier proposition for flight.

Prof Rae led the design of its aerodynamics. According to Prof Rae, the central fuselage is filled with helium, which makes it buoyant so it can ascend like a balloon upon take off. Inside the quasi-airship is another bag that has compressors on it that brings air from outside and compresses it which makes the airplane heavier and enabling it to descend like a glider. This ability to "breathe" - to switch quickly between being heavier or lighter than air - doesn't just make the plane go up and down. It is the key to driving it forward. The quasi-airship shape is based on an aerofoil, meaning it also provides lift like its wings do when the plane moves forward.

The technical term for this type of aircraft is "ultra-long endurance autonomous aircraft" and its use mainly really is to create a cheaper alternative to launching satellites. Prototype testing inside the Drystack in Portsmouth, a huge indoor area which normally stores pleasure boats, has been quite successful and the consortium of universities, government agencies wants to collaborate with major manufacturers to take Phoenix to the next level.