A partnership of universities and private companies led by Andrew Rae of the University of the Highlands and Islands Perth College in Scotland, have developed and flown what is claimed to be the first aircraft powered by variable-buoyancy propulsion. In March, the unmanned Phoenix prototype made an indoor maiden flight at the Drystack facility in Portsmouth, England, where it repeatedly flew for a distance of 120 m (395 ft).

Looking like a blimp with wings, the ultra-long endurance, autonomous Phoenix is a demonstrator aircraft that is 15 m (50 ft) long and has a wingspan of 10.5 m (34 ft). It uses a technology similar to the swim bladder in a fish to generate thrust by shifting from being lighter than air to heavier than air.

Most bony fish maintain neutral buoyancy by inflating and deflating the air bladder with, well, air. It's also the principle behind the buoyancy vests worn by scuba divers, allowing them to balance their weight for neutral buoyancy by inflating it enough to balance out their weight, sit comfortably on the bottom by deflating the vest, or float on the surface by inflating it fully.

The Phoenix uses the same principle to produce a very simple, inexpensive design that allows it to fly while using very little energy.

"The Phoenix spends half its time as a heavier-than-air airplane, the other as a lighter-than-air balloon," says Rae. "The repeated transition between these states provides the sole source of propulsion.

"The vehicle's fuselage contains helium to allow it to ascend and also contains an air bag which inhales and compresses air to enable the craft to descend. This motion propels the airplane forwards and is assisted by the release of the compressed air through a rear vent.

"This system allows the Phoenix to be completely self-sufficient. The energy needed to power its pumps and valves is provided by a battery which is charged by lightweight flexible solar cells on its wings and tail.

"Vehicles based on this technology could be used as pseudo satellites and would provide a much cheaper option for telecommunication activities. Current equivalent airplanes are very complex and very expensive. By contrast, Phoenix is almost expendable and so provides a user with previously unavailable options."

According to the team, the Phoenix could also operate on the edge of space, where it could act as a launch platform for micro-satellites. The current goal is to get the Phoenix to operate at an altitude of 20,000 m (66,000 ft), where it will be powered by its wing-mounted solar panels to allow it to stay aloft for several days.

The three-year project was funded in part by by the British government's Innovate UK agency. The team says it is now looking for major manufacturers to help with the next phase of development.

Video of the Phoenix in flight can be seen below.