Ultra-long endurance UAV flies using variable-buoyancy propulsion

A group of UK researchers led by Andrew Rae, professor of Engineering at the University of the Highlands and Islands Perth College UHI Campus (Perth, U.K.), has successfully flown the first ever large-scale aircraft powered by variable-buoyancy propulsion. The Phoenix unmanned aerial vehicle (UAV) is designed to repeatedly transition from being lighter than air to being heavier than air, generating thrust to propel the craft forward.

The prototype aeroplane has a fuselage containing helium that allows it to ascend like a hot air balloon. An air bag then brings in outside air and compresses it, making the vehicle heavier and triggering its descent. The resulting rising-and-diving motion (aided by the release of the compressed air) propels the aircraft forward as it glides on carbon fiber wings.

“This system allows the Phoenix to be completely self-sufficient,” says Rae. “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.”

The Phoenix is 15 meters long and has a wingspan of 10.5 meters, was flown successfully and repeatedly over a distance of 120 meters during indoor trials at the Drystack facility in Portsmouth, U.K. in March. The test flight was the culmination of a three-year project to prove the viability of a variable-buoyancy powered aircraft. 

The team behind the ultra-long endurance autonomous aircraft includes representatives from academia and industry. National Composites Centre (Bristol, U.K.) and the University of Bristol (Bristol, U.K.) contributed to the carbon-fiber wing and tail structures, wing skins and the gondola.

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

The Phoenix team is now exploring collaborations with major manufacturers to take the technology to the next phase of development. The project has been part-funded by Innovate UK (Swindon, U.K.) through the Aerospace Technology Institute (Bedfordshire, U.K.).