Scientists from the UK’s University of the Highlands and Islands (UHI) and other institutions have developed an aircraft that can remain airborne indefinitely using an innovative propulsion system.

The new Phoenix unmanned aerial vehicle (UAV) is claimed to be the first ever large-scale aircraft powered by variable-buoyancy propulsion.

The ultra-long endurance autonomous aircraft can ‘repeatedly transition from being lighter than air to being heavier than air’ as its design will ensure thrust is generated to propel the craft forward.

UHI professor Andrew Rae led the design of the aircraft.

“Phoenix is almost expendable and so, provides a user with previously unavailable options.”

Comprising representatives from academia and the industry, the team flew the prototype aeroplane over a distance of 120m during indoor trials at the Drystack facility in Portsmouth last month.

The 15m-long prototype has a wingspan of 10.5m. The researchers performed the test flight after working on the project for three years.

The project aims to prove the viability of a variable-buoyancy powered aircraft.

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According to The Times, the aircraft has the ability to fly at an altitude of 70,000ft.

Rae said: “The Phoenix spends half its time as a heavier-than-air aeroplane, the other as a lighter-than-air balloon. 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 aeroplane forwards and is assisted by the release of the compressed air through a rear vent.”

The propulsion system makes the Phoenix a completely self-sufficient aircraft.

The aircraft’s wings and tail carry lightweight flexible solar cells that will charge the battery designed to power its pumps and valves.

Rae added that vehicles using this propulsion technology could serve as pseudo satellites and would offer a much cheaper alternative for telecommunication activities.

He said: “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 research team intends to forge a partnership with major manufacturers to take the Phoenix technology to the next phase of development.

BBC reported that the Phoenix could be fitted with cameras for future surveillance missions.