Experts have created a new type of aircraft that can stay in the air indefinitely by varying its buoyancy.

Phoenix, an unmanned aerial vehicle (UAV), has been designed to repeatedly transition from being lighter than air to heavier than air.

This generates thrust to propel the craft forward.

The development of the aircraft was lead by the University of the Highlands and Islands, with the team made up of representatives from academia and industry.

Phoenix, which is 49ft (15m) long and has a 34ft (10.5m) wingspan, was flown successfully and repeatedly over a distance of 120m (394ft) during indoor trials at the Drystack facility in Portsmouth in March.

The test flight was the culmination of a three-year project to prove the viability of a variable-buoyancy powered aircraft.

Andrew Rae, professor of engineering at the university, led the design of the UAV.

He said that vehicles based on its technology could be used as pseudo-satellites which would provide a much cheaper option for telecommunication activities.

Pseudo-satellites are high-altitude aircraft that operate in the stratosphere and can be used for various different purposes such as communications, monitoring, surveillance, and environmental observation.

Mr 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.

Mr Rae continued: 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 design of the Phoenix allows it to be completely self-sufficient.

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 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, the UK’s Innovation Agency, through the Aerospace Technology Institute.

(c) Sky News 2019: Revolutionary Phoenix aircraft can stay in the sky indefinitely, engineers say