WHAT’S lighter AND heavier than air – and could help revolutionise the telecommunications industry?

The answer is the Phoenix, a prototype aircraft which has passed performance tests thanks to input from Scottish scientists.

But don’t expect to step onboard any time soon, because this high-flying innovation has been developed for business and scientific uses, not passenger transport.

At 15 metres in length and carrying a wingspan of 10.5m, the high-tech creation has been three years in the making.

And it is hoped that the cutting-edge technology driving the large-scale devices could help cut telecoms costs by reducing the use of standard satellites.

Unlike conventional alternatives, its power source is a variable-buoyancy propulsion system which sees it switch between being a lighter-than-air balloon and a heavier-than-air aeroplane.

That change creates thrust to propel the vehicle forwards, and while the system – which makes the craft almost self-sufficient in energy – has been used in remotely-operated underwater instruments, it has never before been used to run a large aircraft.

Without the raft of costly components present in established alternatives, the Phoenix is “almost expendable”, it is claimed – and so could slash spending by telecoms operators, improving profitability and potentially allowing savings to be passed on to customers.

As many as five different UK universities have collaborated with industry bodies on the project.

They include the University of the Highlands and Islands (UHI), where engineering professor Andrew Rae led the design of the craft.

Following a series of successful indoor flight trials, the team behind the Phoenix is now “exploring collaborations” with major manufacturers to take the technology to the next phase of development.

Rae, who is based at UHI’s Perth College campus, explained the science behind the success, saying: “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.

“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.”

Rae went on: “Vehicles based on this technology could be used as pseudo satellites and would provide a much cheaper option for telecommunication activities.

“Current equivalent aeroplanes are very complex and very expensive. By contrast, Phoenix is almost expendable and so provides a user with previously unavailable options.”

Trials spanning a distance of 120m took place at the Drystack facility in Portsmouth last month.

Other universities involved include Bristol, Newcastle, Sheffield and Southampton.

The project, which set out to prove the viability of the technology for aircraft, was part-funded by Innovate UK, the UK’s Innovation Agency, through the Aerospace Technology Institute.