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Phoenix flies using new form of propulsion – variable buoyancy

Phoenix UAV seen in its hangar

On Thursday 21st March 2019, the world’s first large variable buoyancy powered (VBP) aircraft was test flown successfully.

The 15m-long, 10.5m wingspan autonomous UAV, Phoenix, is an ultra-long-endurance aircraft. It is uniquely designed to spend half its time heavier-than-air, the other half lighter-than-air, and the repeated transition between the two provides the sole source of propulsion. Its main anticipated use is as a pseudo-satellite.

The team leader, Prof Andrew Rae of the University of the Highlands and Islands, explained “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. 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. This system allows the Phoenix to be completely self-sufficient.”

The team included:

  • University of the Highlands and Islands
  • CPI – Photovoltaic Cells
  • IQE – Photovoltaic Cells
  • Newcastle University – Fuel Cells, Batteries, Power Management
  • University of Southampton – Fuel Cells, Batteries, Power Management
  • TCS Micropumps – Centre of Gravity, Aileron & Elevator Actuation
  • Banks Sails – Bouyancy, Structure
  • University of Bristol – Structure
  • MTC – Structure, Flight Control System
  • NCC – Structure
  • University of Highlands and Islands – Platform and Flight Control Design
  • Stirling Dynamics – Flight Control System
  • TCS Micropumps – Flight Control Actuation
  • InnovateUK
Phoenix flies indoors

Variable Buoyancy Propulsion

The Phoenix UAV is a unique aircraft – no other aircraft has ever been designed to take advantage of a propulsion system only dreamt up 30 years ago in a completely different field.

Ocean going Slocum gliders were first imagined by Henry Stommel in 1989, who envisioned a worldwide ocean observing system based on “a fleet of small neutrally-buoyant floats called Slocums” that “migrate vertically through the ocean by changing ballast, and they can be steered horizontally by gliding on wings at about a 35 degrees angle.” By changing their volume and buoyancy these gliders cycle vertically in the ocean, creating lift forces on the wings which convert this vertical velocity into forward motion – variable buoyancy propulsion.

It took another 10 years before a 2001 paper Autonomous Buoyancy-driven Underwater Gliders by Davis, Eriksen and Jones, described the development and operation of early slocum gliders. And almost 20 years after that before the first flight of a VBP aircraft – Phoenix – was announced.

Phoenix‘ VBP system relies on the same principle as the Slocum gliders. The vehicle changes its buoyancy – becomes heavier or lighter – and so moves up and down in the fluid in which it is moving. In the case of slocum gliders, the fluid is the water of the ocean, for the Phoenix, it’s the air of the atmosphere.

As Phoenix moves up and down, air flows over the flying surfaces, which are at an angle to the flow – the angle of attack. As the air flows over the surfaces, a force is created which moves the surfaces and therefore the vehicle to which they’re attached. Normally this force is simply thought of as the lift which supports the vehicle, but the force is actually perpendicular to the flow direction, so there are both vertical and horizontal components to the force. The vertical component is the lift, and the horizontal component moves the vehicle forward. Up to a point, the greater the angle of attack, the larger the horizontal force.


Setting the angle of attack of the flying surfaces can be done in a few ways. The surfaces can themselves be rotated, like the pitch of a helicopter’s blades, or the entire vehicle can be angled either by adjusting the elevators like a traditional aircraft, or by altering the vehicle’s centre of gravity relative to its centre of lift, as a slocum glider does.

It’s unclear which mechanism Phoenix uses, altering its centre of gravity, or adjusting the angle of attack of its flying surfaces. But reviewing the publicly available information, it seems to use the same approach as slocum gliders with fixed surfaces and adjusting its centre of gravity.


The first flight of an aircraft is always exciting and engaging. But the first flight of an entirely new form of propulsion is something else again. It feels like we’re on the cusp of something at the moment, with another novel propulsion system – the solid state ET engine – flying at MIT, and the combination of all the AI and electric technologies midwifing new approaches to flight.

Whether or not VBP becomes part of the standard aerospace toolbox, or the Phoenix template results in a product reaching market, the project proves that

  1. radical innovation is not dead;
  2. scanning outside the box for new ideas is always worthwhile;
  3. that it’s not always the ususal suspects who invent the future – would you honestly have predicted it would be the UHI in Perth, Scotland who would hit the headlines with a ground breaking piece of aerospace tech?