World first for UK team as autonomous ‘Phoenix’ aircraft flies

The world’s first large variable-buoyancy-powered autonomous uninhabited air vehicle (UAV) is successfully test flown

Bristol UK, March 2019

A team comprising experts from three High Value Manufacturing Catapults (HVMC), Academia and UK based SMEs is celebrating the successful test flight of ‘Phoenix’ a 15m long, 10.5m wingspan ultra-long-endurance aircraft that spends half its time as a heavier-than-air aeroplane, the other as a lighter-than-air balloon. The repeated transition between the two states is what propels the aircraft which can be quickly deployed as a communications satellite in areas of humanitarian need.


Phoenix is the world’s first large variable-buoyancy-powered autonomous uninhabited air vehicle (UAV) and completed its test flights in March. The ultra-long-endurance aeroplane uses the concept of variable-buoyancy propulsion that has been exploited previously for underwater remotely-operated-vehicles (ROVs) but has never before been used successfully for the propulsion of a large-scale aircraft. The team of UK experts backed with funding from UK Research and Innovation have created a prototype which passed its flight tests in March.


The fuselage is made from a vectran-based woven material and contains Helium, providing buoyancy sufficient to make the complete vehicle lighter than air and ascend like a balloon. Within the fuselage is a separate air bag with pumps located at the mouth of this air bag  that inhale and compress air from outside and thereby add weight (without altering the displacement) sufficient to overcome the buoyancy.  This transition to heavier-than-air flight allows the aircraft to descend like a conventional aeroplane.  The release of the compressed air returns it to a lighter-than-air configuration and the process is repeated. The forward inclination of the lift/buoyancy vectors with respect to the flight path, and the expulsion of the compressed air through a rearward facing vent, provide a thrust force that propels the aeroplane forwards without need of any other form of propulsion.  The energy needed to power the pumps, actuate the valve, and move the flight-control surfaces is provided by a rechargeable battery which, in turn, is supplied by an array of lightweight, flexible solar cells distributed on the upper surfaces of the wings and horizontal tail. 


The prototype aeroplane was flown successfully and repeatedly during indoor flight trials in March 2019 under the command of a fully autonomous flight control system over a distance of 120m (the length of the trials facility) making approximately five transitions in each flight.  The fuselage retains its rigidity through internal pressure and the structure of the flight surfaces uses carbon-fibre sandwich panels for the ribs, carbon-fibre spars and a lightweight skin.  The wings house a pair of ailerons and the cruciform tail includes pairs of rudders and elevators.  A reversible hydrogen fuel cell has been developed to augment the power system on future versions.


The project partners are:


High-Value Manufacturing Catapults:

The Centre for Process Innovation (project management and photovoltaic cells);

The Manufacturing Technology Centre (flight control system and hardware testing); and

The National Composites Centre (carbon-fibre wing and tail structures and the gondola).



Banks Sails (fuselage materials and manufacture, wing skins);

IQE (photovoltaic cells);

TCS Micropumps (pumps and valves, computer aided design, and flight control actuators; and

Stirling Dynamics (flight control system).



University of Bristol (carbon-fibre wing and tail structures and the gondola);

University of the Highlands and Islands (platform and flight control surface design);

University of Newcastle (reversible hydrogen fuel cell);

University of Sheffield (wind-tunnel testing); and

University of Southampton (rechargeable battery).


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