A new type of unmanned aerial vehicle (UAV) has made a successful maiden flight thanks, in part, to the expertise of engineers from the University of Southampton.
The 15m-long, 10.5m wingspan, Phoenix is the world’s first large variable-buoyancy-powered UAV. Resembling an airship with wings, in appearance, the ultra-long-endurance aircraft spends half its time as a heavier-than-air aeroplane and the other half as a lighter-than-air balloon. It is the repeated transition between the two which provides the sole source of propulsion for the Phoenix’s anticipated use as a pseudo-satellite.
Under a project funded by Innovate-UK approved by the Aerospace Technology Institute, and bringing together SME’s, High Value Manufacturing Catapults and Academia, 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 fuselage is made from a vectran-based woven material and contains ~120m3 of 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 of 6m3 capacity. Pumps located at the mouth of this air bag can 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 created under the guidance of Southampton Professor Andrew Cruden, Head of the University’s Energy Technology Group, and colleague, Associate Professor, Dr Richard Wills. The batter pack is charged by an array of lightweight, flexible solar cells distributed on the upper surfaces of the wings and horizontal tail of the Phoenix.
“The University of Southampton team within the School of Engineering developed a specific lithium-ion battery pack for this UAS, capable of operation across the wide range of temperatures found at altitude, and communicating the status of the pack to the automatic flight control system,” Professor Cruden explained. “The battery is designed to capture and store sufficient energy from the flexible photovoltaic arrays to power the UAS during the hours of darkness, with a safety margin for periods of poor weather and emergency use.
“This project was a substantial collaborative effort by all partners and it delivered a real sense of achievement to witness the successful flight after nearly 30 months of design, manufacture, assembly and testing,” Professor Cruden continued. “It is anticipated this unique UAS will provide a substantially lower cost route to providing long endurance, zero emission pseudo satellites for communication, surveillance and humanitarian missions around the globe.”
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 Drystack facility, Trafalgar Wharf, Portsmouth used for the trials) 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 Phoenix project partners are SMEs: Banks Sails (fuselage materials and manufacture); TCS Micropumps (pumps and valves, computer aided design, and flight control actuators; Stirling Dynamics (flight control system). IQE plc led on the development of flexible photovoltaic cell technology.
Three of the UK’s High-Value Manufacturing Catapults were also involved – 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, wing skins, and the gondola).
Joining Southampton as university participants are the University of Bristol (carbon-fibre wing and tail structures, wing skins, and the gondola); University of the Highlands and Islands (platform and flight control surface design); University of Newcastle (reversible hydrogen fuel cell) and the University of Sheffield (wind-tunnel testing).
For further details, please visit the Phoenix project website.
Published: 24 April 2019
