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Lung-Like Propulsion Drives British Long-Endurance UAV Tony Osborne    A British team has flight-tested an autonomous aircraft powered by variable-buoyancy propulsion that engineers say could deliver a novel lower-cost approach for the missions performed by high-altitude pseudo-satellite (HAPS) platforms. The Phoenix, a joint development of several UK universities and small businesses, is the result of a three-year project to study the potential of a variable-buoyancy propulsion system on an aircraft. Until now, such a propulsion system has only been adopted for use on autonomous underwater vehicles (AUV), giving them the endurance to survey the oceans. Essentially a 15-m-long (49-ft.) monoplane with a 10.5-m wingspan, the Phoenix features a large bulbous fuselage filled with 120 m3 of helium that helps the aircraft ascend. Also contained inside the fuselage is an inflatable bag with a 6 m3 volume, that draws in and compresses air, making the craft heavier than air. As a result, the Phoenix descends, and the wings convert that movement into forward movement. When the compressed air is released through a vent in the rear of the aircraft, it then becomes lighter than air and ascends again, with the wings yet again turning that into forward flight. Once airborne, the platform is almost entirely self-sufficient and could stay in the air theoretically for an unlimited period of time, says Andrew Rae, professor of engineering at the University of the Highlands and Islands, who led the design of the aircraft.

Phoenix’s envelope contains helium and a second inflatable bag, which pumps and compresses the air and repeatedly turns it from a lighter-than-air craft, back to a heavier-than-air craft, propelling it forward. Credit: University of the Highlands and Islands

And most crucially, say engineers, it can do it at a much lower cost than other HAPS platforms such as the solar-powered monoplanes developed by Airbus and Prismatic. “There are several types of vehicles that could do the same role, but they are more complex machines and more expensive,” says Rae. “A cheap, almost disposable aircraft like this will mean you can do things with it you would not contemplate with a more expensive aircraft.” The fuselage—constructed from a Vectran-based woven material—retains its rigidity through internal pressure; the wings use carbon-fiber sandwich panels for the ribs, spars and a lightweight skin. Solar panels placed on the wings and horizontal stabilizers charge lithium-ion batteries that drive the pumps and the compressors, enabling the Phoenix to slowly porpoise gracefully through the sky. Its electrical system, designed by a team from the University of Southampton, is capable of providing power at night and has a safety margin for periods of poor weather and emergency use. Bristol, England-based Stirling Dynamics developed the flight-control system using its experience working out control laws for AUVs. Stirling’s system controls the pumps and compressors and also receives data from the power pack. Initial flights of the Phoenix were performed indoors in the Drystack—an enclosed boat storage facility in Portsmouth, England. The first flight took place on March 21, although details were not released until April 23. The aircraft flew a distance of 120 m repeatedly, making about five transitions of the propulsion system during each flight. The team had wanted to fly the aircraft outdoors but was not able to secure certification from the UK Civil Aviation Authority. But in its current configuration, the Phoenix would likely be able to operate comfortably at altitudes of 3,000 ft. Even at these altitudes, the platform could act as a flying cellphone mast in areas struck by natural disasters. However, the team is firmly focused on the HAPS mission, although Rae says the aircraft will likely have to be scaled up considerably to give it stability and inertia to counter wind gusts at high altitude. Hydrogen, a gas given “bad press,” according to Rae, could also be used as an alternative to helium, in part because it allows higher altitudes to be attained and is now considerably cheaper than helium, due to international shortages of that gas. The team has also explored the potential use of a reversible fuel cell developed by the University of Newcastle to complement the batteries and allow the aircraft to recharge the hydrogen gas onboard for future versions. Funding for development of the Phoenix has been provided by the British government’s Innovate UK agency. Other academic institutions involved in the project include the University of Bristol, which worked on the carbon-fiber wing and tail structures, wing skins and gondola, and the University of Sheffield, which conducted wind-tunnel testing. Several of the UK’s manufacturing catapults—innovation centers for composite, manufacturing and process innovation—were also involved. The future of the Phoenix and its technologies will very much depend on market interest and “specific missions,” says Rae, but the team is hoping the project will appeal to major industry players and organizations such as the Ministry of Defense, which is exploring the use of HAPS aircraft with Airbus’ Zephyr.  UK team trials first large-scale aircraft powered by variable-buoyancy propulsion A group of UK experts has successfully flown the first ever large-scale aircraft powered by variable-buoyancy propulsion. A group of UK experts has successfully flown the first ever large-scale aircraft powered by variable-buoyancy propulsion.  The Phoenix is designed to repeatedly transition from being lighter than air to being heavier than air so thrust is generated to propel the craft forward. The team behind the ultra-long endurance autonomous aircraft includes representatives from academia and industry. Andrew Rae, Professor of Engineering at the University of the Highlands and Islands Perth College UHI Campus, led the design of the aeroplane. He said: “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." Pseudo satellites According to the team behind the Phoenix, aircraft based on variable-buoyancy propulsion 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," Rae said. The prototype aeroplane, which is 15 metres long and has a wingspan of 10.5 metres, was flown successfully and repeatedly over a distance of 120 metres 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.  Future development 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. Watch this video of the Phoenix trials: This Drone 'Breathes' Air To Propel Itself and Has Unlimited Range With the rapid rate that drone technology is advancing, we shouldn't be surprised when increasingly complex UAVs hit the scene with fanfare. But the Phoenix, a new drone out of the United Kingdom, is a marvel-and could have major military implications. At 49 feet long and 34 feet wide, the Phoenix looks like a small flying bomb with (relatively) tiny wings covered by solar panels, which makes it plenty imposing on the outside. But it also uses a "variable-buoyancy propulsion system" to move through the air. Phoenix UAV - Unmanned Aerial Vehicle for Satellite Applications As the Phoenix sucks in air and stores it within an inflatable bag, it becomes heavier and uses its wings to steer forward and into an altitude-losing dive. This provides forward movement. The Phoenix then releases the air, rising to altitude again. It also has a supply of helium, or alternately hydrogen, to provide increased buoyancy. The drone essentially spends half of its airborne time as a lighter-than-air vehicle, and the other half as a heavier-than-air aircraft. Since it lacks an internal combustion motor and the need for fuel, theoretically the Phoenix could stay aloft indefinitely and act as a floating sensor or communications node for military forces. The aircraft's fuselage is constructed out of Vectran with wings made of carbon fiber, and it's so inexpensive that its designers-who come from several universities and small businesses in the U.K.-describe it as "near disposable." The Phoenix, which has been in development for three years, would have a broad array of military uses. It could be used as a satellite alternative to provide line-of-sight secure communications, passing along signals across thousands of miles. The aircraft could also serve as a persistent sensor platform, hovering over a trouble spot and using cameras or electronics to keep track of enemy movements and communications. And because it's so low-cost, armed services could keep plenty Phoenixes in reserve, deploying them in emergencies.