Phoenix ultra-endurance air vehicle is first to fly like a fish

Phoenix ultra-endurance air ve...
The Phoenix in flight
The Phoenix in flight
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The Phoenix in flight
The Phoenix in flight

A partnership of universities and private companies led by Andrew Rae of the University of the Highlands and Islands Perth College in Scotland, have developed and flown what is claimed to be the first aircraft powered by variable-buoyancy propulsion. In March, the unmanned Phoenix prototype made an indoor maiden flight at the Drystack facility in Portsmouth, England, where it repeatedly flew for a distance of 120 m (395 ft).

Looking like a blimp with wings, the ultra-long endurance, autonomous Phoenix is a demonstrator aircraft that is 15 m (50 ft) long and has a wingspan of 10.5 m (34 ft). It uses a technology similar to the swim bladder in a fish to generate thrust by shifting from being lighter than air to heavier than air.

Most bony fish maintain neutral buoyancy by inflating and deflating the air bladder with, well, air. It's also the principle behind the buoyancy vests worn by scuba divers, allowing them to balance their weight for neutral buoyancy by inflating it enough to balance out their weight, sit comfortably on the bottom by deflating the vest, or float on the surface by inflating it fully.

The Phoenix uses the same principle to produce a very simple, inexpensive design that allows it to fly while using very little energy.

"The Phoenix spends half its time as a heavier-than-air airplane, the other as a lighter-than-air balloon," says Rae. "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 airplane 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.

"Vehicles based on this technology could be used as pseudo satellites and would provide a much cheaper option for telecommunication activities. Current equivalent airplanes are very complex and very expensive. By contrast, Phoenix is almost expendable and so provides a user with previously unavailable options."

According to the team, the Phoenix could also operate on the edge of space, where it could act as a launch platform for micro-satellites. The current goal is to get the Phoenix to operate at an altitude of 20,000 m (66,000 ft), where it will be powered by its wing-mounted solar panels to allow it to stay aloft for several days.

The three-year project was funded in part by by the British government's Innovate UK agency. The team says it is now looking for major manufacturers to help with the next phase of development.

Video of the Phoenix in flight can be seen below.

Source: University of the Highlands and Islands

Phoenix demonstrator variable-buoyancy aircraft

I like the low power usage but I do see a couple of issues. while the climb/dive concept is really innovative, I do like it, I wonder if it will be able to over come the wind speeds encountered at altitude. I would assume that the aim of this system would be to have the craft transit from it's launch site to its working area and maintain it's position over a fixed point in the sky. I don't see how it will launch satellites. Firstly their is the weight issue of even a very small satellite, this sort of craft would have very little useful payload, also if it did launch something from altitude the residual buoyancy that this craft would have would mean that it would end up with a run away accent, probably until it burst. Having said that I will be interested to see how this project evolves.
Simon, you addressed every question I had while watching the video.
It's a very good concept and they've done virtually everything right, in particular using the combination of proper airfoils and a low-drag shape envelope / hull rather than a expensive and low performance lifting-body design. For a sustainable commercial system it will need to use hydrogen as the lift gas (helium is hundreds of times more expensive and is non-renewable), and be much, much larger. (Lift goes with the cube of length, weight as the square.) To handle the size, need custom hangars built from pneumatic structures and fire-resistant tensile materials (e.g. ETFE); to handle flammable hydogen, need protective gear for ground crews. Another trick they appear to be missing is in the tail-box section, which is unnecessarily high drag when it could be negative drag via boundary-layer ingestion going to a ducted fan coaxial with the hull stern. The fan could be electrically driven, and also driven by the compressed air exhaust via tip-jets or an ejector- type system. (An ejector could also work without a fan, but having an electrically-driven fan allows using it as a generator as well while descending or slowing.) Rather than launching microsatellites on rockets from 20000 m, it makes far more economic sense to use airships as electronic platforms for cell service, data service, imagery, radar, and so forth. I calculated in 2010 that at 60 kft (lower than the 20 km the Phoenix targets) one can see 480 km, which would be as few as 13 airships to cover all of Australia. With double coverage, 40% in the air at a time, and coverage a bit beyond the coasts it would only take 82 airships for all of Australia, which would be between $0.5- $1B/yr (~$3 - $6 / capita / month). 2010 quote from me on drone airship alternative to Australia's then-proposed National Broadband Network (NBN): "With antenna arrays and software-defined radio, the system could support not only mobile high-speed internet but fixed ground-station backhaul links, cell-phone service, radio, TV, and potentially advanced radar for ATC, weather and defense. At the same time it would have much more even coverage and greater resistance to multipath and active interference. At 60,000 feet there would be little interference with optical networking between airships, and with a double-coverage system each airship would have line-of-sight to its six nearest neighbors, allowing a mesh network of vast capacity. Optical downlinks to ground stations would also be possible in clear weather, and because of the extent of the network there would always be multiple spots where the weather would permit a bridge to the fiber network. Such a system would have the high bandwidth, low latency and robustness to allow high-resolution remote mobile video, telepresence, remote consulting, gaming, unmanned aerial and surface vehicles, remote science and utility monitoring, and convenience and accessibility for users throughout Australia" The Phoenix, to power such large quantities of electronics, will -- in addition to being several times as long and hundreds of times its present volume -- need at least on the order of 100 times the solar cell capacity and battery storage of the prototype. A few 100kW baseload power would be more like it, given that a single pair of airships at a time would be handling an entire metropolis' wireless communications. Really, it would almost certainly make more engineering sense to drive a combustion generator using the compressed air, but even otherwise sensible engineers often promote "green" energy in applications where it isn't a workable idea.
This is hardly a new concept, I have seen this mooted for a commercial aircraft, years ago. Cannot think of the name. Used variable bouyancy to charge batteries (dropping) and then propel the aircraft and compessors (to repeat the cycle). Same guy had a similar idea for a ship.