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-satellites (HAPS). 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 been adopted for use only 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 m~3 of helium to help the aircraft ascend. Also contained inside the fuselage is an inflatable bag with a 6 m~3 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 motion. 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 again converting that into forward flight.
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