When it comes to flight endurance for an unmanned UAV, the Qinetiq Zephyr still reigns supreme with a flight of over 336 hours, but its solar-powered, high-altitude design doesn't make it suitable for many applications. A team of engineers at MIT has developed a cheaper UAV design that has the ability to stay aloft for up to five days at low-altitudes on a single tank of gasoline, potentially offering communications support in areas struck by natural disasters.
The long-duration UAV is powered by a 5-hp gasoline engine, weighs under 150 lb (68 kg) and features a glider-like design with a 24-ft (7.3-m) wingspan. As well as being designed to carry communications-support payloads of up to 20 lb (9 kg), the UAV could offer a cost-effective platform for general environmental monitoring.
"These vehicles could be used not only for disaster relief but also other missions, such as environmental monitoring," says R. John Hansman, one of the leaders of the MIT project. "You might want to keep watch on wildfires or the outflow of a river."
The team initially investigated the idea of using solar energy to power the aircraft, but soon realized a fossil-fueled engine would have much more functionality considering the emergency-relief applications being targeted.
"[A solar vehicle] would work fine in the summer season, but in winter, particularly if you're far from the equator, nights are longer, and there's not as much sunlight during the day," Hansman explains. "So you have to carry more batteries, which adds weight and makes the plane bigger."
The team computer-modeled a design for the gasoline-powered UAV using a software tool called GPkit, which was developed by the other leader on the project, Warren Hoburg. GPkit is a tool that takes specific constraints and then models optimal design dimensions for a vehicle. Unlike other similar software tools, which are fairly limited in terms of the number of constraints that can be considered, Hoburg's system can process around 200 constraints simultaneously.
The ultimate design, determined by the software for optimal flight duration, was constructed from lightweight materials such as carbon fiber and can be easily taken apart for shipping, allowing for quick delivery to disaster zones. The software emulations also predict the design will be able to fly at altitudes of 15,000 ft at any latitude in up to 94th-percentile winds (only six percent of flights would encounter winds that are too strong), for more than five days.
Having built a prototype last year, this year the team developed a launch system that consisted of a basic metal frame that attaches to a car roof rack. With the UAV sitting atop the frame, the car or truck accelerates to the UAV's optimal takeoff speed and the remote pilot angles the aircraft upwards, which causes the fastener to automatically release and sends the UAV skywards.
The team is yet to test the UAV under long-distance endurance conditions, but early short-run prototype flights conducted in May proved successful. However, these test flights required the aircraft's weight to be reduced from 150 to 55 lb (25 kg) so as to comply with FAA regulations for small unpiloted craft. Despite success in launching, flying and safely landing the prototype, the team says there are other factors that need to be considered for longer, multi-day test flights, such as ensuring there are enough people to monitor the aircraft for the duration of the flight.
"There are a few aspects to flying for five straight days," Hoburg says. "But we're pretty confident that we have the right fuel burn rate and right engine that we could fly it for five days."
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