A team of engineers from MIT have come up with an economical design for unpiloted aerial vehicles (UAVs) that can remain in the air for more than five days, providing support capabilities in the event of a disaster.
The intention is for these UAVs to be able to hover disaster-affected regions to provide temporary telecommunications coverage, even if phone and Internet systems are disrupted.
Current UAVs that carry out this functionality are often expensive to operate and can only remain the air for a day or two. The US Air Force's autonomous surveillance aircraft have a similar hover time. Thus, in cases where adequate and persistent coverage is required, multiple aircraft would have to fly in relay, landing and refuelling around the clock, costing thousands of dollars per operational hour per vehicle.
The MIT design takes after a thin glider, with a wingspan of 7.3 m. The UAV can carry 4.5 to 9 kg of communications equipment while flying at an altitude of 4.5 km. It weighs just under 68 kg, and is powered by a 3728 W gasoline engine, which keeps it aloft for more than five days, which is longer than any gasoline-powered autonomous aircraft has remained in flight.
The team was led by R. John Hansman, Professor of Aeronautics and Astronautics; and Warren Hoburg, the Boeing Assistant Professor of Aeronautics and Astronautics. The instructors worked with MIT students to design the long-duration UAV.
In spring 2016, the US Air Force approached the team with an idea for designing a long-duration UAV which would be powered by solar energy. In theory, the aircraft could be powered by the sun for potentially unlimited flight times. However, when the team looked into the idea and analysed the problem from multiple engineering angles, they found that solar power was unsuitable for the purpose.
"[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. So you have to carry more batteries, which adds weight and makes the plane bigger," Hansman says.
This would limit the UAV to only deployments in summer, at low latitude.
The researchers modelled the problem using GPkit, a software tool developed by Hoburg that allows engineers to determine the optimal design decisions or dimensions for a vehicle, while taking into account constraints or mission requirements. Using GPkit, the team were able to consider around 200 constraints and physical models simultaneously, in order to create an optimal aircraft design.
The methodology also allowed the engineers to assess the effects of changing any one of hundreds of parameters of the aircraft.
Following their assessment regarding the viability of a solar-powered aircraft, the team looked at gasoline-powered aircraft, and came up with their design. They built a prototype of the UAV following the dimensions determined by the team using the GPkit tool.
To keep the vehicle lightweight, they used materials such as carbon fibre for its wings and fuselage, and Kevlar for the tail and nosecone, which houses the payload. The researchers designed the UAV to be easily taken apart and stored in a box that can be shipped to any disaster region and quickly reassembled.
They also engineered a launch system, using a simple metal frame that fits on a typical car roof rack. The UAV sits atop the frame as the driver accelerates the launch vehicle up to the optimal takeoff speed. A remote pilot then angles the UAV skyward, releases a fastener and allowing lift-off.
According to Hoburg, while the fuel burn rate and the right engine would allow the UAV to fly for five days, there are special considerations that have to be made to test the vehicle over multiple days, such as having enough people to monitor the aircraft over a long period of time.
[Image: JHO first flight, May 4, 2017 at Plum Island (Mass.) Airport. The aircraft is launched from a moving vehicle. Photo: Veronica Padron/MIT.]