MIT Engineers Design Unpiloted Aerial Vehicles that can Fly Over Longer Durations

The Jungle Hawk Owl team. CREDIT: Sally Chapman/MIT.

When phone and Internet systems across a broad area are disrupted during a natural calamity, communications payloads offering temporary telecommunications coverage to the people in need can be carried by means of autonomous aircraft hovering over the affected area.

Yet, these unpiloted aerial vehicles (UAVs), for example, most autonomous surveillance aircraft used by the U.S. Air Force, are normally costly to operate and can fly only for one or two days. Therefore, to offer ample and continued coverage, a number of aircrafts have to land and refuel continuously. This increases the operational costs to thousands of dollars per vehicle, per hour.

Engineers at MIT have recently developed a considerably less costly UAV that can fly in air for longer periods of time and offer a broad range of communications coverage. The engineers designed, constructed, and investigated a UAV that looks like a thin glider with a wingspan of 24 foot. The UAV has the ability to carry communications equipment weighing about 10-20 pounds when it flies at an altitude of 15,000 feet. Powered by a 5-horsepower gasoline engine, the UAV, with a weight of only 150 pounds, can remain aloft for more than 5 days. According to the researchers, this period of flight is longer than that of any gasoline-powered autonomous aircraft.

The researchers will present the outcomes of their research this week at the American Institute of Aeronautics and Astronautics Conference in Denver, Colorado. R. John Hansman, the T. Wilson Professor of Aeronautics and Astronautics; and Warren Hoburg, the Boeing Assistant Professor of Aeronautics and Astronautics headed the research team. Hansman and Hoburg are co-instructors for MIT’s Beaver Works project, which is a student research collaboration between MIT and the MIT Lincoln Laboratory.

A solar no-go

Hansman and Hoburg collaborated with MIT students to develop a long-duration UAV under the Beaver Works capstone project, a two- or three-semester course that enables MIT students to design a vehicle that fulfill specific mission requirements, and to construct and investigate their vehicle.

With a concept of developing a long-duration UAV that operates on solar energy, the U.S. Air Force contacted the Beaver Works collaboration during spring 2016. The idea then was that a solar-powered aircraft can prospectively remain aloft for an indefinite period of time. Companies such as Google have tested this idea and have developed high-altitude, solar-powered aircraft to provide uninterrupted Internet access to remote and rural areas in Africa.

However, the researchers analyzed the concept and scrutinized the practical difficulties from a number of engineering angles, deducing that solar energy will not be suitable for long-duration emergency response.

[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. For the mission of disaster relief, this could only respond to disasters that occur in summer, at low latitude. That just doesn’t work.

R. John Hansman, Co-Instructor, Beaver Work's Project, MIT

The team arrived at their decision by modeling the difficulty with the help of GPkit—a software tool created by Hoburg to enable engineers to find out the ideal design dimensions or decisions for a UAV ‒ with specific mission specifications or constraints.

Although this technique is not explicit among initial aircraft design tools, in contrast to those tools that consider only a number of main constraints, Hoburg’s technique enabled the researchers to take into account nearly 200 constraints and physical models at the same time, and also to use them as a whole to develop an ideal aircraft design.

This gives you all the information you need to draw up the airplane. It also says that for every one of these hundreds of parameters, if you changed one of them, how much would that influence the plane’s performance? If you change the engine a bit, it will make a big difference. And if you change wingspan, will it show an effect?

R. John Hansman,​ Co-Instructor, Beaver Work's Project, MIT

Framing for takeoff

The researchers used their software estimations to deduce that a solar-fueled UAV will not be probable for long-duration flight in any region on the globe. Consequently, they carried out the same modeling for a gasoline-fueled UAV. The team developed a design that was estimated to stay aloft at an altitude of 15,000 feet for more than 5 days, under 94th-percentile winds at any latitude.

During fall 2016, the researchers constructed a prototype UAV by using the dimensions deduced by means of Hoburg’s software tool. In order to maintain the weight of the UAV to be lighter, the fuselage and wings of the vehicle were made of carbon fiber, and the tail and nosecone (housing the payload) were made of Kevlar. The UAV was designed such that it can be easily dismantled and placed in a FedEx box, and transported to any disaster-struck place and swiftly reassembled.

During spring 2017, the students enhanced the prototype and built a launch system, designing a simple metal frame that can be fixed on a normal car roof rack. The UAV is positioned on the frame while a driver accelerates the launch vehicle, which is a truck or car, up to the rotation speed, that is the ideal takeoff speed of the UAV. During this point, the remote pilot angles the UAV in the direction of the sky, thus automatically releasing a fastener and lifting off the UAV.

Early in the month of May 2017, the researchers investigated the working of the UAV by holding flight tests at Plum Island Airport in Newburyport, Massachusetts. At the time of the initial flight investigations, they modified the vehicle to conform to FAA regulations for small unpiloted aircraft, which permit drones weighing less than 55 pounds to fly at low altitudes. They achieved a weight reduction from 150 to less than 55 pounds by just loading it with lesser gasoline and a smaller ballast payload.

During the initial investigations, the UAV took off very well, flew around, and landed in a safe way. According to Hoburg, there are specific considerations to be made to investigate the UAV over many days, for example, the need for adequate number of people to observe the aircraft for a longer duration.

There are a few aspects to flying for five straight days,” stated Hoburg. “But we’re pretty confident that we have the right fuel burn rate and right engine that we could fly it for five days.”

These vehicles could be used not only for disaster relief but also other missions, such as environmental monitoring. You might want to keep watch on wildfires or the outflow of a river. I think it’s pretty clear that someone within a few years will manufacture a vehicle that will be a knockoff of this.

R. John Hansman,​ Co-Instructor, Beaver Work's Project, MIT

MIT Lincoln Laboratory partially supported the research.

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