Power electronics advance lifts prototype aircraft into the history books
Eric Smalley | EECS Contributor
In a historic test, researchers sent an experimental aircraft flying 45 meters (nearly 148 feet) across an MIT gymnasium. The prototype looked like any number of hobbyist model gliders; like a glider, it had no moving parts and made no noise. But this was the first of its kind: the first aircraft to be powered by a new propulsion system since the dawn of the jet engine age in the 1940s.
The aircraft is powered by electroaerodynamics, or ionic wind. Electroaerodynamics results when a pair of electrodes produces a strong electric field that creates a flow of ions between them. The ion flow, in turn, pushes air molecules.
That first flight, in December 2017, was a major milestone in Aeronautics and Astronautics Professor Steven Barrett’s years-long quest to prove that solid-state aircraft propulsion is feasible. When the journal Nature published a paper about the research in November 2018, news of the silent, no-moving-parts aircraft rocketed around the world.
EECS Professor David Perreault and his doctoral student Yiou He made a critical contribution to this historic moment: the aircraft’s power systems. Search YouTube for “ionic wind” and you’ll come across numerous videos of do-it-yourself electroaerodynamic lifters, typically small devices that rise a few feet in the air. Look closely in most of the videos and you’ll see a cord leading from the device to a point off-camera. “Everything you’ll find in the literature or in demonstrations has always been tethered,” says Perreault, who heads the Power Electronics Research Group in MIT’s Research Laboratory of Electronics (RLE). “The power supply and the energy source are not flying.”
Balancing voltage and weight
The major challenge for the team was creating a power system that produced the high voltages needed to create sufficient thrust but were light enough to fly on board the aircraft. The propulsion system is a set of wire-thin, positively charged emitter electrodes at the front of the aircraft and a set of wing-shaped, negatively charged collector electrodes at the back of the aircraft. The strong electric field around the emitters ionizes air molecules, which move to the collectors. As the ions flow they bump into neutral air molecules, creating a wind that blows from the back of the aircraft.
The prototype aircraft weighs 2.45 kilograms (about 5.4 pounds), has a wing span of 5 meters (about 16.4 feet) and flies at a speed of 4.8 meters (nearly 16 feet) per second. To fly, the aircraft needs to produce 3.2 newtons of thrust, which means the propulsion system needs an output voltage of 40,000 volts. The team used a lightweight lithium-polymer battery to deliver 500 watts of power, but batteries that size typically produce much lower voltages. The linchpin of the propulsion system turned out to be a high-voltage power converter that dramatically stepped up the voltage.
Existing high-voltage power converters are typically used with industrial equipment and medical x-ray machines, so there’s been no incentive to optimize them for size or weight, Perreault says. The team’s solution was to build a small power converter that operates at very high frequencies. Imagine moving water from a full bathtub to an empty one, says He, the PhD student. You could use a bucket, but that’s relatively large and heavy, she says. But if you could find a way to move very quickly, you could use something a lot smaller and lighter, say a spoon, to move the same amount of water in the same amount of time, she says.
Before coming to MIT, He earned an undergraduate degree in electrical engineering in her native China. She met Perreault on a visit to MIT during a summer exchange at North Carolina State University, which inspired her to apply to MIT. After earning a master’s degree with EECS’s John Kassakian, He joined Perreault’s lab to work on power conversion systems for her PhD. When Barrett approached Perreault about working on the power conversion system for the solid-state aircraft propulsion, Perreault suggested the project to He. “She has really been at the center point of managing the energy and electrical and electronics for the vehicle,” Perreault says.
The unusual and demanding power system challenge intrigued He. The work provided her with new experiences, including working with such high voltages. “Sometimes I would hear this kind of lightning sound in my lab,” she says. “Especially as we’re shrinking the size of the converter, everything gets closer and it arcs more easily.” To pinpoint where insulation was needed in the propulsion system, Perreault and He built an enclosure surrounded by black curtains to better spot where arcs were occurring, she says.
Despite its novelty, the electroaerodynamic aircraft is part of a larger revolution: the electrification of transportation, Perreault says. The prototype aircraft is cousin to electric skateboards, electric bicycles, electric automobiles, and Cessna-sized electric airplanes. “It’s because we’re able to get the energy storage and the energy conversion and the control all small enough and light enough to enable new kinds of systems,” he says.
It’s unclear whether solid-state propulsion will ever be able to power piloted aircraft, let alone airliners. But the potential for electroaerodynamic drones is tantalizing. Given the expected proliferation of delivery, environmental monitoring and surveillance drones, the threat of noise pollution makes a case for using silent drones, the researchers say. There are also potential applications for electroaerodynamics beyond aircraft propulsion, Perreault says. For example, it could be used to improve the efficiency of conventional aircraft by controlling airflow over surfaces to reduce drag.
In the meantime, the electroaerodynamic aircraft team is working on a new version that incorporates thrust vectoring – directing the thrust – to provide controlled flight, Perreault says. The team is also working on making the electronics lighter and more efficient, according to He: “We’re basically opening up a new research space for this, and there are a lot of things to be done to optimize it. Hopefully, in several years we can fly another version of this that can give us higher efficiency and longer distance, to make it a more practical reality.”
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