MIT researchers have demonstrated that an aircraft with a 5-meter wingspan can sustain steady-level flight using ionic-wind propulsion. The aircraft has no moving parts, does not depend on fossil fuels to fly, and is completely silent.

The researchers describe their proof of concept for electroaerodynamic (EAD) airplane propulsion in a paper in the journal Nature.

Since the first aeroplane flight more than 100 years ago, aeroplanes have been propelled using moving surfaces such as propellers and turbines. Most have been powered by fossil-fuel combustion. Electroaerodynamics, in which electrical forces accelerate ions in a fluid, has been proposed as an alternative method of propelling aeroplanes—without moving parts, nearly silently and without combustion emissions. However, no aeroplane with such a solid-state propulsion system has yet flown. Here we demonstrate that a solid-state propulsion system can sustain powered flight, by designing and flying an electroaerodynamically propelled heavier-than-air aeroplane.

—Xu et al.

Corresponding author Steven Barrett, associate professor of aeronautics and astronautics at MIT, noted that his team’s study marked the first sustained flight of a plane with no moving parts in the propulsion system.

This has potentially opened new and unexplored possibilities for aircraft which are quieter, mechanically simpler, and do not emit combustion emissions.

—Steven Barrett

He expects that in the near-term, such ion wind propulsion systems could be used to fly less noisy drones. Further out, he envisions ion propulsion paired with more conventional combustion systems to create more fuel-efficient, hybrid passenger planes and other large aircraft.

The principle of electroaerodynamic thrust, first identified in the 1920s, describes a wind, or thrust, that can be produced when a current is passed between a thin and a thick electrode. If enough voltage is applied, the air in between the electrodes can produce enough thrust to propel a small aircraft.

For years, electroaerodynamic thrust has mostly been a hobbyist’s project, and designs have for the most part been limited to small, desktop “lifters” tethered to large voltage supplies that create just enough wind for a small craft to hover briefly in the air. It was largely assumed that it would be impossible to produce enough ionic wind to propel a larger aircraft over a sustained flight.

The MIT team’s final design resembles a large, lightweight glider. The aircraft, which weighs about 5 pounds, carries an array of thin wires, which are strung like horizontal fencing along and beneath the front end of the plane’s wing. The wires act as positively charged electrodes, while similarly arranged thicker wires, running along the back end of the plane’s wing, serve as negative electrodes.

The fuselage of the plane holds a stack of lithium-polymer batteries. Barrett’s ion plane team included members of Professor David Perreault’s Power Electronics Research Group in the Research Laboratory of Electronics, who designed a power supply that would convert the batteries’ output to a sufficiently high voltage to propel the plane. In this way, the batteries supply electricity at 40,000 volts to positively charge the wires via a lightweight power converter.

EAD airplane design. a, Computer-generated rendering of the EAD airplane. b, Photograph of actual EAD airplane after multiple flight trials. c, Architecture of the high-voltage power converter (HVPC). The HVPC consists of three stages: a series–parallel resonant inverter that converts 160–225 V direct current to a high-frequency alternating current; a high-voltage transformer that steps up the alternating-current voltage; and a full-wave Cockcroft–Walton multiplier that rectifies the high-frequency alternating current back to direct current. The three stages contribute a voltage gain of about 2.5×, 15× and 5.6×. Xu et al.

Once the wires are energized, they act to attract and strip away negatively charged electrons from the surrounding air molecules, like a giant magnet attracting iron filings. The air molecules that are left behind are newly ionized, and are in turn attracted to the negatively charged electrodes at the back of the plane.

As the newly formed cloud of ions flows toward the negatively charged wires, each ion collides millions of times with other air molecules, creating a thrust that propels the aircraft forward.

The team, which also included Lincoln Laboratory staff Thomas Sebastian and Mark Woolston, flew the plane in multiple test flights across the gymnasium in MIT’s duPont Athletic Center—the largest indoor space they could find to perform their experiments. The team flew the plane a distance of 60 meters (the maximum distance within the gym) and found the plane produced enough ionic thrust to sustain flight the entire time. They repeated the flight 10 times, with similar performance.

Undistorted camera footage from flight 9, with position and energy from camera tracking annotated. Sped up 2x. Credit: Steven Barrett. Click on image to see the flight.

This was the simplest possible plane we could design that could prove the concept that an ion plane could fly. It’s still some way away from an aircraft that could perform a useful mission. It needs to be more efficient, fly for longer, and fly outside.

—Steven Barrett

The new design is a “big step” toward demonstrating the feasibility of ion wind propulsion, according to Franck Plouraboue, senior researcher at the Institute of Fluid Mechanics in Toulouse, France (who was not involved in the research), who notes that researchers previously weren’t able to fly anything heavier than a few grams.

The strength of the results are a direct proof that steady flight of a drone with ionic wind is sustainable. [Outside of drone applications], it is difficult to infer how much it could influence aircraft propulsion in the future. Nevertheless, this is not really a weakness but rather an opening for future progress, in a field which is now going to burst.

—Franck Plouraboue

Barrett’s team is working on increasing the efficiency of their design, to produce more ionic wind with less voltage. The researchers are also hoping to increase the design’s thrust density—the amount of thrust generated per unit area. Currently, flying the team’s lightweight plane requires a large area of electrodes, which essentially makes up the plane’s propulsion system. Ideally, Barrett would like to design an aircraft with no visible propulsion system or separate controls surfaces such as rudders and elevators.

The editors of Nature noted in an editorial in the issue of the journal in which Xu et al. appears that:

Predictions about the future of flight are dangerous because work can be overtaken by events or exposed as wishful thinking. (Just four years before the aerial carnage of the Second World War, Nature solemnly predicted that the risk of attack from the air was remote. And in the 1970s, it reported claims that a hydrogen-powered aircraft could take to the skies by the end of the twentieth century.)

When the Wright brothers made their historic flight in December 1903, it didn’t receive that much attention. In part, that was because their idea was just one of several being explored to achieve flight—with others betting on the success of gliders, airships and even kites. The same is true today. Ion-drive engines are just one much-needed option to improve the efficiency and environmental impact of aircraft engines, alongside tweaks to fuel and design. Let’s hope some of them take off.

This research was supported, in part, by MIT Lincoln Laboratory Autonomous Systems Line, the Professor Amar G. Bose Research Grant, and the Singapore-MIT Alliance for Research and Technology (SMART). The work was also funded through the Charles Stark Draper and Leonardo career development chairs at MIT.

Resources

• Haofeng Xu, Yiou He, Kieran L. Strobel, Christopher K. Gilmore, Sean P. Kelley, Cooper C. Hennick, Thomas Sebastian, Mark R. Woolston, David J. Perreault & Steven R. H. Barrett (2018) “Flight of an aeroplane with solid-state propulsion” Nature volume 563, pages 532–535 doi: 10.1038/s41586-018-0707-9