Electroaerodynamics is a phenomenon first identified in the 1890s and, almost 120 years later, it may finally enable quiet, propellantless thrust involving no moving parts. ROB COPPINGER reports.

Two tractor propellers on 17 December 1903 propelled Orville Wright at the controls of the Wright Flyer for the first time for 3.5 seconds at Kitty Hawk. Later that day the brothers accomplished a 259.6m (852ft) trip over 59 seconds on their fourth attempt. Thirty-five years later, a Heinkel He.178 made the first turbojet powered flight on 27 August 1939, a six-minute journey, with its Hans Joachim Pabst von Ohain-designed jet engine.

Both of those significant events in aeronautical history involved engines with moving parts but, in 2018, 79 years after the Heinkel flight, a 5m wingspan electroaerodynamic (EAD) demonstrator flew 60m in 10 seconds in the Massachusetts Institute of Technology (MIT) duPont Athletic Center. It eventually flew ten times.

“We were constrained by the size of the gym,” says (MIT) Associate Professor of Aeronautics and Astronautics, Steven Barrett, commenting on the range and time. He is also the MIT Electric Aircraft Initiative lead and Director of its Laboratory for Aviation and the Environment Director. British-born Barrett is also a visiting professor at University College London, and he gained his undergraduate and graduate degrees in aerospace engineering at Cambridge University, where he also lectured.

" A plane powered by electro-aerodynamics wouldn’t necessarily look like a normal aircraft"

“We’ve been developing newer versions of the propulsion system that we think will get around some of the limitations of the first test flights,” Barrett explains. “With the first test flight, the aim was to have the minimum viable demonstrations that could sustain steady level flight, though not being necessarily representative of what it (an EAD aircraft) might be in the long-term.”

The thrust efficiency of his prototype is already equivalent to a turbine, Barrett says. “This minimum viable demonstrator ended up at a design point of six Newtons per kilowatt, which is actually fairly reasonable. That’s comparable to a gas turbine engine, something like that.” But the potential speed is far greater than many may imagine. “The time for stripping away electrons is so fast that it could potentially work at extremely high velocities and maybe even supersonic,” says Barrett.

Barrett expects that EAD aircraft, when they arrive, will be drones for urban operations or high-altitude pseudo-satellites that provide communications. For such applications, “having a propulsion system with no moving parts could potentially operate for a very long time,” he says. Like existing high-altitude drones, a high aspect ratio wing, meaning its wing is disproportionately wide when compared with the length of the fuselage, is the preferred design. It would also differ in another way because, “it’s likely to be something like a bi-plane, perhaps a tandem bi-plane, because that would give a convenient configuration within which to structure the ionisation regions and things like that,” Barrett explains.

How electroaerodynamics works

Actual flight time lapse image from the test flight across the gymnasium in MIT’s duPont Athletic Center. (MIT)

Electroaerodynamics is where electrical forces accelerate ions in a fluid, which in this case is air. To be precise, a sufficiently high voltage is applied across two electrodes of different sizes. Once the electric field around the smaller electrode is high enough, it emits a coronal discharge and electrons near it are stripped from their air molecules’ atoms creating positively charged ions. An electric field between the two electrodes draws the ions from the smaller electrode to the larger one, which is called a collector. Collisions between the ions and neutral air molecules transfer momentum, creating an airflow, called an ionic wind, producing a thrust force.

Barrett’s demonstrator is described as a ‘large, lightweight glider’ with a mass of 2.26kg (5lb) and a 5m wingspan. Barrett says that the number five is a coincidence in the mass and wingspan there is no correlation between the two. The fuselage contains the lithium-polymer batteries that provide the power.

With Barrett’s team’s design, the 5m wingspan has an array of thin wires, which are strung horizontally beneath the leading edge of the prototype’s wing. Thicker wires, also strung along the span of the wing, are at the trailing edge. The leading edge’s wires are positively charged, and they create the ions with 40,000 volts, and the trailing edge wires are negatively charged; they are negative electrodes. “The first demonstrator we built was conservative, in the sense that we tried to make it as close to a conventional aircraft as we could, and obviously manage the technical risks,” Barrett says.

“Our focus was on the propulsion system, the airframe, and the power converter,” he adds. A key technology is a voltage converter to step up the batteries’ lower voltage to the tens of thousands of volts that the electron stripping process needs. “The converter that was developed, which was obviously specifically for this application, was between five and ten times more power dense than what you’d get (commercially) off the shelf,” Barrett explains. “We think we can probably get another factor of two or something of that order, which is still useful to have because the power converter is by far the heaviest component of the whole vehicle.”

Barrett’s team included members of MIT’s Power Electronics Research Group in the Research Laboratory of Electronics and Lincoln Laboratory Autonomous Systems line staff. The Power Electronics Research Group created the converter specifically to step up the voltage to 40,000 volts. This research was also aided by the Singapore-MIT Alliance for Research and Technology, and the Charles Stark Draper and Leonardo career development chairs at MIT also helped fund the work, along with a Professor Amar G. Bose Research Grant.

Barrett says of the possibility of Singaporean involvement in ionic propulsion: “Yes, possibly. Some researchers in Singapore have expressed an interest. I’ll certainly look into those options and see if that makes sense to do that from, basically, a project standpoint.”

High voltage movement

Artist's impression of ionic-powered aircraft. (MIT)

The ability of a high voltage discharge to cause the movement of air, ionic wind, was studied as far back as 1899. While Barrett started studying EAD flight nine years ago, before he began his work, NASA had funded an investigation into ionic propulsion. The research was carried out by the Glenn Research Center and an Ohio-based company. The resulting technical paper, dated December 2009, said: “Parametric experiments and theory, showed that the thrust per unit power could be raised from early values of 5N/kW to values approaching 50N/kW but only by lowering the thrust produced and raising the voltage applied.”

In 2013, MIT stated that its researchers had achieved 110N per kW. In the Institute’s description of its work, it stated: ‘In their (MIT researchers’) experiments, they found that ionic wind produces 110 Newtons of thrust per kilowatt, compared with a jet engine’s two Newtons per kilowatt. The team has published its results in the Proceedings of the Royal Society.’ The article referred to Barrett’s involvement and stated that: ‘ionic thrusters’ were silent and invisible to infra-red sensors.

In that 2013 project, Barrett acknowledges an obstacle is the thrust density. With ionic propulsion, because thrust density is related to the gap between the electrodes, the larger the gap, the larger the thrust. He now distances himself from the 2013 figure of 110N/kg. ‘While those very high efficiencies were recorded, they were recorded at a design point that just wouldn’t be practical in terms of the amount of volume that would be taken up.’ Lifting a small aircraft and its electrical power supply would require a very large air gap.

Today, the work is investigating thrust density and actual thrust to discover what realistic efficiency can be achieved. “We’re trying to break the trade-off between thrust density and actual thrust or I should say thrust density and efficiency, by trying to get a bit of both. That’s what we’re working towards.” Another difficulty is the voltages. While a small balsa wood glider like Barrett’s demonstrator needs tens of thousands of volts, a small aircraft would require hundreds of thousands.

In the earlier, NASA work, the conclusion was downbeat. NASA stated: “It was concluded that the use of a corona discharge for aircraft propulsion did not seem very practical.” Barrett’s answer to the thrust problem is to have EAD thrusters that would encompass the entire vehicle.

Boundary layer re-energisation

Indoor flight of the EAD demonstrator (MIT)

“In terms of the propulsion system, the architecture, a plane powered by electroaerodynamics wouldn’t necessarily look like a normal aircraft,” Barrett explains. “We do expect to deviate very strongly from that and have configurations that don’t look like conventional aircraft, which will better leverage the way in which electroaerodynamics works.” In the 2013 statement it says: “Barrett envisions that electrodynamic thrusters for aircraft, if they worked, would encompass the entire vehicle.”

Barrett wants to produce thrust over the length of the airframe and he knows of different techniques that could be used to separate the process for producing ions from what is needed to accelerate them. “(This) enables us to create a thruster that is long, in relation to the length of the aircraft, rather than just essentially a plane, like a vertical plane, like a propeller. We’re talking about having thrust being produced over a significant length of the aircraft.”

He points out that EAD works better if there is a low thrust density over a very large volume. “In some sense, that’s what you want because it gives you a high propulsive efficiency if you have a low velocity change over a large area.” His team is building bench experiments for electrical pulses that are spread over large volumes. “That’s basically what we’re working on. We think we’ll probably have a test flight of that in about two years, he adds.

His team’s near-term goals also include decreasing the average electric field strength. This is because some of their early work showed that, the lower the electric field strength, the higher the efficiency. Essentially, low strength fields across the entire vehicle provides low thrust density for a large volume, for high propulsive efficiency. A drawback to this is that it sets a limit for the efficiency because of the low field strength restriction.

Barrett also wants to integrate the EAD thrusters into the surface of the aircraft. “As well as having thrusting areas that are (on the wing), not on the skin of the airframe, we also want to change the skin of the airframe from being drag producing to being thrust producing,” he says. “That’s more challenging to do because you get all kinds of interactions between [the electrical] charge and the materials.” He compares this approach to boundary layer ingestion, saying that it would have the same advantages. “It’d be more like boundary layer re-energisation because we just never let a boundary layer grow in the first place. You just continuously re-energise it.”

For such a vehicle encompassing source of thrust, Barrett’s long-term goal is to see the wires almost disappear. “The ultimate objective is to make it completely flush with the skin of the airframe, so not having wires and collectors protruding,” he explains. “You could still have an electric field parallel to a flat surface, a flat plate, that collects the electrons. We have early working prototypes that do work but it’s much more complicated when you start interacting with material properties and that sort of thing.”

Wires are not the only feature that would disappear; Barrett sees a time when traditional control surfaces could be replaced with the actions of EAD thrusters. “(We’ll try) to remove control surfaces and instead have all the control done by the main propulsion system by shaping the electric fields. That would be desirable to do on at least a small, quiet take off,” but he draws a line at having only electric field control for passenger aircraft, they would still have physical control surfaces, “as a backup”.

Near-silent, no moving parts, no emissions, IR invisible, more efficient than a jet engine, thrust from boundary layer re-energisation, no control surfaces. The promise of ionic propulsion is great but so is the level of voltage that is needed.

Rob Coppinger

