Researchers successfully fly ion-drive model aircraft

Silent, solid-state, emission-free air travel comes one step closer. Andrew Masterson reports.

A model ion-driven plane with a five-metre wingspan successfully completes a test flight.

A model ion-driven plane with a five-metre wingspan successfully completes a test flight.

Steven Barrett

Aeronautics engineers have succeeded in building and flying a model heavier-than-air powered aeroplane that contains no moving parts and creates no emissions.

The model, which has a five-metre wingspan, uses a solid-state propulsion system in which electrical forces accelerate ions in a fluid. It was built by a team headed by Haofeng Xu of the Massachusetts Institute of Technology (MIT) in the US, and is described in a paper published in the journal Nature.

With the exception of unpowered gliders, all aircraft designed to date require a mechanical propulsion system incorporating moving parts – typically turbines or propellers – and (with the exception of few very early pedal-powered designs) fossil fuel inputs.

Electroaerodynamics, the ion-based system used by Xu and his colleagues, has long been mooted as an alternative, but the physics have proved to be extremely complicated.

The heart of the problem is that the thrust delivered by an electroaerodynamical engine is strongly affected by speed, air density and, hence, altitude. An analysis published in 2017 in the journal of the American Institute of Aeronautics and Astronautics found that the thrust-to-power ratio of an electroaerodynamical aeroplane would decrease by about 80% as the vehicle climbed from ground level to 25 kilometres up.

However, the study’s authors, Christopher Gilmore and Steven Barrett (who are also co-authors on Xu’s paper), found that some design tweaks could be employed to reduce the drop-off. Increasing the size of the thruster, they suggested, would compensate for altitude-based loss.

They also noted that the “thrust to power is also expected to decrease with increasing forward velocity of an electroaerodynamically propelled aircraft due to an increase in the effective mobility of ions generated by the propulsion system”.

This, however, would still boost overall efficiency as speed picked up. In the final analysis, Gilmore and Barrett concluded that an electroaerodynamical thruster could be made to deliver a thrust-to-power balance broadly comparable to that achieved by propeller-based and turbine-based aircraft engines.

Like all work on ion-based aeroplane engines, the 2017 study was theoretical. Until the most recent work, no one had actually built a working vehicle.

Arguably, they still haven’t. The model constructed and flown by Xu and colleagues is large, but not large enough to carry a person. The researchers also conducted their test flights in an enclosed building – free thus of energy-sapping cross- or head-winds – and no more than two metres off the ground.

Nevertheless, the researchers say, the results are significant. Carrying a specially designed ultra-lightweight 40-kilovolt battery onboard, the test plane flew successfully 10 times in a row.

“We show that conventionally accepted limitations in thrust-to-power ratio and thrust density, which were previously thought to make electroaerodynamics unfeasible as a method of aeroplane propulsion, are surmountable,” they write.

“We provide a proof of concept for electroaerodynamic aeroplane propulsion, opening up possibilities for aircraft and aerodynamic devices that are quieter, mechanically simpler and do not emit combustion emissions.”

In an associated editorial in the same journal, Franck Plouraboué from Toulouse University in France points out that the theory behind electroaerodynamics has been known for more than a century.

“When charged molecules in the air are subjected to an electric field, they are accelerated,” he writes. “And when these charged molecules collide with neutral ones, they transfer part of their momentum, leading to air movement known as an ionic wind.”

He describes the work of Xu’s team as a “breakthrough” and points particularly to the design of a fine wire which, when exposed to an electric field, is termed an “emitter”.

“The field is strong enough to induce a chain reaction: free electrons in the region collide heavily enough with air molecules to ionise them, producing more electrons that then ionise more molecules,” he writes.

“These electron cascades give rise to charged air molecules in the vicinity of the emitter — a phenomenon called a corona discharge. Finally, the charged molecules drift away from the emitter and generate a propulsive ionic wind as they are accelerated by the electric field towards a device called the collector.”

Research aimed at refining and improving the design, he concludes, can now begin.

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