Modern aeroplanes achieve flight using propellers and turbines powered by the combustion of greenhouse-gas emitting fossil fuels. A new paper published on 21 November in Nature has reported the first flight of an ionic wind aeroplane (1). The small lightweight device called a ‘lifter’ uses something called solid-state propulsion, also referred to as electroaerodynamic aeroplane propulsion, and does not require any combustion processes or moving parts.
The mechanism behind this type of propulsion has been known for years. In brief, charged molecules in the air are accelerated by an electric field and when they collide with neutral air molecules, the transfer of momentum creates something called ionic wind. However, the authors claim that no other kind of aeroplane with a solid-state propulsion system has ever been flown.
In the proof-of-concept study, researchers from the Massachusetts Institute of Technology (MIT) were able to use ionic wind propulsion to sustain the flight of a small aeroplane. The aeroplane was designed using a method called geometric programming in order to find the optimal design that could minimize the wingspan and therefore, the weight, power requirements, and costs. This ended up being a wingspan of 5 metres, with a mass of 2.5 kilograms, a flight velocity of 4.8 meters per second and a 600 watts of power.
The entire power system and batteries were carried onboard the aeroplane. This included a specially designed ultralight high-voltage (40-kilovolt) power converter. The aeroplane was flown ten times, demonstrating that widely accepted limitations “previously thought to make electroaerodynamics unfeasible as a method of aeroplane propulsion, are surmountable.”
To achieve steady-level flight, an electric field was applied to the area surrounding the emitter ― a fine wire that produces the free electrons that collide with air molecules to ionize them ― thereby creating a region of charged air molecules, a phenomenon is known as a corona discharge. As the charged molecules move away from the emitter, they generate a propulsive ionic wind and are finally accelerated towards the collector, thus generating enough propulsion to fly the lifter.
Whether researchers will be able to scale the propulsion system remains unknown. The authors hope that further improvements in overall efficiency and thrust density will “open up new design spaces and unexplored applications for near-silent electric aircraft based on solid-state propulsion.” One of the main challenges will be achieving sufficient thrust ― the mechanical force required to accelerate the aircraft. Although the researchers were able to generate enough thrust to fly the unmanned aerial vehicle, the current technology would not be capable of high-speed commercial flights. Ionic wind may not be able to sustain the flight of aircraft weighing several tonnes but much lighter solar-powered planes may be more feasible.
In any case, this latest proof-of-concept will contribute to a better understanding of the physics behind ionic wind and has paved the way for further optimization of the emitters and collectors. Moreover, solid-state propulsion could be applied to urban drones to limit the associated noise impacts and may lead to further technological advances toward aircraft that are “quieter, mechanically simpler and do not emit combustion emissions.”
(1) Xu, H. et al. Flight of an aeroplane with solid-state propulsion. Nature (2018). DOI: 10.1038/s41586-018-0707-9
Image credit: Hu, H. et al. Nature (2018)