Fly with the ionic wind

Small lightweight devices called "lifters" can propel themselves in the air without combustion or moving parts. They have become a popular topic of discussion with technology enthusiasts on social media in recent years. And yet, the physical mechanism behind the risers has been known for over a century¹. When charged molecules in the air are subjected to an electric field, they are accelerated. And when these charged molecules collide with neutral molecules, they transfer part of their momentum, resulting in a movement of air called ionic wind. On page 532, Xu et al.² demonstrate that an aircraft with a wingspan of 5 meters can withstand constant level flight thanks to the ionic propulsion of the wind. Improvements are needed, but the proof-of-principle demonstration of the authors could pave the way for the development of improved propulsion systems.

In the plan of Xu and his colleagues, an electric field is applied to the region surrounding a thin wire called a transmitter (Fig. 1a). The field is strong enough to induce a chain reaction: the free electrons of the region collide sufficiently with the air molecules to ionize them, producing more electrons that then ionize more molecules. These electron cascades give rise to charged air molecules in the vicinity of the transmitter – a phenomenon called corona discharge. Finally, the charged molecules move away from the transmitter and generate an ionic propulsive wind when they are accelerated by the electric field towards a device called a collector (Fig. 1b). This process only occurs in gases, and not in liquids, which justifies the authors' use of the term "electro-aerodynamic".

Previous experiments had suggested that Ionic-wind propulsion could allow the hovering flight of an aircraft, but that the possibility of obtaining it was at the limit of what is currently technologically possible. .³. Xu et al. It was therefore necessary to systematically search all possible aircraft designs for a feasible option. They used a technique called geometric programming to find the optimal set of design variables that would also minimize the size of the aircraft and hence its weight, electrical energy requirements and cost.

The optimization technique found a feasible design with a span of 5 meters, a mass of 2.5 kg, a flight speed of 4.8 meters per second and a power requirement of 600 watts. The authors constructed a full-scale plan based on this design (see Fig. 1b of the document²). They flew the aircraft ten times and showed that he was flying at a constant level.

In the 1960s, various studies^4,5 seemed to sound the knell of propulsion based on the ionic wind. They demonstrated that only about 1% of the electrical energy supplied was used for propulsion, which is close to the 2.6% reported by Xu and his colleagues. However, at least three factors make the approach attractive to aircraft.

First of all, it is now known that the energy efficiency improves considerably when the speed of the aircraft is increased. For example, if the speed reaches 300 m s^-1, the efficiency^2,6 can reach 50%. Secondly, many studies have shown that ionic wind can improve wing aerodynamics⁷. Third, the technique could facilitate what is called distributed propulsion⁸, which is considered a major focus of aviation improvement.

The propulsion of an airplane is quantified by the mass flow rate of the free flow, ie the total mass of air that passes through a given area at a given moment. This rate is directly proportional to the section of the propulsion system and the increase in air velocity provided by the system. In distributed propulsion, a set of propulsion systems is distributed over the length of the aircraft. This increases the total area of ​​the cross-section and, therefore, the mass flow rate of the free flow. But this also improves aerodynamic drag (the frictional force between the aircraft and the air). Using fine wires as a propulsion system, like Xu et al. This would significantly increase the total area of ​​the section while having virtually no impact on aerodynamic drag.

The evolution of the authors' propulsion system remains to be seen. Can Ionic wind propulsion drive a multi-ton aircraft? This practical question is still open, but predictions suggest that planes such as the Solar Impulse 2 solar aircraft could sustain constant level flight using only ionic wind.⁹. The advantage of wind ionic propulsion systems, as opposed to propellers, is that they can be interfaced directly with the batteries – the energy storage devices of future aircraft – without affecting the conversion rate of energy. In the coming decades, drones or aircraft using the ionic wind could include ionic secondary wind propulsion systems dedicated to energy saving and potentially coupled to solar panels.

These technological developments should lead to a better understanding of the coupled physics of the production of charged molecules and the resulting ionic wind, which is at the heart of these propulsion systems. The force generated by the ion wind is directly proportional to the electrical current flowing in the system.^2,ten. Since this current is highly dependent on the configuration of emitters and collectors, research on the design and optimization of ionic wind propulsion can now begin, thanks to the breakthrough of Xu and his colleagues.