Small, lightweight devices called lifters can propel themselves into the air without combustion or moving parts, and have become a popular topic of discussion with technology buffs on social media in the past few years. And yet the physical mechanism behind lifters has been known for more than 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 ones, they transfer part of their momentum, leading to air movement known as an ionic wind. On page 532, Xu et al.2 demonstrate that an aeroplane with a 5-metre wingspan can sustain steady-level flight using ionic-wind propulsion. Improvements are required, but the authors’ proof-of-concept demonstration could pave the way for the development of enhanced propulsion systems.
In Xu and colleagues’ plane, an electric field is applied to the region that surrounds a fine wire called the emitter (Fig. 1a). The field is strong enough to induce a chain reaction: free electrons in the region collide heavily enough with 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 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 (Fig. 1b). This process occurs only in gases, and not in liquids, justifying the authors’ use of the term ‘electro-aerodynamics’.
Previous experiments suggested that ionic-wind propulsion could enable the steady-level flight of an aircraft, but that the feasibility of achieving this lies at the limit of what is currently technologically possible3. Xu et al. therefore needed to systematically search through all of the possible aeroplane designs for a feasible option. They used a technique called geometric programming to find the optimum set of design variables that would also minimize the aircraft’s wingspan and, in turn, its weight, electrical-power requirements and cost.
The optimization technique found a feasible design at a wingspan of 5 metres, with a mass of 2.5 kilograms, a flight velocity of 4.8 metres per second and a power requirement of 600 watts. The authors built a full-scale plane based on this design (see Fig. 1b of the paper2). They flew the aircraft ten times, and showed that it achieved steady-level flight.
In the 1960s, various studies4,5 seemed to sound the death knell for propulsion based on ionic wind. They demonstrated that only about 1% of the input electrical energy was used in propulsion — not far from the 2.6% reported by Xu and colleagues. However, at least three factors make the approach appealing for aircraft.
First, it is now known that the energy efficiency improves substantially when the aircraft velocity is increased. For example, if the velocity reaches 300 m s–1, the efficiency2,6 can be as high as 50%. Second, many studies have shown that ionic wind can enhance the aerodynamics of plane wings7. Third, the technique could facilitate what is known as distributed propulsion8, which is considered a major direction for improvement in aviation.
Aircraft propulsion is quantified by the freestream mass-flow rate — the total mass of air that passes through a given area in a given time. This rate is directly proportional to the cross-sectional area of the propulsion system, and to the increase in air velocity provided by the system. In distributed propulsion, an array of propulsion systems is spread along the length of the aircraft. This increases the total cross-sectional area and, in turn, the freestream mass-flow rate. But it also enhances the aerodynamic drag (the frictional force between the aircraft and the air). Using fine wires as the propulsion system, as Xu et al. did, could allow the total cross-sectional area to be greatly increased, while having almost no impact on the aerodynamic drag.
The scalability of the authors’ propulsion system remains to be seen. Can ionic-wind propulsion fly an aircraft of several tonnes? This practical issue is still open, but predictions suggest that aircraft such as the solar-powered plane Solar Impulse 2 could sustain steady-level flight using only ionic wind9. An advantage of ionic-wind propulsion systems, as opposed to propellers, is that they can be interfaced directly with batteries — the energy-storage devices of future planes — without affecting the rate of energy conversion. In the decades to come, drones or aircraft that use ionic wind might include secondary ionic-wind propulsion systems dedicated to energy saving and potentially coupled with solar panels.
These technological developments should provide a better understanding of the coupled physics of charged-molecule production and the resulting ionic wind that is central to such propulsion systems. The force generated by ionic wind is directly proportional to the electric current that flows in the system2,10. Because this current is strongly dependent on the configuration of emitters and collectors, research into the conception and optimization of ionic-wind propulsion can now begin, thanks to the breakthrough by Xu and colleagues.
Nature 563, 476-477 (2018)