Last year, researchers at the Massachusetts Institute of Technology (MIT) made the first sustained flight of a heavier than air aircraft with no moving parts. It was propelled by electroaerodynamic propulsion which requires no moving parts and was, by many definitions, the first ever solid state flight. This article explains exactly how it works and what the potential uses for the technology might be.
The initial test aircraft weighs 2.5 kg and has a wingspan of 5 m. The fuselage contains lithium-polymer batteries and a specially designed power supply that increases the voltage to the required 40,000 volts. The initial flight was just 60 m over 10 seconds, although this was limited by the gym in which the test was carried out. In total, 10 test flights were carried out, and a bungee catapult system was required to launch the aircraft. Once in flight, the solid state propulsion was able to produce sufficient thrust to sustain the flight. A simple photogrammetry system was created to measure the distance of powered and unpowered flights; this demonstrated the effectiveness of the propulsion system. Three GoPro cameras, positioned perpendicular to the flight path, were used to track a red and a green LED light attached to two points on the aircraft. This modest demonstrator may be the first step towards silent drones and more efficient passenger aircraft.
Propelling an Aircraft with Ionic Wind
The aircraft’s solid state propulsion uses electroaerodynamic thrust. The basic principle is to ionize air and then use electric fields to accelerate this ionized air. The researchers decided to start with something that looks much like a conventional airplane but replace the engines with the ionic wind propulsion system. This consists of series of electrodes, with thin wires at the front and an airfoil behind each wire. The thin wires are held at a high voltage of +20,000 V which causes nitrogen in the air to ionize around the wire – the positive charge literally strips away the negatively charged electrons leaving positively charged molecules. The airfoils behind the wires are set at -20,000 V, this negative charge attracts the ions. As the ions flow towards the airfoils, each ion collides with millions of other air molecules, creating a flow of air known as an ionic wind.
“The idea dates back to at least the 1920’s where an eccentric inventor at the time started experimenting with high voltage electrodes and thought he had discovered antigravity, which of course was not the case, but that set some of the initial ground work on mechanisms for creating what’s called an ionic wind.” Steven Barrett, associate professor of aeronautics and astronautics at MIT
The principle is also known as a corona discharge, where a sustained high electric potential difference between two electrodes causes a self-sustaining atmospheric discharge. As electrons are accelerated, they produce a cascade of ionization due to electrons colliding with neutral molecules.
What is really significant about this demonstration is that it has proven that it is possible for electroaerodynamic thrust to achieve significantly greater thrust-to-power ratio and thrust density than had been previously thought. As recently as 2009 an Investigation of Ionic Wind Propulsion was published by NASA which suggested it would to be practical to achieve this type of flight.
Potential applications
Electroaerodynamic thrust will not be able to replace more conventional propulsion systems for high thrust applications. There are some fundamental limits such as the breakdown voltage of air, which varies with altitude. The theoretical limit to the thrust density, the amount of thrust per unit area, suggests that this method is better suited to small aircraft such as drones. It is however, expected that it will be possible to improve the thrust density to the point where the electrodes can be integrated into the normal aerodynamic surfaces of the wings. This could result in an airplane that has no visible propulsion or any moveable control surfaces.
An immediate advantage is noise. In the near future we are likely to have large numbers of drones in urban spaces carrying out functions such as monitoring traffic and air pollution. If they are all producing similar noise levels to current drones this will create significant noise pollution. Ion wind propulsion offers the possibility of virtually silent drones. Because the propulsion system is solid state, it will also enable a much greater degree of miniaturization. Silent and highly miniaturized drones would clearly also have military applications.
Previous laboratory experiments have shown that the thrust-to-power ratio for electroaerodynamic thrust can be much higher than jet engines and at a level equivalent to helicopter rotors. This suggests that in the longer term, it may be used as a highly efficient cruise propulsion for larger hybrid aircraft that also use more conventional propulsion for takeoff. However, this may be limited by the known trade-off between thrust-to-power ratio and thrust density.
A typical single aisle aircraft requires approximately 30 kN thrust for cruise and has a wing area of 125 m2. It had previously been shown that a thrust density of 3 N m−2 and a thrust-to-power ratio of 6.25 N kW−1 could be simultaneously achieved. This was using a two-staged configuration with four sets of parallel electrodes. This level of trade off would mean the electrodes would need to occupy an area 80 times the area of the wings, which is clearly not feasible.
Using electroaerodynamic thrust to create ionic winds is an interesting and novel way to propel an aircraft. It has some practical application for small and silent drones used for surveillance and environmental monitoring. However, fundamental limitations suggest this is unlikely to be suitable for larger aircraft.