|Fig. 1: Typical construction of ionocraft. The corona wire runs in parallel with the collector skirt made of aluminum foil. (Courtesy of Blaze Labs Research. Source: Wikimedia Commons)|
Electrohydrodynamic aircraft, also known as an ionocraft or a lifter, is an emerging aerospace application that uses high voltage to ionize air and generate propulsion.  Ionocraft has a potential to be a new type of aerial vehicle with advantages unseen in conventional technologies such as propeller or jet propulsion. Since its invention in 1959, many ionocraft researches have been focused on demonstrating self-sustained liftoff, but none of those attempts has been successful largely due to the technical difficulty of developing a high-voltage power supply that is light enough to fly.  Recent advances in semiconductor technologies offer a new hope of finally enabling the ionocraft technology.
Ionocraft generates thrust by ionizing the air with a high voltage and propelling ionized air molecules downward. The thrust mechanism consists of a pair of conductors, one with a sharp or pointed edge and the other without, which in conjunction create an ionic wind downward. Those two conductors are separated by an air gap usually a few inches wide. When a high voltage of tens of kilovolts is applied between two conductors, a strong electric field is induced only around the conductor with the sharp edge. Due to the imbalanced electric field, only the air molecules near the sharp-edged conductor are ionized and subsequently propelled toward the conductor of the opposite polarity. As the ionized molecules travel downward, they drag neutral molecules with them, and this macro-scale air movement gives rise to the upward propulsion force on the aircraft. Due to the asymmetric way the ionized air molecules are created and collected, the conductor with a sharp edge is called a corona wire, and the other is called a collector.
Fig. 1 illustrates the typical construction of ionocraft. The aircraft consists of corona wires, collector plates, and structural supports. Just as in many other aerospace applications, reducing the weight of the whole system is of paramount importance. For that reason, when prototyping the aircraft, thin bare copper wires, aluminum foil, and balsa wood are typically used for corona wires, collector plates, and structural supports, respectively.
Since the thrust mechanism requires no combustion or moving parts, ionocraft can potentially bring about a new type of aircraft that has a very low heat and noise profile.  Also, having no moving parts will allow low maintenance need and long service life.  These advantages may find a potential use in military settings where stealthy and reliable operation is of paramount importance. Another potential application is delivery drones in urban environment in which keeping the buzzing noise low is one of the primary issues.  EHD also is suggested to be more energy efficient (thrust per unit power) than contemporary jet engines.  The thrust-to-power ratio of EHD was reported to be as high as 110 N/kW, as compared to a value of approximately 4 N/kW for modern aircraft engines. 
Although it has been more than five decades since the patent of ionocraft has been filed, the self-sustained liftoff of an ionocraft has never been demonstrated so far.  This is largely because ionocraft propulsion needs a voltage near 100 kV in order to have practical thrust density (force per cross-sectional area of the engine), and power supplies that can provide such voltage level have always been too heavy to fly. 
The recent advances in wide-bandgap semiconductor technologies offer a new hope of achieving a practical level of performance and further advancing the ionocraft research. When silicon was the only viable material for power semiconductor devices, high-voltage transistors and diodes had to be made in a large die in order to withstand the high voltage stress. This large die size unavoidably gives rise to large device capacitance and hence limits the switching frequency. Recent improvements made in wide-bandgap materials, namely gallium nitride (GaN) and silicon carbide (SiC), make available high-voltage devices that are small in capacitance, enabling fast switching frequency. Since fast switching frequency generally leads to small inductors and capacitors in the power supply, clever designs that utilize the full potential of those new semiconductor devices are expected to be a key to reducing the weight of the system and realizing the ionocraft application.
© Sanghyeon Park. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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