ion-propelled fixed-wing airplane, the MIT EAD Airframe Version 2 flew 60 meters. It was developed by a team of students led by Steven Barrett from the Massachusetts Institute of Technology. It had a 5-meter wingspan and weighed 2.45 kg. The craft was catapult-launched using an elastic band, with the EAD system sustaining the aircraft in flight at low level. Principles of operation Ionic air propulsion is a technique for creating a flow of air through electrical energy, without any moving parts. Because of this it is sometimes described as a "solid-state" drive. It is based on the principle of electrohydrodynamics. In its basic form, it consists of two parallel conductive electrodes, a leading emitter wire and a downstream collector. When such an arrangement is powered by high voltage (in the range of kilovolts per mm), the emitter ionizes molecules in the air that accelerate backwards to the collector, producing thrust in reaction. Along the way, these ions collide with electrically neutral air molecules and accelerate them in turn. The effect is not directly dependent on electrical polarity, as the ions may be positively or negatively charged. Reversing the polarity of the electrodes does not alter the direction of motion, as it also reverses the polarity of the ions carrying charge. Thrust is produced in the same direction, either way. For positive corona, nitrogen ions are created initially, while for negative polarity, oxygen ions are the major primary ions. Both these types of ion immediately attract a variety of air molecules to create molecular cluster-ions of either sign, which act as charge carriers. Current EHD thrusters are far less efficient than conventional engines. Unlike pure ion thruster rockets, the electrohydrodynamic principle does not apply in the vacuum of space. Electrohydrodynamics The thrust generated by an EHD device is an example of the Biefeld–Brown effect and can be derived through a modified use of the Child–Langmuir equation. A generalized one-dimensional treatment gives the equation: where F is the resulting force. I is the electric current. d is the air gap. k is the ion mobility of the working fluid, measured in A s2 kg−1 in SI units, but more commonly described in units of m2 V−1 s−1. A typical value for air at surface pressure and temperature is 1.5×10−4 m2 V−1 s−1). As applied to a gas such as air, the principle is also referred to as electroaerodynamics (EAD). When the ionocraft