Ion-Propelled Lifters (a.k.a. "Ioncraft") have revolutionized travel since their introduction in 1934. Ion-propulsion is the latest form of transportation within the civilian sector. Currently the civilian aspects of the craft are being explored. Ioncraft are an electrohydrodynamic (EHD) device to produce thrust in the air, without requiring any combustion or moving parts. The ionocraft is a propulsion device based on ionic air propulsion that works without moving parts, flies silently, uses only electrical energy and is able to lift its own weight plus additional payload, with the future prospect of its power supply. The principle of ionic wind propulsion with corona-generated charged particles has been known as from the earliest days of the discovery of electricity, with references dating back to 1709.
A simple ionocraft derivative, also known as a lifter, can be easily constructed by anyone with a minimal amount of technical knowledge. The model in its simplest form has the shape of an equilateral triangle with sides generally between 10 and 30 cm. They basically consist of three parts, the corona wire (or emitting wire), the air gap (or dielectric fluid), and the foil skirt (collector). The electrical polarities of the emitting and collecting electrodes can be reversed. All of this is usually supported by a lightweight balsawood or other electrically isolating frame so that the corona wire is supported at a fixed distance above the foil skirt, generally at 1 mm per kilovolt. The corona wire and foil should be as close as possible to achieve a saturated corona current condition which results in the highest production of thrust. However the corona wire should not be too close to the foil skirt as it will tend to arc in a spectacular show of tiny lightning bolts which has a twofold effect.
In its basic form, the ionocraft is able to produce forces great enough to lift about a gram of payload per watt, so its use is restricted to a tethered model. Ionocraft capable of payloads in the order of a few grams usually need to be powered by power sources and high voltage converters weighing a few kilograms, so although its simplistic design makes it an excellent way to experiment with this technology, it is unlikely that a fully autonomous ionocraft will be made with the present construction methods. Further study in electrohydrodynamics, however, show that different classes and construction methods of EHD thrusters and hybrid technology (mixture with lighter than air techniques), can achieve much higher payload or thrust-to-power ratios than those achieved with the simple lifter design. Practical limits can be worked out using well defined theory and calculations such as those given on the 'Ionocraft mathematical analysis and design solutions' paper. Thus, a fully autonomous EHD thruster is theoretically possible.
When the ionocraft is turned on, the corona wire becomes charged with high voltage, usually between 20 and 50 kV. The user must be extremely careful not to touch the device at this point, as it can give a nasty shock. At extremely high current, well over the amount usually used for a small model, contact could be fatal. When the corona wire is at approximately 30 kV, it causes the air molecules nearby to become ionised by stripping the electrons away from them. As this happens, the ions are strongly repelled away from the anode but are also strongly attracted towards the collector, causing the majority of the ions to begin accelerating in the direction of the collector. These ions travel at a constant average velocity termed the drift velocity. Such velocity depends on the mean free path between collisions, the external electric field, and on the mass of ions and neutral air molecules.
The fact that the current is carried by a corona discharge (and not a tightly-confined arc) means that the moving particles are diffusely spread out into an expanding ion cloud, and collide frequently with neutral air molecules. It is these collisions that create a net movement. The momentum of the ion cloud is partially imparted onto the neutral air molecules that it collides with, which, being neutral, do not eventually migrate back to the second electrode. Instead they continue to travel in the same direction, creating a neutral wind. As these neutral molecules are ejected from the ionocraft, there are, in agreement with Newton's Third Law of Motion, equal and opposite forces, so the ionocraft moves in the opposite direction with an equal force. There are hundreds of thousands of molecules per second ejected from the device, so the force exerted is comparable to a gentle breeze. Still, this is enough to make a light balsa model lift its own weight. The resulting thrust also depends on other external factors including air pressure and temperature, gas composition, voltage, humidity, and air gap distance.
The air gap is very important for the function of this device. Between the electrodes there is a mass of air, consisting of neutral air molecules, which gets in the way of the moving ions. This air mass is impacted repeatedly by excited particles moving at high drift velocity. This creates resistance, which must be overcome. The barrage of ions will eventually either push the whole mass of air out of the way, or break through to the collector where electrons will be reattached, making it neutral again. The end result of the neutral air caught in the process is to effectively cause an exchange in momentum and thus generate thrust. The heavier and denser the gas, the higher the resulting thrust.
