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An ionocraft or ion-propelled aircraft (commonly known as a lifter or hexalifter) is a device that uses an electrical electrohydrodynamic (EHD) phenomenon to produce thrust in the air without requiring any combustion or moving parts.
The term "ionocraft" dates back to the 1960s, an era in which EHD experiments were at their peak. In its basic form, it simply consists of two parallel conductive electrodes, one in the form of a fine wire and another which may be formed out of wire grid, tubes or foil skirts with a smooth round surface. When such an arrangement is powered by high voltage (in the range of a few kilovolts), it produces thrust. The ionocraft forms part of the EHD thruster family, but is a special case in which the ionisation and accelerating stages are combined into a single stage.
The device is a popular science fair project for students. It is also popular among anti-gravity or so-called "electrogravitics" proponents, due to the research of Thomas Townsend Brown, who built these devices in the 1920s and incorrectly believed that he had found a way to modify gravity using electric fields.
The term "lifter" is an accurate description because it is not an anti-gravity device; rather, it produces lift using the same basic principle as a rocket, i.e. from the equal but opposite force upward generated by the driving force downward, specifically by driving the ionized air downward in the case of the ionocraft. Much like a rocket or a jet engine (it can actually be much more thrust efficient than a jet engine), the force that an ionocraft generates is consistently oriented along its own axis, regardless of the surrounding gravitational field. Claims of the device also working in a vacuum have been disproved.
Ionocraft require many safety precautions due to the high voltage required for their operation; nevertheless, a large subculture has grown up around this simple EHD thrusting device and its physics are now known to a much better extent.
An ionocraft is a propulsion device based on ionic air propulsion that works without moving parts, uses only electrical energy, and is able to lift its own weight, not including its own power supply. The principle of ionic wind propulsion with corona-generated charged particles has been known from the earliest days of the discovery of electricity with references dating back to 1709 in a book titled Physico-Mechanical Experiments on Various Subjects by Francis Hauksbee. Its use for propulsion was given serious thought by Major Alexander Prokofieff de Seversky who contributed much to its basic physics and construction variations in 1960. In fact, it was Major de Seversky himself who in 1964 coined[non-primary source needed] the term Ionocraft in his (U.S. Patent 3,130,945). There are also designs by the American experimenter Thomas Townsend Brown, such as his 1960 patents for "Elektrokinetic Apparatus". Brown spent most of his life trying to develop what he thought was an anti-gravity effect, which he named the Biefeld–Brown effect. Since Brown's devices produce thrust along their axis regardless of the direction of gravity and do not work in a vacuum, the effect he identified has been attributed to electrohydrodynamics instead of anti-gravity.
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 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 kilo-volt. 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:
- It degrades the thrust as it is shorting the device and there is current flow through the arc instead of the ions that do the lifting
- It can destroy the power supply or burn the balsa structure of the lifter.
The corona wire is usually, but not necessarily, connected to the positive terminal of the high voltage power supply. In general, it is made from a small gauge bare conductive wire. While copper wire can be used, it does not work quite as well as stainless steel. Similarly, thinner wire such as 50 gauge tends to work well compared to more common, larger sizes such as 30 gauge, as the stronger electric field around the smaller diameter wire results in better ionisation and a larger corona current. 44 gauge stainless steel wire is also known to work very well and has been used with great success by hobbyists. Due to anomalies in construction adding supports made from 3D printed staggered mesh (to prevent surface arc-over) from SLS in resin can work if atmospheric humidity is low enough.
The corona wire is so called because of its tendency to emit a purple corona-like glow while in use. This is simply a side effect of ionization. Excessive corona is to be avoided, as too much means the electrodes are dangerously close and may arc at any moment, not to mention the associated health hazards due to excess inhalation of ozone and NOx produced by the corona.
The air gap is simply that, a gap of free flowing air between the two electrodes that make up the structure of an ionocraft.
The collector may take various shapes, as long as it results in a smooth equipotential surface underneath the corona wire. Variations of this include a wire mesh, parallel conductive tubes, or a foil skirt with a smooth round edge. The foil skirt collector is the most popular for small models, and is usually, but not necessarily, connected to the negative side of the power supply. It is usually conveniently made from cheap, lightweight aluminum foil.
The foil skirt is named simply because it is shaped much like a skirt, and is made from aluminum foil. It is by far the most fragile part, and must not be crumpled to work properly. Any sharp edges on the skirt will degrade the performance of the thruster, as this will generate ions of opposite polarity to those within the thrust mechanism.
Reversing the polarities of the corona wire with that of the foil does not alter the direction of motion. Thrust will be produced regardless of whether the ions are positive or negative. For positive corona polarity, nitrogen ions are the main charge carriers, whilst for negative polarity, oxygen ions will be the main carriers and ozone production will be higher. The slight difference in their ion mobility, results in slightly higher thrust for the positive corona polarity case.
Use of conductive aquadag paint (eg carbon) has been explored but invariably it deteriorates in use unless supplemented by conductive foil underneath. The problem appears to be sputtering and further research is underway to determine if this can be resolved.
The generated thrust can be explained in terms of electrokinetics or, in modern terms, electrohydrodynamics propulsion and can be derived through a modified use of the Child-Langmuir equation. A generalized one-dimensional treatment gives the equation:
- F is the resulting force, measured in dimensional units M L T−2. (In SI units, this is measured in Newtons.)
- I is the current flow of electric current, measured in dimension I. (Amperes in SI units.)
- d is the air gap distance, measured in dimension L (meters in SI units)
- k is the ion mobility coefficient of air, measured in dimension M−1 T2 I (amp-sec2/kg in SI units). (Nominal value for air is 2·10−4 m2 V−1 s−1).
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., 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.
Recent research suggests electrohydrodynamic propulsion is more energy efficient (thrust per unit power) than other means of propulsion, generating up to 100N of thrust per kilowatt of power, compared to 2 N/kW for jet engines. This is mainly due to the much lower air speed of an ionocraft vs a jet engine, as power requirement per unit mass of payload drops with air velocity. However this also means the ionocraft needs a much wider surface area to lift the same payload.
A variant may enable for the first time a self-contained unit that uses a working gas; this is the subject of a patent application but is believed to be similar to ESA's air-breathing ion engine with the notable exception of size. In this case a specific combination of electrodes with very low work function enables radically higher efficiency and as such can be recharged easily with a simple arrangement of gas canisters.
- Hall effect thruster
- Ion thruster
- Magnetoplasmadynamic thruster
- Plasma actuator
- Wingless Electromagnetic Air Vehicle
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|Wikimedia Commons has media related to Ion driven air thrusters.|
- Electrostatic Antigravity on NASA's "Common Errors in propulsion" page
- NASA: Asymmetrical Capacitors for Propulsion
- video on lifter construction