An ionocraft or ion-propelled aircraft is an aircraft that uses electrohydrodynamics (EHD) to provide lift or thrust in the air without requiring any combustion or moving parts. Current designs do not yet produce enough thrust for manned flight or heavy loads.

History

Origins

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.

VTOL "lifter" experiments

American experimenter Thomas Townsend Brown spent much of his life working on the principle, under the mistaken impression that it was an anti-gravity effect, which he named the Biefeld–Brown effect. Since his devices produced thrust in the direction of the field gradient, regardless of the direction of gravity, and did not work in a vacuum, other workers realized that the effect was due to electrohydrodynamics.^[1][2]

VTOL ion-propelled aircraft are sometimes called "lifters". Early examples were able to lift about a gram of weight per watt,^[3] This was insufficient to lift the heavy high-voltage power supply necessary, which remained on the ground and supplied the craft via long, thin and flexible wires.

The use of EHD propulsion for lift was studied by American aircraft designer Major Alexander Prokofieff de Seversky in the 1950s and 1960s. He filed a patent for an "ionocraft" in 1959.^[4] He built and flew a model VTOL ionocraft capable of sideways manoeuvring by varying the voltages applied in different areas, although the heavy power supply remained external.^[5]

The relatively recent Wingless Electromagnetic Air Vehicle (WEAV) is a saucer-shaped EHD lifter with electrodes embedded throughout its surface, studied by the Florida Center For Advanced Aero-Propulsion (FCAAP) at Florida State University in the early part of the twenty-first century. The propulsion system employed many innovations, including the use of magnetic fields to enhance the ionisation efficiency. A model with an external supply achieved minimal lift-off and hover.^[6][7]

Onboard power

Modern developments allow much lighter power supplies and more efficient systems. Practical limits can be worked out using well defined theory and calculations.^[8][9] The first ion propelled aircraft to take off and fly using its own onboard power supply was a VTOL ion propelled aircraft developed by Ethan Krauss of Electron Air. His patent application was initially filed in 2014.^[10] The craft develops enough thrust to rise rapidly or to fly horizontally for several minutes.^[11][12]

In November 2018 the first self-contained ion-propelled fixed-wing airplane, the MIT EAD Airframe Version 2 was flown over a distance of 60 meters by a team of students lead by Dr. Steven Barrett from the Massachusetts Institute of Technology. It had a 5 meter wingspan and weighed 2.45 kg.^[13] The craft did not take off under its own power but 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 and they accelerate backwards to the collector, producing thrust in reaction. During their travel, these ions collide with neutral air molecules and accelerate them in turn, greatly increasing the efficiency of the thruster.

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 to match. Thrust will be produced in the same direction, either way. For positive emitter 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.^{[citation needed]}

EHD thrusters are currently far less efficient than conventional aero engines.^[14]

Unlike pure ion thruster rockets, the electrohydrodynamic principle does not apply in the vacuum of empty space.^[15]

Electrohydrodynamics

The thrust generated by an electrohydrodynamics (EHD) device is an example of the Biefeld-Brown effect and can be derived through a modified use of the Child-Langmuir equation.^[16] A generalized one-dimensional treatment gives the equation:

${\displaystyle F={\frac {Id}{k}}}$

where

• F is the resulting force.
• I is the electric current flow of electric.
• d is the air gap.
• k is the ion mobility coefficient of the working fluid,^[17] measured in amp-sec²/kg in SI units. (The nominal value for air is 2·10⁻⁴ m² V⁻¹ s⁻¹).^{[citation needed]}

The principle applied to a gas such as air is also referred to as electroaerodynamics (EAD).

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 mass in the gap between the electrodes is impacted repeatedly by excited particles moving at high drift velocity. This creates electrical resistance, which must be overcome. 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 air, the higher the resulting thrust.

Aircraft configuration

As with conventional reaction thrust, EAD thrust may be directed either horizontally to power a fixed-wing airplane or vertically to support a powered lift craft, sometimes referred to as a "lifter".

Design

The thrust generating components of an ion propulsion system consist of three parts; a corona or emitter wire, an air gap, and a collector wire or strip downstream from the emitter. A lightweight insulating frame supports the arrangement. The emitter and collector should be as close as possible, i.e. with a narrow air gap, to achieve a saturated corona current condition which results in the highest production of thrust. However the emitter should not be too close to the collector or it will tend to arc across the gap.^{[citation needed]}

Ion propulsion systems require many safety precautions due to the high voltage required for their operation.

Emitter

The emitter wire is typically 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 44 or 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.^{[citation needed]}

The emitter is sometimes referred to as the "corona wire" because of its tendency to emit a purple corona discharge glow while in use.^{[citation needed]} This is simply a side effect of ionization.

Air gap

The air gap provides insulation between the two electrodes and allows the ions generated at the emitter to accelerate and transfer momentum to neutral air molecules, before being stripped of their charge at the collector. The width of the air gap is typically 1 mm / kV.^{[citation needed]}

Collector

The collector is shaped to provide 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. 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.^{[citation needed]}

Pseudoscientific theories

Electrogravitics is a false physical theory proposed by Thomas Townsend Brown, purporting to link electric field effects with anti-gravity. Although long disproved, the theory has remained associated with the idea of ionocraft lifters and has become part of UFO folklore.

See also

• Hall effect thruster
• Ion thruster
• Magnetoplasmadynamic thruster
• Plasma actuator