Lifter
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A lifter is an ion propelled aircraft that produces thrust in air using electrical energy without moving parts.
Contents |
Function
There are 3 forces at work:
- Ion wind (plasma jets from corona wire)
- Electrohydrodynamic
- Radiation pressure
Ion wind is very weak because the mass of the electron is very low. Electrohydrodynamic thrust is significant since it incorporates a charged cloud of particles moving through a gas or liquid. Radiation pressure is extremely weak and is the only force of these that is present in vacuum, it is too weak to have any use for this application.
The small diameter of the corona wire will cause the gradient in the electric field to be very steep. Air molecules close to the wire will become charged as they lose or gain electrons in the field. Charged molecules are attracted to the opposite charge which in this case is the skirt of the lifter. It does not matter if the corona wire is positive or negative it will still work but it works best when the corona wire is positive.
Newton's third law states that "For every action there is an equal and opposite reaction." so if the lifter drags the air down, the air will drag the lifter up. When the force is large enough to overcome the pull of gravity the lifter will become airborne.
Calculations
Consider a small ionocraft/lifter powered at 30 kV, consuming 3.33 mA (100 W), having an air gap of 3 cm.
Ion wind calculation
The speed of single charged particle with mass m and charge e accelerated between two electrodes with voltage V is:
- v = sqrt(2 * V * e / m)
Momentum of single particle is
- p = m * v = m * sqrt(2 * V * e / m)
To calculate the number of particles flying at given power per second, we divide total passed charge by the charge of single particle
- n = Q / (e * t) = i / e
current i we obtain from power P as:
- i = P / V so n = P / (V * e)
Total force applied is equal to total exchanged momentum per unit time (assuming the ideal case where all momentum of accelerated particle is used for propulsion).
As result we have:
- F = n * p = m * sqrt(2 * V * e / m) * P / (e * V)
If you put:
- m = me = 9.1093897E-31 kg (electron mass)
that gives an ion wind force of about
- 1.95E-6 N at P=100 W, V = 30 kV.
This is the kind of thrust one would get from an ionic thruster.
Electrohydrodynamical calculation
On the other hand, the EHD thrust is given by the well established equation:
- F = id / k (Refer to Sigmond's scientific papers)
k = ion mobility coefficient (in air = 2E-4), d = air gap, i = current
- F = 3.33E-3*0.03/2E-4 = 0.5 N
which is what one actually gets from an lifter tested in air. If one wants to find the EHD thrust in oil, just replace k, by the ion mobility in oil.
Conclusion
The above calculations show the difference between an ionic thruster and an EHD thruster. The lack of knowledge and of a precise definition of ion wind, has led many people (including early tests from army research labs, universities,...) to arrive at the wrong conclusion that the measured thrust is too high to be ion wind.
Practical considerations
From some experiments done in 2002:
- The closer the wire is to the foil, the larger the thrust.
- The further away the wire is, the higher the efficiency.
- Thinner wire gives more lift.
- The height of the foil skirt is not important, replacing it with a wire thicker than the corona wire made no difference to the thrust. Possibly reduced the efficiency.
The holy grail of lifters is a self contained lifter that carry its own energy.
How to make
Safety
- The power supply may be dangerous, see electrical safety.
- Corona discharges produces ozone which is irritating and can cause strong headaches and nausea.
