# Electrohydrodynamic Propeller as One of the Alternatives to Replace Conventional Propulsion Systems
## 1 Introduction
A related negative effect is humidity, which significantly reduces the mobility of ions in the air. Humidity also reduces the breakdown voltage [11; 12]. Conversely, the mobility of ions in the air increases as the air temperature increases. Due to its polar nature, a water molecule can form bridges between the coronating electrode and the collector, which results in an electrical breakdown [13; 14]. The Biefeld-Brown effect occurs when electrodes of both polarities are used. However, experimental observations in the majority of studies have indicated that the thrust is, in fact, a little stronger when the coronating electrode has a positive charge. This is attributable to a difference between the ionisation energy and the energy of the electron affinity of the air components. The mobility of anions (O\({}^{2-}\)) in the air is also lower than the mobility of cations (N\({}^{2+}\)). Another factor is a difference in the mechanisms of action of a positive corona and a negative corona [15]. A significant decrease in the momentum of the system occurs when the pressure of the ambient medium decreases. In this case, the number of air molecules around the coronating electrode decreases, and hence, the number of ionised particles also proportionally decreases. Furthermore, the number of collisions between the ionised and the neutral particles decreases. A decrease in the magnitude of the propulsion force represents another consequence of a decreasing breakdown voltage of the air, which causes a decrease in the intensity of electrostatic interactions [10]. At very low-pressure values, detectable results were only obtained in experiments in which the electrode clips were exposed to a very high voltage as a result of a higher probability of ionisation of an extremely limited number of available molecules of residual air. As a result, a cloud of ions remains in the environment, and these ions are pulled by electrostatic forces towards the collector with an opposite charge, where they are neutralised again [10; 16]. At an absolute ambient pressure lower than 1 Pa, systems utilising the principle of extremely increasing the voltage would generate dangerous ionising radiation-or cathode rays [17]. In a hypothetic perfect vacuum, the resulting propulsion force would be zero due to a zero volumetric density of the electric charge [2]. Under the quantum theory of fields, a vacuum does not mean an empty space. The theory considers a vacuum to be a polarisable and magnetisable medium, containing momentarily existing pairs of virtual particles and antiparticles. Those momentarily existing pairs represent the so-called vacuum bubbles. An example of such a pair is a virtual electron-positron pair. Pairs with such a charge function as an electric dipole. In the presence of an electric field, these particle-antiparticle pairs relocate and, hence, partially act against the field. Such a change is referred to as a vacuum polarisation. In the early stages of the development of the relevant technology, the thrust generated by an EHD propeller produced only a negligible amount of kinetic energy relative to the weight of the whole equipment [18]. Therefore, engines that used a current comprised of ionised particles to produce kinetic energy were only used in the field of space exploration. On the Earth's surface, this effect has traditionally only been used for the propulsion of pumps; however, it has recently been increasingly used in the field of accurate printing and food drying. In the field of microbiology, a technology based on electric osmosis has increasingly been used for the dehydration of salt solutions. When combined with electrophoresis, facilitating the separation of ions, this technology may also be widely used in the field of gene engineering [19]. At present, this phenomenon is increasingly being used in the propulsion of unmanned aerial vehicles (UAVs). Compared to conventional propellers, UAVs produce much lower noise emissions. The military industry is particularly interested in the potential of this technology [4].