# Electrohydrodynamic Propeller as One of the Alternatives to Replace Conventional Propulsion Systems
## 1 Introduction  
All conventional types of propelling systems share a common feature--they include rotary components. The rolling and sliding of such components against each other results in certain shortcomings that combine to create multiple negative features. Even the latest progressive machinery technologies are not capable of ensuring the manufacturing of perfectly smooth surfaces that exhibit zero surface roughness or shapes that are perfectly compliant with the designed geometries. Friction causes erosive and abrasive wear of the individual parts of these devices. During friction, a portion of the mechanical energy transforms into heat, with consequent thermal effects on certain zones of the materials that cause a degradation of the mechanical properties of those materials. Rotary components perform oscillations, where the amplitude depends on how perfect the balancing is. However, achieving a perfect static and dynamic balance is technically impossible using the current technologies. The dynamic loading of individual components of devices reduces their service life to a certain number of cycles. Oscillation also contributes to the occurrence of vibration-induced wear-called fretting. Efforts aimed at minimising the presence of rotary parts in various technological devices, as well as maximising the elimination of wear, are currently being made in the related industries. One of those technologies, the application of which has only recently begun, is the Biefeld-Brown effect, which was observed for the first time in 1920 [1]. The principle of the Biefeld-Brown effect is based on the formation of electrically charged particles, which then move from the emitter to the collector with an opposite charge [2,3]. Such a movement of ionised particles causes the formation of thrust. An advantage of electrohydrodynamic propellers is the absolute absence of rotary parts. Unlike conventional propellers, they are comprised of simple and compact structures, consisting of a pair of electrodes and an isolator that determines the distance between them. Moreover, the noise emission is much lower than that emitted from a conventional propeller [4]. The Biefeld-Brown effect may be characterised as an electrohydrodynamic phenomenon based on the formation of electrically charged particles whose momentum is transposedonto the surrounding neutral particles [5; 6]. It describes the force observed on an asymmetrical capacitor when a high voltage is applied to its electrodes [7; 8]. When the electrode clips are exposed to a voltage that is sufficient to form a corona discharge, the charged particles generated around the coronating electrode are accelerated by an electric field towards the electrode with a larger radius (collector), and they then render their momentum to the electrically neutral particles in the surrounding environment-the air [9]. This results in the formation of a stable directed flow of the accelerated ambient medium from the emitter to the collector. The asymmetry of the electric field determines the direction of the preferred movement of the particles of both polarities. The force exerted in the opposite direction on the structure of an asymmetric capacitor is a result of the law of conservation of momentum [10; 11]. The value of the voltage between the electrodes depends on the particular usage, and it can range from several kilovolts to megavolts. An optimal distance between the electrodes is reached when the gradient of the electric potential is right below the value of the breakdown voltage of the air at the current density, which is typically referred to as the corona saturation current density limit [1]. This results in the formation of a high gradient of the electric field around the smaller coronating electrode. Around the electrode, the process of ionisation is running, primarily involving the formation of ions with a polarity that is determined by the polarity of the electrode itself. The maximum achievable velocity of the ions is directly proportional to the mobility of ions in the surrounding gas medium.