First thrust measurements in ionic multi-propeller rotational engines

Rotary ionic engines (RIEs) with multi-coaxial contra-rotating propellers (12.6 cm diameter) and their axial thrust are investigated in laboratory conditions, in air at atmospheric pressure, for the first time. The goal is to evaluate more advantageous configurations that may be scaled up and further help with development of rotary ionic drones. The propellers are designed with regular pin-emitter electrodes placed coaxially inside a cylindrical collector electrode. When high voltage is applied, propellers spin generating conventional axial thrust which is measured with an electronic scale in a “see-saw” setup. Up to 40 mN thrust was obtained in single propeller RIE at 2600 rpm, 0.34 mA, 37.5 kV, and an optimal collector electrode diameter of 17 cm. More thrust can be obtained with two and three propellers at constant current per propeller but at a decreased efficiency. Varying the axial propeller-separation showed that propeller-interaction is minimal above 5 cm. Thrust-to-power and thrust-to-current ratios were calculated and compared. We experimentally confirm here for the first time that within certain limits, the thrust-to-power variation can be assessed by the propeller kinetic energy-to-power ratio and also by the impedance of the gap (voltage-to-current ratio). A comparison of RIE arrays performance with one, two, and three coaxial propellers and the same total number of propellers per array (six) is also performed. RIE arrays with multi-coaxial propellers can provide larger thrust densities than single-propeller arrays. Also, arrays employing two coaxial propeller unit may be more weight effective.