Numerical-experimental evaluation and modelling of aerodynamic ground effect for small-scale tilted propellers at low Reynolds numbers

In recent years, aerial manipulators with fully-actuated capabilities are gaining popularity for being used in aerial manipulation operations such as critical infrastructure inspection or aerial manipulation tasks. Those scenarios usually demand the aerial platform to operate in constrained and narrow scenarios. It is well known that in these situations, the interaction of the wake generated by the propellers with the environment can significantly alter and change the performance of the rotors. Most studies have addressed this problem by considering the ground effect in hover conditions or during the landing maneuver for co-planar multirotor. However, few works analyze the behaviour of tilted rotors, which are used in fully actuated multirotor configurations thanks to their omnidirectional motion capabilities. This paper presents a numerical-experimental evaluation of the aerodynamic ground effect for small-scale tilted propellers at low Reynolds numbers. This aerodynamic effect has been experimentally evaluated through an extensive testing campaign in a testbench designed for this purpose which has been complemented by a CFD-based study. CFD results have been validated through a mesh independence study and a CFD-experimental propeller performance comparison. A numerical model has been also proposed to capture the dependence of thrust with distance to the ground and angle of inclination between the propeller and ground planes. We demonstrate that the proximity to the ground of tilted rotors decreases the thrust increment due to the ground effect as the tilt angle (theta) increases. This means that Cheeseman's classical theory is inapplicable, as it only considers the distance from the ground without reference to how the thrust increment changes with the tilt angle. This outcome enables future aerial robotic applications that strongly demand accurate aerodynamic effect models to operate close to obstacles and narrow environments.(C) 2022 The Author(s). Published by Elsevier Masson SAS.