High Fidelity Aerodynamic Force Estimation for Multirotor Crafts in Free Flight

Abstract

Aerodynamic investigation of multirotor crafts under non-hover conditions plays a vital role in the development of efficient and stable systems. Non-hover conditions include any flight scenario where the craft is no longer subject to to an infinite, stationary and undisturbed fluid. Examples of non-hover conditions are forward-flight, axial descent, and flying under ground effects. This paper presents the experimental framework for estimating multirotor free-flight forces. The presented method does not require rigid mounting of the craft to a load cell. Instead, the forces on the craft are estimated using two complementary methods-- one based on rigid body dynamics obtained by motion capture measurements and the other inferred from rotor rotational speed measurements. By considering these two independent estimates, with the dynamically-derived forces serving as a continuous reference, we are able to evaluate relative rotor performance, even under conditions where net forces on the craft are fluctuating. In order to map rotor rotational rates to rotor forces, we developed a closed-form expression which incorporates Reynolds number and rotor-rotor interaction effects. Using our free-flight force estimation technique, we accurately estimated rotor performance during various vertical flight conditions (i.e. ascent, descent, and hover with and without ground effects) with a high degree of accuracy.