Dynamic response of BLDC-thruster for small scale Quadrotors under aerodynamic load torque

In multi-rotor UAVs, a controlled Thrust is a main factor to achieve a safe and stable flight. Thrust is the result of a coordinated action of a set of symmetrically distributed thrusters whose single contribution is usually described as a square function of its rotor speed; however, aerodynamic forces induce a load torque on the rotor that disturb its expected speed response, thus its effective Thrust, affecting the whole performance of the aircraft. In this paper, we analyse and quantify the effects of the aerodynamic load torque induced on the spinning propeller of a BLDC thruster used in small scale Quadrotors and present some experimental validation. We describe the model of a 3-phase thruster with a two blade propeller of fixed pitch angle, we analyse its numerical simulation to determine the effects of the induced load torque on the dynamic response of the rotor, and we experimentally verify the results assuming ideal conditions of hover flight. Finally we illustrate the convenience of using feedback control to reduce the sensibility of thrusters face to non modelled dynamics applying a PD controller and we discuss about the importance of an accurate dynamic model of the thruster for control purposes.