Heliquad for Mid-Flight Flipping and Full-Attitude Control Under the Complete Actuator Failure

This article contributes to the design and development of a variable-pitch-propeller mechanism and its application on a quadcopter called Heliquad. The Heliquad demonstrates unique capabilities, including mid-flight flipping and full-attitude control, even after the complete failure of an actuator. The input–output relationship is estimated for a generic mechanism. Various singularities and actuator sizing requirements are also analyzed. The mechanism is manufactured, and the validated input–output relationship is implemented in Heliquad's controller. Heliquad is controlled by a unified nonswitching cascaded attitude-rate controller, followed by a unique neural-network-based reconfigurable control allocation to approximate nonlinear relationship between the control input and actuator command. The Heliquad prototype's mid-flight flip experiment validates the controller's tracking performance in upright and inverted conditions. The prototype is then flown in upright conditions with only three of its working actuators. To the best of the authors' knowledge, the cambered-airfoil propeller-equipped Heliquad prototype demonstrates full-attitude control, including yaw rate, on three working actuators for the first time in the literature. Finally, utility of this novel capability is demonstrated by safe recovery and precise landing post-mid-flight actuator failure crisis. Overall, the Heliquad tracks the references well for all the experiments and the neural network-based control allocation output remains bounded throughout.