Modeling and Experimental Validation of Underactuated Rotor Dynamics for Swashplate-less UAVs

This study characterizes the dynamics of a novel lag-pitch-coupled underactuated rotor design that can be incorporated into rotary-wing unmanned aerial vehicles (UAVs) to provide pitch and roll control with effectiveness comparable to that of a conventional swashplate albeit with significantly lower mechanical complexity and weight. The concept integrates a single lag hinge tilted at a 45-degree angle located at the center of the rotor hub with independent flap hinges for each of the two blades. This idea relies on the ability to cyclically vary the angular velocity of the rotor in a 1/rev fashion via motor torque modulation, which induces a cyclic lag resulting in a cyclic pitch variation due to the tilted lag hinge (lag-pitch coupling) and causes the tip path plane (TPP) to tilt in a desired direction for pitch and roll control. To understand this concept, simulations using the Rotorcraft Comprehensive Analysis System (RCAS) were performed to capture the 1/rev response in lag, pitch, flap and moments under steady flight conditions. The predictions were validated using hover and wind tunnel experiments. The results show that the maximum lag and pitch response are delayed by 180° relative to the maximum input torque. Additionally, Blade-1 and Blade-2 have different flap responses, which lag the peak pitch response by phase delays of approximately 15° and 40°, respectively. Wind tunnel data indicated an inherent coupling between pitch and roll moments as a function of the advance ratio. Finally, a single main rotor helicopter UAV was built and flight-tested to demonstrate the effectiveness of the concept in both hover and high-speed forward flight.