Rotor aerodynamic interaction effects for multirotor unmanned aircraft systems in forward flight

<br>Multi-rotor Unmanned Aircraft Systems (MUAS), also known as “drones”, are an emerging technology that have been applied to a growing number of civil and defence applications. Although MUAS use several closely spaced rotors to provide thrust and attitude control, the dynamic and aerodynamic models used to develop these systems commonly consider each rotor to be operating in isolation. Existing studies of conventional helicopters have identified that aerodynamic interactions between closely spaced rotors can have negative impacts on aircraft stability and rotor performance. However, the impacts of these interactions on small-scale rotorcraft operating at low Reynolds numbers in forward flight are not well understood. <br> <br>This thesis presents the results of an experimental quantification of the effects of wake interference on individual rotors within quadrotor configurations in steady state trimmed forward flight conditions. The extent to which these effects could be mitigated with changes to rotor separation distances were explored. Each experiment utilised fixed pitch 9.4” diameter rotors, which provided measurements that are applicable to a range of commercially available MUAS aircraft. <br> <br>An Isolated rotor Wind Tunnel System (IWTS) was developed to obtain rotor load measurements for a range of advance ratios and pitch angles between edgewise and axial flow conditions. An extensive database of rotor force and moment measurements was produced, which provided a baseline for comparison to rotors within MUAS configurations. The rotor loads were found to be highly sensitive to Reynolds number, and were contrasted to the limited available results for similar rotor systems in existing literature. Limitations of existing analytical rotor models were highlighted, and suggestions were made for potential improvements. <br> <br>A novel sting-mounted Quadrotor Wind Tunnel System (QWTS) was developed that provided the capability to adjust the longitudinal (x/R) and lateral (y/R) rotor tip separation well as aircraft pitch angle to represent a range of rotor configurations and forward flight conditions. The rotors were differentially controlled to achieve trimmed conditions, and airframe-induced loads were eliminated to provide a universal rotor configuration analysis. Individual and total rotor loads were captured using integrated load cells, and wake interference factors were developed by comparing to equivalent isolated rotor measurements. To the author’s knowledge, this was the first parametric experimental study of changes in wake interference effects with rotor geometry in trimmed forward flight conditions. <br> <br>The flow induced by the front rotors primarily manifested as reductions in rear rotor thrust coefficient, C_T,rear, and pitching moment coefficients, C_My,rear. As a consequence, increased rear rotor speeds were required for trimmed conditions. C_T losses were most severe at x/R=0 and decayed exponentially as x/R increased, with the amount of mitigation being dependent on flight speed and pitch angle. Losses became proportionally more severe at conditions representing low speed forward flight for systems with low disk loading. In comparison, the power coefficient, C_P for each rotor demonstrated less sensitivity to mutual interference effects or changes in x/R. Aside from limited positive interference effects at low separation distances, the front rotor was found to perform similarly to an isolated rotor for most conditions. The culmination of these effects typically resulted in a proportional increase in rotor propulsive efficiency as x/R increased. The interactional flow mechanisms contributing to changes in rotor performance were investigated using experimental flow visualisation and analytical wake models. Similar performance trends were observed using analytical predictions, albeit with less severe losses at close separation distances. Staggering the rotors laterally demonstrated that relative propagation of the advancing and retreating blades of the front and rear rotors altered the magnitude of wake-induced effects on rear rotor thrust, pitching moment, and rolling moment coefficients. <br> <br>A flying quadrotor system with adjustable geometry and integrated sensors called the Adjustaquad was developed to further characterise trim conditions and rotor performance metrics in forward flight. Measurements obtained in flight combined with an interpolation of airframe-induced loads provided a comparison to outputs from QWTS using contrasting experimental constraints. This demonstrated that QWTS measurements could be applied to flying systems within practical limitations, and both systems displayed similar trimmed rotor speeds and power interference factors. The measurements obtained using the Adjustaquad were also used to explore the bounds of validity for analytical performance prediction models. <br> <br>The outcomes of these studies quantified rotor trim conditions and the limits to which wake interference effects could be mitigated by altering rotor configurations. These factors were used to provide suggestions for MUAS design to improve propulsive efficiency within a range of forward flight conditions. <br>