Scaling of Hovering Rotorcraft Aerodynamics in Hyperbaric Experimental Conditions

This paper analyzes the ability to experimentally measure the aerodynamic performance of rotorcraft blades using small-scale models. If the model scale is reduced significantly enough relative to the full scale vehicle rotor, the nature of the aerodynamic phenomena involved changes drastically, which can significantly alter the overall rotor performance. This stems from the inherent dependence of airfoil performance on Reynolds number, and the inability to keep both Reynolds number and Mach number constant for a scaled rotor under normal atmospheric conditions. By conducting experiments in a hyperbaric chamber, density can be controlled as an additional experimental parameter allowing for better replication of full-scale operating conditions and aerodynamic behavior. Experiments can therefore be conducted using smaller models, better facilitating low-cost, high-fidelity measurements of the complex aerodynamics at play in rotorcraft applications. Performance data from a 1/8th scale model of an Urban Air Mobility (UAM) type rotor is presented to illustrate the dependence of performance on Reynolds number. A UAM rotor was chosen due to the potential applicability of this technique to investigate interactional effects of multirotor systems. Although the present paper focuses only on an isolated rotor operating in hover, the results of this study will be used to inform future work on multirotor systems, such as tandem and coaxial rotor configurations, investigating the interactions between the individual rotors and their wakes. The results quantify the behavior of the rotor over a wide range of equivalent scales, clearly displaying a sharp degradation in performance for low Reynolds numbers, below approximately Re = 100000. This performance decrease can be linked to the dependence of airfoil Cl and Cd on Reynolds number, outlining the need to accurately replicate full-scale Reynolds numbers in an experimental model, and the ability for high-pressure aerodynamic testing to facilitate such investigations at low cost relative to a full-scale experimental study.