Flow Diagnostics of Scaled-Model Coaxial Rotor Hub Flows

Rotor hub parasite drag remains one of the challenges in further improving the forward-flight capabilities of coaxial rotorcraft. Comprehensive datasets on notional coaxial hub configurations are rare, and more so at Reynolds numbers sufficiently high to preserve dominating flow structures downstream into the wake where they interact with the rotorcraft empennage and tail. The present investigation was designed specifically to improve the understanding of interactional aerodynamics effects and wake flow physics of counter-rotating coaxial rotor hubs. A unique dataset is presented on a rotor hub design equipped with the DBLN 526 airfoil at a diameter-based Reynolds number of 1.13x10⁶, corresponding to approximately quarter-scale Reynolds conditions of a coaxial compound helicopter at 200 knots. The experiments measured the time-averaged and time-varying drag on the hub configuration, with focus on a cruise advance ratio of 0.25 and a high-speed condition at 0.60. In addition to measuring hub drag and its harmonic content, a unique aspect of the experiment was the use of multiple non-invasive flow-diagnostics techniques for the particular hub configuration, advance ratios, and Reynolds number. Specifically, stereoscopic particle-image velocimetry (SPIV) measured the three velocity components at two downstream locations in the hub wake, thus providing insight into and visualizing the development of the wake. In addition, a limited laser-doppler velocimetry (LDV) and a tomographic PIV (Tomo-PIV) data campaign were conducted that confirm the earlier SPIV measurements. A subset of all measurements is presented. Some new insight into the flow physics was gained, one example being the dependence of higher-harmonic per-rev flow structures in the wake on hub advance ratio.