Turbulent Wake Scaling of Coaxial Rotor Blades Using Compressed Air

Coaxial rotor configurations are becoming more commonplace on specific aerospace applications, such as electric vertical take-off and landing rotorcraft. These systems are characterized by many highly-interacting rotors that can produce unwanted noise, vibration, and unexpected aerodynamic behavior. However, from a fundamental perspective, coaxial rotors have not been studied as extensively as single main rotors meaning that less is known about their underlying fluid dynamics. In this work, the Reynolds number (or scale) dependence of the coaxial rotor wake is examined using a specialized, high pressure wind tunnel known as the Penn State Compressed Air Wind Tunnel (CAWT). A model of the NASA Dragonfly coaxial rotor system was used for these tests. Verification of model performance includes a comparison of rotor thrust coefficient for different operating conditions and shows good agreement in hover and axial climb. Next, a wake survey was conducted at several downstream locations to determine the Reynolds number sensitivity for both mean and fluctuating axial velocities. Results indicate that only moderate Reynolds numbers are required to before the profile shape becomes insensitive to further increases in the Reynolds number. A preliminary analysis of the wake spectra shows that wake meandering may be present for the coaxial rotor. These findings offer the first detailed insight into the coaxial rotor configuration wake scaling by determining how modifications to upstream rotor loading conditions affect wake dynamics.