Characterizing Rotor Performance Changes with Scale in Compressed Air

Full-scale rotorcraft aerodynamics are challenging to study in the field due to the lack of control over ambient conditions as well as the complexity and cost of operating a full-scale vehicle. Small-scale testing can provide significant insight into rotorcraft operation and aerodynamics but is limited by the scale factor between model and prototype. An alternative method for testing rotor performance is presented that utilizes compressed air as the working fluid. By compressing the air, flight-scale aerodynamic conditions can be achieved in hover (Reynolds number, Mach number, and advance ratio matched on small models). In this way, new and unconventional rotor configurations can be tested easily and at low cost before implementing larger prototype tests. An experimental facility at Penn State known as the Compressed Air Wind Tunnel (CAWT) is utilized to examine the scaling of thrust and power coefficient for the NASA Dragonfly rotor geometry in the single rotor configuration. Trends in the hover and axial climb results agree with the Dragonfly data obtained in the NASA Transonic Dynamics Tunnel. Scale effects are clearly present for the power coefficient at the lower tested Re, with scale invariance observed at the larger Reynolds numbers which is inline with prior work on single helicopter rotors. The utility of compressed air testing is demonstrated by the relative ease of achieving many operating Re, including full-scale, on a single model geometry.