NASA Dragonfly Mission - Coaxial Rotor Test & CFD Prediction

A fixed-pitch speed-controlled coaxial rotor system (Dragonfly Phase B*) was tested in the NASA Langley Transonic Dynamics Tunnel (TDT). The rotors have a diameter (D) of 1.35 meters and an inter-rotor spacing of 0.3375 meters, or D/4. The primary objective of the TDT test was to experimentally measure rotor performance of a candidate full-scale flight rotor for the Dragonfly program, NASA's 4^th New Frontiers Mission, in an atmosphere as close as possible to that on Saturn's largest moon Titan. The TDT heavy gas (HG) test setup provided Mach scaled data at one-third chord-based Reynolds number when compared to Titan condition. These data serve as a validation anchor for computational fluid dynamics (CFD) performance tables used by the Dragonfly team to predict rotor performance on Titan. The present work provides a thorough CFD validation study of coaxial rotor performance estimation with accuracy of order 5-10% over the primary flight envelope using an efficient hybrid BEMT-URANS flow solver, RotCFD. Airfoil lookup tables (i.e. C81 tables) were generated in OVERFLOW, run fully-turbulent using the Spalart-Allmaras turbulence model. The full CFD performance matrix consisted of more than 1,500 cases total, including hover, climb, edgewise flight, descent, vortex ring state (VRS), turbulent wake state (TWS), and windmill brake state (WBS). Significant effort was devoted to quantitative comparisons between experimental data and CFD results, with emphasis on uncertainty quantification and confidence levels of performance predictions. This work has been instrumental in establishing the hybrid BEMT-URANS methodology to provide mean coaxial rotor performance data for Dragonfly Mobility Closed-Loop Simulations.