Measurement and Validation of a Mach-Scale Rotor Performance and Loads at High Advance Ratios

Reducing the rotor tip speed by slowing down the rotor in forward flight is a viable means of extending the cruise speed of a compound helicopter. However, in a high advance ratio flight regime, many aerodynamic phenomena remain unclear, including the thrust reversal and the dynamic stall that occurs in the reverse flow region of the retreating blade. In order to investigate the performance and vibratory loads characteristics of a high advance ratio rotor, wind tunnel tests were conducted with a Mach-scale 4-blade articulated rotor at 900 RPM, up to advance ratio 1.15. Hub loads, control angles, flap angles, blade structural loads and blade surface pressure data were measured from the test. The wind tunnel test data was compared with the predictions from an in-house comprehensive analysis code, UMARC. The performance data correlation was satisfactory for most cases, however it degrades with increasing advance ratio. The hub load and blade structure load showed similar trends; they were insensitive to collective for low advance ratio and increased with collective for high advance ratio. The trend was captured by the analysis, but the analysis tends to under-predict the magnitudes. The pitch cyclic predictions were accurate and could be used as trimming target for future tests. Finally, the pressure data was studied to characterize and understand the dynamic stall in the reverse flow region.