Refined Performance Results on a Slowed Mach-Scaled Rotor at High Advance Ratios

Abstract

Owing to its ability to alleviate the compressibility effect on the advancing side, the slowed rotor operating at high advance ratios is a key feature in high-speed compound rotorcraft. A series of wind tunnel tests were conducted in the Glenn L. Martin Wind Tunnel with a four-bladed Mach-scaled articulated rotor. The objective of the tests was to gain a basic understanding of unique features of high-advance-ratio aerodynamic phenomena, such as thrust reversal and dynamic stall in the reverse flow region. In this study, high-advance-ratio tests were carried out with highly similar, noninstrumented blades and on-hub control angle measurements, to minimize possible error due to blade structural dissimilarity and pitch angle discrepancy. The tests were conducted at 900 and 1200 RPM, advance ratios of 0.3–0.9, and a shaft tilt study was conducted at±4°. Pitch and flap motion at the blade roots, rotor performance, and vibratory hub loads were investigated during the test. The test data were then compared with those of previous tests and with predictions from comprehensive analysis. The airload results were investigated using comprehensive analysis to gain insights into the influences of advance ratio and shaft tilt angle on rotor performance and hub vibratory loads. Results indicate that the thrust benefit from backward shaft tilt is dependent on the change in the inflow condition and the induced angle of attack increment, and the reverse flow region at high advance ratios is the major contributor to changes in shaft torque and horizontal force.

Slowed Hingeless Rotor Wind Tunnel Tests and Validation at High Advance Ratios

“…With this setup, four wind tunnel tests were conducted. In the first test, the blades from a previous articulated rotor test [16] were used. The primary goal of the test was to validate the capability of the setup for high-advance-ratio hingeless rotor tests, so it was carried out without lift offset.…”

“…A data correlation study was conducted with the University of Maryland Advanced Rotorcraft Code (UMARC). Previously, the comprehensive analysis has been validated extensively against UH-60A wind tunnel test data [10], as well as the in-house Machscaled wind tunnel test data [16,23]. The structural model included coupled elastic flap, lag, twist, and axial deformations to second-order approximation [24].…”

“…With zero hub moment trim in Fig. 5a, the trend of thrust resembles that of an articulated rotor: the thrust versus collective curve is close to linear for moderate collective, and the slope decreases with increasing advance ratio (by 61% from μ 0.3 to μ 0.7), as the rotor needs to balance the loss of lift in the reverse flow region, making it less effective in producing lift even on the advancing side [15][16][17]. With the wing trim, the change in thrust curve slopes with advance ratio is small, whereas the curves shift up with increasing advance ratio.…”

“…Previously, a series of wind tunnel tests with slowed Mach-scaled articulated rotors was carried out at the University of Maryland. One of the tests achieved an advance ratio of 1.61 [15], another used highly similar blades without sensors to obtain baseline performance data [16], and some tests featured blade-embedded pressure sensors and strain gauges [15,17]. From the tests, fundamental understanding was gained on some unique aerodynamic phenomena in the highadvance-ratio flight regime, such as thrust reversal (rotor thrust decreases with increasing collective angle), deep dynamic stall in the reverse flow region, and the beneficial effects of backward shaft tilt on rotor performance and hub vibratory loads.…”

“…From the tests, fundamental understanding was gained on some unique aerodynamic phenomena in the highadvance-ratio flight regime, such as thrust reversal (rotor thrust decreases with increasing collective angle), deep dynamic stall in the reverse flow region, and the beneficial effects of backward shaft tilt on rotor performance and hub vibratory loads. It was shown by previous wind tunnel test results [15][16][17] as well as comprehensive analysis predictions [18,19] that an articulated rotor trimmed to zero hub moment generates limited thrust at high advance ratios, because the advancing side of the rotor needs to be trimmed against the retreating side with significant reverse flow, where the rotor is ineffective in generating thrust. Therefore, a hingeless rotor with lift offset can help overcoming this problem by generating more thrust on the advancing side, and thereby achieving high cruise speed (see Fig.…”