Miss distance for rotor blade-vortex interaction noise reduction

Blade-vortex interaction noise generated by helicopter main rotor blades is one of the most important noise mechanisms both in military aspects and community acceptance. Noise generating mechanisms and controlling concepts are studied, particularly in terms of a blade-vortex miss distance. A comprehensive code (CAMRAD-JA) coupled with a finite difference code(FPR) has been used to assess airloads and blade aeroelastic deformations. The results are compared with existing BO-105 test results. Potential methods to control the blade-vortex miss distance are studied by exploiting the blade flapping and torsional characteristics through active blade control technology. Introduction Rotor blade-vortex interaction(BVI) noise is generated from unsteady pressure fluctuations on the blade's leading edge due to interactions with previously generated tip vortices during descent or maneuvering flight conditions. This BVI noise is loud and impulsive in nature and it plays an important role both in military aspects and community annoyance. This paper is work of the US Government and is not subject to copyright protection in the United States. * Associate Fellow Understanding of the noise generating mechanisms has advanced substantially in recent years, but the progress of controlling this noise has been limited. Major controlling parameters for rotor BVI noise have been analytically investigated (ref.l) and several parameters were identified, such as tip vortex strength, blade-vortex miss distance, and blade pitch. Among many efforts to control these parameters, active blade control concepts have recently achieved some success in reducing BVI noise. In particular, the higher harmonic blade pitch control (HHC) and individual blade control (IBC) concepts have been tested and significant mid-frequency noise reductions of 5-6 dB have been reported (refs. 2-10). Meanwhile, several analyses (refs. 2,4,6, and 11) reported that the HHC concept with a proper blade pitch control schedule changes blade-vortex miss distances, vortex geometry, and blade circulation. Results also indicated that pitch control would not only modify the pitch, but would also modify the strengths of the shed vortices as well as possibly the interaction locations. Decreased blade loading in specific azimuthal regions has been identified to reduce BVI noise (ref.5); in addition, blade-vortex miss distances are definitely affected by HHC, and they evidently represent a dominant factor in the BVI noise reductions (refs.4 and 6). A vibration suppression mechanism by HHC was also analytically investigated through the flapping moment at the blade root (ref.6). Results showed that a vibration suppression can be achieved by reducing the blade flapping motion, thus decreasing the blade-vortex miss distance and, in turn, increasing the BVI noise. An international cooperative program, called Higher-Harmonic Control Aeroacoustics Rotor Test(HART), was carried out with a 40%-geometricalIy and dynamically scaled model of a BO-105 main rotor in the DNW in order to investigate the effects of HHC inputs on BVI noise reduction(refs.!2-18). This program concluded that a blade-vortex miss distance is strongly influenced by both a tip vortex trajectory and blade aeroelastic characteristics. In this paper, results of a study of the effect of a blade-vortex miss distance on rotor BVI noise reduction are presented. Potential concepts using active blade control technology to control the miss distance are also discussed. For this study, the in-house CAMRAD-JA/FPR codes (ref. 19) were used; the results are compared with the existing HART test data (ref.18). Blade Deformation and Airload From a blade-vortex interaction geometry and acoustic radiation, it has been known that the major contribution to acoustic radiation comes from near-parallel interactions between a blade and a vortex, which happen in both the advancing and the retreating sides. For example, a fourbladed model rotor of BO-105 shows that the major contribution of noise comes from the interaction of a blade at the azimuthal angle(y) of 50° with a vortex generated at y of 130° in the advancing side. In the retreating side, it comes from the interaction at 310° of the blade position with the vortex generated at y of approximately 230° (refs.14 and 15). Blade tip deflection, blade-vortex miss distance, airload, and vortex circulation will be discussed at these two azimuthal angles of 50° and 130° for the following three flight conditions: baseline case(run 140, without HHC), low noise case (run 138, with HHC inputs of 0.8° amplitude and 300° phase angle) and low vibration case (run 133, with HHC inputs of 0.8° amplitude and 180° phase angle) from the HART test (ref.18). Other common parameters are: advance ratio=0.15, tip Mach number =0.64, shaft angle=5.3° and thrust coefficient(Cr) = 0.0044. Blade tip deflection: Blade tip deflections along azimuthal angles play an important role in determining blade-vortex miss distances during interactions. In particular, blade positions at the time of both vortex generation and blade-vortex interaction are important to determine the miss distance. Figure 1 shows a blade tip deflection history over azimuthal angles from the HART test, along with results from the AFDD in-house analysis for rigid and soft blades. In this figure, measured blade tip deflec-tions show I/rev characteristics for the baseline case and 3/rev characteristics for cases with HHC 3P inputs(run 138 and 133). In the analytical results, the tip deflection history for a rigid blade shows substantially different characteristics compared to the experimental data. However, the results with a CAMRAD-JA elastic blade model show characteristics similar to those with the measured one in mode shapes, but not in amplitudes. This result has a serious implication in determining a miss distance. The measured blade tip positions at v=130° and \|/ = 50° show an interesting phenomenon. For the low noise case(run 138), the blade tip position is 3.5 cm at \y=130°, the point at which the vortex is generated, and the blade tip position is 9 cm at y=50°, the point at which the blade interacts with the vortex. The relative separation distance of blade tip positions at the two azimuthal angles is about 5.5 cm, which is much larger compared to 1.5 cm for the baseline case. This large separation distance of the blade tip positions directly contributes to the large bladevortex miss distance. For the low vibration case (run 133), the relative separation distance of blade tip positions at v = 130° and 50° is much smaller compared to those of the baseline case. The relative separation distance can also be obtained through the blade flapping deformations at y = 50° and Y = 130° as shown in figure 2. These figures suggest that blade tip deflections and flapping deformations can be controlled by adjusting HHC inputs.