Another set of 16 scale model acoustic data refs 4

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Another set of 1/6 scale model acoustic data (refs. 4 and5) that were obtained from the Duits-Netherlandse Windtunnel (DNW) for a rotor similar to that of the test helicopter was also used for comparison with the flight test acoustic data. This set of data, taken at the test condi- tion of CT/ σ =0.07, covers a broader range of airspeed as shown in figure 6 for three microphone locations. The azimuth location of these microphones was: 180 deg for the center microphone, 150 deg for the starboard micro- phone, and 210 deg for the port microphone. The eleva- tion angle and the distance from the rotor hub center are the same for the three microphones, namely –25 deg and r/D = 1.50 respectively. These microphone locations were selected for the purpose of comparing with the selected microphones in the Langley tunnel described earlier. The acoustic data were processed with a passband of 500 to 3000 Hz to capture the dominant BVI noise, similar to that used in the Langley tunnel data described above. The directivity effect of the BVI noise is strong as evident from figures 6(b) and 6(c). The difference in the BVI noise between the starboard microphone and the port microphone exceeds 10 dB at some flight conditions. However, this set of data shows considerably smaller variation in noise level than the other wind tunnel data, as evident from comparing figure 5(a) with figure 6(a), and figure 5(b) with figure6(b), especially in the critical speed range of 50 to 80knots. There is no clear region with noticeably intensive BVI noise for this set of data. Also, lack of data in the low rate of descent region at the air- speed of 50 to 80knots prevents an assessment of the Quiet profile in that region. However, the approach profile appears to be reasonable for the higher speed segment of 80 to 110knots as evident from figure 6. Finally to complete the definition of the Quiet approach profile, the level of deceleration was defined, and the
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4 velocity and altitude profiles, expressed in terms of dis- tance to the helipad, were calculated using dz = [f(V)/V] dx (1) dV = [a/V] dx (2) where f(V) is the defined functional relationship between the rate of descent and airspeed for BVI noise-abatement shown in figure 4, and “a” is the level of constant deceler- ation. A value of 0.7 to 1 knot per second (0.035 to 0.05g) was selected from handling qualities considera- tions. Beginning at the helipad, the integration is per- formed in a backward piecewise fashion, commensurate with the piecewise linear nature of the f(V) relationship. Operational constraints were incorporated in the process, including consideration of extended segments of the profile necessary to obtain desired initial path acquisition altitudes or airspeeds. Example constraints that were con- sidered to define the integration limits were: (1) terminal speed at the helipad is zero, and (2) airspeed at decision height should be close to the takeoff safety speed, typi- cally around 35–40 knots, thus assuring that balked land- ing performance is achievable if on a single engine approach or in the event of engine failure at the landing decision point. Decision height was assumed to be 200ft,
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