The spacing on the four microphone array was that

This preview shows 7 out of 9 pages.

phones used to collect the reference 6 data. The spacing on the four microphone array was that used in the stan- dard ICAO/FAA noise certification test (ref. 8). The microphones were 1/2 in. diameter condenser type fitted with grid caps covered by commercially available foam wind screens.
Image of page 7

Subscribe to view the full document.

6 The acoustic data were recorded on four data channels of a battery operated commercially available digital tape recorder. A separate time channel was used to record a time signal synchronized to the helicopter flight track position time. Each microphone was calibrated with a 124dB tone at 250 Hz and with a constant amplitude white noise before and after data collection. Acoustic data were recorded at a sample rate of 24,000 samples a sec- ond to permit the achievement of a maximum 10 kHz bandwidth. For subsequent BVI noise analyses, the acous- tic data signals were reduced to overall sound pressure levels typically associated with the bandwidth of frequen- cies of the blade vortex interaction noise. This overall BVISPL consisted of an integration of the sound pressures within the frequency band limits of 0.5 to 3 kHz (the 3rd through 46th harmonics of the main rotor blade passage frequency, ref. 6). Data reduction also consisted of obtain- ing a set of five noise metrics compatible with the heli- copter noise database generated by the FAA and industry (refs.9–11). Three of the metrics are obtained from only the instantaneous sound pressure levels, i.e., overall SPL, dBAmax, and the maximum tone-corrected perceived noise level (PNLTmax). The remaining two metrics, SEL (sound exposure level) and EPNL (effective perceived noise level) are time duration corrected single-event met- rics. Additionally, as part of the data reduction for the analysis of each flight, the OASPL, dBA, and PNLT were graphically studied as a function of time and horizontal distance along the flight track to each microphone. A tethered-balloon weather-measurement system was used to collect barometric pressure, dry and wet bulb tem- peratures, relative humidity, wind speed and direction over the altitude range of interest. The balloon was located approximately 2500 ft to the side of the Run- way35 (see fig. 7). It was permitted to ascend and descend from ground level to an altitude of 1200 ft at a rate of approximately 1 ft/s. The weather data were collected at a rate of 0.1 Hz. The data acquisition began approximately 30 minutes before each flight. wind speeds were closely monitored, so that no acoustic data were acquired when wind speeds exceed 10 knots within the altitude of interest. Test Results The main results of the flight tests are presented below in two categories: flight mechanics data and acoustic data. The reduction of the flight mechanics data was performed at Ames Research Center, and some of the results were later merged with the acoustic data, which were analyzed at Langley Research Center.
Image of page 8
Image of page 9
You've reached the end of this preview.
  • Spring '10
  • frando
  • Aerodynamics, Aviation terminology, BVI, Instrument approach, Helicopter rotor, acoustic data

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern