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0 3 1 N72-27 OF WIND 2 6 6 ) COj1PABISON (NASA-TMX-6 OF MEASUREMENTS NOISE FLYOVER AND TUNNEL Jr., A. Atencio, STOL AIRCRAFT yOV-10A THE 's 01B CSCL 28 p Jun.
al (NASA) et 34547 G3/02 NOISE AND FLYOVER OF WIND TUNNEL COMPARISON STOL AIRCRAFT YOV-1OA OF THE MEASUREMENTS Adolph Atencio, Jr., and Paul T. Soderman Ames Research Center and U.S. Army Air Mobility R & D Laboratory Moffett Field, Calif. 94035 Details of illustrations in may be better this docur-enilt nmiorofioh_ studied on Reproduced by NATIONAL TECHNICAL INFORMATION SERVICE U S Deportment of Commerce 'SpringfieldVA22151 June 1972
PRECEDING PAGE BLANK NOT FILME
NOMENCLATURE dB decibel EPNdB effective perceived. noise level - decibels Hz Hertz - cycles per second PNL perceived noise level - decibels PNLT perceived noise level tone corrected. - decibels RMS root mean square revolutions per minute RPM SPL sound pressure level - decibels STOL short take off and landing V/STOL vertical and short take off and landing ~a angle of attack - pitch attitude with respect to horizontal iii COMPARISON OF WIND TUNNEL AND FLYOVER NOISE MEASUREMENTS OF THE YOV-lOA-RCF STOL AIRCRAFT Soderman Adolph Atencio, Jr. and Paul T.
Ames Research Center and.
U.S. Army Air Mobility Research & Development Laboratory SUMMARY The YOV-1OA Research Aircraft was flown to obtain flyover noise 80- data that could. be compared to noise data measured in the 40- by foot (12.2 x 24.4 m) wind tunnel at NASA Ames Research Center.
early morning The flyover noise measurements were made during the California. A number of passes hours on runway 32L at Moffett Field, were made at 15.24 m (50 ft) altitude in level flight with an airplane Two configuration closely matching that tested in the wind tunnel.
passes were selected as prime and were designated for full data reduction. The YOV-1OA was flown over a microphone field geometrically similar to the microphone array set up in the wind tunnel. An acoustic center was chosen as a matching point for the data.
Data from the wind tunnel and flyover were reduced and appropriate corrections were applied to compare the data. Results show that wind tunnel and flight test acoustic data agreed closely.
-2- INTRODUCTION Ames Research Center is actively involved. in advanced programs to develop STOL and. V/STOL transport aircraft.
A very important part of the research effort is concentrated in designing aircraft for low noise emission to the environment. The noise emitted by the aircraft during take off, landing, and. flyover will have much to do with STOL and. V/STOL acceptance by the public.
Ames measures noise generated by large scale research models in the 40- by 80-foot wind tunnel. The noise measurements are used both to predict noise characteristics of full scale aircraft and to evaluate the change in noise with aerodynamic parameters.
To determine the validity of the wind tunnel measurements, an existing flying research aircraft of the STOL type, the YOV-lOA, was tested. in the 40- by 80-foot wind tunnel and then flown over a similar microphone array at Moffett Field., California. The flyover data and wind tunnel data were reduced, analyzed, and compared on the same basis.
This report summarizes those results.
AIRCRAFT AND INSTRUMENTATION Aircraft The YOV-1OA Research Aircraft is a modified North American YOV-10 Navy aircraft. The aircraft was modified for STOL research by incorporating an improved propulsion system with power interconnect and.
a high lift flap system with rotating cylinders. The rotating cylinders are 12 inches (.305 m) in diameter and are driven by hydraulic motors.
The maximum rotation speed. is 1600 RPM. The cylinders provide a means
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for keeping the airflow attached to the wing surface over a larger speed range and angle of attack range than conventional flap systems resulting in increased. lift throughout that range. The basic airplane has a wing span of 10.36 m (34 ft) with an aspect ratio of 4.75 and a The propulsion system consists of modified 642A-3150 airfoil section.
two 4-blade propellers driven by Lycoming T53-L-11 engines. The propellers are 2.87 m (9.42 ft) in diameter and were designed. for low noise emission. A schematic showing the YOV-lOA detail is given in figure 1. Figure 2 shows the YOV-1OA in flight and the wind tunnel installation.
Instrumentation Wind. tunnel noise measurements were made Wind tunnel test.- cathode using 2-inch (1.27 cm) condenser microphones (B&K 4133) with follower (B&K 2615). The microphones and cathode followers were connected. to signal conditioners, and the output from the signal at 30 ips on an Ampex conditioners were recorded on magnetic tape FR-1300A tape recorder. Before each run, each microphone was calibrated with a 250 Hz piston phone to 124 dB at .5 volt RMS.
Overall system error is estimated. at ± ½ dB.
The microphones were attached. to 1.83 m (6-foot) microphone screens (B&K UA 0052). With stands and. had. special bullet nose wind.
the nose cones the microphones had omni-directional response. The microphones were pointed. into the wind. during the wind. tunnel test.
A schematic of the wind. tunnel microphone array is shown in figure 3.
Sound. van.- Flyover noise data measurements were made using a portable sound. data van. The self contained. van had. all necessary site data reduction.
equipment for data recording and. on The sound. data measurements were made with 1.27 cm (½-inch) condenser microphones (B&K 4138) with cathode followers (B&K 2619).
Each microphone and. cathode follower was connected. to a portable signal conditioner at the microphone site, and. the portable conditioner was connected.by long cables to a van signal conditioner. The van-to-portable conditioner arrangement allowed.both on site and remote setting of signal gain. The signal output at the van was recorded. on magnetic tape at 30 ips using a Honeywell tape recorder. In addition to microphone signals; time code, Fairchild. camera signal, operators voice, and pilots voice were recorded.
Prior to testing, the long microphone cables, signal conditioners, and. cathode followers were calibrated. with a sine wave signal generator.
The input to each system from the signal generator was 1 volt RMS at each 1/3 octave center frequency from 50 to 10,000 Hz. The output from each system was recorded on magnetic tape and. was used. for data correction.
Shortly before the day's flights, each microphone was calibrated with a 250 Hz piston phone to 124 dB and. 1 volt RMS. Overall system error is estimated. to be less than + ½ dB.
The microphones were set on 1.83 m (6-foot) stands and. adjusted to receive grazing incidence from the sound. source. Each microphone had.
a wind. screen made of polyurethane foam (B&K UA 0237). The microphone set up on the runway is shown in figure 4.
Wind velocity and. direction, dry and. wet bulb temperature, barometric pressure, and humidity were measured. at a portable weather station located near the van. Weather conditions were obtained. prior to each day's flights and. if the wind velocity exceeded 5 knots, the relative humidity exceeded 90%0 or was below 30%0, or temperature exceeded. 86°F or was below 41°F the day's flights were cancelled..
-5- Radar.- A portable radar was used. to guide the pilot and. aircraft along the flight path and to provide information on aircraft position radar signal was received.
with respect to the microphone field. The YOV-lOA. The radar from a reflector attached. to the nose wheel of the output was aircraft range, altitude above the runway surface, and.
displacement from the runway centerline.
Fairchild. flight analyzer camera.- A Fairchild.Flight Analyzer Camera was used to determine when the aircraft was directly over the a reference acoustic center of the microphone field.. The camera takes series of photos on a single photo plate when swept across a viewing field.. Careful set up of the camera allowed. accurate determination of aircraft altitude and. flight speed.. In order to synchronize the camera sound. d.ata recordings, a pulse signal was emitted. from the camera with the the signal was recorded at the sound. van simulta- at each shutter click, neously with the sound. data. The set up distances for the camera are shown in figure 5. A sample photo plate is shown in figure 6.
DATA REDUCTION Wind. Tunnel Data Data from wind tunnel noise measurements were reduced. through a B&K real time 1/3-octave-analyzer. The analyzer had a parallel filter set and outputs digitized data from the analog signal from magnetic tape.
The data were reduced. using an averaging time of 15 seconds. The output from the analyzer was put on punched. paper tape and. formatted. to be used in a data reduction program.
The data reduction program calculated overall sound pressure level and. perceived. noise level (PNL), and. applied. corrections for reverberations to the data. The output from the program consisted.of overall SPL for
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each 1/3-octave center frequency, corrected. and. uncorrected, overall SPL (total SPL for all bands) and PNdB corrected. A sample sheet is shown in figure 7.
Flyover Noise Data Data from the flyovers were reduced on site using the reduction equipment in the sound. van. The data were reduced. through a General 6adio real time 1/3-octave analyzer with parallel filter set using an averaging time of 1/8 second. (due to speed. of the aircraft).
The out- put from the filter set was input to a mini-computer on board. the van.
The computer applied. the electrical corrections from pre test calibra- tions and. output a punched paper tape and a printed. sheet. The punched.
paper tape was used. for further data reduction as reported in.reference 1. The computer PNL and. PNLT for 80 data points 1/8 second apart. In addition, the 1/3-octave center frequency SPL were printed, for each of the 80 points. An uncorrected. EPNdB was printed. for each set of data points.
The data used. for this report are the 1/3-octave SPL data produced. on site from the van.
TEST PROCEDURE Wind. Tunnel Wind tunnel noise data were taken at selected. aerodynamic data points. Approximately 30 seconds of sound. data were recorded for each
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condition. Voice inputs for airplane configuration, wind. tunnel air velocity, airplane power setting, and. microphone gain settings were sound. data.
recorded simultaneously with the Flyover The sound. data recording equipment was turned. on when the aircraft cue for turning entered. the approach path to the microphone field.. The on the recording equipment came from the radar operator who visually sighted the aircraft from the radar dish. The data recording continued.
until the aircraft lifted. off at the end. of the runway near the sound.
van. Data were recorded approximately 243.84 m (800 ft) on either side of the microphone field. Prior to the day's flights a background noise level was recorded on mag tape for reference when reducing data.
DATA ANALYSIS In order to compare the data from the wind tunnel to the data from flyover, it was necessary to correct both sets of data to free field.
conditions. In addition, it was necessary to extrapolate the flyover noise data back to wind tunnel measurement distances, from source to micro- phone, by applying the spherical divergence law for sound attenuation (6 dB per double distance). Atmospheric absorption corrections were applied when significant.
Corrections to wind. tunnel data were based. on a point noise source test section. An omni-directional horn driver calibration of the located in the center of the test section was driven with pink noise through a 1/3-octave band filter set. Noise measurements were made at selected center frequencies and. distances from the source. Free field.
reported in reference 2. The sound. pressure levels for the horn are -8- differences between the wind. tunnel measurements and free field were used as corrections at each 1/3-octave center frequency SPL. The corrections account for the reverberation and. reflection of the wind.
tunnel. The data used. for the corrections are reported in reference 2.
Corrections to flyover noise data consisted. of correcting the data for reflections off a hard. surface, correcting for frequency shift where applicable, and correcting for distance attenuation. The corrections for reflections were based. on references 3 and 4. The pure tone reflection corrections were based. on reference 3 and all other corrections were based. on reference 4. In order to use the corrections the following assumptions were made: 1) The aircraft was considered. to be a point source with respect to each microphone.
2) The concrete surface of the runway was assumed. to be a perfect reflector with no surface irregularities.
3) Spherical divergence was assumed for distance attenuation.
The corrections to data for frequency shifts were based. on a simple application of the Doppler equation.
Data were compared on an equal basis by selecting the point in time where the flyover microphone data were directly comparable to wind. tunnel data for a geometrically similar condition. Table 1 and Table 2 give information on aircraft configuration, power setting, and.
position with respect to the microphone fields.
RESULTS AND DISCUSSION The reduced. data were compared by plotting SPL versus 1/3 octave center frequency. The final resulting data are summarized in figures 8 through 12.
Sound. data from microphones positions 1 and 3 (figures 8 and. 10) show close correlation between wind. tunnel and flyover data throughout
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the spectrum. These microphones were located. at the acoustic center of the microphone fields. At the selected. analysis time the aircraft was overhead. and. the relative velocity along a line connecting the aircraft and. microphone was zero. Therefore, no frequency shifts took place so that wind. tunnel and flyover data at these microphone positions corrected. to free are directly comparable after each set of data was field. and. equal distance. The slight differences near the blade passing and. second. harmonic are due to the difference in frequency (about 80 Hz) The shorter averaging time used. for the two sets of data during reduction.
flyover data allows lower frequency spikes averaging time used. for the small to be weighted. more heavily when averaged since the sample number is 80 Hz wave is sampled 10 times in 1/8 second. and 1200 times in 15 (i.e.
seconds).
Microphones 2 and. 4 (figures 9 and 11) were affected by frequency the analysis shift during flyover. Microphone 4 location was such that at time the aircraft sound source had relative motion away from the microphone and. as a result the frequencies seen by the microphone were lower than the frequencies emitted by the source. When the Doppler equation was applied at the blade passing frequency and second harmonic, however, the frequencies don't shift out of their respective 1/3-octave bands.
for microphone 4. The analysis Therefore, no shifts of data were made to account for pure tone reflections was, however, made using the Doppler equation calculated shift frequency.
the analysis time, the aircraft Microphone 2 was located. such that at sound source had relative motion toward the microphone during flyover and.
so the frequencies measured. at microphone 2 were higher than those emitted applied to data at the blade by the source. When the Doppler equation was passing frequency and second harmonic it showed that the frequencies did.
shift out of their respective 1/3-octave bands into the next higher band..
The flyover data for microphone 2, therefore, have been shifted. at the -10- blade passing frequency and second. harmonic to account for the Doppler effect. The SPL's used. to replace the affected 1/3-octave band SPL's were the levels measured on both side of the affected. band.. Wind tunnel narrow band data analysis was used. as a guide. Reflection corrections were based. on shifted frequencies.
When the corrections were applied to data at microphones 2 and 4 and. the Doppler affect applied to microphone 2, the flyover data and wind tunnel data showed good agreement. Microphone 2 data has some discrepancies at frequencies below 500 Hz; this may be due again to the shorter.averaging time used. to reduce flyover data. In addition, the reflection corrections are sensitive to airplane position.
Additional analysis of flyover data from microphone 2 was done for the source directly over the microphone. The Doppler effect and reflection correction errors are minimized. for the source in that position. The resulting data are shown in figure 12 and. compared. to tunnel data.
These data show the same close agreement as microphone 1 data. The comparisons made for all data show that closer agreement between wind, tunnel and. flyover data occurs at the non-Doppler affected. micro- phone positions than occurs at the Doppler affected. microphone positions.
The data give encouragement for the continued. measurement of noise data from.research aircraft models installed. in the 40- by 80-foot wind.
tunnel.
CONCLUSIONS i) When appropriate corrections are applied., flyover data and. wind tunnel data show close agreement for 1/3-octave bands.
2) Wind. tunnel tests can be used to estimate flyover type noise to be used to predict the noise emission from future aircraft.
-11- - il- 3) At higher velocities the Doppler effect could become significant for flyover data. Energy shifts accompanying frequency shifts are hard to account for using simple 1/3-octave analysis. It will be necessary to use narrow band analysis to account for these shifts.
-12- REFERENCES 1. The General Electric Company: Inflight Sound Measurements on the XV-5B and OV-10 Aircraft. NASA Contract NAS 2-5462, April 1972.
2. Bies, David A.: Investigation of the Feasibility of Making Model Acoustic Measurements in the NASA Ames 40- by 80-Foot Wind Tunnel. CR 114352, Bolt Beranek and Newman Inc., 1971.
3. Hoch, R.: Acoustics Effects Produced by a Reflecting Plane.
SAE No. 31, September 1970.
4. Howes, Walton L.: Ground Reflection of Jet Noise.
NASA Technical Report, R-35, 1959.
5. Morse, Phillip M., and Ingrad, K. Uno: Theoretical Acoustics, 1968.
6. Anon: Standard Values of Atmospheric Absorption as a Function of Temperature and Humidity for Use in Evaluating Aircraft Flyover Noise. Society of Automotive Engineers Inc., August 31, 1964.
TABLE
CONFIGUlJRATlONK DETAILS , ALTITrUDE AND AIRSPEED COM PAR 1.50 N5 FLAP AIRSPEEC ALTITUDE PROP BLADE AIRCRAFT GIROSS 5ETTING KNOT"S METERS RPM ANGLE PI--tC4 WEi&__T (FEE T ) DIE GG KG,
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FLYOVER 2.
,30/1-5 77.9 16,46 1236
23 Z.7 '114.
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30/1 5
6 6,71 12O5 27 NOT7
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MICROPHONE 2 59.74 (194-.0) 14.3
MICROPHONE 3 z2.ZZ ( 79 Z) 37.3
MICROPHONE 4 ,40 (z126.0)
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MICROPHONE I 4.85 (16.0)
MICROP"O NE 2 1,.90 (62.0) 14.9
MIC-ROPHOE 3 7,80 (25. 6) 38.6
MIC.ROPHONE 4 I 2,2z5 (40.2) z3.4
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