On the Design and Test of a Low Noise Propeller - CAFE Foundation

Report No, 4764Bolt Berurek and Wenun Ino,Three figures are also su?plied with each run. Fig, B1 is aplot of the peak sound level at each harmonic. This 18 a plot ofthe column labeled FASTFT in Table 82. Fig. 82 is a plot of theaound level versus frequency, Each of the peaks in Fig. B2labeled P1 is a propeller harmonic. These peaks are 20 logA 2 x log5) where Ai is the amplitude listed in Table B2.Fig. 83 presents the pressure time signature.There appears to be two errors in the time signature.First, if the period is measured with calipers then the measuredperiod is .0235f.002 aec, which corresponds to a rotation rate ofapproximately 254 1 RPM. However, the measured blade passingfrequency in Fig. B3 is 13624 Hz which corresponds to a rotationrate of approximately 2697 RPM, This is a 6% error. Apparentlythe pressure time signatures are based on the tachometer RPM.Because the tachometer reading is saspected to be in error, theperiod should be corrected as is done in Fig. 8 of this test.The second error in the nteasured pressure time signature isthe sign of the amplitude. The pressure signature in Fig. 83shows a slow drop in pressure followed by a steep rise. Also,the peak negative amplitude is larger than the peak positiveamplitude. The calculated pressure signature shows precisely theopposite effect. In the calculation a slow rise in pressure isfollowed by a steep drop. Also the peak positive emplitude islarger than tbe peak negative amplitude. The error is one ofsign, and the data can be corrected by multiplying all measurementsby -1. The probably source of error is the interpretationof the voltage reading from the microphone. For condensor microphonesa negative voltage corresponds to a po8itive pressure.This correction 1s also made on Fig. 8 in this test.