Small aircraft propeller noise with ducted propeller - CAFE Foundation

the1/8ofanCopyright© 1998, American Institute of Aeronautics and Astronautics, Inc.respectively.increasethistheyhavecurvesEngine Noise (dB, re:orJ96(90° < 9 < 150°) the levels of noise appear fairlyconstant, (within 2 dB's) indicating no distinct lobe atthis position. This isin contrast tothe unshroudedpropeller which appears to a distinct lobe at thislocation ascanbe seen by comparing the inFigure 4and8.help to determineMeasurements beyond 150° wouldisthe case.ifOASPL (dB, re:with shroudFigure 7. Directivity Pattern of EngineTone at 200 HzNoise With ShroudThe same measurements previously discussedwere taken with the shroud in place. Of primaryinterest wasthe OASPL measurements which provideda direct comparison tothe unshrouded configuration.Also of importance were the power spectra, whichproved tobe most insightful when analyzing thepropulsion system noise with the shroud in place.With the propeller blades passing within about one halfan inch behind the trailing edge of the stator vanes, itwas assumed that strong interaction tones would beobserved between the propeller and stators. In fact,this was the case. Other factors thought to influenceoverall noise included shroud shielding, ductacoustical emissions, andan in propellerefficiency due to a reduction in tip vortices. The gapbetween propeller tipand shroud was approximatelyinch.OASPL Measurements The directivitypattern of the OASPL with the shroud follows the sametrends asthe unshrouded case, although an increase inlevels is evident. Increases in higher harmonicsamount to approximately asix decibel increase intheoverall sound pressure level. Upon examination of thedirectionality field in Figure 8 maximum OASPL's areobserved at both 9=45°and9= -45° than with valuesof 110.1 dBand 111.3 dB, This variesonly slightly from the pattern noted without the shroud,as a slightly higher noise level is seen around 9 = -45°with the shroud. Behind the plane of rotation,Figure 8. Measured Directivity PatternWith Shroud in PlaceRotor-Stator Interaction Tones Powerspectralanalysis scans were performed in the samemanner as for the unshrouded configuration. Again,all tones fall under one of two types: either rotor -stator interaction noise or engine noise, except foroneunidentified tone at 520 Hz which appears in all thefrequency scans with the shroud in place.From examination ofthepower spectrumpresented in Figure 9, the increase in OASPL canbeattributed mostly to a set of distinct tones that occur atintervals of 160 Hz and higher. Notice that the tonesappear at the same frequencies as the BPF and itscorresponding harmonics, however, these tones areattributed more topropeller harmonics.rotor-stator interactions than theUnlike theBPFand associated propellerharmonics, the fundamental rotor-stator interactiontone does not appear as the most significant tone.Instead, the highest level in the spectrum appears ataround the4thor5th interval tone, and instead ofthehigher tones diminishing as increase in frequency,they remain at fairly constant levels until a frequency468