Small aircraft propeller noise with ducted propeller - CAFE Foundation

andair filter4) intakeCopyright© 1998, American Institute of Aeronautics and Astronautics, Inc.shroudplottedplaneis greaterimpactengineplace,engineIn Table 2, the results of similar type ofanalysis performed on harmonics through s=6areshown. From these, frequencies through 960Hzareaccounted for and further analysis of higher harmonicsproduce similar results forthe higher frequenciesexamined inthe power-spectrum scans. Bythe ninthharmonic orso, corresponding toa frequency of 1,420Hz, the effects start to weaken significantly and themajority of remaining tones can be neglected.It should be mentioned, that every thirdharmonic (s=3, 6, 9,...) there isno rotating patterndue to interaction tones. Instead, a wave isetup. This occurs whenever the blade harmonics aredivisible bythe number of stator blades. When thissituation is set up, the interactions between the bladeharmonics and the stators occur such that each stator isacted upon at the same exact time. The result is astrong tone at the particular blade harmonic. Details ofthe theory as well as more expermental results arepresented in Reference 5.Engine Tones The tones attributable totheengine with the shroud appear very similar to thosewithout the shroud Other than the shroud providingpossible blockage effects at certain azimuthal angles,there isno further reasoning that the engine noiseshould differ due to the apparatus configuration.Engine NoiseIn the past two sections, the directivitypatterns ofthe engine noise levels are asafunction of azimuthal angles. Results show the enginenoise being generated by two monopole sources: one isthe intake and the other is the exhaust. Since theengine noise is significant enough to affect the overallnoise levels while testing, further investigation into theeffectiveness of the silencers focusing on the intake andexhaust seems appropriate.Recordings were taken at the azimuthal angle9 = 45°, at the standard distance of 25 ft from theengine. Al recordings taken for this comparison wereperformed with the removed. The fouroperating configurations measured were:1) intake silencer on, secondary exhaust muffler on2) intake silencer off, secondary exhaust muffler on3) intake silencer on, secondary exhaust muffler offoff' indicates removal of the additional muffler, or"after-muffler" which was placed in series with themain muffler. Due to the nature of a two - strokeengine, testing with the main exhaust muffler removedwas not feasible since itwas needed forthe torun properly.Based on initial testing ofthe apparatuswithout the intake silencer in itis felt that theimpact ofthe intake silencer onthe noise levelsthan the ofthe after-muffler. Resultspresented in Figure 11 prove this conclusion where thepower-spectrum of the farfield radiated noise for thedifferent intake/exhaust mufflers are presented. Thetone of interest isthe fundamental engine tone at200 Hz. From the two plots in Figure 11 with theintake muffler in place, the primary tone isnotsignificant when compared to the overall levels ofnoise. Also, the second engine harmonic levels both/ft /|A/ KMilv Vvfexhaust on, intake ontocotsooFrequency (Hz)exhaust off, intake on—————PRIM/ RY ENGINE TONE|| 1A A ft^ ^T V7^AAM^"N-x-v-vrt"* V IT0 SOO 1000 1500 2000 25!Frequency (Hz)exhaust on, intake off1000 1500Frequency (Hz)exhaust off, intake ofsilencer off, secondary exhaust muffler of"Intake silencer off' indicates the entire intake silencerwere removed. Essentially noise was freeto propagate out of the carburetor throat into the openatmosphere uninhibited "Secondary exhaust muffler1000 1SCOFrequency (Hz)Figure 1. Spectrum Plots of Engine Noise UsingFour Different Exhaust/Intake Configurations471