1. Adetailed analysis of.variations in harmonic levels for different attitude .angles is based on respective individual harmonic-level directivity-patterns as determined from measurements for different helical blade-tip Mach-numbers. From each two data-points being representative for a constant difference in attitude angle (i.e. AO= 7.3-0 or Ag= 3.6-(-3.8) OK ad= 0-1-7.4)) but otherwise constant propeller operational conditions, respective differences in maximum directional harmonic levels were determined for different helical blade-tip Mach-numbers M It turned out, that in the range kii attitude angles realized in the tests, respective noise level differences only depend on the absolute difference in attitude -anqle. Thus "individual" level differences corresponding to different combinations of attitude angles (but constant A 4 ) were averaged to reduce inherent data scatter. These level differences are plotted in Fig. 15 versus harmonic number. From this representation a general trend is observed, i.e. a slight decrease of AL with increasing harmonic number up to HN z 5 and a subsequent increase of respective level differences for the higher harmonics. dB
2. I 60 40 20 E -20 -40