1. The systems Data Processing. Fluctuating pressure data
2. Fan Speed Sweep. To obtain an overview of the rrifo%e-b?ThFBMTs preliminary tests were conducted in which the speed of the fan was slowly decreased from a supersonic fan tip speed to a subsonic fan tip speed while continuously recording data. In figure 6(a) a spectral analysis of BMT H at various fan speeds Ls presented in a spectral time history format. engine was mounted in the Amewind tunnel but the tunnel was not running. An overview evaluation of this type shows many charaaeristics. Peaks in the pressure spectrum show up at the fundamental fan speed and several harmonics (23 or more) at all fan speeds. There appears to be three areas of characteristic spectral shape. a subsonic region below 11,150 rpm where the fundamental fan speed and the sixth harmonic dominate the spectrum: a transonic region around 12,050 rpm where the fundamental fan speed still dominates but broadband noise below 2000 Hz increases significantly. and a supersonic region above 12,500 rpm where all the harmonics are about equal, although the sixth harmonic is still somewhat dominant and the broadband noise is no longer observable. In comparison the fan speed sweep in flight is shown in figure 6(b). An overview of these data shows that the higher harmonics have all but disappeared from the spectra. sixth harmonic still dominate the spectra at all fan speeds and there still appears to be three regions of spectral response. struts in the bypass duct (e.p. ref. 10). Since this response in the spectra is visible throughout the operating range of the engine, the BUT are probably responding to B potential field of these struts and not an acoustic signal related to propagating duct modes. mechanism are being investigated and data from these studles are now being reported, such a6 in reference 16 and 17.