1. There are few measurements of noise levels in flight that are carried out at sufficiently high frequencies to be useful for transition studies. The available literature wa.s reviewed by Bushnell [7], who cites References. The primary source is the work of Fisher and Dougherty [20, 19], who performed measurements with a 5- degree half-angle cone and a pitot tuhc1in wind tunnels and in flight. B&K microphones were u.sedfor static-pressure measurements at the cone surface, and a Kulite pressure transducer was used for the impact-pressure measurements. The bandwidth for both instruments was roughly 0-25kHz. Both cone and pitot tube were placed ahead of the F-15 aircraft. As might be expected, the rms pressure fluctuations on the pitot tube decrease by a factor of about 5 as the Mach number increases from 0.4 to 1.6. This is presumahly caused by the inability of aircraft noise to propagate forward in a supersonic flow. At the highest Mach number flown, Mach 2, Table I in Ref. [20] shows that P;rms/Pt,merm was 0.020% for flight 346 and 0.0057% for flight 340. Herc, p;rms isthe rrns pitot pressure fluctuatio 11, and JJt,men is the mean pitot pressure; both flights were at about 38kft. altitude.
2. Dona19son and Coulter report measurements of frcestream fluctuations carried out at Mach numbers ranging from 4 to 8, in AEDC tunnels A and B [15]. These measurements are less conservative than Fisher's flight data, for the wind-off noise was directly subtracted from the measurements during flow. A direct suhtraction assumes complete correlation of the two 'random' signals - a more conservative approach would be to difference the squares of the signals and take the square root [3]. The massflow fluctuations at Mach 8 in 1\mnel B are about 1-2%ofthe mean; the total temperature fluctuations are about 0.1%of the mean. The levels measured at Mach 6 are similar. Although the scaling between pitot-prcssure fluctuations, mass-flow fluctuations1and static-pressure fluctuations remains unresolved, available data indicates these quantities normalized by their mean values are generally within a factor of
3. Figure 2: Comparison of transition onset Reynolds numbers for sharp cones at zero angle of attack. From Chen [1989]