It is well known that the high levels of noise present in conventional hypersonic ground-test facilities cause transition to occur earlier than in flight. Flight measurements of incoming noise are reviewed and compared with measurements in ground-test facilities, of both conventional and quiet design, at hypersonic and high supersonic speeds. The low noise present in flight is apparently the reason for the very large transition Reynolds numbers sometimes measured in flight, when roughness, crossflow, and other factors are controlled.
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