Has there been any airplane that has been able to cruise in both the subsonic and transonic regime where the drag is high because the critical Mach number has been reached or surpassed?
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2 Answers
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If we define the critical Mach number as "the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it," then yes, many aircraft cruise above this speed. Any modern jet airliner has some supersonic flow over parts of the wing and fuselage at cruise, placing it squarely in the transonic range. This video and its description provide a great visualization and explanation of the associated phenomena.
It should be noted, however, that there is not necessarily a large increase in drag upon reaching the critical Mach number. More useful in the transonic range is the drag-divergence Mach number, which is always greater than the critical Mach number. Supercritical airfoils are designed specifically to increase the drag-divergence Mach number, and are fitted to all modern jetliners.
answered Dec 31, 2017 at 17:44
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@Peter gives a quite clear explanation. I would like to add that the speed range of 'subsonic' and 'transonic' actually overlaps with each other, for subsonic flight, the speed is less than Mach 1, whereas for transonic flight, the speed the between Mach 0.8 and Mach 1. Nowadays, most commercial aircraft fly at a speed between Mach 0.8 and Mach 0.88, well with the transonic range.
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1$\begingroup$ It's true enough that the literal definition of "subsonic" does mean "less than the speed of sound," but that's not the same as "subsonic regime," which ends at the beginning of the transonic regime. See Wiki: Mach regimes. $\endgroup$– reirabJan 1, 2018 at 10:18