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Comment by ufmace | Hacker News Reader

Well as long as we're getting technical, Mach number is a unitless aerodynamics term, not a speed. As such, it is only valid in the exact medium that a craft is traveling in and relative to that medium, so here air at the exact temperature and pressure the aircraft is flying at. There is no valid general conversion between speed and Mach number, and things like Mach number relative to the ground is nonsensical.

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>There is no valid general conversion between speed and Mach number, and things like Mach number relative to the ground is nonsensical.

Sure, if you also want to say water doesn't freeze at 0C or boil at 100C. In practice if an author casually leaves out the variables needed to calculate ground speed from mach or airspeed, it usually means they're doing engineering calculations set at the ISA standard day model (sea level, 15C, no humidity).

Yes agreed. We should be using ground speed knots here.

I replied above, but knots have a similar problem when flying (indicated air speed).

Not terribly useful to assume supersonic airflows are occuring at sea level.

Sure, but it is more relevant to describe aerodynamic properties of an aircraft. Knots can have a similar problem (look up indicated airspeed). But I guess you’re right, we should be using ground speed knots in this instance.

If you ask a pilot, ground speed only really is important for calculating ETA. Everything relevant to flying is a relative number since atmospheric density is fluid.

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