🚀 go-pugleaf

In article <1992Dec23.173753.14938@mksol.dseg.ti.com> martinn@mksol.dseg.ti.com (niels r martin) writes:

>In article <alien.020z@acheron.amigans.gen.nz> alien@acheron.amigans.gen.nz (Ross Smith) writes:
>>In article <10302@ncrwat.Waterloo.NCR.COM> tjgerman@53iss6.Waterloo.NCR.COM (Trevor German) writes:

>>>       I have been following the thread about the Mach 8 spy plane
>>>       and have a general question. What exactly is MACH 1. I understood
>>>       it to be the speed of sound at sea level (or more accurately at
>>>       an air pressure defined to be sea level).

>>Mach 1 is defined to be the *local* speed of sound, so it varies with
>>altitude, but not in the way you describe. To a rough approximation (from

>If the point of the question is: "can you go faster at Mach 1 at sea
>level or at high altitude?"  The answer is probably not yes or no, but
>rather gets into the max dynamic air pressure that the structure can
>handle--for example, the SR-71 could apparently not go much faster
>than 200 mph at sea level because of the dense air causing structural
>damage if it exceeded that speed--therefore most high-Mach flight will
>be done at relatively high altitude and I suppose that when that is
>translated into mph (or knots for you aviation buffs) it's not as
>impressive as one might imagine.

Actually, the envelop of the SR-71 at sea level is 145 KEAS to 450
KEAS (that's knots equivalent airspeed; at sea level that's pretty
close to _true_ airspeed).

And you are confusing several issues.  Specifically, air speed.

true airspeed: the speed you are actually flying through the air,
regardless of density or Mach number.

calibrated airspeed: the sea level impact pressure airspeed corrected
for compressibility (this one is not used often).

equivalent airspeed: the sea level impact pressure _NOT_ corrected for
compressibility (this one is used in most pilot manuals).

indicated airspeed: the airspeed you read off of the cockpit guage
(sea level impact pressure with errors due to position and
instrumentation errors).

Note that at max Mach and altitude (std day) the SR-71 is at Mach 3.2
and 85,000 feet msl.  The _true_ airspeed is 1835 KTAS (2112 mph), and
the _equivalent_ airspeed is 310 KEAS.  And the airplane is really
covering ground at 2112 mph.

--
Al Bowers                       bowers@rigel.dfrf.nasa.gov
NASA F-18 High Alpha Research Vehicle, Lead Aero
NASA, Dryden Flight Research Facility, Edwards, California