🚀 go-pugleaf

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). Is this the case or is
>	MACH 1 the speed of sound at any pressure, ie is MACH1 a higher
>	ground speed at 100,000 than it is at sea level. 

The reference frame for mach speed comparisons is the ground.  This isn't an
"indicated vs. true" issue.

The speed of sound is hinged on *temperature*, not altitude.  On a cold
day, the speed of sound will be lower; on a warm day, it will be higher.

Therefore, as altitude increases, the speed of sound will drop.  HOWEVER, 
as we enter the thermosphere, the ambient temperature *increases*, so the 
speed of sound will increase.

For your reference, the speed of sound at sea level is about 661.5 knots,
ICAO Standard Day.  At 100K, it's 583.3 kts, same restrictions.  1 knot = 1.15
mph.  The speed of sound can be approximated by the equation:
	a = 29.04 * SQRT(T),
where T is the ambient temperature, in Rankine. 

Mach is simply the ratio of the airplane's true airspeed to the ambient
speed of sound.


If there's any more interest in this, I could dig up the archived articles
from the last three or four times it was discussed.  I believe we got it
right, the last time. :-)


Lastly, a nit:

>       If it is the later (ie pressure dependend) how fast would a
>       Mach 8 plane be going in ground speed.

Ground speed is a navigation term, the resultant of an free-stream (true)
airspeed and the prevailing wind vector.  



---
Robert Dorsett
rdd@cactus.org
...cs.utexas.edu!cactus.org!rdd

 I'm sure I might be corrected by the more elite members of this net, but 
 from my old Fluid Mechanics book, the speed of sound in a medium is

 c = sqrt(kRt) where k is the specific heat ratio  Cp/Cv, R is the gas constant,
 and T is the temp.  For air, k = 1.4 at sea level, R is 53.3 ft-lb/lbm R

    T = 459+70 = 529 R
   so for sea level, c = sqrt(1.4*53.3 * 32.2 * 529) = 1126 fps

    in the stratosphere, where T = -67F, c = 971fps

  hope this helps...

In article <1992Dec16.101747.25799@cactus.org> rdd@cactus.org (Robert Dorsett) writes:

>The speed of sound is hinged on *temperature*, not altitude.  On a cold
>day, the speed of sound will be lower; on a warm day, it will be higher.

I remember reading about when North American/USAF wanted to retake the
world speed record from Douglas/USN's F4D Skyray. They had to fly the
F100 Super Sabre at least 1% faster for it to count. This was in the
fall of 1953. They waited for an especially hot day in the Mojave
desert, so the Hun could go faster without running into transonic drag
rise. I believe they made it by just a couple of mph. The interesting
thing is that they were not allowed to gain any altitude during the
two runs (in opposite directions) and since they flew low to get
maximum TAS, the pilot had to make a 270 degree turn at just under
Mach 1 flying about 50-100 feet above the ground over the desert. 

Now that must have been a hell of a ride, especially in the choppy
desert air. I read they used burning piles of tires to mark the
course.

Gopal

-- 

Gopal Ramachandran <gopal@cirrus.com>
UUCP:  uunet!cirrus.com!gopal
Cirrus Logic Inc, Fremont CA  PH: (510)-226-2138 FAX: (510)-226-2160

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). Is this the case or is
>       MACH 1 the speed of sound at any pressure, ie is MACH1 a higher
>       ground speed at 100,000 than it is at 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
memory, so I may not have the figures exactly right, but it's certainly
something like this), M1 is 1225 km/h at sea level, *falls* to 1062 km/h
at 11,000 metres, remains more or less constant from there to about 30,000
metres, then starts slowly increasing.


--
...... Ross Smith (Wanganui, NZ) ...... alien@acheron.amigans.gen.nz ......
   Mine is the ship bound for Alpha Centauri
   We must be out of our minds
   For though we are shipmates bound for tomorrow
   Everyone here's flying blind                          (Crystal Gayle)
--

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). Is this the case or is
>>       MACH 1 the speed of sound at any pressure, ie is MACH1 a higher
>>       ground speed at 100,000 than it is at 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
>memory, so I may not have the figures exactly right, but it's certainly
>something like this), M1 is 1225 km/h at sea level, *falls* to 1062 km/h
>at 11,000 metres, remains more or less constant from there to about 30,000
>metres, then starts slowly increasing.
>
>

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.


-- 
_____________________________________________________________________________
Niels Martin			   |                      
martinn@mksol.dseg.ti.com	   | Quos deus vult perdere prius dementat!
My opinions don't represent my employer's--but they're right!

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

and various airframe and engine heat limits

>--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--

In 1968 on Armed Forces Day, I was a guest of the father of a high-school friend
at the air show at Pt. Mugu NAS.

*The* highlight of the day was a flyby (suprise!) by an SR-71.  Two, actually:
The first with everything hanging out at a bit over 200 mph.

The second was better: cleaned up, just barely sub-Mach 1 until it passed over
the beach, went into afterburner and climbed away and out of sight in a few
seconds.  Took hours for the grins to dissipate. (Huge noise as the throttles
went forward, too.)

--
-------------------------------------------------------
| Some things are too important not to give away      |
| to everybody else and have none left for yourself.  |
|------------------------ Dieter the car salesman-----|

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

alien@acheron.amigans.gen.nz (Ross Smith) writes:
>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). Is this the case or is
>>       MACH 1 the speed of sound at any pressure, ie is MACH1 a higher
>>       ground speed at 100,000 than it is at 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
>memory, so I may not have the figures exactly right, but it's certainly
>something like this), M1 is 1225 km/h at sea level, *falls* to 1062 km/h
>at 11,000 metres, remains more or less constant from there to about 30,000
>metres, then starts slowly increasing.

I guess when you talk about the speed of an aircraft, using the speed
of sound at a reference state (sea level at a certain temp. and
pressure, etc) makes sense.  But when you talk about the flow
characteristics around the aircraft, the local Mach number should
definitely be used.  

-- 
 Alvin W. Law ........................................... Oracle Corporation
 Senior Applications Engineer ............... 300 Oracle Parkway, Box 659306
 Oracle Manufacturing ............................. Redwood Shores, CA 94065
 Email: alaw@oracle.com ....... Voice: 415.506.3390 ...... Fax: 415.506.7299

> Mach 1 flying about 50-100 feet above the ground over the desert. 

> Gopal Ramachandran <gopal@cirrus.com>
> UUCP:  uunet!cirrus.com!gopal
> Cirrus Logic Inc, Fremont CA  PH: (510)-226-2138 FAX: (510)-226-2160

How low can you go in a supersonic aircraft before things like ground effect
and shock waves bouncing off the ground make flying difficult?

    ++PLS

In article <72541@cup.portal.com> pls@cup.portal.com (Paul L Schauble) writes:

>> Mach 1 flying about 50-100 feet above the ground over the desert. 

>How low can you go in a supersonic aircraft before things like ground effect
>and shock waves bouncing off the ground make flying difficult?

A lot lower than you would want to fly the aircraft.  The Australians
are particularly good at this sort of thing with their F-111As.  They
usually practice their coastal defence exercises by coming in off the
water at 20 to 30 feet (TFR) at Mach 1.2.  Typical fuel burns are just
over 100,000 pounds per hour for _EACH_ engine at those sorts of
conditions (you obviously don't do it for very long).  A fully loaded
-111 at 20 feet off of the water leaves quite a roostertail in the
water...

I don't know of any other larger aircraft that does this
supersonically, which makes the -111 so impressiver.  I imagine a B-1
(A or B) could do it, but I don't know that they are used like this...

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

This is a test. Sorry to bother anybody.