Mach meter

The speed of sound varies with temperature. At sea level Mach 1 is around 742 mph. It decreases with altitude until it reaches about 661 mph at 36,000 feet, then remains at that speed in a band of steady temperature up to 60,000 feet. Because of the variation, it is possible for an airplane flying supersonic at high altitude to be slower than a subsonic flight at sea level. The transonic band extends from around Mach .8 – when the first supersonic shock waves form on the wing – to Mach 1.2, when the entire wing has gone supersonic.
Source: Air & Space/Smithsonian.

So my question is: Does the Mach meter on supersonic aircraft adjust to "local" conditions? Are there other instruments aboard that indicate air speed in knots or miles per hours?

And if the Mach meter does adjust to local conditions and a fighter pilot comes home from work and tells his wife he hit Mach 1.8 today and she asks him, "How fast were you really going?" – how does he do the conversion to tell her in terms of miles per hour?

That's a nice summary. The numbers you quoted assume standard day, as in 29.92 and 15 degrees C. The tropospheric altitude also varies with season and latitude. Also, the transonic band is a function of aerodynamics.

The mach meter is very sensitive to local conditions. In fact, the readout the pilot gets is only a secondary to the CADC (central air data computer), what the aircraft cares most about is giving that data to the motor so bad things don't happen. The FCC will also give you KCAS or TAS, and it can do the math based on present altitude winds to give you GS as well.

If you're still telling your wife you went supersonic to get her to sleep with you, you need to reevaluate your marriage, that trick only works when you are single. But to answer that question, a rough estimate is that your true airspeed is equal to mach number times ten, in miles per minute. So Mach 1.2 is 12 miles per minute, or 720 knots. Roughly. Sometimes.

Nothing cool happens in the cockpit when you go supersonic. In the Block 50, you can do it in mil power even, so your really have to watch where you are. Also, once you get that fast in the block 50 you can pull 9Gs and keep on accelerating. Your body will give in before the jet. Your fuel quantity might give in first actually. As far as cockpit indications go, your altitude readout will skip about 1,000 feet as you hit the mach and then find its way home again. That's it.

Yo ho!

Sheesh, STBYGAINs is starting to gain my respect, even if he is still a young sprog, still full of piss and vinegar!

Your best bet to impress someone is your TAS when no wind, or groundspeed when sailing along downwind in the jet stream. We used to see 140 knots routinely in northern parts of the U.S..

Just flip the switch that displays TAS or GS on the HUD.

Remember that if the airspeed indicator or altimeter is not connected to CADC, then the gauges jump when going thruthe mach. The HUD mach doofer is steady as a rock.

BTW, the temperature is the real deal. Ask Habu about the Blackbird's mach.

later,

Thank you all, especially STBYGAIN, you've answered my question.
There's a list of acronyms here on the web site and I didn't find FCC and KCAS on it. Can you help me out? (I know, which way did you come it?)

Fire Control Computer, which is the central processor for most avionics displays.

Knots Calibrated Airspeed, which is Knots Indicated Airspeed corrected for cockpit installation.

Just a small correction, the speed of sound in a medium varies with, and is proportional to, the density of the medium. Density is affected by both temperature and pressure. Generally speaking, as altitude increases both pressure and temperature drop. Lower pressures mean lower density, lower temperatures mean higher density. The net change due to these changes is a lower density at higher altitudes, so the speeed of sound in the medium is lower. Taken to extremes, the speed of sound in a vacuum approach zero - that's why you can't 'hear' in outer space. It also means your Mach numbers in space approach infinity....

Physics - we put the "ph" in fun...

Ahem, cough, wheez......

I'm not so sure, habu deux.

Check out this site - it's an old one, but still has super stuff for all the newbies to use:

http://www.best.com/~williams/avform.htm

I just checked it, and it's still good. Look for the Mach calculation, hint, hint.

Mach is a function of temperature!!!!!! neeenah neenah neeenah.......

LOL

Sure, go up and temperature goes down. Go up a lot and temperature rises. Hmmmmm. Go up some more and temperature is meaningless, as there ain't no air!!! Indicated speed goes down the higher you go - less of those air molecule doofers impacting you. 'dynamic pressure', so to speak.

Seems to me that when I zoomed all over the place with my hair on fire that the true airspeed wasn't that much different at 50,000 ft than sea level for a certain mach number. Guess the density and temperature cancelled out..... will have to look at some std atmosphere tables. JR can pitch in here, as the Zipper really flies fast and high, like the Habu itself...... STBYGAIN can also help by running an experiment on his next hop, huh?

HABU WARNING HABU WARNING!!!!!!

My roomie, who flew on 4 shuttle missions, said it was really weird during re-entry. Mach meter started at 25, airspeed zero!!!

good question by the young sprog, in any case. And at least we are polite enough to try to add our two-cents worth, regardless of the question.

out,

Gums wrote:Ahem, cough, wheez......

I'm not so sure, habu deux.

Check out this site - it's an old one, but still has super stuff for all the newbies to use:

http://www.best.com/~williams/avform.htm

I just checked it, and it's still good. Look for the Mach calculation, hint, hint.

Checked it out - good site.

Gums wrote:Mach is a function of temperature!!!!!! neeenah neenah neeenah.......

LOL

That's what the first part says. If you read the rest of his derivation you will see that density altitude (aka change in density as a function of atmospheric pressure and altitude) is a factor in determining TAS.

Gums wrote:Sure, go up and temperature goes down. Go up a lot and temperature rises. Hmmmmm. Go up some more and temperature is meaningless, as there ain't no air!!!

....and no pressure...so no density....

Gums wrote: Indicated speed goes down the higher you go - less of those air molecule doofers impacting you. 'dynamic pressure', so to speak.

...lower dynamic pressure because there is less atmosphere == less density...

(small snip)

Gums wrote:My roomie, who flew on 4 shuttle missions, said it was really weird during re-entry. Mach meter started at 25, airspeed zero!!!

Not sure how the shuttle determines airspeed during re-entry, but obviously airspeed isn't zero or they would not need those thermal tiles etc.

To summarize, the speed of sound in a medium is a function of density. Density is a function of material properties, temperature and pressure. Mach is a ratio of the speed of sound to calibrated air speed. It all goes back to PV=nRT....

Hey STBYGAIN, nice that the 50's can do Mach in mil power, of course we can to, but it’s in a 1961 model Lockheed Fighter. If we just had the -19 engine with 4,000 lbs more go juice!

Gums,

When we are down in the weeds, on a “spring” day, Mach is showing about 660 kts on our old steam gauges. But doing an x-cntry at 290 we are showing Mach at about 390 kts and at FL 350 about 340 kts. Attached are some Zipper “steam gauge” examples...

Hey JR, cool pics, and I also wanted to point out to you that you're a tv star now...er...well, sort of. See, I was watching "Legends of Airpower" the other day on Discovery Wings, and the documentary was about Steve Ritchie. Well, they said one of his first assignments out of flight school was flying Zippers at Eglin. The Zipper they showed as an example? Why, yours of course! Whodathunkit? Good looking jet! If any of you have digital cable or satellite, look out for a replay of that documentary. Oh yeah, and I also took my pic of the first time I broke Mach...one of those silly things you do to get the girlfriend all excited.

Habu deux! Give up!

I gotchya by the gonads and I ain't givin' up!

Mach( the ratio of true airspeed to speed of sound) is a function of temp and that's my story - I stickin' to it......

I refer you( and others) to a U.S. Navy site which clearly shows that Mach is a function of temperature.

https://ewhdbks.mugu.navy.mil/mach-as.htm

Note that one chart that shows a CONSTANT mach true airspeed above about 36,000 feet. That's 'cause the temp stays fairly constant from 36,000 until way way up there. If mach had much of a density component, then mach would have a different TAS above 36,000 feet, right? But it doesn't.

Man, you new guys are giving us old pharts a hard time, sniff, sniff.

Gums, I ain't givin' up 'cuz I ain't wrong. And you're not wrong either - you just aren't seeing the whole picture.

Again, I went to the (new) link you provided, and it clearly shows the role temperature and pressure play in determining the speed of sound. All of these examples that show Mach changing with temperature also stipulate the condition of a 'standard atmosphere' (I'm sure you know what that means)

Also, the charts use 'pressure altitude' not AGL or height above MSL because pressure is a factor. The pressure altitudes in the chart are based on a standard day - change the pressure and/or temperature from standard day and you get a different set of pressure altitude curves.

As for why your chart shows a "constant" true mach speed above 36K, that is because density does not change linearly with altitude.



At 36K about 80% of the air is below you, so density changes very slowly from there on up - making temperature the dominant factor. If you only change one variable (temperature) it doesn't mean the other variables (pressure, density) have no effect, it means you are keeping the other variables constant. Hold temperature (and everything else) constant and change the pressure and I guarantee you your Mach number will change too.

Gums wrote:Habu deux! Give up!

I gotchya by the gonads and I ain't givin' up!

Mach( the ratio of true airspeed to speed of sound) is a function of temp and that's my story - I stickin' to it......

He's right Greg. Come on, you should know this....temp devs made all the difference when the Habu was flying a mission. Above avg temps really would cut into their cruising speed, and sometims could not hit the 3.2 Mach nominal crusing speed. Yes, pressure does play a role, but not as much as temp, at the higher altitudes. All the narratives I've read of the Blackbird are consistent in mentioning the temp devs. In fact I can't even recall a description where pressure devs were even mentioned.

OK look I don't want to get into a flamefest - I was 6 hrs away from a physics degree when I got my mechanical engineering degree. I've got an extensive background in physics, thermodynamics, fluid dynamics and aeronautics, so I'm not just basing my arguement on something I read on some website.

Now, the reason everyone "ignores" density in Mach calculations is that the changes in density are small in the altitudes that most airplanes fly at speeds where Mach is measured. But it is still a factor. Don't we have any F-111 guys here who know about flying at high speed at low altitudes?

My original post stated the speed of sound depends on the density of the medium. Mach is derived from the speed of sound. I can't help it that all the fast jet guys only learned that Mach is only a function of temperature because of the reasons I stated above, but the physics of sound pressure waves prove it. The speed of sound in a vacuum is ZERO because the density of a vacuum is zero - doesn't matter what the temperature is. The speed of sound in a solid (yes there is such a thing) is MUCH faster than in air, and it is because of the higher density of the solid. Granted, you aren't going to measure your Mach number while trying to fly through a steel bar, but there is still the physical phenomenon of sound waves travelling through the medium - the speed of sound.

I respect all the guys who actually fly these jets, but physics wasn't invented by pilots. The reason we can design and build jets that can do what they do is because of a bunch of geeky engineers with a thorough understanding of the physics involved. Oh, and taxpayers...

So what's your point?