Author Topic: Question about Mach Numbers (Read 1952 times)

calan · « **Reply #15 on:** November 22, 2006, 05:57:32 PM »

Quote

Originally posted by Traveler
Then you thought wrong, go to Google search speed of sound.

ummm... I don't think so (but we may have a communication gap going here). The density of air does affect it, but the density is determined (mostly) by temperature.

For any given temperature, the pressure and density will remain constant and cancel out of the equation, thus making it possible to say that it is only dependent on temperature. But in affect it is still the density of the air that determines how many molecules are available to transmit molecular motion.

Or I could be full of crap... I dunno...

Traveler · « **Reply #16 on:** November 22, 2006, 06:05:01 PM »

Quote

Originally posted by calan
ummm... I don't think so (but we may have a communication gap going here). The density of air does affect it, but the density is determined (mostly) by temperature.

For any given temperature, the pressure and density will remain constant and cancel out of the equation, thus making it possible to say that it is only dependent on temperature. But in affect it is still the density of the air that determines how many molecules are available to transmit molecular motion.

Or I could be full of crap... I dunno...

if you look at the formula for speed of sound you will see that air density is not part of the formula only air temp. So while your statement about pressure and density is correct and I agree with you. It is not part of the formula. if you know of a different formula for the speed of sound that includes pressure altitude , what is it?

calan · « **Reply #17 on:** November 22, 2006, 06:12:15 PM »

From wikipedia:

*****************

In general, the speed of sound c is given by

where

C is a coefficient of stiffness
p is the density

Thus the speed of sound increases with the stiffness of the material, and decreases with the density. For general equations of state, if classical mechanics is used, the speed of sound c is given by

where differentiation is taken with respect to adiabatic change.

*****************

I'll agree to disagree

NCLawman · « **Reply #18 on:** November 22, 2006, 06:22:05 PM »

Quote

Originally posted by JB88

OH MY GOD! AH has the same temperature at all altitudes??? That's it, I quit, I am never playing this CARTOON game again. I just can't handle the lack of REALISM!

calan · « **Reply #19 on:** November 22, 2006, 06:26:45 PM »

I'm an expert on the relative air temperature at different altitudes in AH...

I have fallen through all of them MANY times with nothing but a chute above me. And it was very cold on my widdle toes until I got closer to the ground.

E25280 · « **Reply #20 on:** November 22, 2006, 06:58:09 PM »

Quote

Originally posted by calan
I thought it had as much (or more) to do with air density....

Air density decreases as altutude increases, so you wouldn't need both. Alt assumes a given density, more or less.

FiLtH · « **Reply #21 on:** November 22, 2006, 07:07:08 PM »

I think the key word was...dry.

Hornet33 · « **Reply #22 on:** November 22, 2006, 07:11:14 PM »

I may be a little off base here but......

WHO CARES????????????

Atoon · « **Reply #23 on:** November 22, 2006, 07:14:53 PM »

BOOOOOOOMMMMMM!!!!!!!!

I'm pretty sure that's what he was lookin for in the first place:p

calan · « **Reply #24 on:** November 22, 2006, 07:22:21 PM »

Quote

Originally posted by Hornet33
WHO CARES????????????

I don't... but I always enjoy expanding my knowledge and learning something a bit more complex than which hat button fires the whoopie bombs.

But to each his own.

dtango · « **Reply #25 on:** November 22, 2006, 07:37:56 PM »

Quote

Originally posted by Traveler
You are correct about "you can't plug in the 262 volocity into the equation" however, you take true airspeed and calculate for velocity. Search Google for speed of sound. The forumla is there.

Traveler -

TAS is the velocity. That's what E6B is telling you. No other conversions needed. The speed of sound is measured on TAS as well. You also can't come up with mach numbers without TAS. Trust me. Stop for a second and think about what I have said above. I'm not trying to prove you wrong, just trying to illuminate the subject.

Thanks for pointing out google, but I have several aero text books btw so no need to look it up on the net

.

Tango, XO
412th FS Braunco Mustangs

E25280 · « **Reply #26 on:** November 22, 2006, 07:40:15 PM »

Quote

Originally posted by Traveler
When I tested the 262 at see level it yielded a Mach of .67 for a true airspeed of 505 mph. That translated to an outside air temp at see level of -24 F. What I should have gotten was a air temp of 58 F.

You are also assuming the .67 and 505mph are distinct and exact. Most likely they are neither, thus you have errors due to rounding. For example:

505.49mph / .665 Mach = Approx 760.1mph speed of sound
505mph / .67 Mach = Approx 753.7mph
504.5mph / .6749 Mach = Approx 747.5mph

Plus you appear to have a small conversion error based on HiTech's table. You posted:

Quote

Originally posted by Traveler
1116.89f,/* Alt 0.000000*/ yields 762.5588812800001 mph at an air temp of 58F
1097.53f, /* ALT 5000.000000*/ 749.3408025600001 mph 40 F
1077.81f, /* ALT 10000.000000*/ 735.87693312 mph 23 F
1016.38f, /* ALT 25000.000000*/ 693.93547776 mph -32.5 F

Converting fps directly to mph (perhaps you converted to meters/second then back to miles / hour?) yield the following results:

1116.89 fps * 3600 seconds (1 hr) / 5280 feet (1 mile) = Approx 761.5mph
1097.53 * 3600 / 5280 = Approx 748.3mph
1077.81 * 3600 / 5280 = Approx 734.9mph
1016.38 * 3600 / 5280 = Approx 693.0mph

All about 1 mph less than what you have.

This, of course, assumes the Bill Gates product I am using is error free, which could be a stretch.

At any rate, the combination of small rounding / conversion / etc. errors that creep into formulas like this will often give unexpected results.

dtango · « **Reply #27 on:** November 22, 2006, 07:45:26 PM »

e2580:

He's making a much bigger error. If it was creep etc. then the SL temp would be 48F, not the -24F he came up with. That's a big difference. Infact for -24F the SL speed of sound should be around 699 mph.

HTC know's their aero physics. Very doubtful there are holes that big in their employment of the physics model.

Tango, XO
412th FS Braunco Mustangs

E25280 · « **Reply #28 on:** November 22, 2006, 08:01:41 PM »

Quote

Originally posted by dtango
e2580:

He's making a much bigger error. If it was creep etc. then the SL temp would be 48F, not the -24F he came up with. That's a big difference. Infact for -24F the SL speed of sound should be around 699 mph.

HTC know's their aero physics. Very doubtful there are holes that big in their employment of the physics model.

Tango, XO
412th FS Braunco Mustangs

I'm barking up the wrong tree, then . . .

ridley1 · « **Reply #29 on:** November 22, 2006, 08:37:30 PM »

what? Is Stephen Hawking playing this game?

He does have a joystick built into his wheelchair, don't he?