Wind drift anomalies?

alinwa said:
Yes, the wall changes with conditions but "the speed of sound" under a given set of conditions is fixed. "Speed of sound" must be defined as XXX velocity @ XXX temp/hum/alt/barom etc.

al
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Al,

Speed of sound in air is directly dependent on temperature with minor influence from humidity. SoS is not influenced by air density. I had thought that SoS increased with air density since we know that it's faster in water and steel but this is wrong. Yeager's team chased the sound barrier at high altitude because of high speed/low drag (low density at altitude) with the side benefit of lower speed of sound because of much lower temperature at high altitude.

I had thought that Federal UM 1 rimfire ammo that was slightly supersonic lost its crack with dropping temperature because the speed of sound increased with the resultant increase in air density. Henry got me past this misunderstanding of the physics of it. Actually, the speed of the ammo drops faster with decreasing temperature than the speed of sound drops and the two cross at about 50 degree F.

It helps to remember that the speed of sound at a temp of absolute zero is also zero.

Greg
 
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To greg culpepper....

The speed of sound in air is equal to the sq. rt of P/r where P=pressure and r=density. There are approximate formulas that involve only temperature for SOS in air but density is in there. Believe me the SOS depends on density in all materials.
 
pacecil said:
The speed of sound in air is equal to the sq. rt of P/r where P=pressure and r=density. There are approximate formulas that involve only temperature for SOS in air but density is in there. Believe me the SOS depends on density in all materials.
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Cecil,

True as far as you go but since pressure and density of air change together in lockstep with each other (ideal gas law) and one is divided by the other in your formula, when applied to earth atmosphere and not some other gas or combination of gasses, the resulting value is a constant and may be ignored as a source of change. Water vapor as a variable does have a minor effect on SoS as its molecular weight is different than the average for nitrogen/oxygen-air. Temperature derived SoS values in earth atmosphere are far better than mere approximations for our purposes, even over very large changes in pressure and attendant density.

Greg
 
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Duh,,

I believe it was Hemingway who said, and I'm paraphrasing,

"Better to remain silent and appear dumb than to open one's mouth and remove all doubt."

Boy, I'm sure glad I stayed outta' this one!

Gene Beggs
 
To greg culpepper....

The equation you use for calculating SOS, which involves only temperature assumes pressure and density are constant. This is why it is only approximate, It's perfectly fine for standard conditions or near standard conditions. It assumes some standard density and is probably accurate enough for everything we're doing here. You must know density and pressure to determine true SOS. You CANNOT do it with temperature alone. Your statement that P/r is a constant is true only if these are the only air properties that were changing.
 
pacecil said:
The equation you use for calculating SOS, which involves only temperature assumes pressure and density are constant.
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Wrong. I assume nothing of the kind. But pressure and density always change together and proportionately to each other, always. If pressure doubles, so does density, always. So the ratio between them is constant and changes nothing.
This is why it is only approximate, It's perfectly fine for standard conditions or near standard conditions. It assumes some standard density and is probably accurate enough for everything we're doing here. You must know density and pressure to determine true SOS.
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Standard air pressure is 14.7 psi at sea level. Standard air density is 0.075 lb/cubic ft at sea level. Travel up until pressure is 14.7/2 or 7.35 psi and density will also be haved to exactly 0.0375 lb/cubic ft.

14.7/0.075=7.35/0.0375

It's the same value, the same ratio, exactly, always, everytime and has no effect on SoS because a change in pressure indicates a change, the same change, in density. Go up or down in the atmosphere and the pressure changes exactly the same percentage as density and exactly inversely to volume.
You CANNOT do it with temperature alone.
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Temperature is the only thing that changes SoS in air (ignoring minor changes from water vapor)
Your statement that P/r is a constant is true only if these are the only air properties that were changing.
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What other air properties are you talking about?

Greg
 
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Lively Discussion!

alinwa said:
Smart Feller!

;)

al
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This thread has certainly been lively and interesting. Some great minds at work here.

Pacecil has probably put me on his ignore list but I enjoy reading his posts and must say he has proven himself to be very smart and very well educated.

Greg and Al, you guys have been doing your homework. Keep up the good work. I love you all and look forward to visiting with you face to face; including you, Pacecil.

Gene Beggs
 
Greg Culpepper said:
Al,

Yeager's team chased the sound barrier at high altitude because of high speed/low drag (low density at altitude) with the side benefit of lower speed of sound because of much lower temperature at high altitude.
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AND...... let's not forget the added benefit of several more minutes and miles before you smoke a hole!

al
 
I'm looking forward to that visit as well, Gene. Maybe we can get Al to Odessa at the same time.

Greg
 
Well, I think I've learned some things......

I've always wondered why you even need wing surfaces on a swept-wing SS jet (other that to support the leading edge!) and pacecil has given me impetus to look into it more. I'm still tempted to argue that the only function of the wing is to change the plane of the shockwave but I'd best shut up and lissen a liddle, aeronautics not being my bailiwick. I'm working on theory alone in this arena.

And Greg...... Temp only huhh..... it's easy to see that pressure and density are in lockstep but ??? Temp only???

So pressure and density effects cancel in a gas but not in a liquid where of course they're ALSO lockstepped...


hmmmmm, fun.


al
 
I'd love the face-to-face...... pacecil too...... but I've learned that TIME is a killer!

Need days not hours

:)

Odessa sounds like the logical place!
al
 
alinwa said:
Well, I think I've learned some things......

I've always wondered why you even need wing surfaces on a swept-wing SS jet (other that to support the leading edge!) and pacecil has given me impetus to look into it more.
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Supersonic landings are tough on the equipment and the pilot. Take offs to. Not to mention rate of turn which is what kills in a dog fight. SS is just for getting to the fight. You need a wing to hold you up the rest of the time.
And Greg...... Temp only huhh..... it's easy to see that pressure and density are in lockstep but ??? Temp only???
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If you take away pressure/density what else is there? Phase of the moon?
So pressure and density effects cancel in a gas but not in a liquid where of course they're ALSO lockstepped...
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Air is elastic so pressure and density change together. Not so for liquids. A quart of water at the depths of the ocean weighs the same as at the surface. But the pressure.....
hmmmmm, fun.
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You betcha!


Greg
 
Nope you are wrong. A quart of water at the bottom of the ocean is more dense, thus weights more. All substances are compressible, some just more that others. In most cases you don't have to deal with compressibility of liquids and solids but there are instances in engineering, and for that matter in practical every day applications, where you do.
 
pacecil said:
Nope you are wrong. A quart of water at the bottom of the ocean is more dense, thus weights more. All substances are compressible, some just more that others. In most cases you don't have to deal with compressibility of liquids and solids but there are instances in engineering, and for that matter in practical every day applications, where you do.
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Really. Can you give us an example in practical everyday life where the compressibility of fluids needs to be dealt with?
 
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If you use silicone based brake fluid you will have a mushy pedal because of the greater compressibility of the silicone fluid. That's about the only instance I can think of at moment for "everyday" life. However compressibility can give problems when engineering high pressure devices such as pumps, compressors, and hydraulic systems. It also has to be considered in lubrication, metal forming, and actually chemical processing, - any place where liquid at high pressure is involved.
 
So, practically speaking, I shouldn't use silicon fluid in the (muzzle) brake of a .22 rimfire?

Geez. And bowling balls in the gunsmith forum?
 
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