Speed of sound?

[flyhi 24]

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Also, the point about the speed of sound in air being based on temperature only is correct. In Meters/second, It is the square root of (1.4*287*T) where T is the temperature in kelvin.

At 125,000 feet (temp 246 kelvin), that gives 314 m/s, or 611 knots.

Mach 1.0 equals 49 times the square root of temperature in Rankine. Rankine equals F + 459.67 (460 is probably close enough for Felix). The units are fps.

Shit happens. And it usually happens because of physics.

[kallend 2,031]

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Drag doesn't just go up as normal when approaching the sound barrier - as soon as you get close, your coefficient of drag increases exponentially due to the shock wave you begin to generate (called wave drag).

You are correct about the big problem being controllability in aircraft. As an aircraft passes through its critical mach number (where supersonic flow begins on parts of the aircraft, but the total flow is not yet supersonic - usually around 0.7-.85 Mach), its center of pressure moves aft, causing a downwards pitching moment, known as mach tuck. Not really an issue with a jumper, as they create drag, not lift.

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The effect of shocks on different parts of the jumper has yet to be determined, so it's hard to say it won't be a stability issue.

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The only sure way to survive a canopy collision is not to have one.

[kallend 2,031]

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Also, the point about the speed of sound in air being based on temperature only is correct. In Meters/second, It is the square root of (1.4*287*T) where T is the temperature in kelvin.

At 125,000 feet (temp 246 kelvin), that gives 314 m/s, or 611 knots.

Mach 1.0 equals 49 times the square root of temperature in Rankine. Rankine equals F + 459.67 (460 is probably close enough for Felix). The units are fps.

Rankine? Jeez, how archaic! I stongly suspect Felix will use Kelvins.

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The only sure way to survive a canopy collision is not to have one.

[timmyfitz 0]

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I'm not a scientist

Yep.

Typical.

[rdufokker 6]

The problem with these charts is that they are based on standard adiabatic lapse rates in each region, i.e. 59F or 15C at sea level, and a loss of some standard amount (I think 1.7C per thousand) up to the Tropopause. The speed of sound at the earths surface is very different at 32F than it is at 100F. In fact the speed of sound at 32F is 741mph and at 100F is 790mph. Kevin Keenan are you out there? You can help!!!

Burke

Irony: "the History and Trivia section hijacked by the D.B. Cooper thread"

[muff528 3]

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The problem with these charts is that they are based on standard adiabatic lapse rates in each region, i.e. 59F or 15C at sea level, and a loss of some standard amount (I think 1.7C per thousand) up to the Tropopause. The speed of sound at the earths surface is very different at 32F than it is at 100F. In fact the speed of sound at 32F is 741mph and at 100F is 790mph. Kevin Keenan are you out there? You can help!!!

Burke

Yeah, that's true if you want to get picky!

[linebckr83 3]

Speed of sound is always the square root of (gamma * the gas constant R * temperature T). For air, gamma equals 1.4. In SI units R is 287 and T is in kelvins, in English units R is 1716 and T is in Rankine. The square root of 1.4 * 1716 is 49.014, hence your equation.

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Flying Hellfish #828
Dudist #52

[kallend 2,031]

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Speed of sound is always the square root of (gamma * the gas constant R * temperature T). For air, gamma equals 1.4. In SI units R is 287 and T is in kelvins, in English units R is 1716 and T is in Rankine. The square root of 1.4 * 1716 is 49.014, hence your equation.

Always? Only if the gas obeys the ideal gas law.

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The only sure way to survive a canopy collision is not to have one.

[JohnMitchell 16]

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Always? Only if the gas obeys the ideal gas law.

That would be ideal.

As I understand it, no elemental gas obeys that law exactly. It's just a very good approximation for all gases. Is that correct?

[linebckr83 3]

Ok that equation is always used to closely approximate the speed of sound in a gas. I thought it was understood that it is not an absolute exact.

"Are you coming to the party?
Oh I'm coming, but I won't be there!"
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Dudist #52

[billvon 3,008]

>I would guess drag. It increases significantly as you approach the speed of sound . . .

. . . but decreases the higher you go. You can easily get to an altitude where there is almost no resistance at any speed; at those altitudes it's a piece of cake to break the sound barrier. There's literally nothing stopping you.

Will he be in the no-drag region long enough to accelerate to ~1000 feet per second? If so then he'll be able to break the barrier, and will decelerate below the speed of sound as he gets to denser air.

[kallend 2,031]

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Always? Only if the gas obeys the ideal gas law.

That would be ideal.

As I understand it, no elemental gas obeys that law exactly. It's just a very good approximation for all gases. Is that correct?

It's a good approximation for air at the temperatures we normally encounter, although even nitrogen and oxygen show measurable deviation. There are some pretty big deviations in some gases, particularly when near their condensation temperature, at high pressure, or if they have particularly large or strongly interacting molecules. Steam (which is a gas) doesn't obey it very well, nor does SO2, ammonia, or CO2.

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The only sure way to survive a canopy collision is not to have one.

[kallend 2,031]

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Ok that equation is always used to closely approximate the speed of sound in a gas. I thought it was understood that it is not an absolute exact.

Don't confuse the equation with the thing it describes.

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The only sure way to survive a canopy collision is not to have one.

[riggerpaul 1]

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>I would guess drag. It increases significantly as you approach the speed of sound . . .

. . . but decreases the higher you go. You can easily get to an altitude where there is almost no resistance at any speed; at those altitudes it's a piece of cake to break the sound barrier. There's literally nothing stopping you.

Will he be in the no-drag region long enough to accelerate to ~1000 feet per second? If so then he'll be able to break the barrier, and will decelerate below the speed of sound as he gets to denser air.

In a vacuum, roughly 31.25 seconds, and 16,000 feet, right?

How high do you have to be for the drag to be so low as to be negligible?

[tjm 0]

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The speed of sound is not related to altitude or air density at all. It is only affected by temperature. This is why he will be able to break the sound barrier.

I'm sure there are some Aeronautical Engineers on here that will give the equations and explanations in further detail.

Burke

So we can't hear good under water because of the temp change? Of course sound is effected by density. Sound travels faster in water, so the human ear hear it as being distorted.

If you're not living on the edge; you're taking up too much room!

[JohnMitchell 16]

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So we can't hear good under water because of the temp change? Of course sound is effected by density. Sound travels faster in water, so the human ear hear it as being distorted.

Okay, the weight of the molecules in a gas do affect the speed of sound through that gas, but changes in pressure do not change the speed of sound transmission in gases. Only changes in temperature do that. This is an effect I deal with daily at work.

The speed of sound in a liquid once again depends on it's molecular weight, which, for a liquid, equates closely to density, and its compressibility. Changes in the speed of sound in a liquid are caused by both temperature and pressure.

[linebckr83 3]

I thought an article on the jump quoted someone in saying that they plan on transonic speeds being reached about 30 seconds into freefall.

I think that's the approximation of when the density is still low but he'll be in freefall long enough to pick up enough speed. Anything before that and he hasn't accelerated enough. Anything after that he begins to decelerate through denser air.

I also seem to recall Kittinger commenting of the buffeting of his body as he approached transonic speed, and how painful it was. It will be interesting to see how that plays out.

"Are you coming to the party?
Oh I'm coming, but I won't be there!"
Flying Hellfish #828
Dudist #52

[linebckr83 3]

So to educate me beyond what they teach us in aerodynamic theory and flight classes, is there a better equation to predict the speed of sound in a gas? Or is it just knowing the deviation some gases make by testing them?

"Are you coming to the party?
Oh I'm coming, but I won't be there!"
Flying Hellfish #828
Dudist #52

[billvon 3,008]

>How high do you have to be for the drag to be so low as to be negligible?

Well, that's the big question. At 100,000 feet the pressure is about 1% of the pressure on the ground, which is almost negligible. After that it starts increasing slowly, then more rapidly the lower you get. Drag is also a function of speed (of course.) So as he descends and accelerates he'll start seeing drag increase.

If he can get to 120,000 feet, that's 20,000 feet worth of acceleration he can get with effectively no air.

[kallend 2,031]

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So to educate me beyond what they teach us in aerodynamic theory and flight classes, is there a better equation to predict the speed of sound in a gas? Or is it just knowing the deviation some gases make by testing them?

You can use van der Waals equation as a better approximation, or even better still is the virial equation.

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The only sure way to survive a canopy collision is not to have one.

[kallend 2,031]

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The speed of sound is not related to altitude or air density at all. It is only affected by temperature. This is why he will be able to break the sound barrier.

I'm sure there are some Aeronautical Engineers on here that will give the equations and explanations in further detail.

Burke

So we can't hear good under water because of the temp change? Of course sound is effected by density. Sound travels faster in water, so the human ear hear it as being distorted.

1. Water is not a gas. For waves in media in general you need to know the density and the elastic stiffness. In ideal gases these are related by the gas law PV = nRT; assume adiabatic conditions then you can get the pressure term to cancel out, and you are left with temperature.


2. There are issues coupling vibrations in water to the ear. Human ears evolved to couple vibrations in air to the cochlea.

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The only sure way to survive a canopy collision is not to have one.

[kallend 2,031]

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>How high do you have to be for the drag to be so low as to be negligible?

Well, that's the big question. At 100,000 feet the pressure is about 1% of the pressure on the ground, which is almost negligible. After that it starts increasing slowly, then more rapidly the lower you get. Drag is also a function of speed (of course.) So as he descends and accelerates he'll start seeing drag increase.

If he can get to 120,000 feet, that's 20,000 feet worth of acceleration he can get with effectively no air.

20,000 ft of gravitational acceleration with no drag gives 771 mph.

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The only sure way to survive a canopy collision is not to have one.

[rehmwa 2]

so

"Degrees Celcius"
"Degrees Fahrenheit"
"Kelvin" (there is NOT "degrees Kelvin", just Kelvin)

What is Rankine? Is it "Degrees Rankine", or just "Rankine"

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Driving is a one dimensional activity - a monkey can do it - being proud of your driving abilities is like being proud of being able to put on pants

[kallend 2,031]

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so

"Degrees Celcius"
"Degrees Fahrenheit"
"Kelvin" (there is NOT "degrees Kelvin", just Kelvin)

What is Rankine? Is it "Degrees Rankine", or just "Rankine"

Degrees Rankine is °R

Real scientists use K.

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The only sure way to survive a canopy collision is not to have one.

[Nutz 0]

I've always liked the sound of it: Rankine. Sounds nasty.

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