Forum: General F-22A Raptor forum

outlaw162 wrote:

The speed of sound decreases as you go up in the troposphere until you hit a wide band of constant temperature called the tropopause where the speed of sound remains constant.

The speed of sound then increases again as you go higher into the next band, the stratosphere, & then decreases once again as you enter and ascend in the mesosphere.

Off the top of my head I don't recall the the specific altitude bands associated with each layer, but obviously 90,000 feet is in the stratosphere.

OL

Why? I don't know. I've never been out of the troposphere. (or very far away from home)

Doesn't air DENSITY play into it just a tad? Question

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Strykerxo:

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Financial constraints may force us to do more with less, but that will come at a different cost as well. Legacy AC are will not fly forever and adversaries are not going to stop thier progress.

Good point IMO, Stryker.. Indeed, USAF/DoD better start actively 'doing more with less', lest they lose a lot more strategic air combat deterrence value than they already will over the next 10 yrs, due to unfortunate political-military misplanning and miscalculations.

Time to get seriously industrious in maintaining our own sufficient progress/superiority with less, employing the best solutions and 'knowhow' possible. (we must prepare for an inevitable program haircut and fat-cutting lifestyle, while minimizing muscle loss in the changing process)

em745 wrote:

Doesn't air DENSITY play into it just a tad? Question

Actually, it doesn't. The speed of sound in an ideal dry gas is sqrt(gamma*R*T), where gamma is the adiabatic index (~1.4 for earth atmosphere), R is the ideal gas constant (8.314 J/molK) and T is the absolute temperature. Again, for earth's atmosphere, this comes out to about 331 m/s, or 1125 ft/s.

Density has nothing to do with it in a gas.

Density (of air) is inversely proportional with temperature.

One can write T in the speed-of-sound equation, in terms of p (density), P (pressure) and R (specific gas constant).

But there is no need to do so...

gtg947h wrote:

em745 wrote:

Doesn't air DENSITY play into it just a tad? Question

Actually, it doesn't. The speed of sound in an ideal dry gas is sqrt(gamma*R*T), where gamma is the adiabatic index (~1.4 for earth atmosphere), R is the ideal gas constant (8.314 J/molK) and T is the absolute temperature. Again, for earth's atmosphere, this comes out to about 331 m/s, or 1125 ft/s.

Density has nothing to do with it in a gas.

Granted, my physics-101 courses are quite a ways back, but I recall learning at some point that sound travels faster in denser mediums (hence why sound travels much faster and farther in water than in air).

Now then, air density starts to taper off (at a logarithmic rate) the moment you climb straight up from sea level... right up until you reach space. And we all know that the speed of sound in a vacuum is nil. So...

*shrugs*

r2d2 wrote:

Density (of air) is inversely proportional with temperature.

The "ideal gas law," yes. I recall THAT too.

Right up until well above the 60,000 feet mark, air temp falls by about 10.8°F for every 3,300 foot increase in altitude (that's why we have snow-capped mountains even in temperate climes). Now, wouldn't that mean an INCREASE in the speed of sound the further up you go?

Thing is, like I said above, air density (and for that matter air pressure, which is directly related) gradually falls the higher up you go, and that decrease has nothing to do with temperature (obviously), and has everything to do with gravity having less "pull" on the atmosphere's molecules at higher altitudes. THAT's why the speed of sound diminishes the further up you go... at least whithin the altitude ranges where air-breathing aircraft fly. And like outlaw162 intimated, at some point, the density/pressure drop becomes so gradual that the sudden and extreme changes in ambient temperature one encounters while rocketing upwards actually starts to have more of an effect on Mach numbers.

As we all know the molecules of a gas are not stationary. The more energetic they are, the more radiply they interact with each other.

Temperature is, actually, an indirect measurement of the free energy content of the molecules. AND the speed of sound -for a given medium at a given temperature- is a reflection of the interaction rate.

So the speed of sound goes with the temperature.

Note: In fact the pressure due to the altitude (for air) or due to the depth (for water) is caused by the weight of column of the mass above, per unit area.
The term 'weight' includes the gravitational-pull variation due to the distance from the earth. While calculating, for incompressible fluids, we take the mid-height of the column.

Even though the USAF won't openly admit it, I think we can surmise that there are a few air-breathers that operate in the stratosphere, like the:

B-47 STRATO-jet

B-52 STRATO-fortress

KC-135 STRATO-tanker

& maybe a few weird recce airplanes.

The stratosphere on top of the troposphere is actually one gigantic temperature inversion due to ultra-violet heating and that's why the speed of sound goes up in this band.

They say the top of the mesosphere is the coldest point in the atmosphere. They obviously haven't dated some of the women I have.

OL

Okay, I get it now. I was unaware that sound speed equations varied depending on the medium (solid, liquid, gas). I also confused the fact that air density affects sound INTENSITY (dBm) and not so much its velocity.

Guess that's why I majored in bio and not physics. Embarassed

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