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Old Mar 02, 2014, 10:58 AM
Launch the drones ...
Ashtabula, OH USA
Joined May 1999
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Way non technical

This got too personal way too fast - BTW boys - I think my physics is sound. But, I can't afford to let my emotions get carried away by snideness and outright insults about my smarts.

So long again - and Nereth, my sweet friend, I do reserve the right to return.

Bye.
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Old Mar 02, 2014, 10:59 AM
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Germany, BW, Stuttgart
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See, when you sweep away all details, any description is as good as any other. The world is just simpler that way... and you can never be wrong.
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Old Mar 02, 2014, 04:01 PM
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Viscosity redistributes momentum within the air (or any fluid).

This redistribution converts large-scale kinetic energy into heat. Despite this conversion, the total energy within the air will not change unless something is adding energy to, or subtracting energy from the air. The redistribution of momentum due to viscosity within the air (i.e. not on its boundaries), will simply convert one form of energy (large-scale kinetic energy) to another (heat), conserving total energy in the process. "Dissipation" within a fluid conventionally refers to this conversion.

The entropy in the air will go up as viscosity redistributes momentum and converts large scale kinetic energy into heat (entropy is not conserved).

Momentum within the air will be conserved unless the air is acted upon by an external force (assuming Newton was correct). Another way of saying this is that the air cannot remove momentum from itself. So if a wing is adding downward momentum to the air, that downward momentum will accumulate in the air unless removed by some other external force. Viscosity acting within the air can redistribute momentum all it wants (as long as this redistribution results in an increase in entropy), but it cannot remove momentum. In order for the air to accumulate downward momentum, its center of mass must continually accelerate downward. It should be apparent that the ground will eventually (if not right away) prevent the air's center of mass from accelerating downward. How does the ground prevent the air from accelerating downward? By pushing up on it (through an increase in pressure).

It's worth noting that the ground can exert an additional pressure even when it is not extracting momentum. All that is required is for the air to be pushing up on a particular element of the air at the same time that the wing is pushing down on it. While this may not be obvious for airplanes, it should be for hot air balloons.

Even though the molecules of air "touched" by the airplane will not impact the ground, any momentum imparted by the wing will result in an eventual increase in pressure on the ground in order to remove the momentum.

Somehow in this discussion large scale kinetic energy (which is removed from within the air by viscosity, and converted to heat) was equated to momentum (which cannot be removed from within the air by viscosity).
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Old Mar 02, 2014, 04:03 PM
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riveting
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Old Mar 02, 2014, 04:11 PM
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I have to agree that most of the made up stuff is more entertaining than the actual Physics
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Old Mar 02, 2014, 08:41 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Shoe, would it be fair to say that the final heat energy dissipated in the air would be pretty much equal to the total BTU payload of the fuel burned during flight?

Some goes into various frictional heating in the airplane but that too will eventually conduct to the surrounding air. Even if it takes place as the airplane cools to ambient on the apron after the flight.
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Old Mar 03, 2014, 01:10 AM
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Bruce,

You could do an accounting of the different paths energy takes from the fuel to the air, but yes, I would agree that all of it eventually ends up in the air.

- Some energy in the fuel goes almost directly into heat because engines are less than 100% efficient.
- The small scale velocity differences across the boundary layers turn into heat very quickly (at a rate equal to the profile drag times the true airspeed).
- The large scale velocities in the wake associated with the trailing vortex system take much longer to turn into heat (they initially break up through the Crow Instability). This creates heat at a rate equal to the induced drag times the true airspeed,
- As you point out, some of the heating will be absorbed by the airframe (as a result of both viscous and compressibility effects). Although the airframe will store some of this until post-flight, in steady state it will be conducting it back to the air at the same rate that it collects it.
- if you had a really efficient airplane, a good chunk of the fuel energy would end up in the brakes (before ultimately ending up in the air).
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Old Mar 03, 2014, 08:51 AM
Herk
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Virginia USA
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EG. Entropy
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Old Mar 03, 2014, 10:17 AM
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http://www.jgsee.kmutt.ac.th/exell/Thermo/EntrTemp.htm
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Old Mar 03, 2014, 10:24 AM
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I got it…!!!:

http://www.isiweb.ee.ethz.ch/archive.../pdf/BI633.pdf
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Old Mar 03, 2014, 10:52 AM
Herk
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The last question!

https://filer.case.edu/dts8/thelastq.htm
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Old Mar 03, 2014, 09:53 PM
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Excellent!
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Old Jul 11, 2014, 02:09 PM
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This was an interesting thread! It reminds me a bit of Newton's rotating bucket experiment, and the notion of absolute space. Worth a Google search!
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