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Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: inonickname » Tue Jul 21, 2009 6:05 am

Ragnarok wrote:I'm sorry, I thought we were having a discussion about plausible theories that actually fit the facts.

When did we stop doing that exactly?


When you used kelvin as a measurement you expect people to know..lol :roll:
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Unread postAuthor: Ragnarok » Tue Jul 21, 2009 6:21 am

So I use the international standard unit for temperature, and I'm the one at fault?

Don't blame me for the fact the US is still using a backwards unit system.
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Unread postAuthor: inonickname » Tue Jul 21, 2009 6:31 am

Ragnarok wrote:So I use the international standard unit for temperature, and I'm the one at fault?

Don't blame me for the fact the US is still using a backwards unit system.


I'm not too bothered with kelvin- except celsius makes more sense to the average person. Those in the U.S. would use fahrenheit..which is weird as.

A linear thing like celsius is easy to understand for most. Water freezes at 0, boils at 100 et cetera.

I was taking the piss by the way, but what I meant that is if you walk to someone on the street who didn't grow up using kelvin, or is a physicist or similar, then rattle of a random number in kelvin and ask them what it is in degrees they will be clueless. (asides from the obvious, like absolute zero and so on)..

Edit: Another example, regardless of what country we're from here we mainly use psi or bar as a measurement of pressure...
How often do we use " of H20, " of Hg, kilo pascals, hecto pascals, barye and so on.

But hell..if Kelvin floats your boat then go for it
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Unread postAuthor: Ragnarok » Tue Jul 21, 2009 6:57 am

Yes, but I'm not talking to be people in the street. I'm having a reasonably serious scientific discussion.
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Unread postAuthor: inonickname » Tue Jul 21, 2009 7:10 am

Ragnarok wrote:Yes, but I'm not talking to be people in the street. I'm having a reasonably serious scientific discussion.


Yeh, alright. Measurement point aside, I made a perfectly valid point.

I was pointing out that he doesn't have such conditions that would allow supersonic with CO2.
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Unread postAuthor: POLAND_SPUD » Tue Jul 21, 2009 7:26 am

lol whoever uses kilo pascals is doing it to show off... serriously i can think of any reason for using anything other than Bar, Psi or atm...

and yeah I know how to convert from pascals to bar... but what's the point in using them if some might not be familiar with them ??
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Unread postAuthor: Ragnarok » Tue Jul 21, 2009 7:58 am

inonickname wrote:I was pointing out that he doesn't have such conditions that would allow supersonic with CO2.

Then tell me where the error in the results comes from.

He's reading a consistent 1090 fps at 30 feet - the distance from the muzzle and the consistency suggests the results are genuine.
I also have little doubt that something as aerodynamically inefficient as a superball at those speeds would lose at least 100 fps over that distance.

Until an explanation for the readings can be found, we have to assume them correct.

And as the old saying goes: "If the theory and the results don't fit, the problem is with the theory."

@Poland Spud: Pascals are an SI unit. Bar aren't SI units, and aren't recommended for serious scientific or engineering use.
For that reason, if I'm doing calculation on a serious level, I'll be using Pascals.
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Unread postAuthor: ramses » Tue Jul 21, 2009 8:53 am

inonickname wrote:A linear thing like celsius is easy to understand for most. Water freezes at 0, boils at 100 et cetera.


Fahrenheit is linear, too. On a graph, if Celsius was "X" its graph could be f(x)=x. The line for Fahrenheit could be f(x)=9x/5+32. you will find that both are lines, and are therefore linear.
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Unread postAuthor: jeepkahn » Tue Jul 21, 2009 9:04 am

Here I go trying to burst my own bubble, but isn't the Cd of a smooth sphere lower at higher velocities or am I confusing Cd with actual drag???

Is it possible that because of the high rate of speed the balls aren't slowing down that much because of the turbulent flow at transonic speeds???
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Unread postAuthor: Ragnarok » Tue Jul 21, 2009 10:13 am

jeepkahn wrote:Here I go trying to burst my own bubble, but isn't the Cd of a smooth sphere lower at higher velocities or am I confusing Cd with actual drag???

No, the Cd and drag are both much higher. Around Mach 1 velocities, everything has relatively horrible drag coefficients.
G<sub>s</sub> puts the Cd of a sphere at Mach 1 at about 0.8 - compare that to Mach 0.5, where the Cd is about 0.5.

Here's a nice chart for you.
Vertical axis is Cd, Horizontal axis is Mach number. G<sub>s</sub> (the sphere) is the cerulean line across the top of the graph.
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Unread postAuthor: Mateo » Tue Jul 21, 2009 10:19 am

Fahrenheit is linear, too. On a graph, if Celsius was "X" its graph could be f(x)=x. The line for Fahrenheit could be f(x)=9x/5+32. you will find that both are lines, and are therefore linear.


but fahrenheit is still a measurement used in mainly in the US. kelvin is the widely accepted scientific measurement and when doing complex equations like this you use kelvin not celsius or fahrenheit. i actually find the metric system way easier to use even though im american. i still dont know why we havnt changed our system yet...
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Unread postAuthor: Ragnarok » Tue Jul 21, 2009 10:32 am

Mateo wrote:I actually find the metric system way easier to use even though I'm American.

It's nothing to do with the country you're from. It's just a simpler and better thought out system, full stop.

For example, two pounds of force on one stone of mass, what's the acceleration?
It's doable, of course, but you need to know a constant and a conversion. The answer is 4.6 ft/s/s

Two newtons of force on one kilogram of mass? No bother at all - it's 2 m/s/s
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Unread postAuthor: jeepkahn » Tue Jul 21, 2009 11:24 am

Ragnarok wrote:
jeepkahn wrote:Here I go trying to burst my own bubble, but isn't the Cd of a smooth sphere lower at higher velocities or am I confusing Cd with actual drag???

No, the Cd and drag are both much higher. Around Mach 1 velocities, everything has relatively horrible drag coefficients.
G<sub>s</sub> puts the Cd of a sphere at Mach 1 at about 0.8 - compare that to Mach 0.5, where the Cd is about 0.5.

Here's a nice chart for you.
Vertical axis is Cd, Horizontal axis is Mach number. G<sub>s</sub> (the sphere) is the cerulean line across the top of the graph.


I remembered seeing the chart before, that's why I thought I might be confused...

Somebody needs to fix the wiki for drag coefficient, because it says
[edit] Drag coefficient Cd examples

[edit] General
In general, Cd is not an absolute constant for a given body shape. It varies with the speed of airflow (or more generally with Reynolds number). A smooth sphere, for example, has a Cd that varies from about 0.47 for laminar (slow) flow to 0.1 for turbulent (faster) flow.

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