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Cd and weight question...

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Unread postAuthor: Ragnarok » Fri Mar 27, 2009 11:32 am

jeepkahn wrote:thanks rag, after looking at those and calculating mv with ggdt, I plugged in Cd of .17(which was g2, because of the similar dimensions, at mach .7)

Actually, because G2 has a boattail shape and long body, it's probably not a good choice of model.

Personally, I would expect a cone's Cd to rise with Mach number (much like the GS and GC models), because of the sharp and flat tail and a lack of body length.
If anything, it would be better to use a constant Cd than try and dub in the G2 model.
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Unread postAuthor: jeepkahn » Fri Mar 27, 2009 12:24 pm

Yeah, I see that now...

All I'm really trying to do is narrow down some variables against real world testing to be able to more accurately establish valve flow rates and such....
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Unread postAuthor: psycix » Fri Mar 27, 2009 3:18 pm

jeepkahn wrote:Actually, the graphs that rag posted shows that for certain shapes, the Cd decreases when speeds exceed .6 mach, so the drag is reduced the faster it goes until it reaches critical mach and creates a shockwave that then increase drag...

No. The drag does not reduce. The drag coefficient reduces.
But the drag (force) will rise as the speed goes up.
The speed is a quadratic factor in the formula for drag while the coefficient is a single term.
Dont be fooled.
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Unread postAuthor: jeepkahn » Fri Mar 27, 2009 3:57 pm

your right, but I was refering to the drag coefficient at speed, not the actual drag... I think we were just miscommunicating what we each meant, Or should i say I missheard what was being typed... :lol:
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Unread postAuthor: Ragnarok » Fri Mar 27, 2009 6:32 pm

Actually, on the note of Cd versus drag, I have another graph that shows the actual drag versus Mach #:

Image
Vertical axis is in m<sup>2</sup>s<sup>-2</sup> (calculated assuming Mach 1 = 340 m/s) - and is proportional to the drag force. Horizontal axis is the dimensionless Mach number.

As you can see, the drag is always increasing with speed, and there is a dramatic increase in drag around the realm of Mach 0.9 to Mach 1 (by approximately a factor of 2)

Interestingly however (well, interesting if you're a bit of a nerd about these things...), if you take G2, G5, G6, G7, and G8, the falling drag coefficient after Mach 1 results in a relationship that is surprisingly linear - although still rising very fast.
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Unread postAuthor: psycix » Sat Mar 28, 2009 3:47 pm

jeepkahn wrote:your right, but I was refering to the drag coefficient at speed, not the actual drag... I think we were just miscommunicating what we each meant, Or should i say I missheard what was being typed... :lol:

No it was not miscommunication. I simply corrected you. :)

@Ragnarok
Interesting graph!
The first two lines which go of the chart, how to they go further?
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Unread postAuthor: jeepkahn » Sat Mar 28, 2009 3:57 pm

psycix wrote:
jeepkahn wrote:your right, but I was refering to the drag coefficient at speed, not the actual drag... I think we were just miscommunicating what we each meant, Or should i say I missheard what was being typed... :lol:

No it was not miscommunication. I simply corrected you. :)

[quote]

yeah, something like that... :oops:
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Unread postAuthor: Ragnarok » Sat Mar 28, 2009 4:08 pm

psycix wrote:The first two lines which go of the chart, how to they go further?

As you might expect, they keep going upwards pretty sharply. The problem is, trying to keep them on the graph squashed all the other curves far too flat to be of any use.

After all, the Cd alone of GS increases until about Mach 1.6 - when it slowly comes back down by about 10% as Mach increases up to the limit of 5 that my data goes to.
GC's Cd just keeps going up, right until the Mach 5 limit.

Combining that with the quadratic nature of drag... they're very poor supersonic shapes, so I figured it wasteful to show them on the graph to the detriment of other more useful data.
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Unread postAuthor: jeepkahn » Tue Jun 09, 2009 4:23 pm

Ragnarok wrote:The weight of a 4" long .40 blowgun dart will be pretty close to 1 gram, give or take about 10%.

The CD is going to be largely defined by the cone shaped tail. An estimation of the Cd of a simple cone shape with it's point into the flow is:
Cd = 0.0056*Angle of point in degrees + 0.162

That cone has an angle of around 25 degrees, so that would give it a Cd of approximately 0.30 (to 2 significant figures)

Hope that helps.


Old topic kickup, but I did get to weigh 10 of these today, individually... they were all 1.1gm each, well actually between 1.100 and 1.102 lightest to heaviest...but most were 1.100 I think 2 were 1.101 and 1 was 1.102...
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