Maximum Length of Spin Stabilized Projectile by Beat Kneubuehl

damoncali

Gunny Sergeant
Mar 19, 2011
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Omaha, Nebraska
#2
Interesting. Not sure it's any better than Miller, though. There is a surprisingly (to me, at least) strong correlation between Cm and bullet length for boattail bullets. (And a very poor one for flat based bullets).
 
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Jim Boatright

New Hide Member
Feb 21, 2018
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#5
Excellent!

Jim, what's your opinion of this calculator based on the "McDrag" program?
Tiro, I think this Swiss approach complements McDRAG quite nicely for circular-arc head-shapes, but they do different things, though. McDRAG is OK for estimating CD0 (plus some CDa thrown in), and the Swiss paper at least attempts estimating CMa (but crudely). The new estimation of Iy/Ix available is the best I have seen and much needed for our CWAJ and Spin-Drift calculations. Gustavo will be pleased with this upgrade.

DC, I have not really tried to evaluate the new initial Sg estimator, but how often do those come along? It certainly looks promising to me.
I am attaching a new version of my working paper on dynamic stability with my errors fixed in the section exploiting the Swiss approach. With these new "adjustments," it fits my bullet design data very nicely.
 

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Jim Boatright

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Feb 21, 2018
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#7
Sorry, I forgot to post the link to the calculator based on the McDrag:

Would you take a look at it? It is on Geoffrey Kolbe (Border Barrels) site.

http://www.geoffrey-kolbe.com/drag.htm
Hello, Tiro;
Now I see what you meant to ask. I played with that calculator some, and it looks good within its limitations. I did the same things in my bullet design spreadsheet (plus a lot more). His BC's are quite close to mine, and his twist-rates match McGYRO and Miller closely. Coupling to a point-mass trajectory propagation is a nice touch. I cannot quite get his solid modelling to match my numerical mass integrations, but it is due to not being able to input the required shape data details. He does not add in the weight of the driving band, for example. A big limitation is not handling hollow cavities, but that is mostly due to how difficult their accurate specification often is.

The main limitation with this calculator comes from McDRAG itself. There is some unspecified amount of yaw-drag included in McCoy's "CD0" calculations. Based n David Tubb's test-firing of 5 of my bullets to 1,000 yards, it seems that McCoy's CD0 calculation is 12.4 percent too large at Mach 2.5. That is what got me to thinking about "hyper-stabilization" in the first place.
 
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