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# revolutions per minute debate

#15
04-21-2005, 07:46 PM
 Platinum Member Join Date: Jun 2001 Location: Potomac River Posts: 5,070
Re: revolutions per minute debate

Brown Dog
You are right about the revs per distance traveled changing as velocity drops with distance from the barrel. I read GG's question several times but never quite grasped what he was asking.

Good job!
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#16
04-21-2005, 07:51 PM
 Platinum Member Join Date: Jun 2007 Posts: 1,515
Re: revolutions per minute debate

I'm not going to disagree with you. I was trying to illustrate that the rifling twist ratio is only part of the picture regarding the stability of a bullets rotation. This ratio must be coupled with MV to equal some RPM. We get used to saying that this or that bullet fired from this or that cartridge will either stabilize or not. What we really mean is that X length bullet of Y weight and Z caliber needs to spin at some approximate RPM. I wrote my illustration with no regard to an actual example and used numbers that had been used before in the discussion.

I also used "somewhat" and "almost" in paretheses for exactly the reason you mentioned. There is the transsonic issue but a bullets stability in crossing it (I believe and I may be wrong)is more a function of RPM because the speed of the bullets crossing it is always about the same. Some remain stable and some not as much. Yes there are other factors that contribute to this transition as well.

Yes, a bullet spinning like a top on a table must spin faster than a bullet in supersonic flight to stay stable. That spin rate is measured in RPM for both bullets. It is not correctly measured as a twist ratio. That's all I meant.
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#17
04-21-2005, 09:37 PM
 Junior Member Join Date: Feb 2005 Location: Westlock Posts: 22
Re: revolutions per minute debate

yes but a bullet leaving rifle at 3000' in a one in 8'twist matching the rifiling as bullet looses speed the rate of twist stays the same but the distance traveled has been extended same twist but longer distance to make the same twist. I think
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#18
04-21-2005, 10:06 PM
 Silver Member Join Date: Nov 2004 Location: GF Montana Posts: 467
Re: revolutions per minute debate

[ QUOTE ]
4ked, that is not right. The rotational angular velocity (spin) required for a particular stability factor is dependant on the drag forces being experienced. Obviously this then depends on the translational velocity. As others have stated, spin decays more slowly than translational velocity, <font color="brown"> increasing stability downrange. </font> .. <font color="red"> This may still not be enough to remain stable during the transonic transition. </font>

[/ QUOTE ]

Slight *apparent* contradiction there. Yes, bullet is over-stabilized as it slows down <font color="purple"> UNTIL </font> it hits transonic where there is huge transient turbulence increase.

Most bullets would require such fast twist rates to stabilize thru the transonic boundary: [*]You couldn't make a barrel with that fast a twist ( 1:4) (guessing here - help)[*]Over stabaliztion causes increased spin drift problems (this is true) - that's why the BR crowd goes for the slowest twist that properly stabilizes a bullet
#19
04-22-2005, 12:31 AM
 Silver Member Join Date: Mar 2005 Posts: 109
Re: revolutions per minute debate

You all fret about some strange stuff.
#20
04-22-2005, 12:48 AM
 Platinum Member Join Date: Jun 2007 Posts: 1,515
Re: revolutions per minute debate

I say we wait for GG to come back.
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#21
04-22-2005, 07:55 AM
 Bronze Member Join Date: Mar 2005 Location: Oklahoma Posts: 47
Re: revolutions per minute debate

Great gobbs a goose grease! I have 'smoked' brain cells trying it keep up with this one. I think.
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