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All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Big Bore AirGun Gate => Topic started by: Paul68 on March 16, 2013, 01:01:35 PM
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I'm not a big bore shooter, but after reading a couple threads thought this might be fodder for those using round shot.
http://www.scientificamerican.com/article.cfm?id=how-do-dimples-in-golf-ba (http://www.scientificamerican.com/article.cfm?id=how-do-dimples-in-golf-ba)
If dimpling will allow a golf ball to travel approximately twice as far as a smooth one, wonder what it would do for round shot in an AG?
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It would cause it to rise or fall in a crosswind.... The "magnus effect" causes lift due to spin.... For a golfball, that spin is backspin, which increases the range it flies.... In a projectile, the spin is caused by the rifling (fortunately the bullet isn't tumbling over backwards).... so there is no increase in range.... Worse than that, if there is a crosswind, the bullet will get lifted if the wind is from one side, and pushed down if it's from the other.... This will cause vertical stringing in a crosswind in addition to the pellet drifting sideways....
Bob
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Bob you're great, always soo much info :)
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It would cause it to rise or fall in a crosswind.... The "magnus effect" causes lift due to spin.... For a golfball, that spin is backspin, which increases the range it flies.... In a projectile, the spin is caused by the rifling (fortunately the bullet isn't tumbling over backwards).... so there is no increase in range.... Worse than that, if there is a crosswind, the bullet will get lifted if the wind is from one side, and pushed down if it's from the other.... This will cause vertical stringing in a crosswind in addition to the pellet drifting sideways....
Bob
I agree on the crosswind and other spin-related phenomenon (they are facts, not opinions), but my understanding, which may be incorrect, is that dimples also reduce the drag coefficient. It would seem to me that this would be a good thing for helping maintain downrange projectile speed (assuming the magnus effect wasn't inducing unacceptable elevation variation due to crosswinds as you pointed out).
A reference (relevant text about 2/3 the way down on the page): http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml (http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml)
edit: the effect on drag coefficient will also be dependent on the Reynold's number of the projectile in air at a given speed, so I reckon that will play a role as well; not sure how much though...
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Reynold's Number in air (STP).... is 6378 x V (fps) x L (ft.) = 532 x V (fps) x D (in.)
For a .45 cal roundball at 900 fps, the R = 532 x 900 x .45 = 215,460.... or 2.15 x 10^6....
For a golf ball 1.68" diameter (min. allowed) at 250 fps (max. allowed) the R = 532 x 250 x 1.68 = 223,400.... or 2.33 x 10^6.... almost identical....
If I'm reading that graph correctly (and the text is no help), to me it looks like the drag of a dimpled ball and smooth is very close at those reynolds numbers.... However, as the sphere slows down (or is smaller), the dimples become more effective, down to a Reynold's number of 3 x 10^5.... If I have this correct, a big bore roundball at high speed wouldn't have a gain from dimples.... while a smaller ball and/or travelling at a lower speed would have less drag.... sperho, would you concur?....
The rifling marks and rapid rotation would likely trip the boundary layer to turbulent anyway, would they not?.... If so, that would make "all" roundballs shot from a rifled barrel behave like it was dimpled behind the maximum diameter....
Bob
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Don't make me crack my fluid dynamics books! I'm going to assume that you have the numbers right for the R calculation, but I do see that you have the decimal in the wrong place in the answer. Should be ~2E5 (200,000) instead of ~2E6 (2,000,000), which puts the dimpled condition (according to the NASA graph) lower in drag than a smooth condition for the same diameter/speed/etc. I also agree with you lower speed for a given diameter decreases R, which helps the dimpling = lower drag argument.
The rifling marks should help induce kick the boundary layer to turbulent, but I suppose it would depend on how much contact area were on the sides of the ball (how much surface area is affected, twist rate, etc.). I wouldn't have a clue as to their contribution.
Surely someone has done the experiment carefully...
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You're right, I goofed on the decimal point on my notation.... but I was in the right place on the graph (3 x 10^5)....
The RPM is very high, but so is the forward speed.... so at the muzzle, the ball could be considered to be "screwing" it's way through the air.... The RPM degrades much slower than the forward speed, however, and with typically 6 lands (ie 60* per land, 1/6th of a turn) it would only take a 17% loss of forward velocity before the rifling would be kicking the boundary layer to turbulent all the way around the ball.... Add in the width of the lands, and I'm pretty confident that from somewhat ahead of the middle (depending on the rifling depth) aft you would have turbulent flow.... with the possible exception of small calibers at low velocities....
For the most part, I think we can consider projectiles (not just roundballs) fired from a rifled barrel to be "dimpled", IMO.... If you measure the Ballistics Coefficient, of course, you are taking into account all those things....
Bob