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Accuracy of GGDT External Ballistics Calculator?

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Accuracy of GGDT External Ballistics Calculator?

Unread postAuthor: JDP12 » Sun Dec 05, 2010 9:24 pm

Hey all, i've been fiddling with some plans for a paintball grenade launcher, and was using external ballistics calculator to mess around with ranges.

Now i plan on creating a set of sights for this, and was going to use the EBC and some basic trigonometry to figure out how to place the sights, so I just wanted to make sure EBC was accurate enough(roughly obviously) to do so. I'm not looking for pinpoint precision, just want to ensure.

Thanks
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Unread postAuthor: mlz3000 » Sun Dec 05, 2010 10:09 pm

I was going to do some test shots soon to check just that. It seems pretty accurate from the shots I did with my sprinkler-valved tennis ball gun.

Two things to consider though:

The EBC does not consider wind, so that's going to change the range, sometimes considerably.

The EBC needs you to input the Drag Coefficient, and I noticed that changing this value changed the range considerably. I use tennis and golf balls as my primary projectile, so it was easy to look up the Drag Coefficient for those, but I don't know how to actually find the Drag Coefficient of something.
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Unread postAuthor: D_Hall » Sun Dec 05, 2010 11:42 pm

There are MANY ways to find the drag coefficient of something and much depends on what you're trying to accomplish. That said, the simplest way I've ever seen is basically to drop the item into a swimming pool. More to the point....


1) Hold item *just* under the surface of the pool.
2) Release item.
3) Measure the velocity of the item as it hits the bottom (many ways to skin that cat).

That's it!

Drag = 1/2 * WaterDensity * Velocity^2 * Cd * ReferenceArea = WeightUnderWater.

Solve for Cd. Measure other variables however you see fit.
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Unread postAuthor: mlz3000 » Sun Dec 05, 2010 11:50 pm

That is very helpful, much thanks!

You didn't address the original question. I suppose you may know better than anyone else: how close would you predict the EBC to get to the actual range?
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Unread postAuthor: D_Hall » Mon Dec 06, 2010 12:03 am

mlz3000 wrote:That is very helpful, much thanks!

You didn't address the original question. I suppose you may know better than anyone else: how close would you predict the EBC to get to the actual range?


It should be good to give you a general idea, but I wouldn't use it to predict an exact impact point. It was intended more as a range safety tool than anything else.

But like any simulation... Garbage in means garbage out. And most people won't have Cd data that's accurate to better than 10% let alone accurate wind data. That means ALL simulations will give MOST people crap for an answer.... depending upon your definition of the word "crap".
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Unread postAuthor: JDP12 » Mon Dec 06, 2010 12:38 am

I'd use the water idea... if it wasn't freezing outside. I think I'll just use a coefficient of drag I found for a rough sphere- .48 until I can try the swimming pool method..
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Unread postAuthor: D_Hall » Mon Dec 06, 2010 7:00 pm

Note that the swimming pool method is easiest to accomplish in a garage if you have - say - a 6' piece of clear plexiglass tubing or similar. Thus, you can do the work and stay dry. The tubing should, however, be on the order of 4X (minimum) the diameter of the object you're trying to measure.

Further, if you go this route you can refine your results by using different fluids. Water. Anti-freeze. Glycerine. All have different densities and viscosities so will result in different Reynold's Numbers.
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Unread postAuthor: jimmy101 » Mon Dec 06, 2010 7:08 pm

The problem with most trajectory calculators, besides garbage-in-garbage out, is that if the round tumbles then it has a variable Cd. And, there are a lot of external variables (wind, barometric pressure, temperature) and gun-specific variables (is the barrel adequately crowned? is it straight? ...) all of which affect the actual behavior of a particular shot. It really isn't possible to get a highly accurate solution without knowing all the variables.

For most ballistic objects really good accuracy is only obtainable with a steerable round.

Another way to determine the Cd is to shoot the round straight up and use a chrony to measure the muzzle velocity and a stop watch (or just do it with a homemade crony) to measure the hang time. Easy enough to calculate the effective Cd, which will include the effect of tumbling.
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 07, 2010 1:09 am

jimmy101 wrote:if the round tumbles then it has a variable Cd.


If you're going for range though, presumably you would have already put the effort into an elongated fin or drag stabilised projectile.
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Unread postAuthor: Technician1002 » Tue Dec 07, 2010 1:30 am

In shooting t shirts in a competition, it was found adding ribbon tails to them helped the flight dynamics by preventing them from turning sideways in flight.

T shirt with ribbon tails..
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