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Distance Calculators

A place to ask general spud cannon related questions.
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Unread postAuthor: D_Hall » Fri Jul 25, 2008 2:38 pm

The hardest part of coding something (excel spreadsheet, stand alone app, whatever) is drag coefficient. Seriously, I doubt anybody here knows their drag coefficients to within 10%.

At that point.... Garbage in. Garbage out.
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Unread postAuthor: POLAND_SPUD » Fri Jul 25, 2008 3:07 pm

you can put specs of your gun into GGDT/chrony it to get muzzle velocity and then fire the gun several times at - let say - 45 deg angle and note how far it landed

then, you can use some balistic programme to model how far your ammo should travel at 45 deg a. for a given cd ratio and modify it till it is the same as results of your 'life firing'

yeah I know that cd changes with velocity and all that stuff and I know that it won't be very accurate but in theory this method can be used to estimate the cd value of ammo, right ?

in fact I supposed that it won't be difficult to modify the programme to do the calculations for us
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Unread postAuthor: jimmy101 » Fri Jul 25, 2008 5:21 pm

Easiest way to get an estimate of the Cd would be to just fire the gun straight up. If you measure the shell mass, shell geometry, muzzle velocity and hang time then the average Cd can be calculated fairly easily.

Firing at an angle, like 45, just adds another value that you have to measure. A vertical shot is fairly insensitive to small changes from perfectly vertical.

For aerodynamically unstable rounds (like a spud) you have to decide how you are going to handle the frontal area. As the spud tumbles the frontal area changes, as does the Cd.

Spherical ammo is a bit easier since the frontal area doesn't change as the round tumbles.

A fin stablized round has a constant frontal area and essentialy constant Cd.

A bullet from a rifled barrel has a non-constant frontal area and non-constant Cd. The bullet always points in the same direction, which is the same as the flight path only at the instant the round leaves the barrel.
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