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<blockquote data-quote="Deleted member 46119" data-source="post: 1292323"><p>The aerodynamic considerations of a projectile</p><p></p><p>Coefficient of Drag</p><p>Center of balance (gravity)</p><p>Center of pressure</p><p>Gyroscopic Stability</p><p>Sectional Density</p><p></p><p>Hopefully I am putting all this together correctly. It's ok to correct me but please, this is not an opening for a slug fest. It's a mix of aerodynamics from high speed RC planes and ballistics. I'm trying to put this into simple terms.</p><p></p><p>Coefficient of drag is all about how the bullet has to push the air out of the way. There are interesting things that happen at the super sonic speeds our projectiles travel at and it changes as the velocity degrades.</p><p></p><p>Center of balance or center of gravity. Imagine balancing your bullet on the edge of a razor blade. This is one of the few things that does not change as the bullet flys. It does effect gyroscopic stability.</p><p></p><p>Center of pressure, this is where the air is pushing on the bullet as it flys. This moves around as the bullet speed changes. If the center of balance is fighting with the center of pressure it can effect accuracy.</p><p></p><p>Gyroscopic stability. This is where twist rate comes in. There is more to it than than twist. With out enough twist you bullet is like a "top" not spinning fast enough. It will wobble. If the center of balance, the center of pressure and the gyroscopic stability are all out of balance this is where you get 10" groups at 100 yards (ha ha). Seriously If these are all on song you are golden. Think about the twist, bullet shape and where the pressure is for the short range bench rest shooters. The bullet is shaped like a "top" as are most hand gun bullets. "Mythbusters" show where a 9mm is shot onto lake ice and it spins like a top for a long time. As far as I can tell with these flat based bullets the center of pressure is actually behind the bullet so does not influence the bullet.</p><p></p><p>All those things have compromises we make for the intended purpose of the projectile.</p><p></p><p>Leaving Sectional Density. </p><p></p><p>An object in motion tends to say in motion. Of course the follow on is "until influenced by some other factor". If bullets traveled in a vacuum they would never slow down. We all know this. Friction from air is our enemy but it will always be there. A method of countering aerodynamic drag is to keep the CD the same but increase the "mass" of the projectile. The more "mass" of the projectile the more atoms of what ever is in the air the projectile has to hit to slow it down. We call this sectional density. </p><p></p><p>Imagine swimming in ping pong balls. You are heavy, they are light. There is a lot more of them and every one you hit is slowing you down. Then think about diving into the ping pong balls. Now think about the perfect shape for getting through the ping pong balls. Except they ping pong balls rub against your sides slowing you down anyway, just not as much so you now have better aerodynamics.</p><p></p><p>One more factor we can add to get through the ping pong balls (air) is weight. If the weight (mass) is increased but everything else is the same you can get farther through the ping pong balls or in the case of a projectile air. </p><p></p><p>If you are still with me, I'm sorry to hear that. This is a bunch of drivel by a coffee starved idiot.</p><p></p><p>Because I have to do do something else right now I will summarize (simplified). CD+SD gives BC. CG+GS gives stability.</p></blockquote><p></p>
[QUOTE="Deleted member 46119, post: 1292323"] The aerodynamic considerations of a projectile Coefficient of Drag Center of balance (gravity) Center of pressure Gyroscopic Stability Sectional Density Hopefully I am putting all this together correctly. It's ok to correct me but please, this is not an opening for a slug fest. It's a mix of aerodynamics from high speed RC planes and ballistics. I'm trying to put this into simple terms. Coefficient of drag is all about how the bullet has to push the air out of the way. There are interesting things that happen at the super sonic speeds our projectiles travel at and it changes as the velocity degrades. Center of balance or center of gravity. Imagine balancing your bullet on the edge of a razor blade. This is one of the few things that does not change as the bullet flys. It does effect gyroscopic stability. Center of pressure, this is where the air is pushing on the bullet as it flys. This moves around as the bullet speed changes. If the center of balance is fighting with the center of pressure it can effect accuracy. Gyroscopic stability. This is where twist rate comes in. There is more to it than than twist. With out enough twist you bullet is like a "top" not spinning fast enough. It will wobble. If the center of balance, the center of pressure and the gyroscopic stability are all out of balance this is where you get 10" groups at 100 yards (ha ha). Seriously If these are all on song you are golden. Think about the twist, bullet shape and where the pressure is for the short range bench rest shooters. The bullet is shaped like a "top" as are most hand gun bullets. "Mythbusters" show where a 9mm is shot onto lake ice and it spins like a top for a long time. As far as I can tell with these flat based bullets the center of pressure is actually behind the bullet so does not influence the bullet. All those things have compromises we make for the intended purpose of the projectile. Leaving Sectional Density. An object in motion tends to say in motion. Of course the follow on is "until influenced by some other factor". If bullets traveled in a vacuum they would never slow down. We all know this. Friction from air is our enemy but it will always be there. A method of countering aerodynamic drag is to keep the CD the same but increase the "mass" of the projectile. The more "mass" of the projectile the more atoms of what ever is in the air the projectile has to hit to slow it down. We call this sectional density. Imagine swimming in ping pong balls. You are heavy, they are light. There is a lot more of them and every one you hit is slowing you down. Then think about diving into the ping pong balls. Now think about the perfect shape for getting through the ping pong balls. Except they ping pong balls rub against your sides slowing you down anyway, just not as much so you now have better aerodynamics. One more factor we can add to get through the ping pong balls (air) is weight. If the weight (mass) is increased but everything else is the same you can get farther through the ping pong balls or in the case of a projectile air. If you are still with me, I'm sorry to hear that. This is a bunch of drivel by a coffee starved idiot. Because I have to do do something else right now I will summarize (simplified). CD+SD gives BC. CG+GS gives stability. [/QUOTE]
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