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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I was having some fun today and was shooting my bench top cg at some cans about 5 meters away and noticed that the projectile was tumbling in flight. I thought about a way to stabilize it and came up for a way for gyroscopic stabilization. So I rigged up a way with a drill and a weak magnet to spin the projectile right when the cg fires. I tried it and it seemed to tumble even more! Does anyone else have an idea if this will work or a way to improve it?
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
For the gyroscopic effect to kick in, you'll need far more then 600RPM or so from a drill. I'd reccomend getting it spinning at several thousand RPM (8kRPM minimum) and then trying. I remember reading somewhere that someone came up with a way to rotationally balance coilgun ammo. You can also try firing drill bits, they have built in rifling and will spin somewhat (however they will break easily at high power).
The long projectile idea is also good, thats how an arrow gets good flight.
Registered Member #1329
Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53
RP181, I would have to respectfully disagree on the length barrel helping to stabilize the projectile. I tried brass tubing barrels from 3" to 3 ' long in an effort to stabilize the projectile. On a 20 ' range the only effect the long barrel had was to cause friction and slow the projectile down. In fact you could actually hear the projectile slide up the barrel. The gyroscopic effect, caused by the rifling in a gun barrel spins the bullet and incredible RPM's. "A 1:9 twist 223 barrel sending a 62gr bullet out at 2900 FPS is also spinning the bullet at about 230,000 RPM" That is a quote from
I am not saying that it has to rotate that fast to help.
I drilled a hole in the back of the projectile and inseted a brass rod and soldered a couple of brass fins to it. That seemed to help somewhat. At least at 20' the projectile always hit point first, albeit on an angle small sometimes. I changed firing 1/2" bearing balls which would hit within an inch reliably at 20 feet. As long as you kept the voltage very consistant. But.. the ball is a lousy projectile from an mass and saturation point of view. These have been my experiences.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Another caveat here, you can _either_ stabilize the projectile gyroscopically _or_ by making it long and thin. The reason real bullets are so short and fat (which is obviously not very aerodynamic) is that they cannot be made any longer and still stabilized by spin.
Registered Member #158
Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282
Dr. Shark wrote ...
Another caveat here, you can _either_ stabilize the projectile gyroscopically _or_ by making it long and thin. The reason real bullets are so short and fat (which is obviously not very aerodynamic) is that they cannot be made any longer and still stabilized by spin.
Actually I thought the longer the bullet the more stable it is (well, at least more aerodynamic)? I'm no ballistics expert but I load my own ammo and have many to choose from. Its a common thought that the longer/skinnier bullets are the better choices, but that’s taking a few different things into consideration. Usually the short stubby bullets are only used in handguns or other applications where the concern is keeping the rounds as small as possible for size reasons. Besides, if the gyroscopic effect doesn't work with longer projectiles than what about arrows?
w1vlf wrote ...
RP181, I would have to respectfully disagree on the length barrel helping to stabilize the projectile. I tried brass tubing barrels from 3" to 3 ' long in an effort to stabilize the projectile. On a 20 ' range the only effect the long barrel had was to cause friction and slow the projectile down. In fact you could actually hear the projectile slide up the barrel. The gyroscopic effect, caused by the rifling in a gun barrel spins the bullet and incredible RPM's. "A 1:9 twist 223 barrel sending a 62gr bullet out at 2900 FPS is also spinning the bullet at about 230,000 RPM" That is a quote from
I am not saying that it has to rotate that fast to help.
A longer barrel isn't going to help prevent a bullet from toppling... it might help guide it in the right direction (accuracy) though but doesn’t stop it from hitting the target sideways or backwards, as long as this isn’t being caused by a too loose fitting barrel and distorted magnetic field that pulls the bullet unevenly.
That link you provided isn't working for me, but that is going to be one of the faster spins out there compared to most bullets. Its not hard to figure, 1:9 twist just means 1 rotation for 9" traveled. 2900FPS x12 is 34,800 inches per second. Divide by 9 gives you 3866 rotations per second, x60 = 231,999 rotations per minute. Of course that's pretty high, stuff I most commonly use (my 50 cal muzzleloader) is 1:28 twist at ~1600fps = 41,142 rpm... much closer to what a dremel could potentially pre-spin your coilgun round.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I'm using a 30 inch barrel now and i know that longer doesn't really help. My old coilgun did the same thing, so i rewound the coils on a longer barrel and the added friction actually added more instability.
Also, the projectile would have to be very long to get very much effect in flight and if fletching are added (like an arrow) it would be more stable but i fear it would reduce the speed -although not too much- due to drag.
At the speeds of a small coilgun (~35 to 50 m/s) would the projectile need to spin as fast? Or in other words is the rotational speed needed for stable flight proportional to the lateral speed of the projectile?
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
I think the speed required for gyroscopic stabilization does not vary with projectile velocity.
The best option for stabilization would be a large bore projectile with most of its mass concentrated around the rim. Rifling on a coilgun isn't that practical, you should try spinning the projectile up say 30krpm on a dental drill or rotary tool and then firing.
Registered Member #222
Joined: Mon Feb 20 2006, 05:49PM
Location:
Posts: 96
You could make a projectile that looks similar to a throwing dart. I think it's the fins combined with length that give it stability. You would probably have to come up with collapsable fins to use in a coilgun.
Crossbow bolts are also similarly short and stubby compared to arrows, and they seem to fly just fine.
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Projectile stabilization
