Tumbling Dart

[boyntonstu]

http://s348.photobucket.com/albums/q339 ... 150psi.flv


150 psi 315 fps looks great exiting the barrel, yet it tumbled within 25 feet.

Question: What is the highest speed homemade nail dart that you know of that flies straight? What speed?

[psycix]

If you get the center of drag and the center of mass right, then any dart can fly straight.

[boyntonstu]

If you get the center of drag and the center of mass right, then any dart can fly straight.

He may be able to throw this dart but i doubt it would fly straight at 900 fps.

http://www.roundofnine.com/my_darts.html

[Insomniac]

Yes, but I get the feeling that those darts owe their stability to the thrower's technique, either by imparting spin to them as they are thrown, or by allowing them to tumble end-over-end at a specific rate, much like a throwing knife.

So far as the center of pressure is far enough behind the CG, the dart will be stable. The only exception I can think of when it comes to high velocity, is darts that arn't structually sound enough to maintain their shape at speed. That, and if you go supersonic or start to approach it a normally stable dart could become unstable. Weird things happen when shockwaves and transonic aerodynamics come into play.

[ramses]

at 900 fps, the pictured darts might tend to cavitate.


The nose would push the air away from the dart for the rest of the length of the dart, so that there is minimal drag on anything but the tip. The rest of the length would not contribute to the drag, so the COP would be the tip, forward of the COM.

[boyntonstu]

The dart shown will usually fly straight at low speeds.

It is a frustrating exercise in trying to design a nail dart capable of 900+ fps.

Even 600-800 fps is not easy.

Low speed darts may be made with rolled paper cones.

However at 600 psi these will blow out.

Sabots for a nail in a 3/4" barrel are another challenge.

[ramses]

I just downloaded Algor. post a dimensioned drawing and I'll try to run it. I don't think I can factor in muzzle blast.

[jimmy101]

One thing to consider; is it the dart or the gun that is making it tumble?

A poorly crowned muzzle will tend to give a big sideways kick to the ammo as it exits. A light weight tail is particular susceptible to getting a big sideways kick as it exits the muzzle.

Similar to a poorly crowned muzzle, if there is blowby on one side of the ammo that can also give the tail a big kick as it exits the muzzle.

If cavitation is the problem then you need a longer projectile. The air stream has to be able to close up around the body of the projectile, or at least close up around the tail, in order for aerodynamic stabilization to work. Supersonic aircraft have to have massive control surfaces for the surfaces to actually work. Standard sized surfaces don't work at supersonic speeds because the air flow doesn't touch the control surface due to cavitation caused by the leading edge of the tail and/or wing.