For someone with a high speed camera, this would be easy to do. All I want to see is a few videos of a dart fishtailing in slow-motion so I can see how they move. A side view alone would be helpful, but multiple views would be best.
Following some recent discussion of Nerf ballistics I thought I should try to develop some simple rules for when a Nerf dart will fishtail or not. Unfortunately, this problem does not seem to have been studied by engineers and scientists in much detail in the past. The closest I could find in engineering journals were experiments and theories about the stability of long and flexible tubes which doesn't really apply to Nerf darts. I tried a very simple model equation to see how stable (i.e., whether or not a dart could begin fishtailing) darts were, but the result was not entirely accurate. The model predicted that darts would only fishtail if the center of gravity is behind the center of pressure regardless of the dart's length or mass. Darts will fishtail then, but that's not the only time.
My theoretical model was very simple and it consequently made many assumptions. If I could see some video I could improve these assumptions and hopefully understand how to make darts that won't fishtail.
Right now the assumptions in my model are:
- No flow instabilities, like this. This is probably my worst assumption.
- No lift.
- The drag coefficient doesn't depend on the angle of attack. Instead, the reference area changes as a function of the angle of attack.
- No bending of the dart. So the center of gravity does not move.
- The center of pressure does not change with the angle of attack. This is wrong, but probably reasonable because I'm focusing on stability, not accurately figuring out how fast the darts will flutter if they are unstable.
- The center of mass of the dart is located at the center of the dart's cross section. So if the mass being slightly offset contributes, this model doesn't see that effect.
- The velocity of the dart does not decrease with time. This is probably fine as it represents the worst-case scenario. We expect fishtailing to improve as a dart slows down.
Edited by Doom, 20 October 2012 - 09:10 AM.