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High-speed footage of fishtailing


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#1 Doom

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Posted 18 October 2012 - 05:38 PM

Would anyone with access to a high speed camera be willing to do some experiments to see how Nerf darts fishtail?

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.

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#2 atomatron

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Posted 18 October 2012 - 09:11 PM

I have the test footage which shows what you're asking for.

HERE

AND HERE

They're from like a year and a half ago so excuse the somewhat poor quality.
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Perche Germolgi. [Because it shoots]

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#3 Blood Angel

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Posted 19 October 2012 - 02:20 PM

I think you maybe on to something there.

The slow motion video somewhat captures the "fishtail". You can see that the darts (even though short) is spiral rotating as it leaves the barrel.

The experience I have with a tighter barrel (17/32 and a stock elite streamline) and a more powerful blaster (around 149 PSI) is the amplified effect of flow separation.

In the link you gave me: http://btrettel.nerf.../ballistics.pdf

Page 33 talks about muzzle burst, in which the expansion of excess air pressure following the dart out of the barrel bumps the tail end of the dart. This bump (in any direction) may be the reason for the "high angles of attack" causing flow separation on the dart. Since high power blasters create a lot of back pressure on the dart to push the dart at faster velocities, it only makes sense. As the dart flies through the air with the muzzle burst unbalancing its flow, parasitic drag and induced drag is increased.

The best why to show the effect is to run a thin ribbon at the tail end of the dart and see if the ribbon spirals or waves as it flies through the air. After one determines whether it is spiraling or fishtailing, then you can either add a muzzle break/barrel porting to relieve the excess pressure or increase barrel length to optimize air pressure on the dart as it leaves the barrel.
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#4 Doom

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Posted 19 October 2012 - 08:56 PM

Excellent work atomatron. Thanks for bringing your videos to our attention.

Some observations and thoughts:

Fishtailing in a real dart damps out with time. In my analysis of my model I treated fishtailing as an instability, i.e., continuous rotation. I've had discussions on NerfRevolution before about fishtailing with regard to whether rifling could help, and I recall considering this before. I have no idea why I didn't consider it this time. So perhaps I should reanalyze my model. Right now I'm thinking about looking at amplification of disturbances. Basically, if the dart-air system will cause an initial bump to grow larger at any time, even if it's damped out eventually, it's not good.

These videos confirm that the darts rotate about their center of mass, which is located near the tip of the dart. The tips of the darts translate forward and the backs fishtail.

It's hard to see, but the fishtailing does seem to start while the dart leaves the barrel. In the first video, the second dart is the worst in terms of fishtailing. The maximum angle this dart reaches is about 45 degrees below flat. In the second video it seems that one dart might make a full 180! That's impressive.

atomatron, if you have the time and resources, could you do some better tests? Figuring out what's going on would be easier with a solid white background, no shadows on the background (top lighting?), and darts that have different colors on the back or front.

Blood Angel, a ribbon at the end of a dart might be a good idea to figure out what's going on. I'd worry about the ribbon affecting the dart, though. It's worth trying.

Edited by Doom, 20 October 2012 - 09:18 AM.

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#5 atomatron

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Posted 20 October 2012 - 12:47 AM

atomatron, if you have the time and resources, could you do some better tests? Figuring out what's going on would be easier with a solid white background, no shadows on the background (top lighting?), and darts that have different colors on the back or front.

I plan on it. Additionally a friend and I have worked out a way to use a fog machine and laser with a line lens to view air disturbances. This should give us a substantially better understanding of how a dart behaves as it exits the barrel and in flight. If you have any suggestions I'd be willing to test them if I am able.

I will be doing the following with a good dart, an intentionally poor dart and a disfigured shit dart;

- Horizontal view, white background
- From the muzzle view, white background
- Cross sectioned air disturbances with fogmachine
- From springer (fogmachine)
- From airgun (fogmachine)
- Really weak blaster

Different darts/ammo;

- Too loose dart/foam (fogmachine)
- Too tight dart/foam (fogmachine)
- Combos of tight/loose from air/springer. (fogmachine)
- Drac's Missle and NERF arrow (fogmachine)

Edited by atomatron, 20 October 2012 - 12:48 AM.

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#6 Blood Angel

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Posted 22 October 2012 - 11:56 AM

I don't know if this will drastic change the idea of dart flight, but this weekend I tried out my new slot vent ported barrel.

I put the barrel porting 4in behind the muzzle of the barrel and dart accuracy improved greatly. The purpose of porting was to increase accuracy with elite streamline darts, and the porting let off enough pressure behind the dart to maintain a good distance to accuracy ratio. The reason behind putting the porting 4in behind the muzzle is that streamline darts are long.

While I don't believe porting is enough to eliminate fishtailing, it certainly decreases the effects of fishtailing in high powered blasters.
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