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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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3 posters

    Would it fly or would it fail?

    Krankenheim
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    Would it fly or would it fail? Empty Would it fly or would it fail?

    Post by Krankenheim Wed Nov 17, 2010 2:55 pm

    Imagine a throwing star (kinda juvenile, I know). You throw it and after some distance you notice it starts to curve. When an object rotates as it travels a distance the two forces acting on it (the force that is moving it and the force that is causing it to rotate) make the object go along a curved path. Going back to that throwing star, what if there was a second throwing star attached to the first by a shaft that was rotating in the opposite direction? Wouldn't it cause the projectile to go along a straight path? As for what gives the "throwing star" it's rotation I was thinking of some kind of rack-and-pinion system. This would entail teeth on a raised portion of the top and bottom of the projectile that would follow a rack on opposite sides. The problems I have come across with this concept are the amount of force needed (say...from a crossbow) to make it go a good distance, if the rack-and-pinion system's teeth could take such force (depending on the material), and the material and size of the projectile. Just to warn you I know very little of this. I honestly wouldn't be surprised if you needed a spring-steel windlass with an 800-900 pound draw weight to fire something like this. Anyway, if there's anyhting (and I would be surprised if there wasn't) that is not understood in this topic I will do my best to answer any questions. Also, I forgot to mention that the current design I have for the projectile would be somewhere around a pound of steel. Eventually I will try to upload an image in case you're wondering what my design actually looks like.


    Last edited by Krankenheim on Wed Nov 17, 2010 3:03 pm; edited 1 time in total (Reason for editing : Very vague about the specifications of the design)
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    Post by jake-owa Wed Nov 17, 2010 3:37 pm

    Uh...whoa!

    More details please. It sounds like you are going for a sort of gyroscopic projectile.
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    Post by Basilisk120 Wed Nov 17, 2010 8:14 pm

    Sounds like an interesting idea, any reason for trying it?

    I would go with both wheels spinning the same direction. Basically in flight the object is going to want to rotate around its center of gravity. If the wheels spin in opposite directions the resulting torque vectors from each wheel are going to be putting the same moment on the projectile resulting in a spin. If both wheels spin in the same direction the resulting torque moments could cancel each other out. This is assuming both wheels are on each side of the CG if the wheels are on the same side then the wheels should spin in opposite directions.

    That brings up a good point. The center of gravity should be in the middle so both moment arms from each wheel would be the same, otherwise the math could be more difficult. This would make getting stable flight more difficult, but might get some stability from the gyroscope principle. which is what I assume your going for. Putting both wheels in front of the CG and having fletching out back could make life a little easier.

    I believe to figure out the torque vector direction you could use the right hand rule.


    Last edited by basilisk120 on Wed Nov 17, 2010 8:21 pm; edited 2 times in total (Reason for editing : typos)



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    Post by Krankenheim Thu Nov 18, 2010 8:57 am

    [img]https://thearbalistguild.forumotion.com/Would it fly or would it fail? Throwi10[/img]
    The "image" I mentioned earlier was only a rough sketch. I'll try to get a working drawing eventually. Anyway, as I read your replies it only increased my interest in this. I suppose your right about the wheels being on either side of the center of gravity. If it were to spin, though, wouldn't it be a very gradual spin? Then again that depends on quite a few factors. Once again, I know very little and I apologize about the lack of imformation. I'll try to give more details about this thing as time progresses.
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    Post by Basilisk120 Thu Nov 18, 2010 1:07 pm

    If I am picturing this correctly its basically going to be a throwing star on a stick shot out of a crossbow?
    In this case I would stick with just the one star on the front with a stick with large fletching out back and not worry about the forces applied from the spinning star. Basically as it looks right now the star wouldn't spin enough to really effect things. The gyroscopic effect and torque induced moments should really only effect things if the star is spinning at High RPM and then really only as a force is trying to act on the spinning bit. So the only time this would be an issue would be during launch if the star is already spinning rapidly. Even then since the force is acting on both sides of the axis of rotation that might change things too.

    Really need to go find my physics and dynamics books and read up on spinning things again..



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    Post by Krankenheim Thu Nov 18, 2010 2:01 pm

    Yes, I know. Thanks for the advice nonetheless. By the way, there is no fletching, no shaft, just what you see in the rough sketch. The idea was to have something of a sling running along a track. The protrusion of the connecting shaft on either side of the projectile would rest in a notch within that sling. The teeth along the top and bottom of the projectile, as I said, would follow the rack and make the projectile spin as it would travel along the track and leave the sling. And yes, you are absolutely right, I should read up on the physics of this thing.
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    Post by Basilisk120 Tue Nov 30, 2010 8:38 am

    Some ideas to induce spin. Instead of having the points contact anything would having some magnets in the rail get the thing to spin with less friction?
    Have a center axel that rides on the rail if done correctly that should cause the star to spin. and since it is of a smaller diameter the star should really get spinning.



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