helix and radius of curvature
I was wondering how to find the radius of curvature of a helix. If it's circling around the z axis, the radius of it's projection onto the xy axis is a circle of radius r. Let one full cycle of the helix around the zaxis cover a distance d along the zaxis, then what is R, the radius of curvature of the helix in terms of d and r? I know it must be larger than d + r... Is there a handy formula for this?

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[tex]\vec{r}=\left<r\cos{t},r\sin{t},\alpha t\right>[/tex] You know the radius projected onto the xy plane, and also that d is proportional to the period. Assuming you know the formula for the radius of curvature: [tex]R=\frac{1}{\left\kappa\right}[/tex] 
hmm, ya. The parametric curve looks good, but what is kappa?
forget the "radius of curvature", what I mean is radius... I guess what I really want to know is what is the radius R of the circle that is made from the length of a string that is wound around a cyninder with radius r as it spans a distance d (along the longitudinal axis of the cylinder) to make one cycle around the cylinder. If i have to integrate the parametric curve to find the length, then I guess thats what I have to do... I just don't like the complexity involved in doing so, and I figured someone has already done that and found a relationship between the variables R, d and r. 
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[tex]2\pi R=\int_{0}^{2\pi}\sqrt{r^{2}+\alpha^{2}}\,dt=2\pi\sqrt{r^{2}+\alpha^{2} }=2\pi\sqrt{r^{2}+\frac{d^{2}}{4\pi^{2}}}[/tex] Which would represent the length of the helix (I calculated that by the definition of arc length). Now you know that the length above (circumference) is really 2piR where R is the radius of the circle you want. Is this what you were getting at or did I misinterpret your question? 
thanks! this is exactly what i was looking for. I reviewed arc length in 3d and checked your solution. It must be correct. I didn't think it would be that easy, I thought there would be a triple integral for some reason. Eh, I got a c in calc 3, so I'm not proficient enough in doing problems like this. Now that I think of it, triple integrals really don't show up unless you're calculating volume, and doubles are usually for area, or to simplify a more difficult single integral... thanks a lot

Helix Radius
I've seen vaiants of formulas such as amcavoy suggests in his second post. They do the job, but it bothered me that a Pathagorean approach was used when trig should offer a streamlined version. This is what I formulated:
R = r(cos)^2 where the cos is derived from the slope of the curve around the cylinder. I recognize that amcavoy did in fact introduce trig into his forms, suggested in his first post, but without squaring the cos, the value for t is unattainable. Regards, Bob 
Re: Helix Radius
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DeltaT 
Re: helix and radius of curvature
Well, the curvature of a curve in R^{3} is [tex]\kappa = \frac{\lvert \vec r' \times \vec r'' \rvert}{\lvert \vec r' \rvert^3}[/tex], and using [tex]R = \frac{1}{\lvert \kappa \rvert}[/tex] should give you the radius of curvature.

Re: helix and radius of curvature
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I would appreciate any help in reaching the R = r(cos)^2 result just using the trig functions and simple algebra if possible. I would just add, I don't need this for any specific purpose, other than personal curiosity. It is a result I've seen stated several times, but so far I have never seen it derived in a way I could follow. Regards DeltaT 
Re: helix and radius of curvature
Hi
Ok, I've found a website that has allowed me to find the solution I wanted. http://ca.geocities.com/web_sketches...er_radius.html From the result given on that site for R, and using the fact that cos(pitch) can be found from the geometry given, it is easy to show that: r = R cos^2 (pitch) which was the result I wanted to be able to find. However, there is still a slight catch. I can follow the math on that page, and I was even able to extend it to reach the trigonometric result. However, I can't see why the opening statement is true: Helix_Length = C * c/Helix_Length I can't think of a justification for that statement, can anyone here see what I'm missing? DeltaT 
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