Conical primary length formula

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: pepperman-at-softhome-dot-net 

Does anyone know a formula to find the length of tubing required for
a conical primary sloped at a given angle?  I found one for a flat
spiral and made a guess at a derivation for a straight helix, but
couldn't figure this one out.  Also, if you have one for a straight
helix, would you mind posting it so I can compare what I got?

Thanks!
Michael Johnson