The helical angle is similar to a right angled triangle wrapped around a cylinder,where the height of the triangle is the interflute distance.The base of the triangle is the circumference and the hypotenuse is the flute itself.
The relationship between helical angle and pitch is
tan (alpha) = pitch/circumference
i.e, tan(alpha) = I/C
When a vane or blade is kept in place of helical path,with flow along the axial direction, the vane angle is to be calculated as,
tan (alpha) = C/I.