The root cutout is at 5ft, then the blade morphs towards a cylinder of
3ft radius. So the blade actually stops at 5ft and not 3.

As described in page 19, the non-dimensionalization uses a factor 1/2.
This factor is associated with the "European" convention, while it does
not appear in the US one (see Leishman, Principles of Helicopter
Aeroydnamics, Eq 2.36).

Interpolation is not needed (and was not working) in order to find the
alpha zero of all elements when the polar is provided for a single
reynolds angle.