fix(config): fix knight config

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).