code: def _D(self, R):
          
          return (0.3 * self._HNn**2 / self._sin_alpha / R
                  + self._HNn + R * self._sin_alpha) / (0.3 * self._HNn + R * self._sin_alpha)
docs: Return numpy array from D1 up to and including Dn. (eqn. 7)