[phi]´ = tan^(-1)(u_o/V_o) = tan^(-1)(Q/[Omega]_o V_o)
be the first or approximate value of [phi], and let t be the
thickness, and n the number of wheel vanes which reach the outlet
surface of the wheel. As the vanes cut the outlet surface
approximately at the angle [phi]´, their width measured on that
surface is t cosec [phi]´. Hence the space occupied by the vanes on
the outlet surface is