Momentum and vortex theory of rotor blade wakes
The customary impulse-momentum theory for a rotor disk in vertical flight establishes a relation between thrust and induced velocity in the far wake. Steady frictionless flow is assumed, and uniform ambient pressure in the far wake, corresponding to infinitely many blades. The exact flow in the wake due to a finite number of blades can be calculated using vortex theory. The customary approximation is to assume a rigid vortex street, which flows down with uniform wake velocity for the ideal rotor operation.