I am doing the research on helicopter aerodynamics. By applying the blade element theory and momentum theory we can determine the induced velocity at the balde, and we knew that, at a distance very far below the rotor, the induced velocity is two time of the corresponding value at rotor disc. how can we determine the induced velocity at a certain distance below the rotor disc. For example, I have a rotor disc, whose diamter is 150mm, and I would like to calculate the induced velocity at the distance 60mm below the rotor disc, but I do not know how to solve this problem. Could you share with me your knownledge in this section. Thanks!