D3, thanks for the clarification. I think the slant is an example of where this can get confusing, depending on whether you reference axis plane control plane or tip-path plane. For the purpose of this discussion it is fair to comment that 1D momentum theory describes a uniform downwash relative to tip-path plane (which would be slanted in axis plane). Once the machine has flapped to equality, we are both agreed that this average velocity goes down with ground effect.
What i am still confused over, and central to the discussion, is how have you accounted for the way the ground plane affects the downwash velocity? Prouty shows how induced downwash Vige/Voge is a function of height, but this data appears to be either from wind tunnels or CFD simulations. I am guessing that you have used an empitical curve, possible taken from very accurate flight test data, to simulate ground effect in your model.
My only concern here is that we are trying to establish the cause of ground effect reducing Vige/Voge in a hovering helicopter. There are two competing ideas:
1. The complex 3D flow means that ground effect reduced downwash velocity through the rotor by reducing wake contraction.
2. The viscosity in the flow means that the ground resists the outwards flow, causing a buildup of stagnation pressure under the rotor.
Although your model is clearly very good, and probably quite accurate, it's reasonable to comment that we can not use it to bust (or otherwise) this myth. I get the impression you had reached a similar conclusion.