To try and get back to the point -
the BERP blade operates at a relatively low NR and creates a greater proportion of its lift at the tip (as too I suppose does the 92?). The 'Donut Effect' has been routinelly commented on by operators in Iraq - not an absence of brownout, but rather a bubble of relatively better vis within the disc.
In light of the lack of other reasonable suggestions I propose this as a solution - but would be delighted to be corrected (I am certainly not an aerodynamicist...and certainly not a Westlands agent!!)
Anybody got a better answer?