Slowrotor,
With all due respect, I strongly disagree with your comments regarding laminar flow and the extent of roughness required to transition to turbulence for the case of a hovering rotor. The article that I referred to describe actual flight tests on a real, full scale Bell UH-1B, with SEVERELY eroded blades. I have (although I cannot describe the specifics) similar reports for flight tests on a Eurocopter AS-365 Dauphin and the BO-105 that show similar extents of laminar flow.
There are a host of complex, interconnected reasons why laminar flow CAN exist on hovering rotor blades - even in the presence of some roughness, not least the relatively low Reynolds numbers seen on rotors. The key to all of this is the fact that in hover, the rotor experiences a steady, largely two-dimensional flow. In addition, the majority of the rotor does not have sweep and therefore the modes by which transition can occur are limited to the two-dimensional modes; Tollmien-Schlichting instability and transition in the free shear layer following a laminar separation. Notice that the troublesome three-dimensional modes, cross-flow and attachment line do not play a part in hovering rotor transition. This is the crux of why laminar flow can exist on a rotor. The three-dimensional modes are generally far more unstable than the two dimensional ones and as a consequence it is much more difficult to sustain laminar flow on a fixed wing. (For small GA aeroplane at low speed I would expect attachment-line contamination to be the main problem, though I have never looked at it in detail.) The attachment line in particular is renowned for its sensitivity to bugs and dirt and hence this, I believe, is where the origins of your views on the effect of surface roughness lie.
Of coarse, in forward flight the rotor is heavily yawed and all modes become important, so for the most part I would expect to see far less laminar flow, but I would be very surprised if 'pockets' of laminar flow were not still present.
I hope this helps
Kind Regards
CRAN
