Is the windmill stopped? Are the blades feathered? I'd think it would be laughable unless the blades are stopped. Leaving them unfeathered would also be easier to to hit the blade cord head on with your water cannon. 20 Knots seems like a lot and it all depends on how close you'd feel comfortable hovering along a blade. It isn't like powerline insulator wash where the things you are washing are small. I'd think that if you are feeling it might be cost effective to do it with a helicopter , it might be less expensive to do one blade at a time from the ground with some sort of lift device that could get you close to being able to reach each blade with a nozzle and brush. How high is the blade root when the blade is straight down? You could have a much larger water tank if pumped from the ground as well. Let us know how this goes for you. It is always nice to see what works with helicopters. Who thought a giant chain-saw suspended from a helicopter would ever work. Surprise, it works pretty well but it is freakin' scary to watch or fly.
