I finally got the chance to use the flail mower last weekend and had mixed results. I am pulling it with a 65hp tractor and had the height set to the middle setting. When i mowed we had just gotten 1/2" rain the night before and i started right at sunrise. when mowing tall but thin grass it did great. however when i got in shorter but very thick grass it would bog the tractor or slip the belt and just roll down the grass. I was able to get about 3 of the 4 acres mowed.
This afternoon i went back down and put on a new belt, and raised it to the highest mowing height of about 5" and the grass was very dry. With this combination it did great, i had a waterfall of grass clippings flowing out of the back. I realize the first time I mowed was less than ideal conditions and a bush hog would have had similar results. I won't be getting rid of the bush hog but will be using the flail in the future.
I do have a leaking output seal on the gear box that I will have to take a look into fixing. I didn't see a parts breakdown so I might have to pull it off and see if I can figure out what it is unless someone knows where to look.
Thanks for all of your help getting this mower back cutting good and I believe this will be a useful attachment in the future.