I still believe that only in light conditions, is a flail gonna be as fast.
This is what I mowed with my 8' bushhog at ~7.5 MPH
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I believe that I could run an 8' flail just as fast with equally as good results.
Jobs like these on the other hand.....which are more common (the last two pics is the same job....before and after)
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I dont think my 42 PTO HP tractor is gonna pull an 8' flail mower across these areas as fast as I pull my 8' woods. I doubt I'd be able to pull a 5' flail at the ~4 MPH that I cut the above at.
Flails have their place. For pasture mowing 2-3 times a year cutting knee high stuff they are ideal. For mowing something that has waste high weeds/grass that is thick, and hasnt been touched in a year or two....flails require ALOT of HP per ft.