I've been following this thread for some time now and reading about belt problems and other issues. Also, I have researched flail mower prices. I recently purchased a 68" flail mower from Titan Attachments, free shipping, attractive price under $1400.00, delivered to my door.
It is of Chinese manufacture and looks to be a copy of popular European brands. It weighs about 675 lbs and is well constructed, thick steel plate, good welds, easy to use and doesn't chew up belts. The 68" uses three drive belts that run cool regardless of load. It came with duck foot fails, just what I was looking for.
I am usually skeptical of Chinese manufactured stuff having made several trips to China for manufacturing services and to purchase machinery. If you are careful, you can get good deals and reliable products. The Titan flails are good. I have used mine for cutting weeds and brush containing some 3 inch limbs and roots from recent clearing. My 45HP M-F handled it well with no stalls, no belts burning and nothing broken. I got some hidden fence wire wrapped in the rotor which might have caused issues with other brands. I cut six acres and some brush was over the hood of the M-F in height.
I found the following;
1- Keep the side skids at a low setting for initial field work. Mine are down the max.
2- Using the top link, raise the front of the mower so that the rear roller engages the ground. Much better cutting this way and clumping is reduced.
3- My mower allows installation of the top link tower with or without offset. I have no need for a hydraulic side shift using the offset setting. Plenty of reach when mowing next to trees and fences.
Some comments regarding Titan; Excellent service, tough flail mower (although, I have no experience with other brands), easy to use and maintain and a great value.
I'll likely sharpen my duckfoot flails over winter due to some contact with rocks. The originals are forged steel not the bent style. They'll last for many years with some occasional sharpening. This will also improve the mulching performance of the mower.
As for power, I mowed my 6 acres in a day with one stop to untangle the wire caught in the flail drum. I believe a 30 HP machine could easily handle this flail especially for routine mowing. First time cut in a rough field might take more time at a slower speed. Keep the PTO speed at or slightly above the 540RPM. Remember too, flails are much easier to maneuver around objects due to their smaller footprint. They are less prone to throw objects, quieter and store inside a small shed. I'll likely pressure wash and repaint the underside in a year or two.