I've always been fascinated by flails, including back when I thought they were all called morning stars... I learned from DnD and farming that the clubbing implement with a chain or hinge was generally called a flail, while a morning star was actually a ball with spikes according to some big dusty tomes I read about weapon types in high school, many, many years ago, and these were normally on a solid handle, and were very nasty devices.https://youtube.com/watch?v=Ox4sCJnCpzo
Anyways, are flails practical weapons? I've watched lots of videos about flails, and most of the creators agree that flails are, if ever used in any numbers (I believe they actually were in a few areas, but not widely and not for long) they weren't very successful or influential, and thus are a silly weapon at best, and dangerously stupid at worst, and largely ineffective.
Now, being out in the country and having a ready supply of wooden handles, old chains, and metal weights, I would occasionally weld up a flail to play with. The first ones required assistance, as I didn't know how to weld, and they were obviously far too heavy for my strength level, but they were a riot to play with, and I was amazed at how hard I could swing the thing. Later, I watched Return of the King in the theaters, and whoa, the Witch King was using a flail JUST LIKE MINE! Only he's using it with one hand, and with more proficiency, but still, that was cool. I felt like Star Wars Kid, and didn't play with flails for awhile. :P
Eventually, I made a new one, and this time I had the idea of making it reasonable in weight, using a lead filled hollow object as the head. This one actually worked, but wasn't quite durable enough, so I retired it, but I did note that it was nice that the chain was short enough that if I didn't choke up on the handle I didn't need to worry about whacking my hand! So that was nice. I had learned a thing!
I also made some flails with very long chains, and no handle, often with a biggish weight. These were fun, and could inflict pretty massive blows, but in a pitched battle these would be impractical in many ways, as you'd need to keep allies at a distance, and just, uh, flail away.
But still, it could be a shield wall disrupter, if you put a 4 to 10 lb weight on a 10 foot chain, which you could really swing for the fences with, that'd bust a shield up pretty badly I bet, and a loose formation of these might actually have messed up a shield wall, though they'd need heavy armour to survive in melee, and would likely need a good side arm. It'd be like how two-handed swords were used to break up pike formations I suppose, but realistically, I think a bunch of guys with sledge hammers or big axes could do the same, and might be more effective in some ways.
So, the most recent flail I made, and a nifty design indeed, it's got two chains with ~1 lb weights on the end, about 2 ft long, and the handle is about 4 ft, and is very heavy ash wood, with a very noticeable kink in it roughly where one's hand should not go past to avoid whacking one's self, a conscious bit of design actually to make it more effective! Anyways, with this device you mostly would strike with flicks of the wrist, and I think even a counter balance on the end, like a pommel, would make this work even better, making it more agile. The nice thing of course is that are striking off target, from a funny angle with this, so it's very awkward to defend against, and those 1lb weights hit REALLY hard quite easily, and could easily kill I'd wager, even with armour on, and without using a big wind up. Now, you CAN use it with a big wind up, and the long handle means you can really get a long level to strike with, giving huge power, but this isn't how I'd fight a duel with one! Note, the handle is quite heavy, so you can also use it to club an opponent, which is a big asset should someone close with you. If you put spikes on it, the clubbing would be even more effective, and it wouldn't likely interfere with gripping, as you don't want to grip where the spikes are anyways. These spikes would get hit by the chains and weights, but that's okay I think, as long as they're sturdy ones they'll survive. Keep in mind that if the chains are striking from the side a bit, and the club can strike directly, this is a very confusing weapon to defend against, and to top it all off, the chains aren't connected to each other, and join the handle from opposite sides, meaning the two won't always strike the same spot! much harder to parry that way, though sometimes it still hits together, so there is that. I think it depends on how I'm holding the handle when I swing. I think if the chains joined the handle, which could have a bit of a T shape, with the chains on the top ends of the T shape would be interesting, thought it might be harder to wield in some ways. Final point, you can do a figure 8 with it that would interfere with attacking, helping to mitigate the lack of a shield in melee, but you'd be really vulnerable to archers!
So, are there any actual advantages to flails? I got to thinking about it, and I came to a few conclusions, one, that the flail's unique acceleration only applies I think when you stop swinging it and let the flail finish the swing, changing it's momentum into a rotation from a point (where the chain ends, with the weight on the far end of the chain, fully extended), and until you stop swinging the chain will naturally form a straight line as long as you're going fast enough. Faster, it'll still be straight, just like a mace or hammer handle, so not much advantage here unless you're very good with the flail and know when to stop your swing and let the flail switch to rotation rather than swinging the whole thing. Still, a flail CAN accelerate very fast if you 'flick' it, which works better with a two handed flail than a one handed, and I think that's the way to fight with a flail mostly, with quick blows that hit hard for their speed, but aren't necessarily as hard as a mace could do with a big swing, because the flail becomes very clumsy in big swings, and you can't defend yourself effectively.
Regarding Shad's point about the flail not transferring force effectively, I have an anecdote he might find interesting... I like to carry logs around for exercise, and do so in a forest, especially when it's not super hot. So one day it began snowing when I was out working, but I kept on, and was carrying a reasonably heavy log (for then, now I can carry much heavier), which I tripped while carrying! It landed on my arm, and bounced off, before bouncing off my leg and ending up on the ground. I had very nasty bruises despite this log only bouncing off me, and I've had a basketball hit my head before too, and that also sucked, despite bouncing, so I assure you, a good bounce can transfer all the energy you need. In fact, in a fist fight, it is the actual first instant of contact that dictates how much injury a punch is likely to cause, not the follow through. Follow through might knock someone down, but fast strikes will cause injury/concussion and win the fight. I learned this the very hard way, but I wasn't well at the time, so I can forgive myself. Some martial arts styles specifically train to not follow through on strikes, to not waste energy, because pushing on your opponent doesn't hurt them much, it just looks good.
So, verdict of folks? Do you think flails are silly and need to be a tiny niche thing in fantasy instead of core material?