Do not underestimate @InfinityzeN's point here!
I've seen a grown man (though a slender one) moved about when he tried to use his (then-)new bo-staff. Weight 2-3 kg (4.5-6-5lbs, I forget the exact mass). He ended up giving it away to a heavier (and more experienced) practitioner, because even he realized that he couldn't control it.
Swinging a sword heavier than yourself is an exercise in loosing control of your own movements. To do so in combat is to die.
To be fair, I didn't specify how much weight to add, aside from my re-statted glaive having one extra load for +5 str requirement to wield... and I figure ars magic physics gives some wiggle room. At least its not anime physics...
To be fair, your answer was the most grounded attempt in the thread to that point and I was was not specifically targeting you.
When I took a class on forging a sword, the ABS master had us swing around a length of spring steel with a 'blade' about three feet long, that looked like an old car antenna. The velocity you could get with it was rather scary and the point was to teach us that weight was the enemy past what you needed for the weapon to survive use.
Compare the arming sword vs the katana. A katana weighs more because it needed more of its weaker steel to meet the structural stress requirements to survive use. Of course the Japanese finishing techniques were really top notch.
Just watch out. Your analysis is very focused on swords even when talking about other weapons, and swords mostly developed toward thrusting when heavier armor was involved. With thrusting weapons, you can put yourself (or your horse) into it, so the sword needn't carry as much mass on the striking end. If you're not thrusting, most swords won't get through heavy armor well. And their length against armor is effectively shortened (as you mentioned), only managing their full reach on swinging with things like tip slashes and the like which are utterly ineffective versus armor. Meanwhile, you want to bring the center of mass very close to the hilt of the sword and keep the blade lighter to make things like parrying easier as well as quick changes in direction for thrusts.
Now look at non-thrusting weapons. Consider a war hammer or a flanged mace. These were much more effective swinging weapons against heavy armor. The war hammer specifically came into common use because it was so much more effective against heavy armor. Yet these are very specifically not designed with "the lightest weight possible to reach the required structural strength to survive use." Noticeably extra weight is put on the far end, moving the center of mass away from the hand and increasing their rotational inertia. They still were not nearly as heavy as many people seem to think, just as swords weren't. While your physics there is decent, this should reveal that it is a bit off.
On a related note, you may be interested in a calculation I did years ago here on just how much impact is lost if you take your hand off an axe (I think I used an axe.) just before striking (this was due to a discussion on ReTe bypassing MR). I'll have to see if I can find it.
My break down of the force calculation in my first post had nothing limited to swords, but covered all swung weapons. Additionally "war hammers" weigh roughly the same or less than swords. In point of fact, pole arm hammers (such as the Lucerne hammer) usually weigh notably less than bladed pole arms while having better balance. These weapons were designed to put the maximum force into the smallest area possible to punch through armor. They were also far better balanced then people believe.
The only reason I moved on to things that are advantages specifically for sword design is because the "Dragonslayer" is technically a sword. But only technically because in reality it is closer to a giant metal club.
None of my data relates to thrust weapons which use a completely different formula for how they generate power. It is the calculation for kinetic energy, combining multiple velocities (you arm movement, your body movement, your mounts movement, etc) and weights (up to and including a horse the welder is riding). The kinetic energy of the target is also applied or deducted.
For thrusting weapons their individual weight beyond required strength is even more of a detriment to use then with a swung weapon. Though taking on extra weight to increase the length can and often was a desired trade off, even if it made the weapon more difficult to use. Being able to kill your enemy before they could reach you was after all a desirable outcome.
What it all boils down to was an attempt to show that "Dragonslayer" is virtually impossible to make into an effective weapon without something like magic or anime physics. In Ars Magica that is easily done with a Re(In)Te effect, though some would argue a ReTe effect would be enough.
On the related note, would that not just be effectively throwing the axe at extremely short range from a rules standpoint? Though the actually calculation would be much more complex. You have peaked my interest. While I have not (yet) forged a sword, I did make an axe. Real ones are closer in design to a modern tactical tomahawk and are insanely destructive. RMJ has a video of them dismantling a car with one.
Yes, I mentioned that they also weigh much less that people tend to think. We both agreed on that.
But in the case of hammers, axes, and similar, you very specifically need to make the head noticeably heavier than necessary for structural integrity. This is an important part of what makes it perform differently than a sword. If you don't do this, then it ends up with a "sweet spot" similar to a sword's, which is less successful at its job. The heads were still not nearly as relatively heavy as an with an axe we use on a tree, for instance, and certainly nowhere near the ratio with a splitting maul. But the issue remains: if the calculations show that you want to make the head only light enough for structural integrity (which can be done with a pole, no head really needed outside of making a small striking surface), then something is wrong because we know reality is not behaving this way.
This is needed for swinging, too. This is what silveroak had hinted at above. If you use only momentum (angular and linear) in your calculations for a swing (as the inverse relationship between mass and velocity, though it needs rotational quantities), then you're assuming all weapons swing at the same rate while identifying that they don't because that inverse relationship requires the amount of time for the swing to be the same if the person applies the same strength. This is, for example, why we dancers, when we want to spin quickly, make ourselves harder to spin at the beginning, not easier to spin; we only switch when we're done applying the force. There are, of course, limiting factors.
If I remember right, that's what my friend was trying to argue against. The thought was that you could actively swing with ReTe right up to the moment before hitting to get the advantages of controlling the blade the whole time while avoiding the disadvantages of throwing it Vilano-style and it should cause damage more like a swung weapon. But the reality ends up much more like a thrown axe, as you point out.
Oh, apparently I actually did it for a katana since I was able to look up breakdowns on it, including a typical sweet spot, center of mass, length, and weight all in relation to each other. I also did with with a very trivial scenario (the axis of rotation wasn't realistic) which was noted to be off because I was just presenting the basic behavior to show that holding on does most certainly matter.
The katana fares worse than a typical sword built for chopping does because of the sweet spot's location so far up it. Anyway, that calculation was just to show that follow-through is an important part of the strike.
I totally agree with that. We may see it a lot in MMORPGs, anime, and the like, but those huge, disproportioned swords don't work at all in reality.
My interest is similarly piqued. I'll do some calculations for what seems to be a reasonably ideal split of mass for an axe or hammer for a reasonably sized person. I'll have to do a whole bunch of approximations, but I should be able to model it pretty well without getting overly complex. With a real example to compare it to, it would be interesting to see. If I do the calculations on a Google Sheet, I could share that here without having to put all the calculations here, so I could focus on the explanation in writing. But I have a ton of work to finish these next two days due to sudden, huge shifts at work.
Different weapons have different "strategies"- for example a lot of pole weapons were less about fine control and more about dropping something heavy with the assistance of gravity at a distance, and the primary attack was a downward slice. Also there were weapons that took something more of the modern SLAP round approach- which is to say that rather than trying to penetrate the armor they simply delivered enough force to the armor that the shock wave would carry through it and injure the person within, which of course was even more effective against flexible armor. So which strategy you are pursuing would probably need to be considered before you finish the design.
Yup. War hammers commonly had two options for employing those varieties. Hammering for the shock or piercing with the other end (or hooking with it). That gave them options. I found it interesting to note that the hammering for shock method was better against thicker armor, but game designers usually seem to make thicker armor better against the hammering for shock approach.
I'd be curious how the composition of the armor fit into that analysis. It makes sense, if you consider- even if the thicker armor reduces the kinetic energy by more some will get through, while failure to penetrate is failure to penetrate.
Yes the full calculations for power of a swung strike are even more complex then what I provided. There is the spring reaction from back swing and pre tension of muscles, acceleration per unit of time per unit of time (yes there are two per unit of time), resistances including air resistance and moment of inertia, the requirement for continued force after contact (follow through), and a whole host of other points, both minor and major. I figured I was being complex enough to prove my point.
I will accept that I over emphasized the desire for lightness to some extent, because in addition to meeting the minimum weight requirements for structural strength you must also meet the design requirements for it to function in the desired manner. I did not believe that I was implying that a simple pole would do the job of a hammer or axe head better because it was lighter. If it comes across that way it was a mistake on my part. I have spent far to much time tying to explain melee weapons to those who have never made or used one.
Weapon design is always about trade offs. However the head design of a fighting axe or hammer, while putting a block of weight at the end of a weapon, is still kept at a far lower weight then on a work axe or hammer. The fighting ones need much more fine control since you are trying to hit a moving target, as well as needing to be able to recover quickly from a strike to allow follow up attacks. They also need to have a short "windup" and "delivery" time to allow the target less chance of successful defensive action.
Side note: The katana fairs worse because it is a draw cut rather than straight cut focused design. Draw cuts favor initial contact closer to the hand, allowing more of the blade to be drawn through the target. Without follow through draw cut blades will always be less effective, because they are designed to cut during the follow through. However with that follow through they will always beat straight cut blades (of comparable size) in the size and depth of the cut inflicted.
You see this in weapon and armor design over the ages. They repeatedly cycled between inflicting/reducing blunt impact, cutting, and piercing. The designs of each just got more complex each time the cycle repeated.
At least we're all in agreement. 
I still want to try out modeling a hammer/axe and see how close I can get with some approximations.
I think we're also all agreeing that doing this crazy sword would be more about sticking in some ReTe effects (probably with great penetration) or similar that might require some high user Str to trigger rather than trying to stat a new sword. This would seem especially so since the giant clubs rules and giant natural weapons rules give exactly the same weapon stats as for the normal-sized ones.
unless of course you are making a giant sized weapon to be wielded by a giant sized person, whether an actual giant or a person grown with MuCo... that provides all kinds of new opportunities...
Yes, that's certainly viable. The pile of stats so far basically say to just keep the weapon the same, probably adjusting load and minimum Str since those aren't listed and it makes sense. For minimum Str, we could probably just add +2 for each Size above a certain point, but that point is a little fuzzy for me with the way Dwarf, Small Frame, Large, and Giant Blood work. Load could be figured out, but it may be tricky separating out weight from awkwardness from the originals to figure it out well, especially with the originals' granularity.
Especially when you sometimes get +2 str for +1 size and other times you don't...
Exactly. That's what drives me nuts with those four V/F compared to all the other Size stuff. With MuCo I allow 1 higher magnitude, matching the MuAn guideline, to have Str and Qik adjust with Size. I believe there is a spell somewhere that grows you without a mass change, which fits not shifting the Characteristics reasonably well. But normal MuCo growth I feel should match normal MuAn growth.
The matter of tradeoffs is actually quite relevant to the discussion here. The main issue that justified the Dragonslayer concept in the source material was that combat with the supernatural threats Guts constantly had to deal with was so much different than fighting people. Like if you're using a sword in melee against a heavily armored opponent, your main plan to do damage is aiming for gaps, and if you have a better pick of weapons you might go for a hammer instead because of reasons already discussed upthread. But when your primary enemy isn't a small, agile, squishy human hoping desperately that the thin sheet of metal he's wearing continues to be in the way of stuff, and is instead a lumbering brute who's just rocking rock-hard flesh and being big enough that you've gotta get through a lot of it, priorities change.
I'm not suggesting a sword heavier than its user is ever practical, but if taken to a much less extreme extent, I can at least understand how "hit hard" and "wider cutting area" could result in "heavier-than-average sword" making sense. Guts in canon tried using regular swords against demons and tended to snap them in half trying to hit hard enough to do damage... Which, I know, not really something Ars represents, but. Makes that whole "maintaining structural integrity" point come to mind.
If I've gotta ditch "heavy mundane sword," it's probably preferable to switch weapon types than to switch to magic. No point to the darn thing if it pings off the primary enemy it was built differently to deal with. Then you've just got something for killing mundanes harder, and nobody needs help doing something that easy.
If it's a special sword rather than a just slightly bigger sword, I would go with Troy's suggestion of a ReTe effect. I'd probably set it to trigger by a very strong swing, so only someone with incredible strength could actually manage it. Give it plenty of penetration so it works against the big baddies. Then you can justify some much better damage with the high Strength while being consistent with all the statted weapons.
Which brings up the question- would an item of quality weapon hit issues with magic resistance?