I was wondering what benefits are granted when a sword is either on fire or emitting flames. I once read that such a weapon would be completely useless, as it would cauterize any wounds it inflicts, but there has to be some benefit.
What are the advantages of a flame sword in melee combat?
While a flaming sword would certainly have a psychological effect on people, unless it was emitting a white hot flame (@ 1600 degrees C), it would not be as effective as a metal sword.
Consider that the mechanism of injury would have to be the transfer of heat to the target. A red hot sword (at a mere 800 degrees C) would have to be held to the target for a period of time, more like a branding iron than a sword, in order to transfer enough heat to cause injury. If the target is protected by armour, then the mass of metal and the protective undergarments need to be heated enough for the heat to transfer through them first (and the masses of metal, leather and quilting would both spread the heat energy and provide insulation to the target as well).
A red hot sword would also be far too cool to melt metal armour, and would have difficulty setting fabrics on fire with a mere swipe or quick thrust. Since the padding was usually sheep's wool, flammability isn't going to be an issue. Even striking a shield isn't going to cause a catastrophic ignition event, try setting a sheet of plywood on fire with a quick sweep of a blowtorch and you will get the idea. Indeed, you can quickly swipe a blowtorch across your own naked skin and feel the heat or suffer a first degree burn, rather than cutting through your arm...
Greater heat allows you to overcome these issues to a certain extent, providing much more energy to transfer to the target, but in general, a flame sword would have to be used differently from a metal one. The best attack would have to be a thrust with the flaming blade to the face, since even the best helmets need to have openings to allow for sight and breathing. A successful thrust could blind the enemy, or they might breath in the flame, both rather horrifying ways to die.
Given the relative lack of effect of a flaming sword, the terrible deaths that it does provide and the pretty instant identification of the user once drawn, I suspect that a flame sword would simply invite the wielder to die under a hail of crossbow bolts, followed by attacks by polearms so the men at arms will be at a safe distance but still able to strike killing blows if the archers hadn't gotten him first.
For a few years I learned how to fight with medieval weapons. Some people I met then had their swords put on fire (in this case, with oil on a wick, so it was for a short time)
It was beautiful and impressive ! But not very effective.
Of course they did not use it to fight an army for glory. It was a choregraphic fight and the "enemy" should not be hurt or frightened. The public was impressed, the noise of the flames and the light were spectacular, but the fight itself has to be quite slow and simple.
The flames are as dangerous for the owner of the sword than they are for the opponent. If you want to protect yourself in a swordfight, you have to keep your sword very close to you. Keep your hair tied and protected, and make sure your clothes won't ignite. And you will have to wear good gloves, as your wrists will stay very close from the heat during the whole fight.
As long as you move quickly there is no problem, neither for you or your opponent (as it has been said, the heat don't hurt you if you don't let it on your body for a moment), but in a real swordfight your swords will meet, and stop, and then move again. Your opponent's moves will bring your own sword to protect you, thus put it very close of your head, your eyes, and you don't want flames near your eyes.
Perhaps, if you manage to hurt your opponent with your flaming sword, it will hurt him more than a regular sword. Burns are very painful, but so are "regular" sword injuries. If you manage to hurt your opponent, a flame sword will be a little more cruel, (burns are painful and take a long time to heal) but it won't help you win the fight, except if your opponent is terrified by your mighty flaming sword and run away as soon as he sees it.
God thought it was a good idea.
So he drove out the man; and he placed at the east of the garden of Eden Cherubims, and a flaming sword which turned every way, to keep the way of the tree of life. Gen 3:24, KJV
You think you are wiser than God? I think the intimidation value of the fact that you are so butch that you do not just use a sword, but a flaming sword has to be worth something.
Humans and animals justifiably have a deeply held fear of fire. It burns us, and often escapes our puny attempts to control it. A weapon that your enemy fears works even when you don't have to wield it in combat.
Whether the flaming sword gambit is a good idea for a human wielder is perhaps a different question.
One clearly negative aspect of a flaming sword is that you are compromising the strength of the metal. At temperatures above 900 C, steel loses about 90% of its strength. This is very bad for battlefield use.
Human soldiers are not as butch and do not have the superior grade weapon material qualities of angels wielding divine swords. Lose the flaming sword.
Lastly, if you are primarily interested in a torch with limited utility as a weapon, well perhaps you are using the right tool after all.
In case English is not your first language. Describing an angel as butch is a whimsical means of describing angels as extremely manly, possibly an irreverent usage. Butch does not necessarily reflect common modern usage as manly lesbians. My only intent was making this light-hearted, no offense was or is intended. I simply intended a light-hearted description to counter the idea that I was engaging in a serious theological discussion. Clearly worldbuilding is not the place for that discussion.
A flaming sword can't be grabbed
Though it's not often shown in movies and fantasy games, grabbing the edge of a sword (your own or your enemy's) was not uncommon.
Sometimes a knight would flip his sword around, grab it by the blade and swing it like a warhammer (the mordhau maneuver).
(source: warosu.org)
Sharp blades would deflect off of full plate armor, but the concussive force of a blunt weapon could transmit through the steel and hurt the knight inside, or perhaps dent the armor and immobilize him.
A sword with a burning edge probably could not be grasped (unless your warrior also has asbestos gloves). This would make maneuvers like the mordhau impossible, restricting the sword's utility.
However, a flaming sword would also be difficult for the enemy to grab, changing the tactics a foe must employ.
Psychological and morale effects. It's going to get people's attention and intimidate and surprise them and make them wonder what's going on, and probably inspire some fear and confusion. Also, metal doesn't usually burn, and flaming oil also doesn't burn well when put on metal and swung about. So if you have a seriously-burning sword, it's going to make people wonder what's going on, at the very least. It's not going to look like something most people will want to risk getting hit with. Not to mention that you may make people wonder if you have some sort of supernatural assistance. Your foes may cringe, flee, falter, freak out, or at least be thrown off their usual behavior. People on your own side may also quite like having you on their side - you may inspire them, or at least they may like that you are attracting so much enemy attention.
Hot metal hurts to touch, so people you cut may suffer additional pain.
You can now easily set flammable things on fire. If your sword produces plenty of fire even while swinging around, it probably will light up cloth and straw quickly, which might be useful. If an enemy is for instance in metal armor under a nice cloth surcoat, for instance. Or perhaps you are pillaging, or want to create a fire obstacle, and there's dry grass or hay around.
You won't have to worry about darkness.
Animals in particular may not want to attack you. You might be able to spook someone's warhorse.
However you are managing a flaming sword, may also be an advantage. For example, if it's covered in Greek Fire or some sort of pitch/tar, that may also add pain and flaming sticky goo to anyone you hit. Or if it's flaming because it's magic, the magic may also make it more effective in other ways. Or if it's flaming because some god of fire has blessed it, his blessing may also have other effects...
Really, the power of a flaming sword beyond the merely psychological would be entirely dependent on the properties of the sword in question. Since the others have basically answered your question with the 'no' answer, allow me to be contrary.
First of all, since the sword doesn't burn itself into a wreck while being ablaze, we can safely assume it is a magical blade, or at least emits magical fire. If so, perhaps it protects its user from its own flame. If it does, this expands its possibilities by a huge amount.
So yeah, maybe a bit too fantastical for your liking, but probably cooler than a normal sword ablaze in burning pitch.
Here's a magic-free (if rather impractical) solution.
Perhaps you could have a sword with a fuel delivery system built in. You'd need a pilot light always burning -- that could be fed by a wick -- then a trigger to release a dose of fuel down a hollow blade. That would mean that you could block without the flames coming too near you, and still deliver serious burning damage. Despite the ancient use of Greek fire (wikipedia) it wouldn't be a good fuel in this case as it needed significant preheating.
Even better would be pyrophoric (self-igniting) liquids (wikipedia). Again you'd need manual control of delivery.
When it comes to how it would be used, either way you'd have a short-range flamethrower combined with a sword, not so very different from a pistol-sword (wikipedia), but with the psychological effects of fire weapons. The flames could be used mainly before joining in close-combat -- perhaps they would spread far enough to be useful against an enemy with pole-arms.
Against an armoured foe, the flames could be quite effective, similar to how flame-throwers were used against bunkers (wp again) to great effect. Essentially because the flame can enter though any opening in the armour and then spread it can cause serious harm without the need to penetrate armour. Flames passing in through existing openings/joints, even if only painful and not lethal, would certainly affect an opponent's ability to fight (especially considering eye-slits). If a sword could open up a new hole in the armour then inject flames, it would be all the more effective.
The fear factor involved with fire might mean it would be used as a torture weapon as well.
A reservoir would have to be reasonably well protected, as would the fuel line, otherwise damage to the fuel line would be a serious vulnerability, especially with pyrophoric liquids.
The overall advantages of each
Normal Sword: simple
Flame Sword: complicated
Flame-thrower: longest range and largest area of effect
Flaming swords, as we currently think of them, involve many complications without any "real" advantage over other weapons which use the same concepts. This makes the complications its strength, because hopefully people won't understand what they are facing and they will be afraid.
But let's say we could have an effective flaming sword, what would it have to be like?
Of course.. the real problem with that is the tech level involved. If a lightsaber just randomly showed up on the battlefield today, the user would still just get shot. If it randomly showed up in a medieval battle, I suppose the person would still get shot with arrows. Randomly showing up at a medieval duel? I guess you've found the right moment to pull it out, so long as you are more likely to be portrayed as the son of a deity rather than an evil demon.
JET PROPELLED CUTS
What if the flames engulfing the sword dinamically flow on the sides of the sword giving aaddictional thrust like a spyke thruster engine (more correctly a linear spike engine like the one made as prototipe for the x project spaceplane... Sorry i forgot the name of the Xproject)
In coordination with the wielder moves it could ad more power to the hit, a more acurate edge alignment (since the sword are automously directing themselves) and if the thrust controll of the wearer are fine enough are possible to perform some "impossible" moves and angles of attack
The issue is how you could have controll over the sword and after that the handling of that sword need a completely new skill set, almost a new martial art
