Center of percussion-calculation.

[Gianluca Zanini]

Does anybody knows how to calculate POP? It is done with a particular formula by measurements and weight or does it involve empiric method which involves strike/cut testing?
Thank you very much in advance for your help!

Gian

[Randall Pleasant]

Does anybody knows how to calculate POP? It is done with a particular formula by measurements and weight or does it involve empiric method which involves strike/cut testing?
Thank you very much in advance for your help!

Gian

Take a look at the article Sword Motions and Impacts by George Turner.
http://www.thearma.org/spotlight/GTA/mo ... mpacts.htm

[Gianluca Zanini]

Take a look at the article Sword Motions and Impacts by George Turner.
http://www.thearma.org/spotlight/GTA/mo ... mpacts.htm

Randall,

thanks I have read it once and I still have to study it better.
I was just wondering if there was a way to calculate COP with a formula by the measurements and weight, but when i post this question on myarmoury.com, I felt to be a bit... blasphemous.
So I am not going to repeat my silly supposition even here, and I content myself of the empiric experimentation proposed even here http://www.myarmoury.com/feature_properties.html as I understand the calculation involves too many variables.
These two essay employ essentially the same pragmatic and empiric experimentation to test sword harmonics?


Gian

[Sean Belair]

“harmonic balance” is more of a marketing ploy. The best way to find the COP is to hold the sword by the grip with one hand and whack the pommel. The part of the blade that doesn’t vibrate is the COP or “sweet spot”.

hope this helps

[Gianluca Zanini]

“harmonic balance” is more of a marketing ploy. hope this helps

Yes thanks

[Randall Pleasant]

What is really sad about the Harmonic Balance (HB) crap is that of all the myths that we as an education organization have to confront, HB is the only myth recently produced and promoted within the WMA community itself.

[Jeffrey Hull]

To my understanding of George Turner's complex and learned article, if I may dare simplify it, then it seems to suggest that this COP / POP is much too hyped and proclaimed than it deserves, as well as misunderstood.

More or less, your best striking may be done with a dedicated cutting sword anywhere in most of the region of the last 1/3 of its blade -- thus from 2/3 up from the cross to within 1 or 2 inches of the point itself. GT uses physics to analyse and explain all that expertly and in detail. And your test-cutting can prove it. And the difference in shock between a strike that impacts exactly upon the COP and a strike that is not exactly upon the COP is trivial, at least in an historically accurate replica sword. Plus, striking beyond that COP and thus even nearer the point makes for a more powerful strike.

And I agree with the other sentiments expressed here about the marketing hype trying to tell you otherswise, insisting upon all the harmonics and so forth. None of the hucksters trying to sell "mystical" swords could present an argment to counter that of GT.

I state all that humbly as someone who made himself read GT's extensive article three times in order to understand it.

[Shane Smith]

It was much easier to understand in person when George explained it to me in Tennessee a couple years ago. What he has to use so many words to express on paper, he can show you in two minutes and it immediately makes sense.

[Vadim Palshin]

The whole point of George Turner's article is that COP has nothing to do with blade vibrations, exactly opposite of the "harmonic balance" theory. As far as vibrations are concerned, your sword hand location should be close to the handle node, to minimize felt vibration no matter which part of the blade hits the target - this is what I would call a harmonically balanced sword (plus, maybe, the blade node being close to the true CoP). COP is calculated from mass and geometry of all components of a sword based on their moments of inertia, or (much easier) is measured experimentally by the pendulum method. Having a theoretical equation is useful when you have to compare swords that you can not handle personaly - e.g. when shopping on-line! And the sword's moment of inertia is a much more important factor than its COP, since MOI tells you how easy it will be to accelerate/stop rotational motion of a sword, in other words how a sword would handle.
I have worked out a more-or-less simple model to calculate MOI of a tapered sword, but it requires a lot of informatoin to be useful - like degrees of profile and distal taper, exact mass and dimentions, and a good idea about relative masses of the sword's handle, pommel, guard and blade. Very recently, I have asked at the SFI if anyone would be interested in it, and got exactly zero responses I thought everyone would want a tool to estimate a sword's dynamic balance without picking it up, shows how much I know At least I did not get flamed for blasphemy.
Regardless, here it is: (x**2 means x squared)
I=2(b**2)*Mtot + (a**2)(Mp + (1/3)Mh) + a*b*(2Mp + Mh) + Mb[((L**2)/2)-L*b] + Mb{(L**2)[((t1*t2)-5)/(30*n)] - L*b[((t1*t2) -(t1+t2))/(6*n)]}
where
n=1+ t1*t2*(1/3) - (t1+t2)*(1/2)
a = handle length
b = distance from pivot point to Point of Balance, or Center of Mass (use the usual PoB-to guard distance to get I relative to cross)
L = blade length
Masses: Mtot=total, Mh=handle, Mp=pommel,Mb=blade (tang not included, it's part of Mh, sorry).
t1 and t2 are distal and profile taper, linear approximation. For example, no taper means t=0, taper to a point - t=1, t=0.5 for 50% taper, etc.
I'd be glad if people tested it, seems to work with my repro saber.
CoP is easily calculated from this, just find Icom=I - (b**2)Mtot, and then for a pivot point (somewhere in the middle of your sword hand for a single hander) at distance X from CoB (balance point), your CoP is on the other side of CoB at this distance from it: Icom/(X*Mtot)
Enjoy!

[Jeffrey Hull]

Well, I appreciate your effort to provide us a helpful formula. You obvviously have taken some considerable time to devise that. Do you find it works with multiple designs?

And of course a commercial-site disguised as a forum like SFI does not want to read or learn anything that may dare threaten their sales of questionable weaponry fabricated by their business partners.

I have done the pendulum testing as per George Turner's article. It was both edifying and fun to find out what-was-what.

(By the way VP: Please reregister yourself with full legal first & last name, as stated in the ARMA Forum rules.)

[Vadim Palshin]

Do you find it works with multiple designs?

That's the thing - it needs extensive testing, i.e. comparing calculated and measured values of MOI, and, more importantly, comparing these values with the subjective impression of how a sword "feels". I am new to this hobby and only have one reproduction saber, 1822 british light cavalry from Military Heritage, which I love dearly and I was hoping that this might generate enough interest that people with knowledge and access to many good swords can tell us if this formula can be used as a "sword feel" parameter.
I can tell you about the model I used to simplify the math: straight blade (but should work with most lightly curved ones), pommel and guard are point masses placed at their respective locations, taper starts at guard, pivot point also located at guard, and handle is a thin rod and includes the mass of the tang. Blade has a density which linearly changes with length, it's proportional to the cross sectional area and can be conveniently integrated (times (x**2)) to produce I. It gives excellent agreement with experiment for my saber, with the pommel mass set to zero. The hardest part was calculating the masses of blade and handle, and the best way may be measuring the blade (same with the pommel), calculating its volume (don't forget about fullers and such), and multiply it by the appropriate density.

And of course a commercial-site disguised as a forum like SFI does not want to read or learn anything that may dare threaten their sales of questionable weaponry fabricated by their business partners.

No comment! I might want to post there again Besides, there are some reasonable people there that deserve respect.

(By the way VP: Please reregister yourself with full legal first & last name, as stated in the ARMA Forum rules.)

Done!