Understanding, transferport-size , valvesize vs pressure behind and barrellength

[rumbleUU]

Yes there is quite a few reading about this subject, I know Bob is an expert in this, and Ive read almost all he has written, I fins this quite challanging to understand, and learning how stuff works is always fun.

But how is the relationship work between:
* Transferport-size (Area)
* Valvethroat-size minus stem area (Area)
* Pressure unregulated unlimited chamber 200bar
* Barrel length

Lets assume:
* Transferport-size is 4.5mm all the way throw, perfect aligned
* Valvethroat-size minus stem area is a little bit bigger then the area of TP, lest say 10% bigger
* Pressure unregulated unlimited chamber, lets say 200bar
* Barrel length is 55cm
* .22 caliber
* strong hammer-spring enough to get maximum power

1) what FPE would that give with a 25 grain pellet?
2) how much more FPE would be achieved with same pellet with TP=5mm and Valvethroat-size minus stem area 10% more the TP area?
3) How much FPE is possible? with 4.5mm vs 5mm TP
4) how does the pressure behind affect how big TP is needed to achive maximun power with a certain length barrel?

Is there a way of calculate this, when is the TP enough?
Yes 11/64" TP is safe, and will be enough for a .22 caliber, but that does not tell us much more............   ? ! ?
Lets skip pellet loading issues talk, cause you can go TP size bigger if oval....

Sometimes I think its easy to understand, but sometimes it gets difficult..

[darkcharisma]

i am sure Bob will chime in soon. but in my limited experiments with the Nova Liberty. i find that 3/16 TP size is best for under 60fpe. once you want to hit that limit, i usually open the TP to bore size.

I don't like to calculate anything and usually just approach it by trial and error opening the TP and test in small increments from 4.5mm to 5.5mm to 6mm and such. i suggest you do the same, thats where i find the fun in tinkering air air passage ways

knowing those calculations do come in handy though

[rsterne]

I use a formula to calculate my "lofty goal", which I consider to be 50% of the maximum theoretical FPE based on the energy INPUT from a given pressure, caliber and barrel length.... To understand why, I should explain a bit how that can be calculated....

The maximum possible FPE is the force available (in lbs.) to accelerate the pellet times the barrel length in feet.... If you had 100 lbs. of force and a 1 ft. barrel, there is no possible way to exceed 100 FPE.... The force is the bore area (sq.in) times the pressure (psi)…. so for your .22 cal, if you had an infinitely large reservoir (so no pressure drop) at 200 bar (2900 psi) the maximum force would be (0.22 x 0.22 x PI/4) x 2900 = 110 lbs.... With a barrel 55 cm long (1.80 ft.) the maximum FPR would be (110 x 1.8 ) = 198 FPE.... The problem is, that represents the TOTAL energy, including that required to accelerate the mass of the air, and overcome all friction and other losses, and for an infinite reservoir.... The very best of our PCPs struggle to reach 50% of that value, which would be 99 FPE.... Most of the time, if you get within 10% of that (ie 90 FPE), you have done a VERY good job....

As you make the transfer port (or whatever is the smallest restriction) smaller, you can reduce that figure further, roughly in proportion to the diameter of the smaller port.... Drop down from 5.5 mm to 4.5 mm, and you will do well to get (4.5 / 5.5) x 90 = 74 FPE.... This is only a very rough rule of thumb, but perhaps you may find it helpful....

Bob

[rumbleUU]

Thanks, here is where I got lost

4.5/5.5x90=74FPE, why do you use diameter and not area? when going down...

And is the doable 90FPE with a 5.5mm TP acceptable efficiency, or is it an airwaister?

[darkcharisma]

Efficiency is affected by many factors. TP being one factor, hammer force, valve springs, pressure and volume and many other things. i too seek a one answer to efficiency. i am sure Bob has an equation

i still rely on trial and error to figure out efficiency...

[rumbleUU]

Efficiency is affected by many factors. TP being one factor, hammer force, valve springs, pressure and volume and many other things. i too seek a one answer to efficiency. i am sure Bob has an equation

i still rely on trial and error to figure out efficiency...

You are right, you have to test and see what happens, and where to be at for any gun, but I think the efficiency should not suffer to much even when reaching power around 70FPE for a .22 caliber pcp

[Motorhead]

Efficiency is affected by many factors. TP being one factor, hammer force, valve springs, pressure and volume and many other things. i too seek a one answer to efficiency. i am sure Bob has an equation

i still rely on trial and error to figure out efficiency...

You are right, you have to test and see what happens, and where to be at for any gun, but I think the efficiency should not suffer to much even when reaching power around 70FPE for a .22 caliber pcp

As I read the original inquire and thought about ALL the nuances involved with each part, applies here and when, tho not here if ... Whoa a lot to cover that would in reality be a small book.
As one who has done extensive testing, loads of  R&D having run with more shared ideas than most have come to a conclusion ... there is no sure fire recipe, only general guidelines for those seeking cutting edge performance.

Of late have my now dedicated .22 cal SLUG shooter launching NSA 24.8's at 975 fps at just shy of 53 ft-lbs and doing so with IMO an outstanding efficiency of @ 1.29 Ave FPE / Cu In.

[rumbleUU]

I understand the complicity of all this, but if I ask it this way:

Is it doable? :
65FPE avarage for 15 shots with a 34 grain pellet
55cm barrel in .22 caliber
11/64" hole all the way throw (+10% area @ valvethroat-steem)
250cc airtube @200bar

[Motorhead]

Click here:  http://calc.sikes.us/1/
I'll let you do the math first by running your basic numbers & perhaps manipulate them to find what works threw an efficiency calculator ....
Once you / we know whats required in efficiency we can better guide you in how to achieve it.

[nervoustrigger]

That kind of power from a .22 cal is out of my wheel house so I can’t offer specific advice but the way I see it, you’re looking for .25 cal power from a .22 cal bore so it’s going to need a good kick in the pants (large porting and high pressure).  So one thing I am confident of is that 11/64” porting (roughly 80% of the caliber) isn’t going to cut it.  Bear in mind that 80% on the diameter is only 64% of the cross sectional area.  I think you’re going to need to move closer to full bore porting.  In doing so, the competing issue that arises is it becomes harder to tune for a bell curve over a large enough pressure range to get the shot count you want.
 
And with that set of sweeping generalizations, I’ll step aside and let one of the experts set me straight

[Atom]

Sebastian (666?) read this
https://hardairmagazine.com/ham-columns/now-its-easy-to-find-bob-sternes-airgun-technical-posts/

[rsterne]

You stated enough hammer strike for MAXIMUM power.... so what do you think the efficiency will be?.... Yup, that's right, HORRIBLE.... Probably about 0.4 FPE/CI.... When you choose to maximize power, you must have valve dwell that keeps the valve open until the pellet exits the muzzle.... That's what maximum power requires, and the result is wasting a lot of air....

I can't give you a mathematical reason why the FPE is ROUGHLY proportional to the port diameter (instead of area).... but I have done enough testing in enough guns to see that is the relationship, rather than area as you might expect.... My reasoning (which could be wrong) is as follows....

When you calculate the bore area, for use in calculating the force in lbs. (by multiplying by the pressure), you are working in square inches (bore squared times PI/4)…. We already know that when you increase the caliber, the potential FPE goes up by the area (the square of the caliber)…. However, if you leave the port size constant, and just increase the bore, you would not expect the force to go up by the square of the caliber, because the air still has to go through the original port size.... Perhaps instead of using the bore squared, we can substitute (bore diameter times port diameter times PI/4)…. The answer is still in square inches, so when multiplied by the pressure in psi, we get the force in lbs., as required to calculate the FPE.... The same thing would then apply if you kept the bore the same, and installed a smaller transfer port, right?.... So, if the caliber is a constant, and the above seems to work in testing (which it does)…. then the FPE is proportional to the transfer port diameter.... It's kind of a convoluted way to explain it.... but it's the best I can do.... and the "units" still work out, which is critical for any formula or theory....

As far as what can be done in an EFFICIENT manner (which was not your original question), obviously that 74 FPE number is out of the question.... In addition, the size of the reservoir or plenum feeding the valve governs the AVERAGE pressure you will see during the shot.... If the plenum is small, the pressure drops more during the shot, the average pressure is less, so that 74 FPE number goes down.... In addition, think of that 74 FPE as being the "plateau" (maximum) velocity.... We already know that to get reasonable efficiency we have to drop the velocity 3-5% below that, and for good efficiency, about 10%.... That reduces the FPE by 6-10% at least, and possibly 20%, to get efficiency you deem acceptable.... Then derate that again to allow for a small plenum.... etc.etc.etc…. It isn't rocket science to see that pretty soon achieving 60 FPE in a .22 cal at 2900 psi with a 22" barrel and a decent shot count is not that easily achieved....

Bob

[rumbleUU]

Thanks for the clarification all, Special thanks to you Bob, you always makes me "understand" things more ....

I will remember that diameter increase = about same FPE increase..

Using that calculator: 2900psi down to 2000psi, gave me efficiency 1.0 , that with 15 shots @ avarage 65 FPE.

reading this: https://www.gatewaytoairguns.org/GTA/index.php?topic=134964.0
"We didn't really do that much to the stock guns.
 First we took the original M10's and converted them to M11 repeaters. Then we used to supplied larger transfer port, adjusted the valve for maximum power, then we added an extra spring (from the local hardware store) placed inside of the stock hammer spring.
 The results are pretty impressive.
 A .22 cal airgun, (using retail sold pellets), touching on 70 FPE, is pretty extraordinary in my book.
 Then to get a fairly decent shot string, and deliver them quite consistently on target is the icing on the cake !"

that M10/11 above is about the specs I did use, also even a bit shorter barrel, dont know what efficiency he did get, and cant read what TP size he did use, but seems that 65PFE is doable reading the above.. 10 shots 3000psi down to 2325psi avarage: +65FPE, thats not to bad at all

[rkr]

You can go over 100 fpe in .22 size, I built a 100 fpe BSA with .224 barrel and Marko built a 110 fpe .224 gun from a scratch.

[gabi.nechita]

Can you tell us how long is your .224 barrel?
Thank you.

[rsterne]

Yes, over 100 FPE is possible in .224 cal, it all depends on pressure and barrel length.... My latest .224 build will shoot 61 gr, slugs at 1000 fps (135 FPE)…. However, it takes 3800 psi and a 28" barrel to get there.... Detuned to 960 fps (125 FPE) I am getting an efficiency of 1.24 FPE/CI.... which is excellent.... 

Bob

[rumbleUU]

10 shots 3000psi down to 2325psi avarage: +65FPE with a 250cc airtube, what efficiency is that? It cant be that low as horible..`?

[Motorhead]

10 shots 3000psi down to 2325psi avarage: +65FPE with a 250cc airtube, what efficiency is that? It cant be that low as horible..`?

need pellet weight and speed .. not the FPE figure to run the calculations

[rumbleUU]

JSB Jumbo BEASTS in 33.9 grain, 945 fps

[rsterne]

Pressure drop = 675 psi (46.2 bar)…. 250 cc = 15.2 CI.... 46.2 x 15.2 = 702 std, CI of air....

10 shots @ 65 FPE = 650 FPE.... Therefore (650 / 702) = 0.93 FPE/CI.... Certainly not great efficiency, but not "horrible" either.... My comment about "horrible efficiency" was in response to your previous question about what the efficiency would be at 74 FPE.... and yes, horrible, is the correct answer, as you would get a lot fewer shots from the same pressure drop.... That last little bit of power takes a LOT of air to achieve....

RE your next post, is 945 fps the average velocity, or the peak?.... Since the gun is unregulated (and therefore not constant velocity) it makes a big difference to the TOTAL FPE from the 10 shots, which is what we need to use to calculate the efficiency.... The "proper" way is to calculate the FPE of each shot and add them up.... OR, is this just a theoretical question, where you are making the ASSUMPTION that you can get 10 shots at 65 FPE?.... 

Bob