GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Topic started by: dkwflight on July 31, 2011, 04:46:54 PM
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Hi
My experiment now has me wondering.
Is there a fomula for the piston size and port size for a specific caliber.
Or is all guesses and experience?
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More than likely experience of who did the original design with possibly some experimenting thrown in.... Then in the case of clones probably just copying the original.... In many cases, the port size doesn't change even when they change the caliber, which may or may not be correct.... Then throw in different pellet weights and that's yet another variable....
Most experienced shooters don't fool around with the transfer port as the chance of losing performance, and even damaging the gun, is greater than the likelihood of any significant gain.... as you already found out.... Having said that, if I were going to play with it, I'd drill out and tap the transfer port for screw-in inserts that could be easily changed.... Even that will only get you close, because changing the taper on both sides can change things as well....
The Cardew brothers published one set of experiments where they found the optimum size was 1/8" but that was for a gun that only shot 428 fps in .22 cal.... A 1/64" change either side of that only lost ~3-4 fps.... but a 1/32" change lost 5-8 fps, and a 3/64" larger port lost 14 fps and a 1/16" larger port (ie 3/16") resulted in the piston hitting the end of the chamber.... 3/64" smaller lost 40 fps, but that port was only 5/64" which is tiny.... The point was that there was indeed an optimum but that a 15% change either side made very little difference.... Greater changes made BIG differences in either velocity and/or longevity....
Piston size (actually the swept volume) determines the maximum potential performance of the gun.... Some numbers I calculated just the other day indicated that typical Springers can produce about 1 FPE for each 3cc of swept volume.... or about 5.5 FPE per cubic inch.... Several designs seem to have about the same numbers, so I can only assume that is a good balance between smoothness and power....
Bob
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Here is a link that may be of interest to you...
https://www.pyramydair.com/blog/2010/02/changeable-air-transfer-ports/ (https://www.pyramydair.com/blog/2010/02/changeable-air-transfer-ports/)
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Interesting that roughly the same dimensions worked on a 19 FPE .22 cal R1 that Cardew's used on a much less powerful gun (~ 6FPE) many years previously!.... 1/8" to 9/64" (3-3.5 mm) worked well in both cases....
Bob
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Hi
Interesting the relationship.
I have nee thinking that this an area that nneds more time and thought.
Thanks for the link to the article
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BTW, just measured the transfer port on my .22 cal B-26.... it's 3mm (0.118")....
Bob
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Curiously, has anyone seen a trans-section of the transfer port? I'll see if I can dig up the picture. Not at all what you guys might think.
OK, took a while but I found it.
(http://i36.photobucket.com/albums/e20/RedFeather2/sectioned34.jpg)
Please note the transfer port on this 34. While you can bore it out at the pellet side all you want, you will find that it does not effect 90% of the port. The Cardews found that many guns had flat transfer ports on the piston side and a slight chamfering increased velocity. However, some manufacturers already did this. Not so sure about today but I'll bet many are. What you end up doing by increasing the hole on the port side is introduce a little vortexing. Air's a fluid and the shape of the port can increase or decrease efficiency in ways that seem contradictory. For example, certain shapes tend to create turbulence that impedes air flow even though you would think they ought to promote it. Others that look "wrong" have the opposite effect.
Something that the Cardews also discovered is that a long transfer port becomes less efficient since it provides more space for the combustion to take place in and, hence, a lower resulting pressure. Opening the port too much on either side adds to that volume.
But, they are your guns so have fun.
Oh, and that apparently inefficient offset port up there really isn't.
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HI
so far my limited examples of spring guns use straight holes.
I have been thinking that the hole act like a valve under the high speed increase in pressure.
It is possible there is a hypersonic flow for a short time.
The hole might be improved by polishing and rounding the piston side to promote smooth flow.
The pressure time values could be studied with the proper instrumentation. Beyond me at this time.
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I've seen "drawings" of various designs, just a drilled hole, both ends tapered (countersunk/beveled), only the breech end tapered, straight and on the centreline of the chamber, straight and offset above the center of the chamber but in line with the barrel (my B-26 is that way), angled from the centerline of the chamber upwards to the bore.... I would think the design varies gun to gun, no?....
Bob
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I hadn't seen one drilled from both ends before like the one in that photo.... The angled one I saw was straight but angled (in the drawing anyway).... My B-26 is parallel to the bore, straight, no kink, and offset upwards from the center of the chamber....
Bob
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-------------|
|___________
A1 ___A2______
| V2
-------------|
V1
Calculation is simple: A1v1=A2v2
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-------------|
|___________
A1 ___A2______
| V2
-------------|
V1
Calculation is simple: A1v1=A2v2
OK... give us a working example of all yer A & Vs
A = area?
V = volume ?
V = Velocity ?
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a=area; v=velocity
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Hi
I have been thinking about these guns.
What I am thinking now is the small port delays pressure build up and when the piston flies to the port the pressure flow to the base of the pellet is relatively slow and low pressure until the wave front builds to a point when it bursts through the port to the base of the pellet like an explosion.
in an oversize port the air flows too easily through the port allowing a slow build up of pressure at the base of the pellet, the pellet starts too early and does not build the speed the original constricted port does.
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Cardew's claim that up to 60% of the power of springers that are over 500 fps is created by the combustion of some of the thin film of lubricant on the wall of the chamber.... If the pressure rise is too slow, the temperature needed to create that combustion would not occur.... Springers are not quite as simple as people think....
The formula give by dimitri would only be valid if there were no compression heating, let alone combustion, taking place.... and also no frictional losses....
Bob
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I may disremember, but I thought that the Cardew nitrogen (inert gas) experiment only showed a decrease of about 150-200 fps when no combustion was allowed. Not 60%.
Is the B26 a break barrel? Are you sure the port isn't angled upwards from the center of the compression tube? The reason for the angle is to keep the barrel more or less in line with the tube. If it were centered, your barrel's bore would be quite low, necessitating a very high front sight (not to say looking a bit odd.) Now, guns with a retracting breech like the 48 are directly in line and, thereby, a bit more efficient.
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I believe it was "up to 60% of the energy".... which would be up to about 26% of the velocity.... which would agree with your recollections (156 fps on a 600 fps gun - 756 including combustion)....
Yes, the B-26 is a breakbarrel, a copy of the HW95.... and yes I'm sure that the port is parallel with the barrel, not angled downward to the center of the compression tube.... The port is above center in the compression tube.... which is one of the variations I have seen a sketch for....
Bob
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Googling "cardew airgun" yields
http://airgunner.org/?p=50 (http://airgunner.org/?p=50)
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Well fellas have any of you done port work before? :P
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a Bob you are correct the port is in line with the barrel! :P
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I'm not necessarily sold on Cardew's research.... but the fact that they tested a springer by charging it and firing it with Nitrogen and found a huge loss in power.... which came back as soon as they operated the gun in air again.... is pretty hard to refute.... It certainly seems to indicate that a small amount of combustion is taking place.... I dismissed it out of hand when I first heard about it.... Then when I read about how careful they were in their experiments I couldn't find a way to explain around it.... I guess you can say I'm a Cardew "agnostic".... I find it hard to believe, but I don't have any other explanation....
One other thing I discovered relatively recently also bothers me.... If there is no combustion taking place, then why is a springer 4 to 5 times as efficient as a PCP?.... They typically deliver 1 FPE for about every 3 cc of chamber swept volume.... A very efficient PCP is lucky to deliver 1 FPE for every 10cc of air used... and most are in the 13 -15 cc range.... Powerful ones are even less efficient.... using up to 20cc of air for each FPE or even more....
Bob
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One other thing I discovered relatively recently also bothers me.... If there is no combustion taking place, then why is a springer 4 to 5 times as efficient as a PCP?.... They typically deliver 1 FPE for about every 3 cc of chamber swept volume.... A very efficient PCP is lucky to deliver 1 FPE for every 10cc of air used... and most are in the 13 -15 cc range.... Powerful ones are even less efficient.... using up to 20cc of air for each FPE or even more....
Bob
Bob, My intuitive grasp(or how I see it in me head anyways) is with a PCP your driving that pellet with the expanding gasses just like a powderburner.... But with a springer(and I am sure you can figure out the science behind it better then me) I belive its not so much the volume of air thats pushing the pellet its more the peak pressure spike(shockwave??) thats pushing the pellet down the barrel... thats why in a PCP just like a firearm the longer the barrel (given enough expanding gasses) you will pick up speed while in a springer generally the pellet has already reached its max speeds in the first 7-12 inches of barrel....
Now of course there is some combustion going on but even then I see it adding to the peak spike and not so much the overall expanding gasses....
Engine terms...
I see a PCP working like a good performance engine... getting its power from the length of stroke and the over all average cylinder pressures.... where springers work on that engine killer rod twisting peak pressure spike... LOL anyways thats how my simple mind grasps it in as few words as possible :)
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Cardew's claim that up to 60% of the power of springers that are over 500 fps is created by the combustion of some of the thin film of lubricant on the wall of the chamber.... If the pressure rise is too slow, the temperature needed to create that combustion would not occur.... Springers are not quite as simple as people think....
The formula give by dimitri would only be valid if there were no compression heating, let alone combustion, taking place.... and also no frictional losses....
Bob
Not true.
If you know one of them, say..expected velocity you can very well calculated the required force. Or if the force is constant, the velocity is measurable. combustion doesnot come here but the friction is. Then you have other stuff like, gravitational force, density of A1, A2 etc. but if you want the formula that is what the parent formula.
-Dimitri
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Hi
If combustion is a proper part of the shot cycle, How do we ensure/promote consistency between shots?
I have heard some say they have had thousands of shots before overhaul/tune.
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" A little learning is a dangerous thing;
Drink deep, or taste not the Pierian spring. "
I think it's far to say that, unless you have access to a complete research lab, some of these questions cannot be satisfactorily answered through DIY experimentation and observation. That is, why it works as opposed to simply it works.
What I was getting at by posting the picture of a complete transfer port is you mostly likely have to bore completely through to really change the configuration. I must admit that, until recently, I thought reconfiguring the exit or entry points affected the entire port but, upon reflection and reading this thread, I can see that it is, more or less, changing only one variable. If you leave the mid-section the same diameter, you are still up against that part of the port.
Something else you don't want to discount is the texture or surfacing of the port. I think Gaylord's blog mentioned JM's experimentation with different shapes. Smooth doesn't always work with fluid dynamics. Sometimes a stepped profile yields greater flow, as does a rough pipe surface. And, if you make an orifice plate (basically the screw-in ports that Gaylord produced), you can sometimes get a hammering effect. When I worked for our local gas distribution utility, one customer was a large, multi-fueled power plant with it's own high pressure natural gas dedicated line. At certain times of the year, the inlet pressure at the plant would be changed by installing a larger or smaller orifice plate. These were stainless steel about the size of a small dinner plate and maybe a half inch thick with the orifice hole in the center. When the plant switched fuels to natural gas, they were to bleed the high pressure gas into the feed line. Someone there got lazy and decided to open the valve all at once. The resulting slug of high pressure gas hit the plate with enough force to actually dish it about two inches. Not saying that an air gun will create this much air hammer but an under-sized transfer port plate must take some of it on the exposed back side (that part between the port walls and the smaller hole). You are bound to get turbulence here which may reduce the flow instead of increase it. Perhaps a hole tapered from the rear and matching the diameter of the original port hole would help, but then you're getting into vortex shapes that don't always do what you might logically think they will.
As to PCP's vs springers, I like to think of them in terms of black powder guns and smokeless guns. In a black powder gun, the burn rate is very slow so that a longer barrel is needed to capture the expanding gasses. In the smokeless, the pressure curve is sharp and skewed near the left or beginning, so that peak pressure occurs much earlier.
Anyway, there are just a TON of variables at play with these transfer ports. Experimentation can be fun but also kind of expensive unless you have have access to a good machine shop where you can resleeve the head of your compression tube. Then there's the piston face and seal, two more variables that affect pressure. I can't juggle three balls, let alone that many! If you still want to take a stab at it, might I suggest picking up a couple of cheap Chinese springers? Might want to hold off on the R-1. ;)
Almost forgot. Why does a springer that is combusting (per the Cardews' observations) maintain a fairly consistent fps? They reasoned that, as the piston sweeps forward, it gathers a bit of spring lube. In their book is a picture of a piston with a small band of lube just behind the seal, formed by the lube migrating forward when the piston stops. This band supposedly feeds the "fuel" ahead of the piston. It could also be that the seal pushes lube from the tube wall as it goes forward. In any event, the "fuel" is coming from the lubes. A good lube will stay sticky for a long time, hence the tendancy for newly tuned guns to settle in and "run" at a certain level for several years. At least, that's how I interpret it.
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I think you are correct that the pressure spike in a springer is higher and of shorter duration than in a PCP.... The fact that all of the acceleration of the pellet occurs in a short distance pretty much confirms that.... Cardew's calculated the peak pressure at about 1400 psi due to the mechanical compression, but they also determined that the peak temperature in the transfer port was over 800*F.... As the pellet accelerates, that additional heat energy would cause the air to expand as well as providing an ignition source for any fuel present....
I too am puzzled at how consistent a springer can be between rebuilds.... The only theory that makes any sense is that a thin film of lube is distributed by the seal shot after shot.... Otherwise they could not possibly be consistent if combustion is taking place.... Like I said, it's VERY difficult to believe that combustion is indeed taking place in a springer.... However, they simply develop more power than they should without it.... and the Cardew's research still has not been proven wrong....
Bob
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Hi
I have noticed that my springers seem to vary the first and second shots. Some times they seem to "Warm up " and the second shot is faster. other times this is not the case.
I have chronied several shot strings and the variance is hard to predict.
The lube in the cylinder has a big influence on the speed and variance.
Any recommendation on lubes?
I have used a moly paste and seen bigger variance in speeds compared to the original factory goo.
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I have a theory that when a springer sits some of the volatiles in the thin layer of lube in the chamber evaporate.... and there is less fuel for the first shot.... One shot is (usually) all it takes to replenish the microscopic layer of lube and get the combustion process going again fully....
Of course for that to be the case.... I would have to believe that combustion is actually happening, wouldn't I?.... Am I really that gullible?....
Bob
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If you go back to the Cardew book, you will see where they built a window in the side of the tube and saw flashes when the piston reached its furthest forward travel. Why guns diesel when they are over-lubed or petroleum based products are used. Medoza even offers springers designed to diesel by adding oil to a fiber washer which sits behind the piston seal (RM600). Same principal as a Mercedes engine. Even in a properly lubed gun, it's not unusual to see a whisp of smoke curling out of the breech when you go to reload. Why they stink up the basement. "Honey, I can smell that you've been shooting 'cause I'm sitting by the floor vent." No kidding. She told me this yesterday.
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I believe it was Weihrauch that made a gun (the HW35 Baracuda?) with a chamber on the side for Ether injection.... I saw a photo of one not long ago belonging to a collector in Canada.... IIRC, it was a 25 FPE gun made in .22 cal in the 1950s-70s.... unbelievable power at the time....
Bob
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Those are collector's items now. The idea was to inject a drop of ether just ahead of the piston. Didn't work as well as planned. Guess it was erratic. You can shoot them without the ether but, from what I've read, they don't have the power of a regular springer.
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Looked up orifice plates (same as those interchangeable transfer ports) and found a couple of good links. Found something interesting in the Wikipedia link. When the air is forced down through the exit hole, it continues to narrow down, with an increase in flow rate and pressure, for a short distance beyond the hole. (See the illustration in the upper right of the Wikipedia link.) Change the size of the hole and you move that point forwards or backwards. This might change how much pressure the base of the pellet is receiving. Maybe more focused into the skirt or more focused outside the bore (back of the barrel). If you want to visualize this, I think a butane torch or lighter might be a good example, where the flame is focused to a point. Anyway, lots and lots of math in them there "simple" equations. And you can't do the math since you don't have before/after pressure observations or a compressibility factor since it's a gas and not constant flowing liquid. Made my head hurt just to look at it.
Ooops, almost forgot!
http://www.physicsforums.com/showthread.php?t=249013 (http://www.physicsforums.com/showthread.php?t=249013)
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I'm right now doing my own testing using a TF87 I got and some of these ideas stand true , this rifle is a lot more powerful then a stock one a local has and that one crony's at 812fps to about 822fps or so. The size of the port directly behind the pellet does play a big role in how fast the pellet comes out the barrel .For example if I use 24.5 grain pellets at a piece of 3/4 pine I have set at 30 yards it goes right through it with ease and the noise is not that bad.Now if I use a crosman 14.5 pellet the rifle sounds about as loud as a rimfire and the pellet also goes through the board with ease . I now use only the 21.5 baracudas and the 24.5 Jumbo pellets to keep noise down. I hope to have a crony here soon to get actual numbers , I'm planning on testing at about 24" from the barrel and at 10 yards , I will use ONLY the 10 yard numbers for MY own log's - I figure that for me a good number at 10 yards should be the ONLY way these should be tested but thats just me.
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I'm right now doing my own testing using a TF87 I got and some of these ideas stand true , this rifle is a lot more powerful then a stock one a local has and that one crony's at 812fps to about 822fps or so. The size of the port directly behind the pellet does play a big role in how fast the pellet comes out the barrel.
Are you saying the port size differs between your gun and the local gun? If so, what are the diameters? Is yours bigger or smaller than the stock one? TIA.
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I'm right now doing my own testing using a TF87 I got and some of these ideas stand true , this rifle is a lot more powerful then a stock one a local has and that one crony's at 812fps to about 822fps or so. The size of the port directly behind the pellet does play a big role in how fast the pellet comes out the barrel.
Are you saying the port size differs between your gun and the local gun? If so, what are the diameters? Is yours bigger or smaller than the stock one? TIA.
I'm sure that some rifles will have ports which are already smaller then what comes in a TF87 ( thats my only point of reference) so just saying that its a smaller port means nothing because the actual shape of the inlet can also change. I will post some pictures later on today of a Dish Network dish I have out that that I've been shooting at since the day I got this rifle.The first day ( the rifle was broken and some parts not installed correctly ) the pellets would mark it and bounce off leaving VERY little markings on it.Then with a new spring and new seals it began to dent it up pretty good , but now it punches holes right through and goes into the wood behind it right through a 1/2 I guess pine board . This is using a 21.5 grain baracuda at 22 yards. I will get those pictures up later on tonight as soon as I find my cable for my phone .
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I believe it was Weihrauch that made a gun (the HW35 Baracuda?) with a chamber on the side for Ether injection.... I saw a photo of one not long ago belonging to a collector in Canada.... IIRC, it was a 25 FPE gun made in .22 cal in the 1950s-70s.... unbelievable power at the time....
Bob
https://www.pyramydair.com/blog/2006/05/weihrauch-barakuda-el54-ether-injection-air-rifle/ (https://www.pyramydair.com/blog/2006/05/weihrauch-barakuda-el54-ether-injection-air-rifle/)
As a child I was subject to the occasional ear ache resulting from infection and having to be lanced under anesthesia induced by drippng ether onto a gauze-covered mask held over my nose by one nurse while several others held me down. I envision a special level of H*** for those people regardless of their good intentions. Thankfully, anesthetics have come a long way since then...an no more ear aches in old age. :D
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BTW, just measured the transfer port on my .22 cal B-26.... it's 3mm (0.118")....
Bob
I measured my Trail NP (not XL) transfer port today during my first ever home tune ;D. Anyway a number47 drill bit shank was a slip fit at 0.124thou = 3.15mm.
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airgun-academy.pyramydair.com/blog/2010/02/changeable-air-transfer-ports/
Also the current (Sep'11) Airgun World issue has some articles regarding combustion...pp.68-78.
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Great discussion guys,the answer lies in linear flow,getting all the air molecules to flow smoothly,try looking up linear flow in water and air,very eminent chaps have been working on this problem for more than a century now.also very complex as everyone has pointed out.the stroke ratio's relationship to the transfer ports ratio,piston weight,spring design,breech and barrel design,and pellet weight and design,all inter- react to create a mine field for the airgun designers and tuners alike.So tend to stick to safe knowns.