GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: lloyd-ss on November 10, 2012, 10:50:43 AM
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Here is the math geek stuff of what is going on inside a PCP. I did a similar post a while back, but this one I hope is better organized and explains things more clearly.
Here are a few graphs that tell a story and illustrate what is probably happening inside the barrel of a PCP. These graphs are from a model I have been developing for over 5 years, adding variables, tweaking, and refining. Questions and suggestions for improvements have come from a handful of people, in particular Bob Sterne. Thank you Bob. The spreadsheet model matches real-world data fairly well and can be used to predict how modifications to a PCP might impact its performance.
These 8 graphs use a .25 cal 20" barrel operating with a 3000 psi air supply, but the model will work with anything. They start with some raw inputs and each graph adds a level of refinement to the example.
The first graph uses a one half grain pellet, with absolutely no losses in the system for friction or flow or anything else. This is to demonstrate the theoretical max velocity if there were no constraints. It is an impossible case, but we will work back to reality from here. I have graphed the velocity in FPS in green, labeled on the right vertical axis. The acceleration is the red line, values on the left axis. The horizontal axis is the length inside the barrel as the pellet travels through it. Because the mass of the pellet and the force acting on it IN THIS EXAMPLE are constant, the acceleration is constant (slight drop because the reservoir is actually 2500ccs, not truly infinite). 3000psi acting on a .25 cal pellet is actually pushing the pellet with 147 pounds of force.
The numbers are pretty ridiculous. Read the values from the appropriate vertical scale. (the kink at the top of the velocity curve is just because of the increasing increments in the calculations because the velocity has gone through the roof). (Remember, an unlimited air supply and zero system losses.)
(http://i226.photobucket.com/albums/dd79/loyd500/Velocity%20GTA/V1.jpg)
OK, so we know that the theoretical value is crazy, so let's add in the weight of the column of air that is behind the 0.5 gn pellet. That makes a tremendous difference as the weight of the column of air increases linearly with the pellet travel. Atmospheric air has little weight, but compress it to 3,000 psi and it is 200 times as dense! The analogy of the weighted sleds used in tractor pulls, where the weight of the sled is transferred from the wheels to the skid as it is pulled down the course, comes to mind. (Note- it looks like the accel drops close to zero, but that is only because of the expanded scale of the accel values.)
(http://i226.photobucket.com/albums/dd79/loyd500/Velocity%20GTA/V2.jpg)
In this third graph, we have the same setup, except with a 25.4 gn pellet with an unlimited (2500cc) supply of 3000psi air, but again NOT adding in the weight of the column of air. Making the pellet 51 times heavier brings the numbers into something that is fathomable. Still way too high, but heading in the right direction.
(http://i226.photobucket.com/albums/dd79/loyd500/Velocity%20GTA/V3.jpg)
In this 4th graph, I have again added the steadily increasing weight of the column of 3000psi air accelerating behind the pellet. I have calculated the column of air moving as a single entity with the pellet, so the air in the reservoir is accelerating itself and the pellet. Not exactly real world, but it is what I have modeled at this time. Looking at the velocity, this is actually starting to get believable.
(http://i226.photobucket.com/albums/dd79/loyd500/Velocity%20GTA/V4.jpg)
In this fifth graph, I have closed the valve when the pellet is half way down the barrel. You can barely see the inflection point in the acceleration curve when the valve shuts at 10" of travel. But the air trapped between the valve and the pellet keeps expanding and accelerating the pellet. Also, at that time, the weight of the pellet and air column becomes a constant.
So it is no big surprise that the pellet lost very little velocity as a result of shutting the valve at half travel. Think about that. Shut the valve half way down the barrel, use half as much air....
The velocity is still too high because I have not yet subtracted any deductions for the many losses within the system, such as: the pellet breakaway force, any barrel friction, any flow losses, or any general system inefficiencies, but that is for another time. I think this model is narrowing in on what the empirical (experimental) data shows.
(http://i226.photobucket.com/albums/dd79/loyd500/Velocity%20GTA/V5.jpg)
This next graph gets even closer to the real world. It has additional variables modeled in: transfer port volume, pellet breakaway and drag, system efficiency, reservoir volume reduced back to 100cc, with the valve still closing at 10". Getting believable?
(http://i226.photobucket.com/albums/dd79/loyd500/Velocity%20GTA/V6.jpg)
This is same as the previous graph, except the valve now closes at 2.5" of travel down the barrel. Not a great loss of velocity, but a tremendous savings in air. Look at this graph and see what you loose if you chop the barrel 3 inches, and then look at the previous graph and see what a 3" chop would do. Do you understand why the difference?
(http://i226.photobucket.com/albums/dd79/loyd500/Velocity%20GTA/V7.jpg)
This final graph is the same as the one above except it shows the pressure inside the barrel, the velocity, and the energy. Make sure you are reading off of the correct scales.
Not perfect, but I feel it reflects the real world empirical data fairly accurately. Comments and questions are welcome. Lloyd-ss
(http://i226.photobucket.com/albums/dd79/loyd500/Velocity%20GTA/V8.jpg)
Lloyd-ss
www.airgunlab.com (http://www.airgunlab.com)
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Suffering temporary Brain freeze ..... Must look away :o
Dang Lloyd some serious thought going on upstairs in your head !!
Nice data indeed.
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Lloyd has been working on the spreadsheet used to develop these graphs for much longer than I have know him.... It is incredibly complex, but we are getting some insight as to how it grew right here in this post.... You start with the (relatvely) simple physics of how an object accelerates over distance when a constant force is applied (F=ma).... Then you add an input to change the weight of the object (pellet), stir in the increasing mass of the air while the valve is open, add in what happens when you close that valve, and then once it starts becoming a reasonable model, you start adding refinements which degrade the theoretical potential, like the transfer port volume, the pellet breakaway and drag, and an efficiency factor.... I have only had a very few suggestions on refinements, but I'm certainly honoured that Lloyd trusted me with his "baby", and gave me that opportunity....
I would like to comment about the efficiency factor, and the valve dwell.... Most of the inputs can be measured directly (pressure, reservoir volume, barrel length, pellet weight, etc.) or closely estimated.... Once you put those into the spreadsheet, you are faced with what values to use for the dwell and efficiency.... It turns out that if you have some empirical data (ie a muzzle velocity in fps for a given pellet and the amount of air used in psi, volume, or FPE/CI).... that there is only ONE combination of dwell and efficiency that produces a pressure drop in the tank (and hence a value for FPE/CI) that matches reality.... By adjusting those two "unknowns" until the velocity and air use match what you measured, I feel you have a very accurate representation of what is going on inside your PCP (or pumper, or CO2 gun)....
Once you have an efficiency factor that works for "your" gun, then changing the dwell (only) gives a very accurate representation of how the velocity and air use track with each other.... Likewise, changing the pellet weight gives a very accurate prediction of the velocity and energy with the new pellet.... You can also change the barrel length and see the effect on velocity and energy.... Changing the pressure provides good results as well, and if you leave everything else the same and change the pressure and dwell, you can show how things change during the shot string of an unregulated PCP as the pressure drops and the dwell (through self-regulation) increases to keep the velocity relatively constant, producing our beloved "bell-curve".... The FPE/CI output in this case very much tracks with real world results....
I have also found that you can even predict how a caliber change will change the FPE output, although sometimes the efficiency needs to be tweeked to agree with reality once you actually do the conversion.... but it gives you a goal you can likely realize.... More obscure things like how the volume of the air reservoir (or in the case of a regulated gun, the plenum between regulator and valve seat) affect the results can also be explored with relative ease and the results are very accurate.... It even works well (as I recently found out with my .25 cal pumper) for quite small valve volumes.... Lastly, it allows you to see if that latest change you made increases the efficiency of your gun.... as making a gun "breathe" better generally increases the efficiency factor required to make the spreadsheet "balance"....
Overall, I think Lloyd's spreadsheet, which was used to produce these graphs, it a wonderful model of what is happening inside our PCPs (also pumpers and CO2).... It is an honour and a privilege to have been able to use it, and I have enjoyed watching it mature....
Bob
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LLoyd
What factors are used in consideration of transfer port volume?
Also, to Bob's explanation of efficiency, I assume volumetric efficiency. Do you feel the starting pressure at initial Pellet accel. is nearly equal to reservoir pressure? The graph depicts as much. i.e. 200Bar
And Greetings, first post here, though I have been dropping in at times for some while
Cheers
Cal
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Heh. If you want your head to really break, start doing this for CO2 or firearms. Adding expanding gasses to the equation is a whiole other mess to deal with. :)
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The transfer port volume used should be the total volume from the valve seat to the base of the pellet.... It produces an intantaneous drop in pressure when the valve opens and before the pellet starts to move....
The "efficiency factor" is a collective one, not aimed at any one specific thing, from what I understand.... Lloyd, please correct me if I'm wrong....
This will work for CO2 providing you know the temperature (and therefore the pressure).... and providing there is no liquid CO2 present in the valve at the moment of firing.... If the valve is full of liquid, all bets are off.... Since firearms PRODUCE their own, very hot, gasses, this calculator won't work.... nor will it work for a springer in which any form of combustion is taking place....
Bob
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Wow, that is some serious @$%^.... Respect .. wouldn't mind having a copy of that... :D
Is the flow of air also included in the chart, the disturbanse in flow through the port and barrel?
It also explains what I have found when I was tweaking, the best result in speed vs. airconsumpsion was having the hammer move shorter with a harder spring, and a higher reg. pressure.
Especially with light pellets... heavier bullets preffered a little less reg. pressure, and a harder hammerblow.
Could it be that this opened the valve a bit longer, giving the heavy bullet time (doesn't accellerate as fast) to move long enough to have "a good shot"?
Sorry for my English.... :-\
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Thanks for the interest, everyone,
Bob,
Thanks for the kind words and explanation on the usage of the spreadsheet. You are pretty much spot-on. Your high level of interest in this has keep it fresh and evolving.
Cal,
Welcome aboard. Bob's explanation of the transfer port volume is correct. In application, the instantaneous pressure drop causes less force to applied than the gauge pressure would imply. The push on the back of the pellet is reduced. Pellet breakaway force immediately comes into play at its full value and then decreases linearly to zero over a distance equal to half the caliber. The distance of 1/2 caliber is just something I chose that seemed appropriate.
The efficiency factor shows up in 2 places with 2 totally different meanings. The output from the spreadsheet has a whole listing of output data including the traditional efficiency in FPE/cuin, (and also barcc/FPE). The other efficiency factor is for the mechanical system as a whole. Most mechanical systems have efficiencies in the neighborhood of 40% to 80% (choose what ever value you like) for a collection of a multitude of small losses. The airgun is the same. All sorts of flow losses, heating and cooling, valve transitions, etc, etc. Plus many we don't have a clue about. So call it a fudge factor or whatever you want. What I have found over time as this spreadsheet has evolved, is that as I have added new quantifiable force reductions, say pellet drag for instance, the magnitude of the adjustment applied by the efficiency factor has become less and less. So over the years, the results have gotten better, but there is still a ways to go.
Bentxbetjent,
No, I have not modeled in the flow dynamics, but I am sure they are significant. That is not within my skill set, LOL, or at least being able to develop it it accurately into excel.
Lloyd-ss
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LLoyd.
Thanks for the welcome, and the response.
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I am sure that the following extraction from another form has it's complications, but wondering how we might explore the topic (and if it could be introduced to the spread sheet of course)
My "gut feeling" is that the actual expansion is neither purely adiabatic or isothermal.... In the first case, there would have to be NO interchange of heat between the expanding air and the surroundings.... In the second, the temperature of the expanding air would be constant.... While I don't believe the latter for a second, I can think of several things that could instantaneously add heat to the expanding air column....
1. The compression of the air in the "transfer port", ie the air lying between the valve seat and the pellet, which goes from 15 psi to (say) 2980 in a fraction of a second, heating rapidly.... [end quote]
TIA
Cal
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Yes, I recognize the text from my math compadre, LOL. I am all for trying things that will make this model more accurate.
The heart of the calculator (well....actually it has multiple hearts) is F=ma, or a=F/m. Different phenomenon either add to the mass or deduct from the force. These variables are applied as appropriate during the travel of the pellet through the barrel. So how would the heating in the transfer port manifest itself in the form of an anomaly in the force??? Flesh this out as some sort of function and I will see if I have a way to quantify it in the model. I may not, but you never know.
BTW,
I have been trying make some chips in the shop all day, but this is awfully distracting!
Lloyd-ss
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Lloyd
I know the feeling. I too had some goals set "in the shop" But....
I keep a good work shop, Three good lathes, H/V mill. surface and T&C grinder, press etc.
Plus supporting "stuff", then the wood working assortment. Hard to get anything done for the options ;-)
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The modeling tool that I have been developing produces very similar results. Lloyd and I have compared notes and it seems that we get results that are extremely close, but have different starting places. My model utilizes relationship of pressure and volume applied to know variables to render velocity, as a heart of the calculator. As Lloyd said, it is made of many hearts. Also, Bob is very accurate in the observation on knowns. Using empirical shot strings along with start and end psi, when the calculated and actual are the same, you are obviously on to something. I think that we both use the same approach in the fact that we start with the absurd and figure in reality until it matches begins to accurately depicts results. Then we change guns, cal, ports, ect... to see if the calculated will accurately depict the calculated. Most of all, this thing never seems to end. These calculators are really a sort of comparing/reasoning engine. A method of using math to prove or disprove the previous series of math. For me, The biggest problem is to keep yourself in check and not falling victim to confirmation bias. Lloyd and Bob have been the greatest of help in leading me into the right direction. They have both enlightened and refreshed me on many approaches but with out teaching me to just regurgitate information (real teachers).
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to paraphrase.... "the best teachers teach you how to think".... All of this leads me back to what my Dad used to say....
"The man who knows HOW will always be found working for the man who knows WHY".... I always strive to know WHY things work the way they do....
Bob
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hey-Hey!!!,
I'd like to see the early silly velocity dealt with by applying the average molecular velocity speed limit.
cheers,
Douglas
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hey-Hey!!!,
I'd like to see the early silly velocity dealt with by applying the average molecular velocity speed limit.
cheers,
Douglas
Next step helium... ;)
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hey-Hey!!!,
I'd like to see the early silly velocity dealt with by applying the average molecular velocity speed limit.
cheers,
Douglas
Next step helium... ;)
ummmm, no. No need to go for half measures. Hydrogen will do just fine...and with a well purged tank poses no great risk.
cheers,
Douglas
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couldn't have stated it better Bob.
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That's extremely interesting. Could you try some more graphs with pellet weights matching .22 and .177 calibers. Also pressure around 2000 psi would be more like the middle of the power curve and closer to operating pressure of regulated guns. If you ever get that spreadsheet finalized it would be a wonderful tool for those who like to tinker with regulator pressures and barrel lengths.
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From Bob's Hydrogen thread:
"i agree, i tried helium in my mrod once and saw about 23% increase in velocity but, the helium gas molecules being much smaller then oxygen/nitrogen mix (air) leaked out of my pressure cylinder in the rife. "
With the acceleration of the pellet in the bore measured, say a high-speed x-ray camera( and I happen to know where one resides) you can determine the pressure behind it. If that 23% increase is valid( assuming it is for the moment ), then it looks like two things are going on. One, Helium flows through the valve better, and two at the higher velocity it delivers more pressure behind the pellet.
If the air pressure drop at high velocity isn't accounted for, the other numbers will be less-than-accurate. Judging by how well it works, that last bit will generate better numbers...I'll bet heavily that the model will do quite adequately on low velocity running heavy pellets( say less than 700 fps ). I suspect there is some curvature to the valve behaviour as MV is increased that should not actually be there( or maybe its response is linear when it should be quadratic for example ).
cheers,
Douglas
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I think you will find that it's not the "air pressure drop" per se, but simple diminishing returns.... If you use Lloyd's spreadsheet and keep increasing the dwell, you will find that the efficiency drops off, and the muzzle velocity tends to plateau.... I don't know if you saw this in my other thread.... but I'll repost it here.... "Pellet Distance" is measured from the breech....
(http://i378.photobucket.com/albums/oo221/rsterne/Millenium%20Pumper/PelletDistance.jpg)
That graph was done using Lloyd's spreadsheet, only changing the valve dwell.... You can see that when the valve is open past the half way point, you gain very little velocity, but you sure use a LOT more air.... The graphs are VERY consistent with real-world results, which is why I am such a big fan of the spreadsheet.... I'm sure you have noticed that providing I can develop enough hammer strike to see it, the velocity plateaus in virtually every regulated PCP I make/test.... just as predicted by the spreadsheet.... Not only that, but the efficiency in FPE/CI tracks almost perfectly as well....
I have seen claims about Helium, but most of them have been when guys are trying to push air past Mach 1.... I have only seen a couple of shots with air over 1700 fps (1650 is the theoretical max), and they were done with a .177 Condor running full power on 3000 psi shooting a 1.7 gr. plastic sabot.... Just as air outperforms CO2 at the same pressure, it wouldn't surprise me that Helium is better again.... I'm guessing, as you are, that it flows better through the restrictions of the valve and ports.... probably even the barrel.... Lower "viscosity" ? ?
Bob
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Past a certain point, won't get a hose to squirt more water.
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hey Bob,
I believe the spreadsheet tracks the real world fairly well. I am also pretty darn sure that the claimed dwell is something like proportional to what is actually going on. It just looks like a more trustworthy method to run with a solution method that doesn't start out on drugs until you add enough other loses to get close. That massless, frictionless pellet should only come out at mean molecular velocity, and from there add in the losses.
I am quite comfortable believing that as speed increases the solution will depart from reality since that effect is becomming a larger and larger 'loss' that is getting ignored. How it departs is unclear, as I don't have enough information.
cheers,
Douglas
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hey Bob,
I believe the spreadsheet tracks the real world fairly well. I am also pretty darn sure that the claimed dwell is something like proportional to what is actually going on. It just looks like a more trustworthy method to run with a solution method that doesn't start out on drugs until you add enough other loses to get close. That massless, frictionless pellet should only come out at mean molecular velocity, and from there add in the losses.
I am quite comfortable believing that as speed increases the solution will depart from reality since that effect is becomming a larger and larger 'loss' that is getting ignored. How it departs is unclear, as I don't have enough information.
cheers,
Douglas
Douglas,
All airgun velocities that i am familiar with stay below the average speed of the molecular mixture of air. Ignoring the first three graphs, which were only included to demonstrate unbounded conditions, what is your concern with the application in the remainder of the graphs. What do you think the maximum speed of 3000 psi air expanding into a barrel with no restriction would be? Also, what do you think the pressure of the advancing face of the column of air is?
If the air has advanced 20" down the barrel, what do you think the velocity and pressure of the air that is 10" down the barrel is. In other words, what do you think the velocity and pressure gradient in the unrestricted column is?
Now if you essentially plug the barrel with a pellet, how are the velocity and pressure gradients affected? Given that the speed of the pellet is quite a bit less that the avg speed of the mixture of molecules in air, does that speed really matter? Are we back to just dealing with the force applied by the air pressure?
Thanks,
Lloyd-ss
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That massless, frictionless pellet should only come out at mean molecular velocity,
that assumes that you subscribe to that theory.... and build that limit into the spreadsheet.... Since I've seen (on video, and know the guy that made it) a couple of shots at over 1700 fps on air at room temp.... I'm not so sure it's 100% accurate.... I can't remember where, but I saw a paper on the Internet that said the maximum velocity of air molecules was Mach 5 (yes, five time the speed of sound!).... The argument presented was just as compelling as the RMS speed theory.... IMO....
Don't forget, the first "real" graph Lloyd did, with a 0.5 gr. pellet but INCLUDING the mass of the expanding air.... peaked at "only" about 2900 fps (about Mach 2.5).... All the graphs with air plus pellet had velocities well under 1650 fps, so even the RMS "speed limit" isn't exceeded....
Bob
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Interesting topic. It seems there is some difference of opinion out there on just what the maximum velocity limit really is. Check this out from the Physics Forum and see what you think. http://www.physicsforums.com/archive/index.php/t-200607.html (http://www.physicsforums.com/archive/index.php/t-200607.html). Bob, maybe this is the same information you were referring to?
The equations come up with a limit of 5 times the speed of sound (for the gas at NTP). I suspect the real answer is somewhere between the RMS free space molecular speed and the ideal conditions for the speed inside a barrel as determined by the (Seigel, 1965) equations. Anyway... FYI.
I would sure like to get a copy of Lloyd's spreadsheet to see the equations used. I was working on the same kind of thing but using MathCad instead of Excel. I was digging into details of poppet valve flow based on this work:
http://www.mathworks.com/help/physmod/hydro/ref/poppetvalve.html (http://www.mathworks.com/help/physmod/hydro/ref/poppetvalve.html)
and that is as far as I have taken it. The laminar /turbulent air flow dynamics will have an effect but with as close as Lloyd has come with his spread sheet, all that may be overkill. Ideal on/off valve design with some assumed flow rate during the open/close and dwell periods might be all that is ever needed. It is amazing how close the data matches reality from what Lloyd has done. Engineering kudos Lloyd! ;)
Eric
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ericw,
Thank you! I just rushed thru the first link, and it is sooo helpful. I will study it more carefully tonight. But I think the jist of it is, that at the velocities we are talking about with airguns, we don't even get near the max velocity of an expanding gas mixture.
This is good stuff. I will also explain the methodology I use in my spreadsheet.
Thanks,
Lloyd-ss
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The short suggestion is to build into the model one more loss that accounts for molecular speed. Bob, you mention that little plastic pellet and compare it to Lloyds 2900 fps...and I'll make an additional wager that Lloyd would not be using a model the way he is( and we for that matter ) that was off by 29/17.
cheers,
Douglas
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The short suggestion is to build into the model one more loss that accounts for molecular speed. Bob, you mention that little plastic pellet and compare it to Lloyds 2900 fps...and I'll make an additional wager that Lloyd would not be using a model the way he is( and we for that matter ) that was off by 29/17.
cheers,
Douglas
Douglas, Thank you, and I agree. I want the model to be as accurate as possible while working withing the velocity range of PCPs. But I don't want to bother with modeling in anything that only has an affect above, shall we say 1500 fps?? But the dilemma is that there is so much conflicting information about what that max air velocity is and how that limit should be applied. The model is very close to being correct now, so how do I model this additional loss(es)? The burning question that I have is, given the significant, and verifiably real, losses that are already modeled into the spreadsheet, if the pellet does not even get close to that velocity ceiling, does it have any effect on the pellet at all? So far, I have not seen any compelling arguments that would demonstrate that it applies below 1500fps.
If there are any, I would love to hear about them.
Thanks,
Lloyd-ss
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Exactly what I saw trying to say.... If (unproven) limits don't kick in until the velocity is above 1500 fps, is it really worth worrying about them.... Additionally, that 2900 fps was at 100% efficiency with a 0.5 gr. pellet in .25 cal.... The test was done in .177 with a 1.7 gr. "pellet" (two big changes right there, the SD is nearly 7 times as high).... and the efficiency was (ready for this?).... 3%.... Don't forget, that 1650 fps number is the RMS velocity, which means that some air molecules are at rest, and others moving twice that fast, or more.... My "guess" is that IF the 1650 number really means anything, there are "enough" molecules travelling over 1700 fps to still provide "some" acceleration.... maybe 3%?.... Either that, or maybe the few cc's of air in the Condor transfer port (it is unusually large) was hammered by the sudden compression to 3000 psi that it was "burning" the back of the plastic pellet, and that combustion was adding to the velocity.... In any case, velocities in that region remain a curiousity, and an anomaly, in airguns that aren't using Helium....
EricW, thanks for that Seigel link, I had forgotten where I saw it.... I'll have to study it, as it may very well be that the 1650 RMS limit is not applicable at all.... we are looking at more like 5650 fps with air.... which really means WHO CARES ! ? !
Regarding the modelling of specific valve shapes, and trying to model the "shape" of the valve dwell, I would suggest that it is extremely difficult, and compared to the "square pulse" dwell used in Lloyd's spreadsheet are probably relatively unimportant.... Don't forget, the way you use the spreadsheet is to put in the "knowns" and then jiggle the dwell and efficiency numbers until the output equates with the real world.... So, you might end up using a "square" dwell of 0.002 sec, but in reality the "actual" dwell is 0.003 sec with less flow on both ends.... My opinion is that the differences are small, hard to measure, even harder to predict, and therefore beyond what is reasonable to look for in a model.... especially when you consider how well Lloyd's works already....
Bob
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Just out of curiouslity, I used Lloyd's spreadsheet to model the .177 Condor shot with 1.7 gr. pellet at 3000 psi.... I used infinite reservoir, air mass included, but no other degrading factors.... I can't be bothered publishing the graphs, but the MV worked out to 2157 fps.... Adding 1 lb. pellet drag and 2 lbs. breakaway did little, but adding in a 1cc transfer port dropped the velocity just below 1700 fps.... For such an unrealistic set of conditions, I think that is excellent agreement with the empirical results....
Bob
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Now y'all have me considering a Hydrogen powered, Mach 3+ 'pneumatic'. Somethig capable of slinging a 30 cal, 50-60 gr pellet out at powder-burner velocity. The only safety issue I see worth paying attention to would be the muffler volume filled up with a combustible mix...and so it would be loud, which defeats a significant portion of the sub-sonic airgun alure( and that completely neglects the pellet's sonic boom ). An interesting experiment for a hunter...but since I am having trouble with something simple like a tank adapter it isn't really worth spending any more energy on.
cheers,
Douglas
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Just tossing this out for the entertainment value ....
As tests above read, velocity was based off a 1.7Gr pellet and with this said ...
Would not a tuner with a chronograph and some featherweight FELT cleaning pellets get a clue of the expanding air columns speed shooting felts point blank across chrony ?
There in bore drags lower than a pellet, there minimal mass should accelerate fairly close to the speed of the escaping pressure pulse ... No ?
I know when I launch the occasional felt in the PCP the Supersonic CRACK is Really pronounced.
Just a silly question really :o
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I just weighed some Beeman .177 cleaning pellets, and they run 0.45 gr.... I'm not 100% sure, but I think the guy who tested the plastic 1.7 gr sabot tried felts and they were slower.... He suspected blowby.... so he used the plastic base from (IIRC) an RWS Hypermax....
BTW, I just realized I had the weight wrong.... the sabot was 1.4 gr.... Here is the video....
http://www.youtube.com/watch?v=2jxT8y8-9j8# (http://www.youtube.com/watch?v=2jxT8y8-9j8#)
Bob
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Great video! Thanks for posting that Bob. I certainly have never been able to get those sorts of speed. I believe that part of the success of the Condor comes from the fact the air comes straight in line with the pellet, no corners to turn.
Thanks for sharing that with us.
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I've never seen anthing over about 1250 fps.... but on the other hand I've never shot anything lighter than 6.9 gr. in .177 or 11.9 gr. in 22 either....
Bob
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Perhaps only the recording, but where was the sonic crack on those two shots?
I listened for them, but no....
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The sound on videos seldom record gunshots well.... I've heard lots of sonic cracks, but none on a video.... I have also tried measuring sound levels with an $80 dB meter and it was useless.... totally inconsistent, shot to shot.... I assume the sampling rate is way to slow....
Bob
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Like taking a photograph at an arena event with flashes seen everywhere. Just not in the photo you have taken. unless by some freak chance of timing, on other "click" is recorded.
Data point. The Evanix AR-6 .22 spits out "lightened *'CPHPs at 1232 fps without anything more than putting the pell in the bore and firing from a 200 bar fill.
* the head cut off , and shot the skirt ;-)
cheers
Cal
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I did shoot a 21 gn .25 cal at 1354fps once. I will try post the video tonight. It is on my youtube channel.
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Recording sonic cracks is problematic because of the inadequate dynamic range of the microphones , A/D converters (number of bits converted) used and possibly even the amplifiers between them. If you can hear it, the sample rate would have to be very low (less than even cheap junk converters would have) to not sample it at least 2x per cycle of the highest freq you can hear(20KHz if your hearing is extrememly good). If the microphone is far enough away, you could record the crack sound if the gain between microphone and A/D converter were not too high.
Eric
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This video is from several years ago. It has the fastest shot that I have ever recorded.
http://www.youtube.com/watch?feature=player_embedded&v=mIpBdP9RQ5I# (http://www.youtube.com/watch?feature=player_embedded&v=mIpBdP9RQ5I#)
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Wow, 42.6 gr. supersonic (141 FPE!).... Looks like the pressure after the shot was 2000-2100?.... How big was the reservoir?.... Did you ever figure the FPE/CI?....
Bob
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Bob, 280 cc and I didn't want to know the efficiency 'cause it was a dump shot.
The valve on that gun is very large, and coaxial with the barrel bore, and dumps directly to the back end of the pellet. Picture pulling the stopper on a sink drain, where the drain has a radiused opening, and not an orifice-in-a-plate shape. In that configuration it was difficult to close the valve until after the pellet had left the bore and relieved some of the back pressure.
I am sure the stroke limit was set very high too. The test was just for maxing the thing out.
Lloyd
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About 0.13 FPE/CI.... *LOL*.... sounds like some of the recent maxed out shots with my .25 cal....
Bob
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Gentlemen,
If I may cause a slight direction change for discussion ?
Just to make clear where I am coming from, My background is one of being a 2 stroke engine tuner where we work with a lot of Time/Area perimeters. Ports opening and closing, sonic wave activity within engine and especially within the Expansion chamber / Tuned pipes.
So ... What is just nagging my brain is why I read so little about AMPLITUDE ( The strength and duration of the pressure pulse )
See: http://search.yahoo.com/r/_ylt=A0oGdN1Bh6ZQCzwAKEIPxQt.;_ylu=X3oDMTByZWgwN285BHNlYwNzcgRwb3MDMQRjb2xvA3NrMQR2dGlkAw--/SIG=11qvv7f7c/EXP=1353119681/**http%3a//en.wikipedia.org/wiki/Amplitude (http://search.yahoo.com/r/_ylt=A0oGdN1Bh6ZQCzwAKEIPxQt.;_ylu=X3oDMTByZWgwN285BHNlYwNzcgRwb3MDMQRjb2xvA3NrMQR2dGlkAw--/SIG=11qvv7f7c/EXP=1353119681/**http%3a//en.wikipedia.org/wiki/Amplitude)
Threads dealing with valving, transfer ports etc ... in at least threads I have read and understood ( Some serious metal masturbation at times in many of these threads ) The realm of making Amplitude of the pulse propelling our pellets a direct design attribute ... Or it does and have missed this because use of less descriptive nomenclature ?
Reason this is increasingly nagging my brain is tests I have been doing with hammer weights and increased hammer velocity getting valves open faster and shut sooner also. Transfer passage area poppet to pellet quite large too.
Pulse AMPLITUDE has to my deductions were greatly increased resulting in greater pellet velocity on less air ... Go figure huh :o
Anywho .. Just tossing it out and see what others may think, debunk, support to make me see my errors or insight on said subject.
Regards,
Scott
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Slightly hard to understand your thoughts Scott.... but if you are talking about the difference between a short sharp blast compared to a long slow push.... you are right on the money.... That is why higher pressures are more efficient for a given FPE level.... and also why larger ports tend to allow less pressure for a given FPE.... or conversely a shorter pulse (dwell) for a given pressure level....
Call it amplitude if you like, but what it amounts to is delivering more MASS of air right at the beginning of the pellets travel.... I don't think it much matters whether that comes from high pressure or high flow rate for a give pressure.... the trick is to get the duration (dwell) as small as possible to get the valve closed before the pellet has moved very far.... That gives the air in the barrel a longer time, and distance, to expand.... This results in a lower pressure at the muzzle, less muzzle blast, and therefore increased efficiency....
Bob
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Slightly hard to understand your thoughts Scott.... but if you are talking about the difference between a short sharp blast compared to a long slow push.... you are right on the money.... That is why higher pressures are more efficient for a given FPE level.... and also why larger ports tend to allow less pressure for a given FPE.... or conversely a shorter pulse (dwell) for a given pressure level....
Call it amplitude if you like, but what it amounts to is delivering more MASS of air right at the beginning of the pellets travel.... I don't think it much matters whether that comes from high pressure or high flow rate for a give pressure.... the trick is to get the duration (dwell) as small as possible to get the valve closed before the pellet has moved very far.... That gives the air in the barrel a longer time, and distance, to expand.... This results in a lower pressure at the muzzle, less muzzle blast, and therefore increased efficiency....
Bob
I know ... convoluted thinking putting into words.
I relate it to taking a Party balloon and blowing it up to a given size. It's size is Volume and the pressure within created by the elasticity of the stretched rubber.
* So we have a vessel that contains confined energy. The potential of how reactive this energy is comes down to how it is purged/released from containment.
Just let the air leave out the fill neck it fly's across the room because the Dwell / Time air takes to leave has little amplitude, but great duration.
Other extreme is simply POP the balloon and ALL the contained energy is released at once. Amplitude is at MAXIMUM, dwell / pressure duration release is minimum.
So guessing that in A REGULATED gun where the pressure and volume for the shot are FIXED, releasing it to drive the pellet is best done Fast as possible with short duration as practical ?
And what we see in NON regulated PCP's is a Very Large Sacrifice of efficiency so the pressure pulse driving pellet is more uniform over a greater pressure variance within valve.
I do realize K.I.S.S. go's to the unregulated design ... But in terms of efficiency, being able to predict what tunes changes will effect, the regulated design is vastly Superior is it not ?
Was goofing around last night and did nothing more than some subtle changes in transfer tube I.D. changing nothing else.
.165" - .140" - .125" driving a 10.5gr .177 pellet.
Velocity were all within 35-40 fps ( 925 fps average) ... but what was interesting was what shot report sounded like :o
Larger the transfer tube size, quieter the report, Smaller got louder.
Figure this due too the Amplitude/Duration and distance pellet traveled down barrel when valve got closed.
* Bigger tube let the valve dump faster, smaller a tad slower.
Do realize these transfer passage sizes are LARGE for a .177 ... but did somewhat enlighten me to the issues address in my query above.
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Scott,
Yes, I agree with what you are saying. Working toward the "instantaneous open--dwell for a few millisecs--instantaneous close." A square wave. We might have to explore that 2-stroke wave/pulse application. :D It is probably pretty relevant to what we are doing.
The loud report with the small transfer port makes sense, just as you describe it. A slower release of air thru the skinny port means the vavle closes later, which means their is more residual pressure coming out of the muzzle, which means less efficient air usage.
Now, about the light hammer with a heavy spring. ??? How do you think that accomplishes getting the valve open and closed faster? I would think that a heavy slow hammer would take longer to make a U-turn and a light one could do that faster. What about the return spring? Does it come into play? Please share what you think.
Lloyd-ss
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Scott,
Yes, I agree with what you are saying. Working toward the "instantaneous open--dwell for a few millisecs--instantaneous close." A square wave. We might have to explore that 2-stroke wave/pulse application. :D It is probably pretty relevant to what we are doing.
The loud report with the small transfer port makes sense, just as you describe it. A slower release of air thru the skinny port means the vavle closes later, which means their is more residual pressure coming out of the muzzle, which means less efficient air usage.
Now, about the light hammer with a heavy spring. ??? How do you think that accomplishes getting the valve open and closed faster? I would think that a heavy slow hammer would take longer to make a U-turn and a light one could do that faster. What about the return spring? Does it come into play? Please share what you think.
Lloyd-ss
Lloyd,
Not implying a LIGHT hammer and Heavy spring, But thinking if hammer fitted is already on the heavy side, getting it into motion faster with higher spring rate may work better than making an even heavier hammer.
My initial thought being with More spring energy stored in a higher rate spring, it would accelerate an otherwise Over Weight hammer faster.
* Such is the case I found with the M-rods hammer once going regulated. Spring rate was Too little for the mass of hammer and to get valve open far enough to get the sufficient volume, valve opened too slowly and dwell too long for efficiency. Thus going down the path to find sufficient lift using minimal hammer mass. Speed of the hammer strike and just enough hammer mass to match spring rate for job at hand gave better results.
Not sure where you read Now, about the light hammer with a heavy spring. ???
Maybe had been a type error on my part being that statement is contrary to my findings .... Did I brain fart someplace ???
* Hey I'm new to this tuning thing and only thinking out loud here :P
Regards,
Scott
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And what we see in NON regulated PCP's is a Very Large Sacrifice of efficiency so the pressure pulse driving pellet is more uniform over a greater pressure variance within valve.
Actually, the reverse is true.... Non-regulated PCPs tend to be MORE efficient, especially at the beginning of the shot string.... Not only that, but the pressure pulse is NOT more uniform over pressure, the opposite occurs.... The pulse is VERY short (and the gun consequently quieter and more efficienct) at the beginning of the shot string.... and gets progressively longer (and the gun louder and less efficient) as the pressure drops.... That is what self-regulation of the velocity is all about....
Bob
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[/i]
Not sure where you read Now, about the light hammer with a heavy spring. ???
Maybe had been a type error on my part being that statement is contrary to my findings .... Did I brain fart someplace ???
* Hey I'm new to this tuning thing and only thinking out loud here :P
Regards,
Scott
Maybe I misread, or assumed too much, but in your thread about lightweight hammers, you went much lighter on the hammer weight, but you did not increase the spring rate?
Thanks,
Lloyd-ss
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An interesting side note with Scott's two stroke relationships.
Keven Cameron, a noted competition motorbike tuner, wrote a series of technical articles for the periodical "Cycle World" May I date myself this was the 70's 80's and perhaps the 90s. (
Pertinent to our interest was one column devoted to the Acoustic signature of the exhaust note of a racing motorbike.
It was all in the SPEED of valve opening and closing. Low and slow......fast and high. Not the engine RPMs, the valve event rate of opening!
Visualize a two cycle IC engine has a wide exit port that exposes a considerable gas transfer area with small displacements of the descending piston on opening.
Pulses on every stroke, coupled with high piston speeds, we end up with the characteristic "two stroke scream".
Compared , at the time, with the relatively slow moving "Hardly- Able sons", with slow moving valve trains and less crisp exhaust opening that took place only 1/2 as frequently for the same engine RPM. That was the true source that returned a thundering baritone so dear to America's heart.
The article included the comparison with "high speed internal combustion engines" (and I don't mean Sir Harry Ricardo style) and how the four cycle valve event timing was approaching that of the two cycle engines so recently relegated off the tracks. Couple the astronomical engine speeds with technologically advanced valve actuation, it is not hard to connect cause an effect resulting in a similar acoustic signature. A screaming howl.
Not a bad sound either very close up, or quite far distant ;-)
So, Valve event timing and evaluation by acoustic characteristics. might be a tuners ear that turns the trick.
Regards
Cal
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Lloyd and Bob
Would you care to enter a discussion on valve event timing vs power and efficiency? I would love to hear your views.
Perhaps the topic has been throughly covered on previous posts.
Perhaps also, such a discussion should have it's own thread. The mention has come up here already ;-)
Regards
Cal
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[/i]
Not sure where you read Now, about the light hammer with a heavy spring. ???
Maybe had been a type error on my part being that statement is contrary to my findings .... Did I brain fart someplace ???
* Hey I'm new to this tuning thing and only thinking out loud here :P
Regards,
Scott
Maybe I misread, or assumed too much, but in your thread about lightweight hammers, you went much lighter on the hammer weight, but you did not increase the spring rate?
Thanks,
Lloyd-ss
True ... never addressed MORE spring.
Test as it transpired was too try and detect a peak of kinetic energy of a known spring rate, known travel against a fixed resistance ( Valve poppet with 1850# behind it )
Velocity target tho mentioned really only was a fictitious number based upon knowing gun could shoot that fast with the factory valve/hammer components in there upper power range of adjustment.
Worst case scenario as hammer weight was altered is screwing up the power potential all together. This however did not prove to be the case allowing some discovery to show up.
That being conflicting in my mind at least with what Bob has said within that thread As to the issue of this being a "magical tune" because of the "ideal" hammer weight.... there is no question that you achieved it by that method.... However, you could also have done it with the standard hammer and less spring preload....
Which does puzzle me because less spring preload on the factory weight hammer causes the power to be greatly reduced ?
Where as using the SAME SPRING at minimum preload & certain lesser weights of hammer rifle shot substantially faster.
Thus I am missing/lost something in translation ??? ... or not.
So staying with Lloyds original topic ....
What I see has many parallels with our Powder burning brethren in the scope that Light weight small bore projectiles are most efficient when fueled by Very fast burning high amplitude propellent of short burn duration. As calibers get larger, burn speed of propellent slows down so it creates a greater volume of pressure over a longer duration with the peak pressure happening shortly after or just at the point projectile go's into motion.
( If we stay for arguments sake in CF rifle and there operating pressures ... small bore to large bore the peak pressure of the pulse all are very close )
So ... our PCP's, pick a pressure and let just use that as a baseline to wrap our collective minds around propelling a pellet/bullet with it.
My take for what it's worth understanding basic PB ballistics is that we are not so different, small calibers would seem to want a propellent shove that hits it's peak pressure very quickly with low duration. projectile is dislodged and well on it's way with not much need for volume / duration of the pressure pulse.
Larger calibers still need the same peak of pressure within shot cycle, but likely just a tad later ( Slower valve opening perhaps ) In theory it would seem the pressure peak also should correspond to right as pellet starts moving. Don't know ... only guessing.
* I know this is basic academic knowledge, but just wanted to be on the same page thought wise :o
Hammer SPEED getting valve up off it's seat for the Sharpest/Faster pressure wave rise behind pellet/bullet does appear to be most important.
Volume of air flowed, duration / dwell falls upon hammers mass holding valve open, pressure on stem from air pressure and poppet spring within valve, port area etc ... again basis academic PCP knowledge.
Again ... just thinking out loud and now talking in a circle :-X
Scott
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What I see has many parallels with our Powder burning brethren in the scope that Light weight small bore projectiles are most efficient when fueled by Very fast burning high amplitude propellent of short burn duration. As calibers get larger, burn speed of propellent slows down so it creates a greater volume of pressure over a longer duration with the peak pressure happening shortly after or just at the point projectile go's into motion.
( If we stay for arguments sake in CF rifle and there operating pressures ... small bore to large bore the peak pressure of the pulse all are very close )
So ... our PCP's, pick a pressure and let just use that as a baseline to wrap our collective minds around propelling a pellet/bullet with it.
My take for what it's worth understanding basic PB ballistics is that we are not so different, small calibers would seem to want a propellent shove that hits it's peak pressure very quickly with low duration. projectile is dislodged and well on it's way with not much need for volume / duration of the pressure pulse.
Larger calibers still need the same peak of pressure within shot cycle, but likely just a tad later ( Slower valve opening perhaps ) In theory it would seem the pressure peak also should correspond to right as pellet starts moving. Don't know ... only guessing.
* I know this is basic academic knowledge, but just wanted to be on the same page thought wise :o
I'm not so sure. The primary reason we use slower powders for heavier calibers is to control the pressure with in safe limits of the gun ( it is more efficient only because we are pressure limited) . A heaver projectile is going to require more energy to accelerate than a lighter one. If the powder is to fast the pressure will build beyond safe limit before the bullet get moving. The burning rate of a powder is selected to match the bullet movement down the bore and prevent an overpressure. If the action of a PB could take 2 or 3 times the pressure faster faster power would produce better performance or the same at a higher efficiency ( smaller charge ).
There may be some correlation between propellent and air but I don't see it as direct.
Tom