How Can We Shape The Lift Curve?

[rsterne]

Partly as a results of my experimenting with the bstaley O-ring buffer, and partly as a result of my lift measurements on various PCPs.... and also from seeing the excitement of Steve in NC when I mentioned that the lift at the low end of a shot string in an unregulated PCP was about twice the lift at the high end.... I started thinking about what would happen if there was some way for us to "shape" the lift curve instead of just relying on the relationship between hammer momentum/energy and air pressure.... which is the basis of self-regulation in today's PCPs.... I plunked some numbers into a spreadsheet, nothing specific, just generic, and realized that it would only take a few thousandths of an inch change in the center of the lift curve to flatten the shot string.... Here is a graph showing what I mean....



The solid lines are a typical bell-curve as we now get, and a typical lift curve associated with that.... Based on the ones I have measured, the lift is not quite linear, it is slightly less in the middle than a linear decline with pressure.... Here is how I manipulated the data....



The pressure is obvious, start, middle, and end.... The lift is typical for what I have measured over a string.... The velocity is about 5% ES over the 1000 psi, again typical.... "P x L" is pressure times lift.... "V/(PxL)" is the velocity over that number.... Now you will see why I chose 952 fps, it was to make all those numbers come out equal.... If we reduce the value of "P x L" for the middle pressure to the same values as the ends (100, which should produce a constant 952 fps) and then divide that by the pressure, we get the theoretical lift required to produce a flat velocity curve, the "New Lift".... It's kind of a "what if" scenario, but I think it gives a pretty accurate idea of what might be required to flatten the velocity curve.... That lift is then graphed as well, as the dotted black line.... The remarkable thing is how little you have to reduce the lift in the middle of the curve to flatten out the velocity.... in this case about 0.003"....

If we think about how the bstaley O-ring buffer works, while it may have some tendency to flatten the shot string, in fact the "progressive" spring rate that the O-rings give (higher spring rate the more they are compressed) is the opposite to what is required (IMO).... I think their benefit is mostly from stiffening the valve spring rate.... We need a valve spring that starts with low preload, has a high resistance at small deflections, and then gets easier to push as the lift increases, to modify the lift curve as shown.... Either that or some other way to "shape" the lift curve.... I'm wondering if the relationship between the diameter of (and hence the area and forces on) the throat and stem may be usable in this regard....

Bob

[bstaley]

Personally I think you're barking up the wrong tree.  In my opinion lift is already too high once you get past the peak of the curve.  Velocity loss is due to the pressure no longer being sufficient to move the pellet any faster (governed by restriction).  Once the pellet starts moving, continuing to blow more air RAPIDLY reaches a point of diminishing returns.  You might gain some velocity, at the cost of a great deal of added air.

The better approach is to decrease restriction, raising the entire curve, then use something like the o-ring buffer to control the change in lift to flatten the string, clipping off the peak that is created by the decreased restriction.

It's quite simple really, to get the most efficient usage of air you want to (as seen by the pellet):

1. provide the highest pressure possible
2. provide the greatest flow possible
3. stop the flow of air as soon as possible after the desired velocity is reached.

Restriction acts counter to all of the above.  Restriction has typically been the means to flatten a curve, but it does so inefficiently.  The o-ring buffer can't completely overcome this inefficiency until this restriction is reduced.  Restriction is the real cause of inefficiency, not insufficient lift and dwell....quite the opposite in fact.

[rsterne]

Perhaps you're right.... but if it takes, let's say, 0.100" of lift at 1000 psi to get 952 fps (in the above example), how can reducing that lift leave us with the same velocity?.... I think you will have to admit that the buffer has MORE effect (ie removes more of the hammer energy) as the pressure drops, correct?.... The more I work with the buffer, the more I think of it as a very stiff valve spring, in that it reduces the lift (progressively) throughout the shot string.... That is accompanied by a reduction in FPE from whatever you started with.... The reduction in FPE is accompanied by an increase in efficiency because you are using less of the same pressure air than without the buffer, hence gaining shots.... You are trading off power for shot count, just as in any other method of detuning....

I agree that adding restriction between the valve and the pellet appears to reduce the efficiency.... Dropping the pressure range by reducing hammer strike tends to give better efficiency, but that too has it's limits, as eventually the pressure range drops, and the efficiency with it.... The buffer eliminates the restriction, while allowing the use of the higher pressure, hence improving efficiency if you detune far enough.... However, all three of these are a method of DETUNING a gun from what it is capable of.... This thread was not intended to bash your mod, it's already well proven.... It was intended to suggest there may be a way of flattening the curve and increasing the shot count and efficiency, all while maintaining FPE.... The only reason I mentioned the buffer was that it started me down this path, and it's progressive spring rate, IMO, appears to be the opposite of what we may need....

Bob

[bstaley]

What I'm trying to say is almost all stock guns use restriction as a means of controlling velocity.  This increases shot count, but is in fact an inefficient means of doing so.  With some other means of limiting power, such as the o-ring buffer or some other device with a different properties, you can remove this inefficiency from the equation, giving you a better starting point from which to begin shaping the curve.

Not trying to say that the o-ring buffer is the ideal solution, just trying to say that in my opinion the restriction found in most stock guns is actually a very large source of inefficiency in these guns.  If we decrease this restriction and then use dwell control to limit power, throughout the ENTIRE curve, you will get better results than trying to improve what is already an inefficient use of the available power. 

[rsterne]

There is one inherent problem with making a gun breathe better.... The shot string gets SHORTER.... It's like putting a hot cam in a racing engine.... you get more power over a narrower RPM range.... If you start with that free breathing gun, and are determined not to restrict it artificially by strangling the transfer port (which I agree is not ideal, although it does flatten the shot string).... we are (currently) left with two ways of detuning the gun, and both of them involve reducing the lift and dwell....

1. We can dial back the hammer strike by reducing the hammer weight, spring rate, preload, travel, or any combination of those.... That results in less FPE, more shots, a flatter string, and increased efficiency unless you detune it severely.... Eventually, the efficiency drops as the pressure range drops....

2. We can install a buffer, and adjust the amount of engagment and/or the buffer stiffness.... That results in less FPE, more shots, a flatter string, and rising efficiency which tends to increase the more you detune the gun.... So far, in my experiements, I have found little difference in the two methods for detuning in the 20-25% range.... but the buffer is superior for detuning a gun 50% or more, it has greater efficiency in that regime....

Would it not be a better solution if we would always have a completely flat shot string over, say, half the reservoir pressure (heck I'd settle for a third of it).... at whatever FPE level we chose to set the gun up for?.... That's what you can essentially get in a regulated gun, and in fact the efficiency just keeps climbing as you detune it (and the opposite, it becomes and air hog if you go overboard on the hammer strike).... The purpose of this thread is to figure out how to get that kind of shot string but without the regulator.... The advantage of doing it unregulated is that the overall efficiency is higher because you aren't always operating at the bottom of the pressure range.... Unregulated guns are about twice as efficient at the beginning of the string as at the end because the pressure is higher.... If we can "shape" the lift/dwell curve to produce a completely (or nearly) flat string, without losing FPE, I can only see that as being a very good thing....

Bob

[bstaley]

The proof is in the pudding.....looking forward to seeing your results.

[rsterne]

Oh, I don't have ANY idea how to achieve what I would like, short of electronic actuators like Lloyd uses in the ePCP system he developed for the Rogue.... Steve in NC has one idea using a spring loaded plunger to absorb energy from the hammer in the middle of it's stroke and then return it during the last half once the lift passes a certain point.... minus friction, which may be the downfall....

http://www.network54.com/Forum/275684/message/1371662557/Thanks%2C+Bob%21++That+IS+very+interesting.++As+for+where+%28I+think%29+I%27m+going...

The purpose of this thread is to hopefully kickstart the thinking process by presenting a challenge to all those clever guys out there who are interested in trying to make a basic design improvement to what has been a static situation (that actually works remarkably well) for decades.... As you say, dwell/lift control is the key to efficiency, whether you're talking a 6 FPE Challenger or a 1000 FPE Big Bore....

Bob

[bstaley]

Well, I'd say that reducing restriction is about as basic a desgin improvement as we can make.  It's easy to do and when combined with the o-ring buffer, or any other lift/dwell control device, really does provide significant improvements.  Even just opening up the transfer port, exhaust port and relieving the 90% turns are enough to see results, so anyone can do it. simple mods that have worked for decades.

[rsterne]

I guess you're right.... we need to start thinking of making the ports in all airguns equal in area to the bore size.... Anything less than that is a restriction.... The fact that it increases the cocking effort, which must be further increased to have enough energy to overcome the losses in the buffer is irrelevant.... It's all a matter of degree, right?.... Is a Disco a tuned down 100 FPE airgun (I mean it could be, right, opened up and operating at 4500 psi) which is not Commercially available.... or is it a free flowing Challenger?.... I prefer to think of it as a baseline, because that is what you can buy over the counter....

I'll say it one more time.... The O-ring buffer is a great method of detuning an airgun.... It is one method of limiting the dwell/lift, just as reducing the hammer spring preload is.... We both agree that controlling that dwell/lift is the key to improving how our PCPs shoot.... Maybe, just maybe, there is a better way to do that nobody has figured out yet.... Just think of what would be possible if you combined that with completely unrestricted, bore size porting....

Bob

[bstaley]

There is no need to increase cocking effort to achieve improved flow.

[rsterne]

I disagree.... The moment you enlarge the ports to an area greater than the (existing) throat area, you need to enlarge the throat because THAT is now the restriction.... When you do that, providing the pressure is the same, the hammer energy (and hence the cocking effort) need to increase.... It's all a matter of degree....

Bob

[bstaley]

Apparently a common misconception.  Pressure loss in a pipe is just as dependant on the length of a restriction as the diameter of the restriction. Any portion of the flow path that you can give a larger diameter will improve flow and decreasepressure loss.  True the throat will provide A limit, but it is not THE limit.  Sharp turns are especially restricive as well, so relieving them can also yield significant flow improvements.

[rsterne]

Not a misconception at all.... I can tell you for a FACT that once you go big enough (and smooth enough) on the porting between the valve seat and the barrel, no further improvements in flow will gain you power.... but if you increase the throat area you WILL get an increase.... The throat area (throat minus stem) must be AT LEAST as large as the port areas, and in fact 10% larger seems to work even better.... Further, once you increase the area of the sealing surface, the force required goes up in proportion to that area.... The only way to get that extra force to crack the valve is to increase the hammer energy/momentum....

An example, my .25 cal Hayabusa.... I originally made the all ports 3/16" or equivalent, including the barrel port, and acheived about 60 FPE.... At that point, the barrel port was the limiting factor, as making it wider than 3/16" could cause loading problem.... So, I made it longer (oval shaped) and increased the transfer and exhaust ports to 7/32" and the valve throat to 1/4" and slimmed the stem to 1/8".... All ports had the same area, and the FPE increased to over 100.... The gun required significantly more hammer energy because of the larger throat.... so I increased the hammer travel to 1" and fitted a stiffer spring with longer travel.... I then lengthened the barrel port even more, and increased the transfer and exhaust ports to 1/4" and that required making the throat 9/32", with a correspondingly larger seat area.... The hammer energy and momentum once again had to be increased, this time to more than double what I started with.... The gun now reached 160 FPE.... The hammer had gone from 100 grams travelling 0.75" and driven by a spring capable of storing 1.2 FPE, to one weighing 115 grams, travelling 1.20", driven by a spring capable of storing 2.8 FPE.... Part of that huge increase in hammer energy was required to just crack open the larger throat.... the rest of it was required to hold open that larger valve for the required length of time to produce 2.7 times the FPE.... Interestingly, all three versions of the gun achieved over 1.0 FPE/CI....

If you go for the bare minimum of restrictions (eg. bore sized ports), you will pay for it with the requirement for increased hammer energy.... My original hammer and spring would barely even crack the valve on the final version of the gun, let alone reach 60 FPE.... and far from the 160 FPE the gun was capable of....

Bob

[bstaley]

Of course there is a limit to everything, and going beyond a certain point will of course reverse any trend.  Before you were insisting that we discus 'stock' guns, but when that doesn't work to the advantage of your argument, you suddenly change the rules and start talking about a maxxed out gun...of course that will change things.  Since you seem to be more obsessed with having the last word, and being right, rather than actually considering ideas that don't fit your preconcieved notions, I'll exit this conversation.

[rsterne]

Preconceived notions?.... Needing to be right?.... Perhaps a look in the mirror is required?....

I really regret this thread has degenerated to this point.... It was a legitimate attempt to stimulate the possible development of a NEW way of looking at the problem of flattening a shot string through reshaping the lift curve.... I sincerely regret even mentioning the bstaley buffer in it, even though learning about what it can and can't do (IMO) had a large influence on my thought process.... I'm going to lock the thread, perhaps at some point in the future the topic may resurface, I hope so, as I think it needs exploring....

Bob