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.... 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

[sholo]

Interesting thoughts for sure, Bob.

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....

I could very well be talking outta my a$$ here, but I wonder if instead of slimming say a Disco valve stem for example down to .112" for the whole length of travel in relation to the TP after hammer hits it, what if you started at .112" (at the poppet), then tapered it up to full diameter so that when valve is at .070" lift (going by your chart here) the "full diameter" part of stem is in line with TP, thus restricting air flow which should lower top FPS a bit. Once at this point then taper stem back down again so that when at maximum lift (at lower pressure) the stem is once again .112" (where you started at beginning of the string). Just not sure if the "bulb" in the center would have a negative effect on the lower part of string.

Hope that confusing description made some kind of sense to you! (It sounded good in my head...not so much when I typed it out...)

I have no idea if this is even feasible or not, and if in fact it did work you would most likely have to play around with different stem diameters to find the perfect combo, but do you think it would work???


Cheers,
Todd

[rsterne]

By "stem diameter" I mean the sliding part where the seal occurs in the valve body.... There is air pressure on the inside, and only atmospheric on the outside, so quite a bit of force.... In a Disco at 2000 psi, there is 45 lbs. of closing force (38 lbs. from air pressure, 7 lbs. from the spring).... Compare that to the rougly 130 lbs. force required to crack the valve, and then the unknown forces due to flow past the various parts of the poppet and stem.... I haven't given it much thought, but I was wondering if making the stem larger or smaller (and hence changing the ratio of opening force to closing force) could be used to advantage to "shape" the lift curve vs. pressure....

As an example, with a 1/8" stem (same valve spring) the closing force drops to 32 lbs. (from 45).... but with a 3/16" stem it increases to 62 lbs.... If the same throat size was used, the opening force remains at 130 lbs, so the ratio changes.... Of course the throat would have to increase a bit with the larger stem to provide the same flow area.... and it could be reduced with the smaller stem.... so more variables to look at....

Bob