How Can We Shape the Lift Curve?
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....
http://i378.photobucket.com/albums/o...ps0b60a6fc.jpg
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....
http://i378.photobucket.com/albums/o...psd09d63f6.jpg
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
