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PostPosted: Wed Jun 19, 2013 2:11 pm 
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Location: Coalmont BC
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....

Image

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....

Image

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

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
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Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
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PostPosted: Wed Jun 19, 2013 2:29 pm 
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Sounds to me that you are looking for a square wire here Bob. Check thru Trakar's on-line catalog. See if there is something there that your maybe able to work with for what you are suggesting.


www.trakar.com

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PostPosted: Wed Jun 19, 2013 2:31 pm 
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Would you have a link to Steve NC's excitement?
curious about that one. :)

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PostPosted: Wed Jun 19, 2013 3:29 pm 
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http://www.network54.com/Forum/275684/m ... t+may+be...

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Wed Jun 19, 2013 11:01 pm 
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I don't actually know what I'm talking about but twin turbo(the analogy) came to mind when I saw this. The beginning and end part of a spring travel is often non-linear. If two springs with different actuation timing are coupled in some way, could they flatten out the overall curve?


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PostPosted: Wed Jun 19, 2013 11:24 pm 
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We effectively have (at least) two "springs" closing the valve now, the air pressure acting on the stem area, and the valve spring.... There is also some (hard to quantify) force from the air drag passing the poppet.... In addition, we have a much larger "opening force" required to conuteract the air pressure acting on the valve seat area....

The problem is, that compared to what we have now, we need more force counteracting the hammer in the mid-range of pressure but the same at the top and bottom pressures.... or alternately less hammer impact in the middle pressure / lift range....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Thu Jun 20, 2013 2:33 am 
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Location: Winnipeg, Canada
Could probably done with great variable control using air- gun would probably end up looking quite Buck Rogers with a tube feeding an "air bag" in front of the hammer... :roll:

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Doc Sharptail

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PostPosted: Thu Jun 20, 2013 2:27 pm 
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Bob I still think your an MIT Physics professor masquerading as an old fart from Coalmont, BC!!!

How are the high end PCP match guns regulated? Would that be of some help on your quest?

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PostPosted: Thu Jun 20, 2013 3:06 pm 
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No idea, I've never owned a "high end" PCP.... If they are regulated, this is not an issue.... If they aren't, they likely use a combination of a restricted transfer port, well controlled hammer strike, and a fill range narrow enough to maintain the velocity within 2% ES or less....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Thu Jun 20, 2013 6:55 pm 
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I'd love to play with an electronic valve setup, but that's my background as a micro-controller/software guy showing. I was wondering if a fuel injection valve could be re-purposed. Instead of working indirectly on the released gas volume through valve lift you could work directly with pulse duration, which could be modified shot by shot by the micro-controller.

From a practical and aesthetic point though, I don't think I'd want all that tech in a gun - I'd rather put my faith in springs :)


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