How to move the peak on a bell curve?

[rkr]

Well, over the past few days I've played with added weights and spring tensions in the hammer and that bloody peak of the bell curve won't budge. Here's three shot string starting at 220 bar. Two with 5 grams of weight added with different spring tensions and one with about 1 gram added. I can flatten out the curve if I release the spring tension but then I end up with too low power as the lower most curve is only at 10 fpe level with H&N FTTs. What I'm left with is the transfer port diameter. If I were to trim the power curve as flat as possible and then open up the port diameter, would that then give me the same flat curve with more power - or would that cause the peak to move to a different pressure region? Undoing the port opening is hard for me so I'd rather know what I'm doing before I screw things up.

[rsterne]

To move the peak of the curve to a lower pressure you have five choices....

1. Decrease hammer spring tension
2. Decrease hammer travel distance
3. Decrease hammer weight
4. Increase transfer port size (ie how the gun flows air)
5. Increase the pellet weight

Note that the first three will reduce the peak velocity and energy reached.... The 4th one will only work if the flow in the gun is somewhat restricted.... and unfortunately it has the effect of making the curve "steeper" (ie less flat).... The last one will likely increase the energy (FPE) even though the velocity (fps) is less....

Bob

[Ribbonstone]

Some generalized truths: (changing one variable in the system): The reverse is true for each:

1, The harder you hit the valve, the higher the pressure for the peak of the bell curve. That harder hit could be from more spring tension...a longer stroke...or a heavier striker. that harder hit generally increases speed for two reasons: 1. The gun works at higher pressure and 2. the valve is staying open longer.

2. The larger the transfer port system, the more air it can move per shot. Between the hole in the valve body, the hole in the barrel, and the transfer port in between, what ever the smallest diameter is your restriction. Small ports conserve air (increase shot count) and slow speeds...but generally do nothing much to the operating pressure.

3. The fatter the valve stem seal, the slower the gun shoots.  A wide valve stem seal is held closed by more force than a skinny one. (its air pressure X the cubic inches of the valve stem) and a fat stem limits the air flow between it and the inside of the valve body.  Crosman has used (the 2300) a brass liner inside the valve to decrease the space between a standard valve stem and the inside wall of the valve to slow guns down and conserve gas....Hatsan (Pneuma) uses different diameters of valve stem seal to make their guns UK or Canadian legal. 

What i would do:

Pretty much retune from scratch,
Keep your desired fill pressure as your constant.
Decrease spring tension until there is a SLIGHT up slope to the bell curve.


LOTS of tests and graphs.

LEts say you can live with 20fps (you can pick nay number you can live with) variation.  Would want the gun to start off about 20fps from its peak velocity.  That way you'd get 20fps on the way up your  shot count curve...a level spot..and 20fps on the way down, which would maximize your shots.

It may be slower than you want...but that would be the  "happy place" for the system.   

Now lets say that turned out to be in the 10foot pound area, and that you really really wanted  to be at 12 foot pounds.  Realizing that nothing comes for free, you'd have to accept a shorter shot count to get that extra 2 foot pounds.  To get there, I'd look to the transfer port ( a bit more open) or the valve stem ( a bit smaller in diameter) and hope that the bell curve still looks about the same, just starts at a higher energy.

But that's just me.

An example.

Recently put together a HiPAc 177/2260 using a mystery barrel that had been hanging around my shop. Knowing I wanted a low speed (middle to low 600's) when i made the port into the barrel, made it .072".  Makes no significant difference what the transfer port size is so long as it is larger than .072"...they all shoot about the same as that barrel hole size is the restriction.


Made sure the valve has all the "slack" out of the system towards the front before pinning the valve (that slack is not much, but wanted all the stoke the striker could give).

Made a Delrin valve stem, but made it very slightly LARGER than a standard valve stem. Figured if it was too slow, would go back in and decrease its diameter a bit. Was only trying to use that slight extra diameter to slow it down a touch (knowing that air tends to flow better than thick co2).

Set the striker spring to about as much effort as I felt was comfortable...didn't want to yank the bolt handle to cock it, but didn't need to have it one-finger lite either,

Started testing by filling to 100BAR, shooting a string.  Then to 110 bar, shooting a string.  The to 120...130...140.

Turned out that the gun gave the best bell curve (20fps) at 135BAR.  Left it there as I am also a pump filler, and the speed (620fps) and shot count (40shots) was acceptable.

Now if I wanted to go faster with that gun, would open up the transfer port in the barrel...use a standard co2 transfer port...either crank in more spring tension or switch to a Discovery striker...decrease the valve stem seal diameter slightly....and start testing at 130BAR...140BAR...150BAr...etc.

[rkr]

I finally got the peak on the curve moving:



I started opening up the transfer port as this allowed me to ease up the spring tension thus moving the paek. The hammer weight is about 50 grams so adding few more grams don't do much. The 50 shot string is with 2.2mm port while two others are 2.0mm with two spring tensions and one with the stock 1.8mm port. All shot with H&N FTTs. Now I'm wondering what would happen with 2.4mm port, it's just that those buggers are PITA to make without lathe

[rkr]

Here's what I think is the final shot string:



I turned the power down a bit so it's doing 11.8 fpe now. A 60 shot string starting 220 bar and with 20 fps variance. 40 shots within 11 fps so I think I'll settle for that. 2.4mm transfer port might give more shots at lower pressure but I might loose the flatness of the curve so I don't think it's worth it. I think this is very good for unregulated little carbine and I learned a lot doing it

[rsterne]

That's a GREAT result.... Congratulations on sticking with it and learning the process.... How many shots do you get with 4% of the peak velocity ?

Bob

[rkr]

That's a GREAT result.... Congratulations on sticking with it and learning the process.... How many shots do you get with 4% of the peak velocity ?

Bob

Those are all within 4%. Peak is 784fps and the lowest one 759fps at the end of the string. I could also get some 5-7 additional shots if I filled to 232 bar that's the max. fill, the string starts at 766 fps. The reservoir size is 100cc if you want to do the math

[rsterne]

Approaching 70 shots within 4% is EXCELLENT....

Your current string is 60 x 11.8 = 708 FPE total
Air usage is 220 bar - 130 bar?? = 90 x 100/16.4 = 549 CI.... (was the ending pressure 130 bar?)
Efficiency is 708/549 = 1.29 FPE/CI.... Not too shabby at all....

Bob

[rkr]

Approaching 70 shots within 4% is EXCELLENT....

Your current string is 60 x 11.8 = 708 FPE total
Air usage is 220 bar - 130 bar?? = 90 x 100/16.4 = 549 CI.... (was the ending pressure 130 bar?)
Efficiency is 708/549 = 1.29 FPE/CI.... Not too shabby at all....

Bob

Ending pressure is 125 bar, the air usage went up slightly as the pressure got lower. I can now fill to 200 bar and get a flat curve of 30 shots for the times I shoot HFT. The middle of the curve looks almost as good as regulated guns for that purpose with less money invested. I could probably get few more useful shots with a blast tamer but I don't think I'm going to bother as it increases the cocking effort.

[lloyd-ss]

It's great to see how this progressed over time.  Excellent results!

[rkr]

Thanks guys. Now here's potentially another interesting bit of information http://varmintair.typepad.com/varmintairs_blog/2012/02/aa-510-crbsl-22-carbine-has-adjustable-power-what-exactly-does-that-mean.html I asked the guy if he could post the graphs of those shot strings as he closed the transfer port opening. It would be really interesting to see if the peak on the curve moves or not as the spring tension is kept the same. I'm starting to think that the peak of the power curve can be moved by adjusting the spring tension (less tension moves the peak down) while the power can be adjusted by opening and closing the transfer port. In practice this would mean that we first tune the gun foe a curve we want by adjusting the spring tension and then open up the transfer port to achieve the power we want. What's your opinion?

[rsterne]

Basically, your statement is correct.... Set the location of the peak velocity with the hammer strike (weight, stroke, spring preload).... and the dial the velocity down by strangling the transfer port.... It flattens the shot string in the middle, which usually means you get more effective shots within 4% (or whatever criteria you use) of the maximum.... You may see some drifting of the position of the peak of the curve as you restrict the transfer port which may need slight correction with the hammer spring preload, however, especially if you are really trying to restrict the power....

Hatsan take their AT44 down from a 30 FPE gun to under 500 fps in Canada (~7 FPE) by using a larger valve stem to restrict the flow.... same fill pressure and hammer spring settings.... The shot count goes from 30ish to about 180ish.... If you are doing it the other way, however.... ie starting with a set curve (in a reasonably efficienct gun) and then opening the transfer port (and in fact all the porting in the gun) to improve its "breathing".... you will usually find that the peak velocity moves to a slighty lower pressure as you make it breath better.... We can therefore assume that the opposite will occur.... As you strangle the transfer port, the peak of the velocity curve should move up slightly....

Bob