Beeman Chief - Merry Christmas to Me !!!

[rsterne]

I reassembled the Chief this afternoon, and had time to shoot a few pellets through the Chrony.... I had thought that although I increased the throat diameter, since I did not change the OD of the poppet I would not need much increase in hammer strike.... While that may be true to "unstick" the valve, it takes more dwell to get more velocity.... I started out with LESS velocity with the same settings at 2000 psi, and had to crank the preload to the max.... When I did that, I got 974 fps with the 18.1 gr. (38 FPE) and 874 fps with the 25.4 gr. pellets (43 FPE).... but with the peak velocity occurring at only 1500-1600 psi.... Pretty good power at that pressure.... 

I will be pulling the valve out again and changing the poppet to one made from Delrin.... I'm pretty sure that will not only reduce the hammer strike required to "unstick" the valve.... but will also increase the usable pressure range as well, giving more shots per fill....

I thought I should at least shoot one string before I changed out the poppet.... Here it is, added to the previous data....



It is shown over to the right because only the shots between 1750 psi and 1300 psi were within a 4% ES.... It sure shows what big ports do....  .... The 10 shots used 450 psi, so that works out to 1.38 FPE/CI.... pretty good at an average of 958 fps (36.9 FPE)....

Bob

[WesBob]

Great efficiency for such a low pressure! Interesting to see how the delrin poppet changes things!

[skorec]

43 FPE by using only 1550 PSI is really nice power/result  done  only  via  increasing/improving air flow way especially by  using  21,5” barrel only.

Which is your assessment . Can balanced/Travis valve push the power  up to 60FPE at the same pressure ?

[nervoustrigger]

874 fps with the 25.4 gr. pellets (43 FPE).... but with the peak velocity occurring at only 1500-1600 psi.... Pretty good power at that pressure.... 

Very nice results, Bob!

That matches up pretty closely with a .22 cal QB78 that I regulated to 1675psi.  It was just shy of 45fpe with the heavy 28.5gr Eun Jins. 



Unfortunately it wasn't a happy shooter at that state of tune so I ended up backing it down a bit.

[rsterne]

A balanced valve won't make more power.... it reduces hammer strike at the same power....

Jason, I need more hammer strike.... or an easier to open valve.... to get the power to max. out at 1675 psi.... let alone 2000 psi.... Right now max. power is only being achieved at about 1500 psi....

Bob

[nervoustrigger]

Bob, yeah I'm sure you'll have my regulated figure bested by quite a bit once you change to a rigid poppet.  Your 0.200" porting is massive compared to the 0.170" I was running...a difference of 38%. 

[rsterne]

Once I find out where this gun maxes out.... I may change down to a smaller transfer port.... we'll see.... First of all I have to find out where the plateau is at 2000 psi.... and also tethered at 1800 like I tested on the stock gun (26.4 FPE with the 18.1 gr. @ 811 fps)....

Bob

[skorec]

A balanced valve won't make more power.... it reduces hammer strike at the same power....

Jason, I need more hammer strike.... or an easier to open valve.... to get the power to max. out at 1675 psi.... let alone 2000 psi.... Right now max. power is only being achieved at about 1500 psi....

Bob

I was thinking that balanced valve is opening much quicker and therefore increase power significantly. But probably it can help only at very short barrel and high pressured PCPs or even not help absolutely for increase power ?

[Ribbonstone]

Just musing (easy to do on someone else's project)....but I'd kind of like to see Bob get to 60 foot pounds...and something about the Chief has been bothering me.

Chief does (at least as issued) seem to have a pretty short stroke....just not a whole lot of striker distanced between valve contact and full cock.

There's is about .23" of "slack" bolt movement from lifting the handle and moving back until the bolt contacts the striker resting on the valve stem.

There is about .134" of "over cock" (bolt still moves back to contact the read of the bolt slot in the brrech after the striker has caught the sear).

And about .65" of actual travel of the striker between sear and valve strike.


Has seeemed to me that at least some of that distance could be used for actual striker stroke....which if looking for something nearer to 2K as the balance point of a fill, might be useful.  A little shorter valve stem...a little shorter (from the front) striker..a little more spring?   Whatever distace you save might be useful when an anti-bounce device gets installed.

[rsterne]

Yes, I measured only 5/8" of actual working striker travel.... Some of the 0.23" of slack at the front is necessary so that the cocking pin won't hit the bolt at maximum valve lift.... but not all of it.... I have already been thinking about the extra bolt movement available at the back after the sear catches.... Between the two, getting 0.70" of working stroke should be pretty easy.... I don't know if a full 3/4" can be obtained or not....

Bob

[Ribbonstone]

I'll see how you do with that...can see that there is more stroke to be had, so more "opening force", but with your work to ease the opeing force needed, then the stroke mod might be  over-kill...or you might use any "overage" on energy to retune into more shots at that nergy (although I suspect, with a 2K fill, even a 1 shot gain in shot count would be a significant percentage gain).

[rsterne]

Had a couple of days helping around the house, taking down Christmas decorations, touching up paint in the Motel.... but got back into the shop today and made a PEEK poppet for the Chief.... I have never used one in such a low pressure gun before, and wasn't sure if it would seal OK.... because the material is so much harder.... but I faced it off square to the stem with a razor sharp tool bit in the lathe and the lapped it to the seat in the valve with Solvol Autosol like I usually do, and it sealed up immediately.... Here is the new poppet, below the stock one.... I removed the brass spring seat/spacer, so there is less spring force closing the valve as well....



I made the spring seat slightly larger in diameter for a better fit inside the spring, and to keep it centered better on the smaller (3/8") OD poppet.... In addition, I tapered the poppet down to only 5/16" at the seat, still quite a bit larger than the 1/4" throat, but a lot smaller than the soft stock poppet, which had an OD at the seat of nearly 7/16".... Not only does this provide a lot better flow around the poppet near the seat, but the much smaller area should take a lot less hammer force to "unstick" at the same pressure.... and in addition the much harder PEEK will compress a lot less, so the distance the poppet travels when hit by the hammer before it breaks the seal is much less also.... The hammer energy (inch.lbs.) used up to "unstick" the valve is the force (lbs.) times the distance (in.) the poppet compresses.... The force should be reduced from about 277 lbs. at 2000 psi to about 156 lbs.... and I'm guessing the compression on the PEEK is less than half.... so the hammer energy required to "unstick" the valve should only be about 1/4 or less that of a stock poppet....

Well, on reassembling the gun, I found that all the above is NOT just theory.... With the stock poppet, even at maximum hammer spring preload I could not max. out the velocity of the gun.... With the new PEEK poppet, I can't even get down to the knee of the curve, even at MINIMUM preload.... Here is the comparison....



Now we're beginning to see just how much the larger ports increased the performance.... The plateau velocity with the same 18.1 gr. JSB Heavies increased from 811 fps (26.4 FPE) to 1053 fps (44.6 FPE), both with the gun tethered to a regulator set to 1800 psi.... That is a 69% increase in FPE from a port diameter increase of 45%.... Frankly, I'm a bit shocked, that surpassed my expectations.... Part of the unexpected gain may be due to the much larger passages on the inlet side of the valve.... which means that ALL of the reservoir volume is available to feed the valve in an unrestricted manner.... instead of having to flow through that 0.138" hole in the gauge block.... which likely caused some pressure drop inside the valve during the shot.... Normally, if your FPE increases in proportion to your port diameter increase, you have done well....

I am basically to the point now that the 18.1 gr. pellets are too light, so I changed over to the 25.4 gr JSB Monsters.... The plateau velocity with those was 955 fps (51.5 FPE) at only 1800 psi.... so my power goal has been met with 200 psi to spare....  .... I left the preload adjusted to the minimum and shot a string, starting at 2000 psi, with each pellet.... Here are the results, along with the previous data for comparison....



The black line is the way the gun came stock.... the green line with the big ports and stock poppet (at max. preload).... and the red line with the big ports and PEEK poppet (at min. preload).... all three with the 18.1 gr. Heavies.... The blue line is that current setup, but with the 25.4 gr. Monsters.... Obviously I need a weaker hammer spring in order to get back some adjustment.... time to go looking.... 

Bob

[rsterne]

Well I didn't need to look far for a spring.... I set the stock spring by compressing it to coil bind (shortened it about 1/4") and then closed up the last coil on the cut end.... I ended up with a spring that was 2.38" long instead of the 2.66" I started with, with 16 coils of 0.046" wire and an OD of 10 mm (0.394").... BTW, a spring calculator says this is about a 12 lb/in spring.... The hammer weighs 80 grams and has a stroke of 0.63".... When I installed the set spring in the gun, I could still get to maximum velocity at 1800 psi, but I could adjust it down all the way to 600 fps with the 25.4 gr pellets.... Here is the data added to the previous graph....



The dotted line is with the set spring.... you can see it reduced the preload by about 8 turns.... The knee of the curve, at 1800 psi, is at about 5-6 turns out.... I filled the gun to 2000 psi, and using the 25.4 gr. JSB Monsters and the preload set to 5 turns out I shot a string.... It would have been great for tethering the gun at 1800-2000 psi, because the first 6 shots were within about a 1% ES, just over 900 fps (46 FPE).... However, it was missing the first part of the bell curve, so I turned the preload out 1 more turn and shot another string.... This time I got 12 shots within a 4% ES.... I have added both strings to the previous graph to make comparisons easy....All the 25.4 gr strings are in blue, using a 2000 psi fill pressure.... The 6 turn out string ended at just over 1300 psi, so I am getting a wider usable pressure range with the PEEK poppet, as I expected.... nearly 700 psi instead of 500....



The last tune above (6 turns out) was 12 shots averaging 871 fps (42.7 FPE), with an efficiency of 1.27 FPE/CI.... The last part of the string I can hear some valve bounce, and the pressure and velocity is dropping pretty quickly, so the gun could benefit from a shorter, stiffer spring with a gap, or an SSG.... I don't have an appropriate spring that will slide over the spring guide pin on the hammer, but I plan to look and see if I can make an SSG fit.... At 6 turns out, there is about 1/10" of preload, which is just about the worst condition to encourage hammer bounce.... I'm pretty sure I can increase the shot count.... but I'm already getting more total energy, at about the same efficiency, and I've gone from 16 shots at 25 FPE to 12 shots at nearly 43 FPE, and from 794 fps with an 181 gr. pellet to 871 fps with a 25.4 gr.... My best string is 68% higher FPE than the best string I had on a stock gun....

Incidently, I did test the power at 2000 psi at maximum preload with the stock spring.... I got 966 fps (52.6 FPE) with the 25.4 gr. Monsters.... and 882 fps (58.0 FPE) with the 33.6 gr. Beasts....

Bob

[WesBob]

Wow!

[Ribbonstone]

Seems like you got to goal....that's a lot of energy, but still pretty efficient.

[rpopeye]

That's some great tests! I didn't realize one could get almost supersonic with an airgun.

[skorec]

Bob,

The  question is why new  PEEK poppet increase the velocity of 18gr pellets from 960 FPS to 1020 FPS.  Do you think that it  is only via  better flow around the new shaped poppet.
Congratulation because you are probably the firs who own  low pressure  PCP able to shooting  Jumbo Monsters.

[tkerrigan]

Bob, do you use a friction fit for the peek poppet or use adhesive?  Regards, Tom

[Ribbonstone]

Soft stuff has compression...so to lift it off the valve face and open the airway takes more time/more energy to both over come the compression and lift ehe sealing face clear of the air flow?  So basically it's a slightly slower lift off for the same strike to the valve?

[rsterne]

Peter (and Ribbon), the reason for the increase from 975 fps to 1050 fps for the peak velocity with the 18.1 gr. when I changed poppets (green line to red line on shot strings chart).... is because even at MAX. preload I could not get the soft poppet to open fully with the big throat, it was absorbing so much of the hammer energy just to "unstick" from the seat.... That is why the green curve is shown shifted way over to the right, because it only started at 1650 psi, down to about 1300.... With the much harder PEEK poppet I was not able to dial down the velocity to get a curve with the original spring, even at MIN. preload, which is why I had to shorten the spring to get some adjustment.... The PEEK poppet was, indeed, absorbing only about 1/4 of the hammer energy to "crack", maybe even less, compared to the very soft, stock poppet....

Tom, I simply drilled the hole in the PEEK to 0.120" and then pressed the 0.125" stem into it....

Bob