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Question One for Loki, cloverleaf, balisticboy and the more technically minded, reference shot cycle

Discussion in 'Anything Airgun Related' started by bigtoe, May 11, 2015.

  1. bigtoe

    bigtoe Donator

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    As you know im testing 23, 22 and 21mm pistons, Loki and cloverleaf tested 2 of my guns, with the .22 97 recording an unbelievable recoil trace on loki's accelerometer...thats when the quest really got started.

    The .22 trace showed no secondary bounce of the piston, and yet its not crashing, it in fact has seriously low recoil and near zero surge.
    The .177 trace however showed seriously reduced recoil BUT there was bounce, this is what I have been working on.

    Right now im almost at the point of dialling out the 2nd bounce with an inertia weight which I can "time" by adjusting the travel distance, its working very very well.

    Now on to observations and questions.

    I did not have to apply this inertia system to the .22, both the 22 and 21mm piston .22 rifles shoot extremely well with very similar shot cycles, here is why i feel they do. The compression tubes have similar transfer ports to the .177 rifles set up, so port flow has to be very similar, yes they are pushing less air than the 23mm set up, however i have come to this conclusion. On the .22, the secondary valve (the pellet) is just the optimum size to release with a good balance of friction vs port dia, for every mm a .22 moves down the barrel the transfer port is being expanded at a greater rate than it is on a .177, so the peak pressure drops much quicker within the compression cylinder and so the spring is able to overcome the secondary bounce and the piston just comes to rest. On the .177 the surface area of the rear of the pellet for the air to push on is smaller, the peak pressure is a little higher at pellet release and the pressure drop is not fast enough with each mm the pellet travels to stop the second bounce...pressure stays high enough for the piston to move backwards a little before coming to rest.

    So, piston weight has to come into it, i have gone lighter and its helped....this is on the 23mm 77k. However i have a second rifle that running 22mm .177, a Diana 440TH, the shot cycle on this one is for me the best .177 out the 2, fast with no slam, and i find it seriously hard to feel any bounce. The difference between the Diana and the HW is the TP size, on the Diana its 4mm dia x 6mm long, on the HW77 its around 3.8mm and 6.5mm long...here is where it gets complicated.
    The Diana does not like 4.52 Exacts, power is low, it much prefers FTT 4.50, superfield 4.51 and 4.52 and superdomes...the 77 does ok with FTT, awesome with exacts and express and hates the RWS pellets, so im wondering if is alter the port to match the Diana will i rock the apple cart to much?
    The Diana has a straight piston with no inerta, right now i doubt it needs one, however i would love to see a trace to prove whether im just not feeling what really is there.

    I already thought about .25, i have already observed .25 needs as much spring as .177, however the shot cycle there after can be smooth and soft, there has to be increased friction hence a higher peak pressure is needed to get the pellet moving, however once moving the pressure drops seriously quickly in the compression cylinder, hence they feel nice to shoot.

    Also got talking to Matt here on the forum about the super light weight piston TX's you hear about (older design not NickG's), aluminium pistons that fly seriously fast, issue is the springs keep breaking, this got me thinking as too why...it has to be piston face surface area and the TP on the guns being too small. Reduce the piston face cross sectional area, less area to push on at peak pressure, much less bounce, less spring is needed so the shot cycle is not violent at all...smaller pistons have to be the key here.

    Is there a way we can work out the pressure drop with a compression cylinder, or the rate of pressure drop, so we can apply some thinking to how we get it to act the same on a .177 as it does on a .22?
    All I have done right now is build a stalling system with the inertia weight, that stalls the piston as it starts to bounce back, hopefully long enough for the pressure to drop that little extra to allow the piston to move forward to rest instead of heading backwards them forwards again.
     
    Last edited: May 11, 2015
  2. richardscar

    richardscar Donator

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    open the transfer port a tiny tiny bit more ? educated guess . lol ???
    my thinking is if the cycle is that much quicker, peak pressure will build more quickly and the smaller(even longer) transfer port with strangle the air flow and cause bounce . The speed/pressure of the air is all becoming a bit pcp like :) If this is the case could you not end up with the pellet accelerating up the barrel that fast that more of the air blast is wasted after the pellet has in fact exited the muzzle ?
    maybe total b*ll*cks , but just a thought .
     
  3. bigtoe

    bigtoe Donator

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    well im getting to the point of....is there enough air?

    Known this for a while, also tighter fitting pellets cause way different shot cycle...so im in the realm of tuning for specific pellets (not a bad thing)

    Im thinking 22mm piston for the next one, I have a 25mm comp tube totally untouched, so modify this to take removable transfer ports and see what works...however it would have been nice to apply some science to it if possible.
     
  4. Nickg

    Nickg Busy Member

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    Who has experienced broken springs in ally piston TXs ?, none of mine have broken, believe me I would have heard about it. Just interested to know as I might be able to help.
     
  5. Darren Petts

    Darren Petts Temporarily Alive

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    As you say the issue is the volume generated behind the pellet being insufficient on the .177 as it traverses the barrel. Perhaps a hollow piston with a micro hole to the "reservior" within from the piston face. This micro-hole acts as a metering valve in the same way as a pellet does and the hollow part makes up the missing volume from the .177 v .22 difference. Volume of the hollow part can be calculated according to barrel length but the hole size would be anybody's guess.
     
  6. loki_79

    loki_79 Donator

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    I can't help with any useful suggestions on what to change mechanically, but I am currently in the process of building a second accelerometer device. I could certainly lend you one of them for a bit, so that you can make measurements for each incremental modification. I really think this is the best way to proceed, since how else can you compare the changes. The ideal experiment would be to capture traces for the full range of transfer port dimensions, piston mass and stroke length (not too much work there then!).
     
  7. bigtoe

    bigtoe Donator

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    Not your guns Nick, edited my post to reflect this...sorry it may have caused confusion
     
    Last edited: May 11, 2015
  8. bigtoe

    bigtoe Donator

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    That may be the best option...we should talk more
     
  9. bigtoe

    bigtoe Donator

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    Now that is an interesting concept...it would need to weep continually (without a valve) but could weep to a small reservoir.

    Really interesting....will think on this one a little more. The hole would be tiny, 0.5mm or so
     
  10. Darren Petts

    Darren Petts Temporarily Alive

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    The best way would be to sweep less volume in the first place for the .177. Given you are only just making enough power as it is that is going to be hard to do.

    As I see it you need the same peak pressure to shift the pellet but for less time

    Another possibility is to dish the piston face. So long as the dish is small enough (less than the difference between .177 and .22 barrel volumes and probably by some margin) so the piston can still generate the same peak pressure (just further down the tube) you are then creating the same peak pressure but for a shorter time as the piston will be nearer the end of it's travel. It's the maintenance of the peak pressure that bounces the piston so reducing the time of it should help. Go too large on the dish and the peak pressure will be reduced.
     
  11. hmangphilly

    hmangphilly Posting Addict

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    if you went that route tony , why limit yourself to a fixed flow hole.
    a needle air adjuster like on a carburettor would give you adjustment either side of a datum.
    a pointy screw,partially bunging up the air passage to be crude.

    just early morning thoughts....
     
  12. bigtoe

    bigtoe Donator

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    Room is the issue, the ideal would be a pressure relief valve, with settable pressure, issue again is room to fit it.

    It is a cracking idea though, if you could set it to bleed right at the piston stall point (peak pressure) it would kill all bounce.
     
  13. bigtoe

    bigtoe Donator

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    Thought of that also, on one of the .22's (21mm piston) I did actually machine some lost volume (similar to the face of an LGV seal) into the face of the piston, power went up slightly and the piston rested a bit softer, maybe i missed a trick and should recreate this on the .177.
     
  14. hmangphilly

    hmangphilly Posting Addict

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    build it into the stem of your piston top hat , stick a hole right up the middle .
    seal it to the end of the piston with an oring machined in the top hat.
    you should be able to adjust without removing the piston ............or anything at all if you're clever
     
  15. bigtoe

    bigtoe Donator

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    Not sure if the following is what you mean BUT i have to thank you for getting me thinking.

    Right now on the .177 I have a sliding inertia weight, when the piston is travelling forward the weight is at the rear of the piston body, as the piston creates peek pressure it stalls and stands still, at this moment the weight travels forward, crashing into the forward O ring buffer on the piston body, lengthening the duration of the stall.

    What im thinking now is use the sliding inertia as a sliding valve also, in the rear position have a drilling thru the body of the piston to a depth of around 4mm, then drill thru the face of the piston (where i recreate the recess much like an LGV piston seal face) to meet up with this drilling...just thinking its a hard ask with a sub1mm drill but we shall see if I get some time. either side of this hole thru the side of the piston body i have an O ring of 1mm CS. With the weight in the rear wards position the hole is sealed, the lost volume is tiny (hence use a tiny drill)

    So as the weight shoots forward is opens the valve and lets the pressure in front of the piston bleed behind the piston...as long as the piston is travelling forwards its building pressure, only when it stalls or is moving backwards does it release pressure.

    I think this would work...

    So not sure if this is what you had in mind or did i just develop your idea even further?

    Edit, obviously the pressure acting on the sliding inertia would act as a pneumatic lock, however i think there would be enough force applied to the inertia as the piston stalls to break the lock.
    It is going to be interesting trying this.
     
    Last edited: May 12, 2015
  16. hmangphilly

    hmangphilly Posting Addict

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    i think you developed it 10,000 times .

    i forgot about the latching rod on the hw and was thinking of a hole right up the centre of the piston with a metering screw built into the top hat / piston guide,.
    the hw hasnt got that setup though
     
  17. matt goodson

    matt goodson Donator

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    Kevin Turners ally tx has broken springs. I was told Mr Pope knows about the issue, and doesnt do that conversion any more.
     
  18. matt goodson

    matt goodson Donator

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    So, are we talking about a piston that has a knock open valve so the pressure will go down quicker, which is activated by an inertia weight?
     
  19. Nickg

    Nickg Busy Member

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    Cant see how this would work, as soon as the reservoir is pressurised it would stop " bleeding" in fact it would only bleed if the piston was open to atmosphere, which I would call a leak, I could be wrong:confused:
     
  20. Nickg

    Nickg Busy Member

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    This sounds good tony, the only draw back is the possibility of the valve opening being to sudden and giving you sudden slam, You have to try it
     

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