So. Were the Cardew...
 
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So. Were the Cardews Wrong?


Steve in NC
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In Trigger to Target, the Cardew's cite both experiment and theory demonstrating that pellet exit always follows -- never precedes -- piston bounce, as in this illustration from page 117.

image

Were they right?


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nervoustrigger
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Following.

When I see the word always, it makes me think, “Really?  Always?”

Not stacking the deck, right?  I.e. no short barrel, super light projectile (plastic), or atypical transfer port.


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Steve in NC
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Posted by: @nervoustrigger

Following.

When I see the word always, it makes me think, “Really?  Always?”

Not stacking the deck, right?  I.e. no short barrel, super light projectile (plastic), or atypical transfer port.

LOL!  Correct.  Only the broad class of spring piston airguns that the Cardews discussed are under discussion.  Unrelated sorts of piston projectile projectors need not apply. 

E.g.

image

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Steve in NC
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My money's on the Cardews.  Here's one reason I think so. 

In actual experiments with actual springers, realistic plots of acceleration vs time look a lot like the one in this post by Jim-in-UK. 

https://airgunwarriors.com/community/airgun-talk/a-scientific-look-into-the-dynamics-of-the-shot-cycle-of-three-spring-piston-airguns/#post-47650

image

The big negative-going spike to the left of center measures acceleration -- therefore force against the chamber face and therefore pressure -- generated by piston bounce.  The negative peak corresponds to the instant when the time integral of force of rising chamber pressure exactly cancels the forward momentum imparted by the mainspring to the piston.  That is to say:  The moment of bounce when the piston stops and then reverses direction.

The interesting thing is this: The same pressure decelerating the piston in the chamber is accelerating the pellet in the barrel, increasing its momentum (and velocity) in direct proportion to decreasing the piston's.  The constant of proportionality is given by the ratio of piston area to pellet base area -- typically 16 to 44:1 depending on caliber and cylinder bore.

Because of the approximate symmetry of the bounce spike, half the pressure x time product occurs before bounce, half after.  Therefore, if the pellet exits before the moment of bounce, it must leave with less than half the velocity and momentum it could have had if it stayed in the barrel for the full duration of the spike.

Furthermore, basic physics tells us that the kinetic energy of a mass is proportional to the square of its momentum, so 1/2 the momentum equates to only 1/4 the energy.

Therefore, since allowing the pellet to exit before bounce would squander more than half the MV and 3/4s of the ME otherwise available, it's my belief that no successful springer design ever does it.

And the Cardews were right.

 

 


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JW652
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 Very interesting, fun exercise.   Thx


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Hector J Medina G
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Yes they were correct in this, Steve.

John has re-done some of the experiments and has re-written Chap 9 you will see it in a few weeks.

Now, that does not mean that I do not think their work is now "dated".

What we know now about the flow of super-hot air through short transfer ports was unknown back then.

What has been done with FUNCTIONAL anti-bounce pistons was discarded by them as an "unworkable idea"

It was great work; for the time and resources available, it was fantastic work.

But we SHOULD be moving on, and I do not know in reality if we ever will. Given the forces of the market, the Spring-Piston airgun may soon be relegated to the status of Black Powder shooting, or Primitive Bow and Arrow.

JMHO

 

 

 

 

 

HM


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Jim in UK
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As a generalisation, reducing the available piston stroke pushes the pellet exit further into the forward surge stage of the recoil cycle post piston bounce, and vice versa.

The pellet exit times that follow were recorded over six years ago, when my test methods and equipment were not so refined as they are now, so they might not be exact, but do serve to illustrate the effect of available piston stroke on pellet exit timing.

My Tx200 has 85mm of available stroke. With the lowest start pressure .177" pellet (7.3gn JSB round head), pellet exit was ~1.1 ms post piston bounce. When the available stroke was the normal 96mm, the same pellet exited the muzzle ~0.5 ms post piston bounce. 

The Cardew's test rifle was the HW35, which has a rather shorter available stroke of 65mm which, on its own, would push pellet exit further into surge, exacerbated, of course, by the considerably longer barrel. 

If the graph posted by Steve in the opening post is of a .22" rifle, I'd say yes - it will be correct in terms of pellet exit. 


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Steve in NC
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Posted by: @hector-j-medina-g

Yes they were correct in this, Steve.

John has re-done some of the experiments and has re-written Chap 9 you will see it in a few weeks.

Now, that does not mean that I do not think their work is now "dated".

What we know now about the flow of super-hot air through short transfer ports was unknown back then.

What has been done with FUNCTIONAL anti-bounce pistons was discarded by them as an "unworkable idea"

It was great work; for the time and resources available, it was fantastic work.

But we SHOULD be moving on, and I do not know in reality if we ever will. Given the forces of the market, the Spring-Piston airgun may soon be relegated to the status of Black Powder shooting, or Primitive Bow and Arrow.

JMHO

 HM

I'm happy if we've finally put this question to rest.  Again.

As for whether any anti-bounce contrivance will ever succeed, we now have a readily available and infallible test.  Thanks to Jim's VelociMeter invention combined with cheap PC audio CODECs, anyone can assemble inexpensive instrumentation and join in the hunt for the singular signature of that holy grail: A realistic piston pressure spike that lacks the tell-tale "surge" of reverse piston acceleration, which (by definition) would be prevented by a (FUNCTIONAL) anti-bounce device.

image

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Steve in NC
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Posted by: @jim-in-uk

As a generalisation, reducing the available piston stroke pushes the pellet exit further into the forward surge stage of the recoil cycle post piston bounce, and vice versa.

The pellet exit times that follow were recorded over six years ago, when my test methods and equipment were not so refined as they are now, so they might not be exact, but do serve to illustrate the effect of available piston stroke on pellet exit timing.

My Tx200 has 85mm of available stroke. With the lowest start pressure .177" pellet (7.3gn JSB round head), pellet exit was ~1.1 ms post piston bounce. When the available stroke was the normal 96mm, the same pellet exited the muzzle ~0.5 ms post piston bounce. 

The Cardew's test rifle was the HW35, which has a rather shorter available stroke of 65mm which, on its own, would push pellet exit further into surge, exacerbated, of course, by the considerably longer barrel. 

If the graph posted by Steve in the opening post is of a .22" rifle, I'd say yes - it will be correct in terms of pellet exit. 

Thanks Jim, as always, for your expert contribution.

I'm seeing bounce-exit intervals a bit longer than that -- e.g. ~2ms in the acceleration plot below from a 14fpe Cometa 400 spitting CPLs, but in that case the pellet was a tight fit in the breech (suggesting a relatively high start pressure), and perhaps I'm not adequately compensating for post-exit delays like the muzzle-to-backstop flight time (~90us/inch) of the pellet.

image

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Jim in UK
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How about 2.135ms; .177" TX200, 85mm available stroke, RWS Supermag (start pressure >600psi)?

Supermag

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Steve in NC
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I'll take it!


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Hector J Medina G
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Posted by: @steve-in-nc

I'm happy if we've finally put this question to rest.  Again.

As for whether any anti-bounce contrivance will ever succeed, we now have a readily available and infallible test.  Thanks to Jim's VelociMeter invention combined with cheap PC audio CODECs, anyone can assemble inexpensive instrumentation and join in the hunt for the singular signature of that holy grail: A realistic piston pressure spike that lacks the tell-tale "surge" of reverse piston acceleration, which (by definition) would be prevented by a (FUNCTIONAL) anti-bounce device.

image

Yes, Steve, but real life is 3-dimensional.

And, as comic as it may sound, some form of the "uncertainty principle" applies:

Vibrations are affected by the mass distributions along the gun and, especially, the barrel; AND by HOW the gun is held/supported.

To measure vibrations, you need to add masses somewhere; and to measure in 3D you need to support the gun somehow. 

So, between setup and instrumentation, you ARE affecting the phenomenon you are trying to measure.

Besides, measuring the "suspended in the air" gun would not reflect what REALLY happens once the gun is held in a shooter's hands against a shooter's shoulder.

We've advanced much, but there is still a LONG way to go, and THAT is going to be the fun part.

😉

Thanks, Keep well, and shoot straight!

 

 

 

HM

 


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James Perotti
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I would think that piston bounce and hammer bounce are parallel reactions taking place in the same time frame and for the same reasons, so why is the pellet exit time different when the basic mechanics seem to be the same?


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Steve in NC
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Posted by: @jpsaxnc

I would think that piston bounce and hammer bounce are parallel reactions taking place in the same time frame and for the same reasons, so why is the pellet exit time different when the basic mechanics seem to be the same?

Hi, James!

I agree that hammer and piston bounce are similar in this sense.  The strike-and-rebound of a pneumatic's hammer from the valve stem, and a springer piston's rebound from the compressed charge, must be complete before the pellet exits.  Otherwise efficiency takes a nose-dive.  You and I certainly proved it for AC MSPs!

It's great to hear from you, by the way!

Cheers,

Steve


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ribbonstone
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Have read “Trigger to Muzzle”….have not read the revised “Trigger to Target” that was 20 years newer…..last one seems rather costly on the used book market, not sure it has enough new information to make me want to hunt up a copy.  If any one has the 1995 "Trigger to target"...what got added in?

Reread parts of Trigger to Muzzle….considering the rifles he was working with then are not the rifles we have now, might be reasonable conclusions then that don’t fit the situation now.  Had to have been doing the work before the publishing date, so the early 1970's...back when the fastest springers were around 12 foot pounds.


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Steve in NC
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Posted by: @ribbonstone

Have read “Trigger to Muzzle”….have not read the revised “Trigger to Target” that was 20 years newer…..last one seems rather costly on the used book market, not sure it has enough new information to make me want to hunt up a copy.  If any one has the 1995 "Trigger to target"...what got added in?

Scribd.com has Trigger to Target available for download.  A subscription is $10/mo, first (trial) month free.


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James Perotti
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@steve-in-nc Hi Steve, Thanks, I didn't realize that it was the same in both cases.


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