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[Sticky] A scientific look into the dynamics of the shot cycle of three spring-piston airguns

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(@hector-j-medina-g)
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Not an April's Fools joke.

This will be a NINE part series.

We use three of the best recoiling spring-piston rifles ever made.

We will publish one part every two weeks.

https://www.ctcustomairguns.com/hectors-airgun-blog/shot-cycle-dynamics-in-3-spring-piston-airguns-preface

Hope you all enjoy!

 

 

HM


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(@hector-j-medina-g)
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Steve;

Let me be clear: We are not friends.

I respect everyone in the airgun community. It's civil to do so.

You are free to believe what you want, but you are definitely "NOT GETTING IT".

I will repeat it here: The MAIN DRIVER of these experiences is to allow EVERYONE to build something that will allow EVERY TUNER AND TINKERER to compare a "before and after" rifle of HIS doing.

We are not claiming to produce absolute results, we are not aiming to achieve a different design of internals. We are just making it possible for everyone with the will to have an instrument to record the results of his efforts.

You seem to be attached to the idea that the airguns should be redesigned. That's a possibility, but a really long shot one. NO manufacturer is at present doing anything worthwhile as far as the internals are concerned. Architectures have been frozen in time since the 19th century for the breakbarrel and the 20th century for the sliding compression cylinder guns.

Other architectures have been explored and have ended by the wayside.

The sad story of the Walthers (LGU and LGV) is a sobering experience for the industry. Walther listened to the vociferantes in the community that assured they would pay $800 for an OoB "tuned airgun". Sure! about 300 of them paid the full price, ¿after that?  You know the result. Apart from the limited supplies of the LGU Varmint still available here and there, everything else has been, or is in the process of being, discontinued.

So, trying to get a "pure" view of the behaviour of the airgun is rather futile. There is no future in that research.

Where there is future is in creating the ability for all to "see" what truly is happening in a gun set in a condition AS CLOSE AS POSSIBLE to the field usage.

Does the human body allow a completely rigid fixation of the gun?

Do your arms and hands have no weight?

Does your torso's mass play no role?

We have already established that whatever happens after the pellet exit is important only to the shooter's frame of mind, or do you still think that the recoil post-pellet-exit has some effects on the pellet's trajectory?

The consistency of whatever happens between the moment of piston release, and the moment of pellet exit is the all crucial time lapse at which the "tuning" efforts should be focused on.

The much vaunted "Smoothness of shot cycle" has been proven to be irrelevant to the accuracy, the consistency, and the trajectory of the pellet.

Again, taking into account a much bigger frame of reference: The pellet's POI relative to the initial "state of rest" will depend more on the architecture of the rifle, than on the internals.

Slight offsets between piston axis, barrel axis, point of support at the rear, and hand support will create rotational momenta in the X-Z plane that will make the barrel to be "looking" at different spots when the pellet exits.

Harmonics in the barrel now become more important than ever, that is an x-Y-Z space problem, and they will depend more on the architecture, than on the internals.

Efficiency is also now more evidently important because the less energy is wasted, the less energy is available to move the gun in those critical 8-10 ms between trigger pull and pellet exit, and the less the gun will move to change the POI of the pellet.

To close this: We KNOW this to be a first step. You want to go one better than us? By all means! Go ahead! Build a better sled, get better results, publish them for everyone to see. I am sure you are more than capable.

In the next chapter we will discuss the statistical tools needed to analyze the performance of rifles, and why the isolated 5 or even 10 shot group is quite a poor evidence of good performance.

When we swap the powerplants between the LGU and the LGV it will be even more apparent that the actual accuracy and precision of airguns depend more on things that are NOT the internals.

The road is still long ahead, so you have the time to get your improved device built and start putting in the hundreds (if not thousand +) shots that these experiments require to be statistically significant.

Keep well and shoot straight!

 

 

 

 

 

HM


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

Steve;

Let me be clear: We are not friends.

I respect everyone in the airgun community. It's civil to do so.

You are free to believe what you want, but you are definitely "NOT GETTING IT".

I will repeat it here: The MAIN DRIVER of these experiences is to allow EVERYONE to build something that will allow EVERY TUNER AND TINKERER to compare a "before and after" rifle of HIS doing.

We are not claiming to produce absolute results, we are not aiming to achieve a different design of internals. We are just making it possible for everyone with the will to have an instrument to record the results of his efforts.

You seem to be attached to the idea that the airguns should be redesigned. That's a possibility, but a really long shot one. NO manufacturer is at present doing anything worthwhile as far as the internals are concerned. Architectures have been frozen in time since the 19th century for the breakbarrel and the 20th century for the sliding compression cylinder guns.

Other architectures have been explored and have ended by the wayside.

The sad story of the Walthers (LGU and LGV) is a sobering experience for the industry. Walther listened to the vociferantes in the community that assured they would pay $800 for an OoB "tuned airgun". Sure! about 300 of them paid the full price, ¿after that?  You know the result. Apart from the limited supplies of the LGU Varmint still available here and there, everything else has been, or is in the process of being, discontinued.

So, trying to get a "pure" view of the behaviour of the airgun is rather futile. There is no future in that research.

Where there is future is in creating the ability for all to "see" what truly is happening in a gun set in a condition AS CLOSE AS POSSIBLE to the field usage.

Does the human body allow a completely rigid fixation of the gun?

Do your arms and hands have no weight?

Does your torso's mass play no role?

We have already established that whatever happens after the pellet exit is important only to the shooter's frame of mind, or do you still think that the recoil post-pellet-exit has some effects on the pellet's trajectory?

The consistency of whatever happens between the moment of piston release, and the moment of pellet exit is the all crucial time lapse at which the "tuning" efforts should be focused on.

The much vaunted "Smoothness of shot cycle" has been proven to be irrelevant to the accuracy, the consistency, and the trajectory of the pellet.

Again, taking into account a much bigger frame of reference: The pellet's POI relative to the initial "state of rest" will depend more on the architecture of the rifle, than on the internals.

Slight offsets between piston axis, barrel axis, point of support at the rear, and hand support will create rotational momenta in the X-Z plane that will make the barrel to be "looking" at different spots when the pellet exits.

Harmonics in the barrel now become more important than ever, that is an x-Y-Z space problem, and they will depend more on the architecture, than on the internals.

Efficiency is also now more evidently important because the less energy is wasted, the less energy is available to move the gun in those critical 8-10 ms between trigger pull and pellet exit, and the less the gun will move to change the POI of the pellet.

To close this: We KNOW this to be a first step. You want to go one better than us? By all means! Go ahead! Build a better sled, get better results, publish them for everyone to see. I am sure you are more than capable.

In the next chapter we will discuss the statistical tools needed to analyze the performance of rifles, and why the isolated 5 or even 10 shot group is quite a poor evidence of good performance.

When we swap the powerplants between the LGU and the LGV it will be even more apparent that the actual accuracy and precision of airguns depend more on things that are NOT the internals.

The road is still long ahead, so you have the time to get your improved device built and start putting in the hundreds (if not thousand +) shots that these experiments require to be statistically significant.

Keep well and shoot straight!

 

 

 

 

 

HM

I'm sorry, Hector, that you have chosen to descend from an objective discussion of technical issues to subjective personal remarks. 

But the facts are that you have chosen to publish results that were, from the getgo, in complete and obvious contradiction (e.g., claiming only 10s of g's of piston-bounce driven acceleration instead of 100s of g's) of decades of existing study and analysis of springer ballistics.

So in truth, you have produced "absolute results."  Absolutely nonsensical and useless results.


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 nced
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@hector-j-medina-g

"The much vaunted "Smoothness of shot cycle" has been proven to be irrelevant to the accuracy,"

Agreed!

Years ago I found that the new "outta tha box" .177 Beeman R9s I bought would shoot "snug fitting" 7.9 grain boxed Crosman Premiers into a 1/2" group at 30 yards, even in "factory twang mode" when sitting on a bucket resting the guns on cross sticks.

Anywhoo........if accuracy was the only issue, for me tuning isn't worth the effort with the HW springers!


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Steve in NC
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Posted by: @nced

@hector-j-medina-g

"The much vaunted "Smoothness of shot cycle" has been proven to be irrelevant to the accuracy,"

Agreed!

Years ago I found that the new "outta tha box" .177 Beeman R9s I bought would shoot "snug fitting" 7.9 grain boxed Crosman Premiers into a 1/2" group at 30 yards, even in "factory twang mode" when sitting on a bucket resting the guns on cross sticks.

Anywhoo........if accuracy was the only issue, for me tuning isn't worth the effort with the HW springers!

Words of wisdom -- at least if group size alone is the sole definition of "accuracy."

But if "accuracy" is also meant to include how closely today's zero matches yesterday's and last week's, then maybe the stability of MV a good tune can provide is worth a significant investment of effort because it pays off in improved likelihood of actually hitting what we aim at, instead of merely missing -- albeit with admirable consistency.  😎 


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@steve-in-nc

 

"at least if group size alone is the sole definition of "accuracy."

LOL, guilty as charged! I'm thinking that if a springer can't GROUP WELL then other definitions of accuracy doesn't matter. 🤣 I'm thinking that consistent the "inside 1/2" @ 30 yard accuracy with an untuned springer would probably be adequate for a large percentage of shooters but I do use some "adjustments" to make my springers more consistent. Still.......I've found that this SHOOTER (me) sitting on a bucket resting the gun on cross sticks, atmospheric conditions, etc, on any particular day has as much (or more) to do with accuracy than the hardware tune level!

A couple 25 yard groups using a home tuned Chinese .177 B3 cobbled together from two guns bought from a Cummins Truckload Sale for $19.95 each a couple decades ago using "irons" and a 6x scope..........

 

 

Here are a couple .177 HW95 groups shot more recently.........

 

LOL.....even the pellet brand used affects the accuracy. Notice the CPL group in the upper left corner of this target compared to a couple other brands, all shot at 50 yards...........

 

 

 


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DavidEnoch
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What would it take to rigidly attach the rifle to the sled.  I was thinking about a trigger guard clamp but you also need access to the trigger.

David Enoch


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Steve in NC
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Posted by: @davidenoch

What would it take to rigidly attach the rifle to the sled.  I was thinking about a trigger guard clamp but you also need access to the trigger.

David Enoch

That's a very good question, David, that I was hoping someone would ask.  Piston bounce generates tremendous chamber pressure and thereby force, as is estimated in the excellent paper that Hector cited and I lifted some numbers from...

...3303lbs of force against the face of a 1" chamber = 182g of reverse recoil...

That works out to 182g x 2.4lb = 437 pounds (typical weight of a baby grand piano!) of force acting against whatever anchors the gun to the 2.4lb sled, and I doubt a trigger guard could ever be expected to stand up to it!

One possibility I was pondering might be to clamp a 1" (or 30mm, depending on the rings mounted) rod in the scope mount, flip the gun upside down, and have a massive pin that would link the rod to the sled.

I really do like the magnetic inductive piickoff of sled motion that Hector and his friend devised -- very clever and elegant!


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Something like this (with apologies to Ed for pinching his B3 photo)...

 

TEMP

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Steve in NC
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Posted by: @davidenoch

What would it take to rigidly attach the rifle to the sled.  I was thinking about a trigger guard clamp but you also need access to the trigger.

David Enoch

Actually, David, recently I'm being nagged by an alternative goofy idea.  Ready?  Don't laugh.  Here goes.  Why not dodge the whole issue in its entirety?

Why not just lay the rifle on its side, entirely unconstrained, on a smooth flat surface so it can recoil freely without marring its finish, and let the rifle be its own sled?

At least you gotta' admit -- you can hardly imagine a more rigid attachment than of the rifle to itself!!

Then the only custom fabrication needed would be a miniature free-standing version of Hector's inductive velocity sensor, and data recording system to sense the rifle's movement.


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How about suspended like a pendulum?  Trigger actuated by a solenoid (pneumatic/electric).

 

Grabbity sux.


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Steve in NC
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Posted by: @scratchit

How about suspended like a pendulum?  Trigger actuated by a solenoid (pneumatic/electric).

 

Grabbity sux.

That would work.  One complication is you'd need some kind of rig to independently capture the maximum amplitude of the recoil swing to get the calibration constant used to scale the magnetic sensor's signals.

Every ballistic pendulum has one.  See the "catcher pawl" in this classic design.

image

http://hyperphysics.phy-astr.gsu.edu/hbasees/Class/PhSciLab/balpen.html


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@steve-in-nc

Dear Steve,

Thanks for your insightful comments. Your questions have made think more deeply about our measurements, and as a result I think that I may understand things a bit better now. Please find below some more thoughts on what may be going on.

1. I agree that the accelerations and forces can be quite large here. As the rifle accelerates it pushes/pulls the sled applying a force that is the acceleration times the MASS of THE SLED (to get the sled moving with the rifle). The sled weight is about 2.4 pounds while the rifles weigh over 10 pounds with the scope. Did you use the sled weight or a typical rifle weight when you estimated the forces?

2. The high accelerations occur for a very short time, otherwise, as you aptly put they would send "the entire apparatus flying away." This makes them hard to observe, even with a perfect apparatus and I'm pretty sure our system is rounding the edges and probably missing the peak accelerations. On the other hand, the small impulses (ma*dt) that these accelerations give to the momentum of the rifle will not strongly affect the velocity or the position of the rifle as functions of time. There will be some sharper edges in the velocity traces that our apparatus has smoothed over.

3. I was curious if I could estimate some of the forces on the sled and I think we can do this for the initial force pretty easily. The spring has a spring constant of around 7 N/mm is compressed about 80 mm, so the initial force of the spring pushing on the piston and the piston pushing back on the rifle is around 560N (125 lbs). This causes the rifle to accelerate backward at 71 m/s^2 (assuming 8 kg rifle plus sled). To get the sled to accelerate with the rifle at this acceleration, the butt of the rifle has to push the 1.1 kg sled with around 78 N (1.1 kg*71 m/s^2) which is around 18 lbs. Here you can see the importance of Point 1! Unfortunately, it's much harder to estimate the accelerations later, but we can compare the measured accelerations. The later acceleration peaks are around a factor of three bigger than the initial acceleration dip, but that still puts the forces on the sled at around 50 lbs, which of course is still significant and makes it clear that the rifle needs to be well-secured to the sled. If I could do these measurements over, I would definitely use a stronger attachment system!

4. I saw very different behavior (no oscillations!) when I forgot to tighten the strap sufficiently, so having the rifle move from the sled can be a real problem. However, when I tightened the strap with all my strength, the results were pretty reproducible. Of course, one could argue that the rifle was reproducibly separating from the sled, but I think the separation, if there was any, was pretty small.

5. The key question is whether the sled (whose motion we're measuring) is moving differently than the rifle. I agree that it would be best to mount the motion sensor rigidly right on the rifle, as Jim Tyler does in his articles in Airgun World (please see first figure). Jim mounts the magnet to the rifle in a scope ring and lets the rifle recoil in a cradle. This is an excellent setup with no added weight from a sled to damp recoil and with a motion sensor that definitely moves with the rifle.

Fig 1 Tyler setup

In the next figure I compared rifle position vs time from Jim's setup with ours. The results look pretty similar, both qualitatively and quantitatively.

Fig 2 Tyer comparison

6. My analysis of the piston motion provides results that are pretty reasonable, so I think that our data are also pretty reasonable. We may have missed some accelerations spikes, but since our goal was to compare three rifles using the same testing system and to look at the overall behavior, I think that we've done a reasonably good job. The goal of this work was not to determine the peak acceleration of airguns during recoil!

7. Mounting a sensor rigidly can also cause problems. If you mount motion sensor rigidly into the airgun, it can vibrate at high frequency (small mass on a very rigid spring) and report accelerations that are much higher than the actual acceleration of the center-of-mass of the rifle. Try mounting an accelerometer on a scope ring on a rifle and then hit the rifle with a wooden mallet. I would bet the accelerometer would go off the charts without the rifle moving much. Hector has a lot more experience with this than I do, so this is more of a question than an answer.

I agree that the backward acceleration dip near 0.02s could be due to the rifle moving forward off the sled's rear bracket and then getting pulled back onto the bracket by the strap, which causes the sled to recoil back from the collision. However, there are some arguments for this being a real effect. First of all, I would expect that the spring constant of the Velcro strap is pretty low and I don't think it could snap the rifle back in a few ms. Also the fact the the acceleration keeps ringing, even when the accelerations (and forces) are really small (and there's no question that the velcro should be holding the rifle firmly against the sled bracket) at later times, suggests that the entire sled is actually moving back and forth.

On the other hand, there looks like there's a bit of a discontinuity in the slope of the acceleration for the FWB 124 and LGV right at the start of this negative acceleration dip in Fig. 2.12, so something weird could be happening? I think the LGU acceleration in that figure looks pretty reasonable. Now that I'm looking more carefully, both the LGU and LGV exhibit a small shoulder (maybe even a peak) as the acceleration heads toward the second dip. Maybe we should ask Jim Tyler to share some of his data at longer times? His published traces usually focus on the first ~20 ms of recoil. In terms of practical use of airguns, we don't really care that much what happens after 10 ms, since the pellet has already left the barrel.

All measurements have limitations, but as long as it is clear how the measurement was done and what the limitations are, useful information can be extracted. If nothing else, we can explore fundamental ideas, gain some insights, and figure out how to do things better next time!

Best wishes,

John

 


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(@hector-j-medina-g)
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@steve-in-nc

Sorry for the late reply, I was doing some serious and real gunsmithing.

I have not turned this into a personal thing, on the contrary, I have disconnected the personal from the objective.

You call "respecting everyone" and being civil a descent?

Again, Steve you are free to think and believe whatever you want. You dodged the simple questions of HOW A GUN IS USED IN REALITY, and I have not seen even a "hammock" test from you.

Challenge still stands: Build your own device and publish.

Keep well and shoot straight!

 

 

 

 

 

HM


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Steve in NC
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Posted by: @johnc

@steve-in-nc

Dear Steve,

Thanks for your insightful comments. Your questions have made think more deeply about our measurements, and as a result I think that I may understand things a bit better now. Please find below some more thoughts on what may be going on.

1. I agree that the accelerations and forces can be quite large here. As the rifle accelerates it pushes/pulls the sled applying a force that is the acceleration times the MASS of THE SLED (to get the sled moving with the rifle). The sled weight is about 2.4 pounds while the rifles weigh over 10 pounds with the scope. Did you use the sled weight or a typical rifle weight when you estimated the forces?

2. The high accelerations occur for a very short time, otherwise, as you aptly put they would send "the entire apparatus flying away." This makes them hard to observe, even with a perfect apparatus and I'm pretty sure our system is rounding the edges and probably missing the peak accelerations. On the other hand, the small impulses (ma*dt) that these accelerations give to the momentum of the rifle will not strongly affect the velocity or the position of the rifle as functions of time. There will be some sharper edges in the velocity traces that our apparatus has smoothed over.

3. I was curious if I could estimate some of the forces on the sled and I think we can do this for the initial force pretty easily. The spring has a spring constant of around 7 N/mm is compressed about 80 mm, so the initial force of the spring pushing on the piston and the piston pushing back on the rifle is around 560N (125 lbs). This causes the rifle to accelerate backward at 71 m/s^2 (assuming 8 kg rifle plus sled). To get the sled to accelerate with the rifle at this acceleration, the butt of the rifle has to push the 1.1 kg sled with around 78 N (1.1 kg*71 m/s^2) which is around 18 lbs. Here you can see the importance of Point 1! Unfortunately, it's much harder to estimate the accelerations later, but we can compare the measured accelerations. The later acceleration peaks are around a factor of three bigger than the initial acceleration dip, but that still puts the forces on the sled at around 50 lbs, which of course is still significant and makes it clear that the rifle needs to be well-secured to the sled. If I could do these measurements over, I would definitely use a stronger attachment system!

4. I saw very different behavior (no oscillations!) when I forgot to tighten the strap sufficiently, so having the rifle move from the sled can be a real problem. However, when I tightened the strap with all my strength, the results were pretty reproducible. Of course, one could argue that the rifle was reproducibly separating from the sled, but I think the separation, if there was any, was pretty small.

5. The key question is whether the sled (whose motion we're measuring) is moving differently than the rifle. I agree that it would be best to mount the motion sensor rigidly right on the rifle, as Jim Tyler does in his articles in Airgun World (please see first figure). Jim mounts the magnet to the rifle in a scope ring and lets the rifle recoil in a cradle. This is an excellent setup with no added weight from a sled to damp recoil and with a motion sensor that definitely moves with the rifle.

Fig 1 Tyler setup

In the next figure I compared rifle position vs time from Jim's setup with ours. The results look pretty similar, both qualitatively and quantitatively.

Fig 2 Tyer comparison

6. My analysis of the piston motion provides results that are pretty reasonable, so I think that our data are also pretty reasonable. We may have missed some accelerations spikes, but since our goal was to compare three rifles using the same testing system and to look at the overall behavior, I think that we've done a reasonably good job. The goal of this work was not to determine the peak acceleration of airguns during recoil!

7. Mounting a sensor rigidly can also cause problems. If you mount motion sensor rigidly into the airgun, it can vibrate at high frequency (small mass on a very rigid spring) and report accelerations that are much higher than the actual acceleration of the center-of-mass of the rifle. Try mounting an accelerometer on a scope ring on a rifle and then hit the rifle with a wooden mallet. I would bet the accelerometer would go off the charts without the rifle moving much. Hector has a lot more experience with this than I do, so this is more of a question than an answer.

I agree that the backward acceleration dip near 0.02s could be due to the rifle moving forward off the sled's rear bracket and then getting pulled back onto the bracket by the strap, which causes the sled to recoil back from the collision. However, there are some arguments for this being a real effect. First of all, I would expect that the spring constant of the Velcro strap is pretty low and I don't think it could snap the rifle back in a few ms. Also the fact the the acceleration keeps ringing, even when the accelerations (and forces) are really small (and there's no question that the velcro should be holding the rifle firmly against the sled bracket) at later times, suggests that the entire sled is actually moving back and forth.

On the other hand, there looks like there's a bit of a discontinuity in the slope of the acceleration for the FWB 124 and LGV right at the start of this negative acceleration dip in Fig. 2.12, so something weird could be happening? I think the LGU acceleration in that figure looks pretty reasonable. Now that I'm looking more carefully, both the LGU and LGV exhibit a small shoulder (maybe even a peak) as the acceleration heads toward the second dip. Maybe we should ask Jim Tyler to share some of his data at longer times? His published traces usually focus on the first ~20 ms of recoil. In terms of practical use of airguns, we don't really care that much what happens after 10 ms, since the pellet has already left the barrel.

All measurements have limitations, but as long as it is clear how the measurement was done and what the limitations are, useful information can be extracted. If nothing else, we can explore fundamental ideas, gain some insights, and figure out how to do things better next time!

Best wishes,

John

 

John,

Thanks so much for your generous and thoughtful reply to my comments.  First of all, I have been inexcusably remiss in failing to adequately compliment you for the ingenuity and elegance of your design and implementation of your inductive sensor and of the data acquisition and reduction hardware and software that process its signal.  Congratulations!

Then let me see if I can respond to some elements of your post.

1. ... Did you use the sled weight or a typical rifle weight when you estimated the forces?

I used a combined mass of 18.1lbs (I hope I copied that correctly from your Chapter 1) for rifle + sled, (15.7lbs for the rifle and 2.4lbs for the sled, so that forces generated by piston acceleration would divide between between rifle and sled in a 15.7:2.4 ratio.  Taking a number for peak compression chamber of 29Mpa from the Tavella paper that Hector kindly linked to (which by the way is entirely consistent with other studies of spring piston thermodynamics), produced the estimate of 3303lbs of force acting on the whole apparatus, hence 3303 / 18.1 = 182g x 2.4 = 438lbs coupled to the sled.

https://airgunwarriors.com/community/airgun-talk/a-scientific-look-into-the-dynamics-of-the-shot-cycle-of-three-spring-piston-airguns/#post-47164

2. ... On the other hand, the small impulses (ma*dt) that these accelerations give to the momentum of the rifle will not strongly affect the velocity or the position of the rifle as functions of time.

Sorry, but I don't follow.  The impulse delivered by piston bounce to the rifle is indeed brief, but in total magnitude is actually larger than the total impulse delivered by the mainspring, since it not only stops but totally reverses the initial recoil motion of the rifle.  Therefore, since dt is small, but ma*dt is large, f = ma is therefore huge.  Accurate measurement of these quantities would therefore seem to be vital to accurate understanding of springer dynamics.  Right?

3. I was curious if I could estimate some of the forces on the sled and I think we can do this for the initial force pretty easily. The spring has a spring constant of around 7 N/mm is compressed about 80 mm, so the initial force of the spring pushing on the piston and the piston pushing back on the rifle is around 560N (125 lbs). This causes the rifle to accelerate backward at 71 m/s^2 (assuming 8 kg rifle plus sled). To get the sled to accelerate with the rifle at this acceleration, the butt of the rifle has to push the 1.1 kg sled with around 78 N (1.1 kg*71 m/s^2) which is around 18 lbs.

Although it may not take into account the amount the mainspring is compressed during assembly (a.k.a., preload) I agree it's a good working ballpark number.

Here you can see the importance of Point 1! Unfortunately, it's much harder to estimate the accelerations later, but we can compare the measured accelerations. The later acceleration peaks are around a factor of three bigger than the initial acceleration dip, but that still puts the forces on the sled at around 50 lbs, which of course is still significant and makes it clear that the rifle needs to be well-secured to the sled. If I could do these measurements over, I would definitely use a stronger attachment system!

Here we must part company.  Both established modelling and measurement of springer ballistics set typical ratios of acceleration due to mainspring vs piston bounce forces at, not ~3:1 but ~30:1.  The Tavella paper cited above is a good example.

4. I saw very different behavior (no oscillations!) when I forgot to tighten the strap sufficiently, so having the rifle move from the sled can be a real problem. However, when I tightened the strap with all my strength, the results were pretty reproducible. Of course, one could argue that the rifle was reproducibly separating from the sled, but I think the separation, if there was any, was pretty small.

Well, John, not to minimize the significance of "all your strength," but how closely would you estimate the force so generated equates with the Tavella-derived estimate I make above of 438lbs of tension in the strap at the peak of piston bounce, strap tension being the only force that, at that instant, opposes the departure of butt pad from vertical stop?

Given this point of divergence between our understanding of the issues involved, I'll defer further comment for the moment, and urge instead that you peruse perhaps in somewhat greater detail the analysis in the Tavella paper, and better reconcile the numbers found therein with your measurements.

I look forward to your observations.

Thanks again for a fascinating discussion!

KR,

Steve


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Hi All, 

I'm the Jim whose recoil measuring rig is in the photograph posted by John. First, thanks for letting me join the forum, and a brief description of my progress in measuring recoil.

I started off in 2013 with the rifle action mounted in a sliding cradle with the magnet attached to the cradle but, try as I might, I could not prevent the action from moving within the cradle, so I devised the rig shown. It comprises a static wooden cradle with an upright at either end, one with a small 'U' for the barrel to slide in, the other a larger U for the cylinder; both are lined with short pile carpet. There is of course some friction between the steel and carpet, but it damps out non-axial vibration which proved a problem with low friction bearings.

I currently have a 400G accelerometer, which I use only for recording accelerations, and much prefer my linear generator to record velocities, which integrate into much more accurate displacement graphs, and which comprises a neodymium rod magnet (set as near the action as possible in a scope mount), coil and oscilloscope.

Here's an example of a UK LGU recoil derived from velocity; it differs hugely from John's largely due to the UK available piston stroke being 88mm, which explains why the 7.87 gn JSB pellet exits (at 818fps) post piston bounce (the red line on the graph). In fact, every pellet I have ever tested in a UK springer exits post piston bounce, and therefore during surge, and I am very grateful to John for opening my eyes to the fact that things are different with much longer piston strokes.

LGU Recoil

As there is some discussion on accelerations, I'll post a graph of the accelerometer data from a .20" HW95. In this, I drilled a tapped an axial hole in the trigger block of the rifle to accept the accelerometer, and tested the bare action (no stock) in my cradle. With 5 mV representing one G, the maximum G at piston bounce is a little over 240. Because of the dangers of trying to cock a bare HW95 action which you cannot brace against your body due to the accelerometer, and mindful of not trapping your fingers between the cocking lever and cylinder, I do not recommend trying this at home. 

Accelerometer

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@jim-in-uk

Dear Jim,

Thanks for contributing to this thread. This forum is greatly enriched by your participation! I looked more carefully at your LGU plot and have realized that there is another important limitation on the system I built, that is probably more important and fundamental than the strap problem. In order to maximize the sensitivity of the pickup coil, I put a lot of turns in the coil. The induced voltage grows with the number of turns, but so does the response time of the system, which is proportional to the inductance (and therefore the number of turns) of the coil. If you look at the first two dips in Jim's plot on the top of the figure below, you'll see that the dips are much sharper and have a bigger curvature (more tightly curved at the bottom, as shown by red "u" in plots). On the other hand, the curvature in the two dips in my data is much lower. The acceleration is proportional to the curvature of the position vs time trace, so clearly my system is recording much lower accelerations. I think that this is due to the slower response time of my system, which averages the signal out over longer times and misses the sharp features that Jim's system measures. Jim told me that he uses a single layer of wire in his coil, so I'm sure that he has far fewer turns and therefore a much faster response time which can catch the sharper wiggles (to use a technical term!) in the recoil response. I should add that my LGU is pretty heavy, over 18 lbs, which would also smooth out the wiggles. If you look at the FWB 124 and LGV plots in Fig. 2.12, you'll see some sharper features, but they probably also have been smoothed out a bit by my slower system.

My data also shows larger amplitude oscillations which could be due to the lower friction of the ball bearing slides on the sled compared to Jim's setup with the rifle sliding on the carpeted cradle, which damps the motion more. Jim, does this sound reasonable?

Fig 2 Tyer comparison LGU

I also noticed that the pellet exit time in my data occurred well before the first dip. I think that this is real and I found Jim's explanation very enlightening and interesting. I assumed that the UK LGUs use the same piston stem length as US LGUs and therefore have the same piston travel. Assuming that the piston bounce disturbs the muzzle orientation, this could be a big advantage of the longer piston travel of US LGUs and makes me wonder why people in the UK don't use the longer piston travel with weaker springs, which would still keep them under 12 ft lbs. Thanks for bringing up this point, which shows that our sled data may have some uses after all!

Again, I'd like to emphasize that Hector and I are not claiming definitive measurements of recoil acceleration, but just wanted to compare the recoil behavior of three springers. I still think the results are useful and hopefully interesting. For future recoil measurements, we should try fewer turns in the coil and attach the magnet directly to the rifle.

Thanks,

John

 


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Dear John,

Many thanks for your kind comments, and I think you’re right about the response time.

Just to give everyone an idea of the problems in gaining precise recoil data, this is my 400G wide band accelerometer mounted low on a cut down scope mount. You might think that this arrangement, with the accelerometer axis some 8.5mm above the cylinder, would be rigid, but you would be wrong when the accelerations of the springer are at work.

Accelerometer scope mount

This is the same graph of my HW95 as in my post above, but with both the trace from the accelerometer mounted direct on the trigger block (blue) and on the scope mount (red). The flex in the mount loses some vibrations from the initial acceleration (slightly reducing apparent recoil displacement), but just look at the traces post piston bounce.

Accelerometer mounting

Integrating the acceleration data once for recoil velocity, and again for recoil displacement shows how misleading data can be if there is any less than absolute rigidity between the rifle and sensor.

Displacement

With the sledge system I would be concerned with the possibility of the recoil pad and the magnet mount compressing minutely, delaying the onset of voltage generation. The light gate would know nothing of this, and faithfully record the passing of the pellet a fraction earlier relative to the compression stroke. Compression stores potential energy, which is later realised as kinetic energy at the onset of piston bounce. 

Why do we in the UK persist with shorter strokes? It’s a long story that goes back to the formative years of field target, and not pertinent to this thread, so maybe some other time.

Best wishes,

Jim

Edit: I forgot to add that methods of recording recoil might fall some way short of absolute precision, but can be very revealing when used for comparative purposes, as you and Hector are doing.


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Posted by: @johnc

@jim-in-uk

... there is another important limitation on the system I built, that is probably more important and fundamental than the strap problem. In order to maximize the sensitivity of the pickup coil, I put a lot of turns in the coil. The induced voltage grows with the number of turns, but so does the response time of the system, which is proportional to the inductance (and therefore the number of turns) of the coil.... I think that this is due to the slower response time of my system, which averages the signal out over longer times and misses the sharp features that Jim's system measures. Jim told me that he uses a single layer of wire in his coil, so I'm sure that he has far fewer turns and therefore a much faster response time which can catch the sharper wiggles (to use a technical term!) in the recoil response. 

John,

I'd like to specifically address your concern about inductance-related response times.

1. The timeconstant t of a inductance L (Henrys) working into a load resistance R (Ohms) is: 

t = L / R

https://www.electronics-tutorials.ws/inductor/lr-circuits.html

 

2. The inductance of an air-core (u = 1) coil is given by:

image

 

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indsol.html

I think if you plug the dimensions and number of turns of your pickup coil (e.g., cm and single-digit 1000s of turns) into this formula, you'll likely get an inductance well below 0.1Hy.  Working into the 1M input impedance of your 'scope therefore predicts a timeconstant less than:

0.1H / 1M = 0.1us

Therefore perhaps not such an important limitation, after all?

I realize I'm getting monotonous, but I really think the issues here lie more with the "timeconstants" of buttpads and velcro than electronics.

Thanks for looking,

Steve


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@steve-in-nc

Hi Steve,

I agree. I should have calculated the response time before posting any speculations. On the millisecond timescale, the sled-rifle system is pretty wobbly and we're only recording the average position of the rifle. I don't think that this creates any new structure in the traces, but simply smooths out the finer structure that happens at shorter timescales. Of course, this finer structure is needed to get the peak accelerations, so it's unfortunate that we're missing it.

Best wishes,

John


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Posted by: @johnc

... I don't think that this creates any new structure in the traces, but simply smooths out the finer structure that happens at shorter timescales...

Hi, John

Unfortunately, I doubt even that conjecture is true.  As I posted earlier in this thread...

https://airgunwarriors.com/community/airgun-talk/a-scientific-look-into-the-dynamics-of-the-shot-cycle-of-three-spring-piston-airguns/#post-47442

image

I find it unlikely that these peculiar artifacts are telling us anything about what's going on inside the guns during the firing cycle. 

Instead, I suspect what's really happening is, in each case, the buttplate is being pulled away from the vertical stop by piston bounce acceleration of the rifle stretching the velcro strap.  Subsequently, the butt slams back into contact with the stop, jolting the sled backward and creating the spikes.

Which therefore have nothing to do with any real events in the actions of the guns per-se, and are in fact (LARGE!) "new structure" created by lack of rigidity in the connection between rifle and sled.


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Alejandro O. Martinez
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Gosh, it never fails to impress-me to-no-end, just-how complex it is to capture-and-measure the dynamic forces involved-with a "simple machine" at work.
It is pure physics ... plain and simple.
To me, it's a wonderment that an engineer can design a relatively-light spring-and-piston air rifle that a person can expect to shoot accurately with regularity.

 


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Posted by: @aom22

Gosh, it never fails to impress-me to-no-end, just-how complex it is to capture-and-measure the dynamic forces involved-with a "simple machine" at work.
It is pure physics ... plain and simple.
And, it's a wonderment that an engineer can design a relatively-light spring-and-piston air rifle that a person can expect to shoot accurately with regularity.

Me too, Alex!  The deceptively simple spring-piston action accomplishes an (almost) miraculous trick by (reasonably) efficiently coupling energy released by the slooow expansion of the (relatively) massive mainspring into the (MUCH) faster velocity of the flea-weight pellet.  The phenomenon that makes possible bridging between these two events separated as they are by an order of magnitude difference in speed and time scale is, of course, our old friend (and scopes' nemesis) piston bounce.


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Well, parts are on order (mainly Amazon), so with luck I hope to have some results from the KISS free-sliding-airgun method sometime in the next few weeks.  I have at least 5 springers (2 Chinese, 1 Spanish, and 2 British) available for initial testing

I intend to use "Audacity" s/w for data acquisition, at least initially.

Here's a (rough) sketch of the pickup I'll be using, that being the only hardware required -- besides a laptop computer with the usual built-in audio, a rug for the gun to slide on, and a nice thick catalog for backstop duty.

image

 


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Posted by: @jim-in-uk This is the same graph of my HW95 as in my post above, but with both the trace from the accelerometer mounted direct on the trigger block (blue) and on the scope mount (red). The flex in the mount loses some vibrations from the initial acceleration (slightly reducing apparent recoil displacement), but just look at the traces post piston bounce.
Accelerometer mounting

Hi Jim,

Brilliant stuff!  Thanks so much for sharing it.

A question:  Would you agree that the second forward recoil spike at ~21ms is created by hard contact of the piston against the end of the compression chamber at the end of its to-fro-to motion cycle, most of the chamber air having been expelled through the transfer port and empty bore (the pellet having exited), leaving too little chamber pressure to stop the piston a second time before impact?

Thanks,

Steve


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@steve-in-nc

Hi Steve,

After looking at Jim's data (thanks Jim for posting and sending me the data!), it looks like there should be two large and sharp acceleration spikes due to the piston going backwards (and/or stopping its forward motion), first at the piston bounce and then at the piston landing at the front of the compression tube. Both of these spikes cause the rifle to move forward. On the other hand, the accelerations of the piston moving forward (and the rifle accelerating backward) are much more gentle. It seems to me that this asymmetry is due to the fact that there are much "harder" media in front of the piston (highly compressed gas at the bounce and the steel cylinder front wall at piston landing) compared to the softer spring behind the piston. Jim, please let us know if this makes sense. Please see below some beautiful data that Jim sent me from his LGV recoil measurements:

Tyler LGV

I agree that our acceleration data misses these features and probably adds artifacts such as the Velcro strap pulling the rifle butt against the sled rear bracket. I think our initial acceleration dip may not be too far off. In the LGV, which is closest to the weight of Jim's test rifles, albeit it has another 2.4 lbs of sled moving with it, the first acceleration dip goes down to around -20g. The rubber buttpad will decrease the measured acceleration, but I've tried measurements with the LGU's steel buttplate hardware pushing directly against the steel rear sled bracket and haven't seen much of a difference compared to the rubber buttpad pushing on the bracket. If I'm reading Jim's acceleration plot (posted on May 9) correctly, he sees about 40g's in the initial acceleration maximum (I think he's using positive acceleration to indicate the rifle is moving backward, please correct me if I'm wrong here) and then -240g's at the piston bounce and similar value at the piston landing.

My earlier point discussed the smoothing out of features in the velocity and position traces. I wasn't claiming that our acceleration plots were simply smoothed out versions of the actual acceleration. The velocity and position traces look qualitatively (and even quantitatively, in terms of magnitudes) similar to Jim's results, but the the fine structure in the velocity trace is really critical in determining the acceleration and our velocity data are clearly missing that structure (and may have some extra kinks due to artifacts such as the rifle bouncing in the sled). I think the first 10 ms of data, when the rifle is pushing back on the sled, are pretty reasonable, but we're really missing the rifle's forward acceleration at the piston bounce and piston landing at later times. I think the smaller acceleration oscillations at later times may be real, since the forces are much smaller and the Velcro strap is probably strong enough to handle those forces. Fortunately, the pellet is out of the bore before the piston bounce, so in terms of accuracy tuning, maybe the first 10 ms are the most critical?

I'm glad to see that you're building your own setup. I'm sure that you've already thought of this, but please be aware that pc sound cards are ac-coupled, so you won't be able to use the calibration technique that I described in Ch. 1.

Best wishes,

John

 


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"Fortunately, the pellet is out of the bore before the piston bounce,"

John, on what do you base this statement?  I don't think it's true.

Please note Jim's comment: "In fact, every pellet I have ever tested in a UK springer exits post piston bounce,"

Also please consider the following elementary argument:.

1. The pressure acting on the base of the pellet can never be greater (in fact due to the flow resistance of the transfer port, it must in fact be at least somewhat less) than the pressure acting on the face of the piston.

2. Assuming ~30% springer energy efficiency, while the pressure decelerating the piston toward the instant of bounce is absorbing (at most) ~70% of mainspring energy, similar pressure accelerating the pellet is delivering ~30% of the same mainspring energy to the pellet. 30/70 = 0.43.  Therefore, if the pellet is assumed to exit the bore no later than the instant when the piston stops, the average force accelerating the pellet down the bore must at least 43% of the average force decelerating the piston.

3. Force = pressure x area.   Therefore the ratio of the area of the base of the pellet to that of the piston face would have to be at least 43%.

4. Typical piston face area is ~0.78in^2, but the base area of a .177 pellet is only 0.025in^2.

5. Calculation of the ratio 0.025:0.78 is left as an exercise to the reader.

6. Do you still think the pellet can exit prior to bounce?


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Posted by: @steve-in-nc
Posted by: @jim-in-uk This is the same graph of my HW95 as in my post above, but with both the trace from the accelerometer mounted direct on the trigger block (blue) and on the scope mount (red). The flex in the mount loses some vibrations from the initial acceleration (slightly reducing apparent recoil displacement), but just look at the traces post piston bounce.
Accelerometer mounting

Hi Jim,

Brilliant stuff!  Thanks so much for sharing it.

A question:  Would you agree that the second forward recoil spike at ~21ms is created by hard contact of the piston against the end of the compression chamber at the end of its to-fro-to motion cycle, most of the chamber air having been expelled through the transfer port and empty bore (the pellet having exited), leaving too little chamber pressure to stop the piston a second time before impact?

Thanks,

Steve

In that instance, Steve - yes. 

In other cases, though, there's enough air left to cause a second piston bounce.

The mainspring can also be throwing its mass backwards and forwards enough to possibly emulate a piston bounce. 

 


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Posted by: @steve-in-nc

"Fortunately, the pellet is out of the bore before the piston bounce,"

John, on what do you base this statement?  I don't think it's true.

Please note Jim's comment: "In fact, every pellet I have ever tested in a UK springer exits post piston bounce,"

Also please consider the following elementary argument:.

1. The pressure acting on the base of the pellet can never be greater (in fact due to the flow resistance of the transfer port, it must in fact be at least somewhat less) than the pressure acting on the face of the piston.

2. Assuming ~30% springer energy efficiency, while the pressure decelerating the piston toward the instant of bounce is absorbing (at most) ~70% of mainspring energy, similar pressure accelerating the pellet is delivering ~30% of the same mainspring energy to the pellet. 30/70 = 0.43.  Therefore, if the pellet is assumed to exit the bore no later than the instant when the piston stops, the average force accelerating the pellet down the bore must at least 43% of the average force decelerating the piston.

3. Force = pressure x area.   Therefore the ratio of the area of the base of the pellet to that of the piston face would have to be at least 43%.

4. Typical piston face area is ~0.78in^2, but the base area of a .177 pellet is only 0.025in^2.

5. Calculation of the ratio 0.025:0.78 is left as an exercise to the reader.

6. Do you still think the pellet can exit prior to bounce?

Steve, pellet exit is determined, all other things being equal, by the length of the compression stroke, shorter strokes push pellet exit further into surge (piston bounce), longer strokes advance pellet exit. 

All the rifles I test are UK spec. The UK LGU available stroke is 88mm, the LGV 90mm, so pellet exit is post bounce - but not a lot. The longer strokes of US spec rifles can indeed see pellet exit precede bounce.

To add a bit of detail, in my TX200 with 85mm of stroke, the piston has 13mm of available stroke left as the AADF pellet starts to move, but in John's LGU, I believe the stroke is 130mm (please correct me if I'm wrong), so there's a tad under 20mm of stroke. 

The LGU piston is decelerating over a greater distance, and the pellet only needs a couple of milliseconds and a bit to get clear of the muzzle.


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@johnc "It seems to me that this asymmetry is due to the fact that there are much "harder" media in front of the piston (highly compressed gas at the bounce and the steel cylinder front wall at piston landing) compared to the softer spring behind the piston. Jim, please let us know if this makes sense."

In a word, yes, John.

The highly compressed air in front of the piston at the end of the compression stroke is highly energetic, manifest by its highly elevated temperature (itself a measure of the air's internal kinetic energy). The piston has run out of momentum, and the spring is down to its preload plus a mm or two of potential energy - a fraction of a foot pound. 

The air wins. 


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Thanks as always, Jim, for the expert commentary.

One caveat:   Please be careful not to confuse folks by inadvertently conflating "pellet start" (when breech pressure first becomes high enough to create the force required to overcome obturation, static friction, etc.) with "pellet exit" (when breech pressure becomes irrelevant to MV). 

I'm totally confident that you're using these (and other) terms absolutely correctly, but when they appear in the same post, as here, some folks might lose sight of the difference.  Especially if they want to.


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