A scientific look i...
 
Notifications
Clear all

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

Page 1 / 3

Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

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


wx4p thanked
Quote
DavidEnoch
(@davidenoch)
Joined: 4 years ago
Posts: 474
 

Hector, I look forward to reading about this research in your blog.  I wish you could test multiple transfer port sizes in one gun.  I think that would be insightful.  We need some new research to update what Cardew did years ago.

David Enoch


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

@davidenoch

Thanks, David.

Sadly, that is something that cannot be done easily. AND it has been done. What we will show is that the TP geometry is NOT THAT important if other factors are taken into account.

The Cardews' work was exceptional for its time, but it is, now, dated. AND, more importantly IMHO, is that we will provide anyone that's reasonably handy with the tools and knowledge to create a machine to measure the vibrations in THEIR rifles

This will be an experiment in "Crowd-researching" that HOPEFULLy, will de-bunk some "theories" ("truths"?) that have been accepted and prevalent since the old days and that have hampered the real development of the spring-piston airgun.

 

So, stay tuned . . .    😉


ReplyQuote
pruitt
(@pruitt)
Joined: 2 years ago
Posts: 7
 

This is great information! thanks for sharing.


ReplyQuote
DavidEnoch
(@davidenoch)
Joined: 4 years ago
Posts: 474
 

Hector,

I look forward to the research.  I know Tom Gaylord played with a set of changeable transfer ports at one time.  I don't think he had a mechanism to measure movement though.

One thing that has puzzled me is that occasionally I come across a gun that jumps vertically when fired.  I don't think it is a rotational movement but that is the only thing I can think of that would cause a springer to bounce vertically.

David Enoch


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

David;

Most modern spring-piston airguns have a "jump" or "upwards flip" as the Brits call it.

It obeys the fact that the pistons are  moving forwad, the rifle is moving backwards,  but it is supported by the shoulder at a point where that reaction is ABOVE and offset vertically, so that the gun seems to "jump".

If you have ever shot a Baikal IZH 60 or the copy, or the SPA SLR900, you will note the real absence of this "flip". That is because the piston runs in the stock, from the buttpad to the pistol grip. SO the support point is ABOVE the reaction of the gun.

To the shooter it is a flip, but in reality is a rotation around a point that is relatively far away from the origin of the reaction.

😉

Keep well and shoot straight!

 

 

 

 

HM


ReplyQuote
straitflite
(@straitflite)
Joined: 3 years ago
Posts: 529
 
Posted by: @hector-j-medina-g

What we will show is that the TP geometry is NOT THAT important if other factors are taken into account.

The Cardews' work was exceptional for its time, but it is, now, dated

I have read Cardews cover to cover and I believe the only thing that is dated are the tools and technology. Same could be said for Einstein's theory of relativity; Given proper context. But dated? That is an EXTREMELY strong statement -given your own context. Same for TP geometry. Are we to believe that Bernoulli's principle no longer holds water as well?

Hector, you are more knowledgeable than I in airgun related terms so my hat is off to you sir. Your efforts are to BE applauded and I will enjoy following along....seriously! Nothing comes without science and its mathematics and using a "thing" in a controlled test also constitutes experience. I personally would love to see someone reinvent the wheel. That would be a pretty cool thing to see....

Best wishes in your endeavors 😀 😀 

Bo

 


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

@straitflite

Bo;

It's not a question of who's "right" or "wrong". There can be more or less precise measurements. Sometimes, as in quantum mechanics, the simple FACT of taking a measurement, affects the outcome ("uncertainty principle").

In airguns things happen so fast, that the human senses have NO WAY to really unravel the whole picture, so SOME FORM of EASILY ACHIEVABLE instrumentation is important to the furthering of airguns themselves.

And AAMOF, I am NOT saying that our measurements are absolute and exact. On the contrary (and we can talk about this later, if you want).

What I am ASKING of everyone that is truly interested in airguns is to build the instrument, and test his/her guns. Even if they do not follow the calibration procedure that John came up with, that is amazingly simple to follow, just the fact that ANY "home tinkerer" can now COMPARE in very effective terms two different rifles in a non-subjective manner is a huge step forward

My HOPE is that by "Popular Mechanic-izing" the INSTRUMENT, we can all start talking in concrete terms about things that, so far, have been only subjective concepts, like "smoothness of shot cycle", "efficiency of the compression phase", "efficiency of the barrel", and even "pellet dwell time".

In the same way that the advent of, relatively inexpensive, Chronographs on the late '80's  came to dispel a LOT of "MYTHS" surrounding the reloading, archery, and airgun world, I do hope that someday, we can all be talking in concrete terms about the firing characteristics of our guns, instead of the old "it kills deader than dead" of the old timers when they were talking about their favorite deer cartridge, or "it is a tack driver with a flat trajectory" of the target shooters. My other favourite one was the "this one bucks the wind, like it's not there" .

After the chronos became available, a lot of those expressions simply fell into disuse, and more solid approached could be followed.

Thanks for the kinds words, I do realized we ALL learn here, nobody is born with innate knowledge, nor with infallibility.

We, together, will drive the future with concrete evidences of what is really happening inside.

😉

 

Keep well and shoot straight!

 

 

 

 

 

 

 

HM

 

 

 

 


ReplyQuote
straitflite
(@straitflite)
Joined: 3 years ago
Posts: 529
 

Gotcha. The excitement in your words and contributions to this sport are nothing short of well, exciting!

I agree, since the dawn of the internet ambiguous and arbitrary words/terms are/have been thrown about like snowflakes in the wind...guilty here LOL

I'm glad you did not interpret my response in a totally negative way. Large rocks that have been sitting for years can be hard to move. Who's to say who can move them?

With that, I'll shut up and listen since I have nothing to contribute -except that my R7 is "smooth as butter" 🙂 

Wish you well,

Bo

 

 


ReplyQuote
chrisT
(@christ)
Joined: 4 years ago
Posts: 43
 

for a springer


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
Posted by: @hector-j-medina-g

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

Hector, I'm very much looking forward to your treatment of this complex topic, and in particular to learning some of the details of your "INSTRUMENT!"

 

Thanks in advance, Steve


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

First Chapter has been posted, where we look at Instruments we can use and instruments we can make to understand the dynamics of the spring-piston airgun:

https://www.ctcustomairguns.com/hectors-airgun-blog/shot-cycle-dynamics-in-3-spring-piston-airguns-chap-1

Hope you enjoy!

 

 

HM


TomR_here thanked
ReplyQuote
Bigbore
(@bigbore)
Joined: 4 years ago
Posts: 173
 
Posted by: @hector-j-medina-g

 

 AND, more importantly IMHO, is that we will provide anyone that's reasonably handy with the tools and knowledge to create a machine to measure the vibrations in THEIR rifles

This will be an experiment in "Crowd-researching" that HOPEFULLy, will de-bunk some "theories" ("truths"?) that have been accepted and prevalent since the old days and that have hampered the real development of the spring-piston airgun.

 

So, stay tuned . . .    😉

 I'm looking forward to this portion of the series. I made an instrument with an Arduino and a GY521 accelerometer that can be rubber-banded to a gun to test vibration in 3 axes. I can get 4 millisecond readings in its current configuration.

 

 Below is one shot cycle from a .22 rimfire. The axes that should show the most movement do, so it looks like it is performing as it should. I want to get the data point times lower but I haven't fooled with it for quite a while.

 

[img] [/img]

 

[img] [/img]


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
Posted by: @bigbore
...I made an instrument with an Arduino and a GY521 accelerometer...

Nice packages, but I think you'll find the 16g fullscale limit of that accelerometer isn't adequate (like by a factor of 10x) to cope with legendary (scope-busting) springer recoil.


ReplyQuote
Bigbore
(@bigbore)
Joined: 4 years ago
Posts: 173
 
Posted by: @steve-in-nc
Posted by: @bigbore
...I made an instrument with an Arduino and a GY521 accelerometer...

Nice packages, but I think you'll find the 16g fullscale limit of that accelerometer isn't adequate (like by a factor of 10x) to cope with legendary (scope-busting) springer recoil.

 Although I didn't consider that, this is one of two that are widely available and cheap. I'll try it and see what shows. It won't show the entire amplitude of the pulse but I'm curious to see what it does show. If it breaks, eh. I put it together for rimfire but I'll give it a whirl.

 I'm curious to see Hector's solution. Arduino sensors are dirt cheap and the code has been written for almost any conceivable package and configuration--or slight modification to existing code-- so that was my starting point.  


ReplyQuote
DavidEnoch
(@davidenoch)
Joined: 4 years ago
Posts: 474
 

Hector,

Will you be providing any better plans for your device?  Maybe someone can draw it up in  Sketch-up.  I am willing to try to make a devise and check my airguns on it if I understand how to build it.

Thanks,

David Enoch


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

@ Bigbore.- VERY interesting. Problem with airguns is that things happen REAL fast. 4 msecs is about 1/4 of the WHOLE shot cycle  (taking out the residual spring reverberations). So, apart from the saturation level of the g's, the time resolution is simply not there.

You also have to remember that those of us that were born before the 80's IN GENERAL, cannot "code". For US, it is much easier to rely on a commercially produced, fast-acting, inexpensive, instrument (the pocket oscilloscope), than to try to learn how to code in any of the 1,000's of "languages" that exist (I used to write really neat Fortran IV programs, but I cannot begin to unravel Android, ROFL!).

In THIS case, the instrument of "capture" is the oscilloscope and data is transferred to the computer for analysis and presentation. And, as demonstrated by the experiments done for the AC/DC coupling, the best, most uniform way is to keep all inputs in the oscilloscope, as there are channels enough to do it.

 

@ Steve in NC.- One of the limitations of using the mathematical approach to obtain acceleration and displacement is that, in effect, you are "flattening" the spikes. One of the things we tried was to reconcile the data obtained from these "rudimentary" experiments with experiments performed with "proper and professional" accelerometers of the correct response speed and range of detection and the best we could do was a factor of 2.

We may talk about that separately, when we get to that point, but one of the conclusions we arrived to, in conjunction with a trained and respected industrial vibrations expert that does this for a living, is that scopes are affected NOT by the huge recoils of the spring-piston airgun, but by the residual reverberations of the spring that has enough harmonic frequencies as to shake the erector out of zero, or shake the reticles out of place (think Tacoma Narrows bridge). ONCE things are loose, yes, the recoils have the time and the space to throw around the parts and destroy them. But it is a "jab-punch" process that takes more than one shot to complete.

 

@ DavidEnoch.-  I do believe there is enough information in Steve's (NitroCrushr) drawings to build the device. If you have specific questions, let us know and we can address them. Many dimensions are there just because that is where Steve started, but if you take into account that airguns will not "see" displacements greater than ¾" then you can modify whatever material you have on hand to accept the holding of the rifle on the top end, and the displacement on the lower end. Apart from those two numbers, you can get creative. Using light materials would also be a step forward because, as discussed, the WEIGHT of the moving portion is added to the weight of the rifle and so, to SOME degree the measurements are not as exact as they COULD be.

Now, if you plan to extend the use of the sled to a 0.577" NE double rifle, then you will need more displacement capacity, LOL!  😉

 

Thanks to everyone for their kind words and for reading!

 

 

 

 

HM


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
Posted by: @hector-j-medina-g

@ Steve in NC.- One of the limitations of using the mathematical approach to obtain acceleration and displacement is that, in effect, you are "flattening" the spikes. One of the things we tried was to reconcile the data obtained from these "rudimentary" experiments with experiments performed with "proper and professional" accelerometers of the correct response speed and range of detection and the best we could do was a factor of 2.

Yes, Hector.  I noticed that the peak forward acceleration plotted in your fig. 1.6b is a bit less than 150m/s^2 = ~15g.

A little arithmetic shows this is less than 1/2 of the ~39g acceleration expected from the ~700 pounds of force that would result from your estimate of ~900psi of peak pressure acting on the ~0.8in^2 area of the face of a typical 1" diameter compression chamber, accelerating 18.1lbs of rifle+sled.

So it would seem that the results of this study may be more qualitative than quantitative.


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  
Posted by: @steve-in-nc
Posted by: @hector-j-medina-g

@ Steve in NC.- One of the limitations of using the mathematical approach to obtain acceleration and displacement is that, in effect, you are "flattening" the spikes. One of the things we tried was to reconcile the data obtained from these "rudimentary" experiments with experiments performed with "proper and professional" accelerometers of the correct response speed and range of detection and the best we could do was a factor of 2.

Yes, Hector.  I noticed that the peak forward acceleration plotted in your fig. 1.6b is a bit less than 150m/s^2 = ~15g.

A little arithmetic shows this is less than 1/2 of the ~39g acceleration expected from the ~700 pounds of force that would result from your estimate of ~900psi of peak pressure acting on the ~0.8in^2 area of the face of a typical 1" diameter compression chamber, accelerating 18.1lbs of rifle+sled.

So it would seem that the results of this study may be more qualitative than quantitative.

In a way, Steve, you are right.

And in the same way, MANY things in ballistics are more "informative" when used in COMPARISONS.

For example, if you take the field of forensics, the reality is that things like bullet expansion, rifling marks and primer indentations are ONLY Useful when comparing VERY CLOSELY MATCHED cases.

The other aspect is that working with pressures and areas is looking at the peaks of spikes. And if spikes are really fast, then the equivalent "smoothed"  function will be much lower.

As far as scopes are concerned, may modern manufacturers are testing their scopes to 3 and 5k g's in both directions, so we KNOW that the impact/recoil is not what is still killing/making scopes jump zeroes.

From the purely epistemological (Differentiating between justified belief and opinion) POV, we are also not too concerned, on this occasion, about WHY/HOW things happen; that is why these articles deal with the "dynamics" of the shot cycle, not the kinematics of the shot cycle. The MAIN interest here is to learn WHEN things happen because, after all, whatever happens after the pellet has exited the barrel, can really have no effect in the pellet, or its trajectory.

One more note: the 900 PSI's are not our estimate, those numbers come from other scholarly work already published by Dr. Tavella.

Keep well and shoot straight!

 

 

 

 

 

HM

 


ReplyQuote
JiminPGH
(@jiminpgh)
Joined: 3 years ago
Posts: 610
 

Fascinating stuff, from the both of you. Tough to pick a favorite in this race, since it is shaping up to be one between two of my favorites. Keep at it, guys, and keep each other honest. We, the airgun-shooting public will come out winners.

Sincerely, I am thankful for this sort of analysis, even though I may only understand half of it. But I do understand the difference between theoretical analysis and practical. In my opinion, neither one would be any good without the other.


ReplyQuote
Bigbore
(@bigbore)
Joined: 4 years ago
Posts: 173
 
Posted by: @steve-in-nc
Posted by: @bigbore
...I made an instrument with an Arduino and a GY521 accelerometer...

Nice packages, but I think you'll find the 16g fullscale limit of that accelerometer isn't adequate (like by a factor of 10x) to cope with legendary (scope-busting) springer recoil.

Steve, MANY years ago there was another thread showing accelerometer results from a springer, I was thinking of that thread when I concocted mine. I would bet you have it somewhere on your hard drive, No?


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 

 

 

 

Steve, MANY years ago there was another thread showing accelerometer results from a springer, I was thinking of that thread when I concocted mine. I would bet you have it somewhere on your hard drive, No?

 

Sadly, you'd lose that bet, just as I lost my implicit one that the Yellow, with its seemingly bottomless treasure trove of cross-linked airgun knowledge and expertise, would go on forever.


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
Posted by: @hector-j-medina-g
Posted by: @steve-in-nc
Posted by: @hector-j-medina-g

@ Steve in NC.- One of the limitations of using the mathematical approach to obtain acceleration and displacement is that, in effect, you are "flattening" the spikes. One of the things we tried was to reconcile the data obtained from these "rudimentary" experiments with experiments performed with "proper and professional" accelerometers of the correct response speed and range of detection and the best we could do was a factor of 2.

Yes, Hector.  I noticed that the peak forward acceleration plotted in your fig. 1.6b is a bit less than 150m/s^2 = ~15g.

A little arithmetic shows this is less than 1/2 of the ~39g acceleration expected from the ~700 pounds of force that would result from your estimate of ~900psi of peak pressure acting on the ~0.8in^2 area of the face of a typical 1" diameter compression chamber, accelerating 18.1lbs of rifle+sled.

So it would seem that the results of this study may be more qualitative than quantitative.

In a way, Steve, you are right.

And in the same way, MANY things in ballistics are more "informative" when used in COMPARISONS.

For example, if you take the field of forensics, the reality is that things like bullet expansion, rifling marks and primer indentations are ONLY Useful when comparing VERY CLOSELY MATCHED cases.

The other aspect is that working with pressures and areas is looking at the peaks of spikes. And if spikes are really fast, then the equivalent "smoothed"  function will be much lower.

As far as scopes are concerned, may modern manufacturers are testing their scopes to 3 and 5k g's in both directions, so we KNOW that the impact/recoil is not what is still killing/making scopes jump zeroes.

From the purely epistemological (Differentiating between justified belief and opinion) POV, we are also not too concerned, on this occasion, about WHY/HOW things happen; that is why these articles deal with the "dynamics" of the shot cycle, not the kinematics of the shot cycle. The MAIN interest here is to learn WHEN things happen because, after all, whatever happens after the pellet has exited the barrel, can really have no effect in the pellet, or its trajectory.

One more note: the 900 PSI's are not our estimate, those numbers come from other scholarly work already published by Dr. Tavella.

Keep well and shoot straight!

 

 

 

 

 

HM

 

Thanks for the link to the Tavella paper.   I thought this paragraph was particularly interesting.

Peak temperatures and pressures occur during pellet travel in the bore and are significantly higher than the corresponding values at bounce time. The highest chamber pressure and temperature are 29,248 kPa and 1452 K, respectively,
 
I notice that 29MPa = 290bar = 4205psi = 3303lbs of force against the face of a 1" chamber = 182g of reverse recoil if Tavella's modelling were applied to your 18.1lb INSTRUMENT.  Would a factor of 10 error in recorded acceleration still qualify as "forensic" accuracy?
 
 

ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

My dear friends;

 

I've done BOTH ("Proper" equipment tests and our "rudimentary") tests:

I started doing tests with EXTREMELY professional grade equipment with professional grade equipment operator (not me) on some rifles:

HW98:

image

 

DIANA 56:

image

 

LGV:

image

And others (HW95, ASP, DIANA 48, Trail XP, etc.) 

We even went to the length of purchasing a faster response, higher limit "g's" accelerometer to review the whole thing because the initial one, just didn't "cut it".

This was the first effort:

image

 As you can see, at a sampling rate of 2 ms, the time response was completely inadequate.

The "seemingly simple" equipment above displayed (industrial grade computer, accelerometer, and software) is worth well in excess of 15k $

The person operating the equipment, normally charges $350 per hour on industrial projects, working on machines and installations that are worth a BUNCH of money.

In brief, believe me when I tell you that we both know what we're doing.

Results are interesting (note that the time scale is now 0.001 sec with sampling rates of 2 kHz), and that the output is in "g's" directly

The ASP shot cycle (g's have not changed dramatically with the sample rate, but the detail of the curve is now dramatically better):

image

 The 56 shot cycle:

image

 A tuned 54:

image

And, yes, some solid numbers were obtained (do note that the "real" g's are well in excess of 10 times the calculated g's from the pressure/area equation):

image

BUT, after MUCH analysis, the conclusions were  not completely satisfactory (sorry that the labels are in spanish):

The 56 shot cycle with our annotations:

image

The tuned 54 shot cycle with our annotations:

image

Labels mean:

Gatillo Rompe.- Sear breaks

Primer Golpe.- First piston strike

Segundo Golpe.- Second piston strike

Salida del diábolo.- Pellet exits

Tercer Golpe.- Third piston strike

Muerte del Twang.- Death of the Twang

 

As you can see, our (WRONG)  conclusion was that the pellet had to exit after the Second piston strike because at that point, the chances were much higher of exhibiting the accuracy we had been observing from the two most accurate of the guns tested.

And yet, John's experiments proved us wrong.

So, with that background, why go to the problems and time and effort that took us where we are now?

Because 

1.- Even VERY expensive equipment cannot clarify for us some CRITICAL aspects that are exclusive to airgunning

2.- We do not own every airgun in the world

3.- We are humans and we cannot see all the angles, analyze all the issues/points, raise all the questions.

HOWEVER, as a community, we CAN, and we SHOULD

BUT I cannot even begin to wish the airgunners in the community invest 10-19 k$ in equipment for testing airguns.

Expecting SOME to spend $150, yeah! THAT is doable!

It may not be as precise/exact, it may not be as "refined" but, in the long run, the whole dynamograph concept gives us a MUCH better "view" of what TRULY matters.

We now know exactly when the pellet exits the barrel in the shot cycle, now we know WHERE to look into to improve the accuracy of the shot cycle.

Now we know what is important to the gun, and what is important to the shooter.

Now, we can address those two aspects separately, with different tools and different approaches (materials, technology, design, etc).

 

And, IMHO, THAT is a goal worth pursuing.

 

Hope this clarifies the intent.

 

Keep well and shoot straight!

 

 

 

 

 

HM


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
Posted by: @hector-j-medina-g
 
image

 The 56 shot cycle:

Hector, I applaud your obvious dedication and persistence to this topic, especially in the face of such daunting instrumental artifacts as the (self-evidently bogus) forward acceleration recorded in the blue rectangle of the figure "56 shot cycle" of ~70g for at least 40ms which, if real, would have sent the entire apparatus flying away at 0.04 x 70 x 32ft/sec^2 = 61 miles per hour!

I assume no such dramatic departure was observed.


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

ROFL!

No, no "liftoff" was present. In part because these set of experiments were done trying to mimic the behaviour of the rifle in the hands of a shooter, and that included simulating the weight a grown man's torso/arms at about 30# of lead in the sled (a lead sled type of gun rest/vise with metal tips running on smooth concrete floor).

And yes, that shows that even the best of instruments/operators/applications can yield "bogus" info.

MOST tests were done multiple times to try to establish repeatability, and most of the time, there was a solid repeatability in the large numbers (like the peak accelerations), but there were some aspects that proved elusive.

Well spotted!

Keep well and shoot straight!

 

 

 

 

 

HM


ReplyQuote
Chuck Howard
(@chuck-howard)
Joined: 8 months ago
Posts: 8
 

Hector, I applaud your work and look forward to subsequent updates. Thanks for sharing.


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

Chapter 2 has been posted:

https://www.ctcustomairguns.com/hectors-airgun-blog/shot-cycle-dynamics-in-3-spring-piston-airguns-chap-2

Hope you enjoy!

 

HM


bf1956 and TomR_here thanked
ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 

Friend Hector,

I wonder if you would kindly indulge three more questions about the construction of the INSTRUMENT, regarding in particular how the rifle is attached to the sled. 

In the first Chapter it's said...

"Figure 1.2 shows a photo of an air rifle mounted on the sled. The rifle is supported on the sled by u-shaped vinyl-coated hooks (labelled 1 in Fig 1.2) under the forearm and under the stock near the buttplate. A long hook and loop strap (labelled 2 in Fig 1.2) is wrapped around the pistol grip or through the back of the trigger guard to pull the rifle tight against a vertical stop at the back of the sled."

This seems to say that rearward (i.e., toward the butt) acceleration of the rifle is coupled to the sled only by the buttplate recoil pad pushing against the vertical stop, and forward acceleration (e.g., toward the muzzle) only by the hook and loop strap.  Is that in fact how the INSTRUMENT is configured?  Or are there additional (and far more rigid) provisions for coupling the motion of the rifle to the sled, compared to which the strap and stop are only incidental?

Otherwise, how are you preventing the elasticity of strap and buttplate from significantly (or even drastically) absorbing and attenuating the large forces of acceleration, and distorting the millisec timescale and millimeter linear displacements of those forces, that must be accurately communicated between rifle and sled if recorded data are to accurately reflect the dynamics of springer interior ballistics?

I suspect this may explain why your measured forward recoil g-force is 10x less than expected, as well as other equally implausible features of the recorded "data."

Thanks in advance,

Steve


bf1956 thanked
ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
Posted by: @hector-j-medina-g

Chapter 2 has been posted:

https://www.ctcustomairguns.com/hectors-airgun-blog/shot-cycle-dynamics-in-3-spring-piston-airguns-chap-2

Hope you enjoy!

 

HM

Thank you, Hector.  Particularly interesting, I think, are the large secondary backward acceleration peaks recorded at ~0.02sec in all three data sets in fig. 2.11 that are as large (in two cases actually larger) than the initial backward motion.  Unfortunately, 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 the forward force of piston bounce.  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.

Sorry to say, but I truly think a significant redesign of your instrumentation will be needed before the data it's producing will be meaningful.


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

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


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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.


ReplyQuote
nced
 nced
(@nced)
Joined: 1 year ago
Posts: 119
 

@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!


Steve in NC thanked
ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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.  😎 


ReplyQuote
nced
 nced
(@nced)
Joined: 1 year ago
Posts: 119
 

@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...........

 

 

 


ReplyQuote
DavidEnoch
(@davidenoch)
Joined: 4 years ago
Posts: 474
 

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


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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!


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 

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

 

TEMP

ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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.


ReplyQuote
scratchit
(@scratchit)
Joined: 3 years ago
Posts: 195
 

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

 

Grabbity sux.


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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


ReplyQuote
JohnC
(@johnc)
Joined: 3 years ago
Posts: 25
 

@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

 


ReplyQuote
Hector J Medina G
(@hector-j-medina-g)
Member of Trade
Joined: 4 years ago
Posts: 710
Topic starter  

@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


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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


ReplyQuote
Jim in UK
(@jim-in-uk)
Joined: 3 months ago
Posts: 72
 

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

Citizen K thanked
ReplyQuote
JohnC
(@johnc)
Joined: 3 years ago
Posts: 25
 

@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

 


ReplyQuote
Jim in UK
(@jim-in-uk)
Joined: 3 months ago
Posts: 72
 

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.


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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


ReplyQuote
JohnC
(@johnc)
Joined: 3 years ago
Posts: 25
 

@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


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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.


ReplyQuote
Alejandro O. Martinez
(@aom22)
Joined: 4 years ago
Posts: 205
 

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.

 


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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.


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 

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

 


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 
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


ReplyQuote
JohnC
(@johnc)
Joined: 3 years ago
Posts: 25
 

@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

 


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 

"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?


ReplyQuote
Jim in UK
(@jim-in-uk)
Joined: 3 months ago
Posts: 72
 
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. 

 


ReplyQuote
Jim in UK
(@jim-in-uk)
Joined: 3 months ago
Posts: 72
 
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.


ReplyQuote
Jim in UK
(@jim-in-uk)
Joined: 3 months ago
Posts: 72
 

@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. 


ReplyQuote
Steve in NC
(@steve-in-nc)
Dealer
Joined: 4 years ago
Posts: 570
 

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.


ReplyQuote
Page 1 / 3