Sonic Choke? Maybe not. 2162 FPS

[Buldawg76]

Bob

Thanks for confirming what I felt I knew to be true but just don't have the math background to be able to prove it on paper in theory. I have however in 45 years of repairing air, hydraulic, electrical, mechanical and all other systems used in todays ultra high tech vehicles gained a wealth of real world knowledge and what Steve shows and states just does not make sense much less even plausible.

Yours and Lloyds explanations and numbers to me are spot on as to what and how thing happen and occur in a HPA gun when fired be it a PCP or Pumper.

Just simple plain common sense if you ask me but then I guess there are some that never opened that door to absorb that common sense when growing up.

Mike

[lloyd-ss]

Bob and Mike,
Thanks for stirring the pot again, and making the big chunks float to the top.   

I certainly have a hard time believing such things on faith, and require some empirical evidence to back up statements that don't seem intuitive.  My on collected data goes contrary to any sort of hard limits on velocity.  Here is one example from data that was collected from shots taken back in January. Yes, it is a rehash of previous data, but presented in a slightly different manner.  The purpose is to show that continuing to add HPA to the barrel as the pellet/projectile travels through the barrel, despite the rapidly diminishing returns,  will always add some additional velocity to the pellet.  There might be a point where the velocity ceases to increase, but I have yet to see evidence of that happening.

Although no conventional valve was used, here is evidence to demonstrate the continuing increase in velocity.  This consists of data from several shots that has been averaged and normalized to yield a comparison between a low volume shot (representing an early valve closure) and a high volume dump shot (representing a late valve closure).

SET-UP INFORMATION-
Constants between the 2 shots:
.22 cal, 23.3" barrel, 10.2 gn projectile, 2100 psi, dump shot.
Variables between the 2 shots:
Shot 1 = 6.2cc dump reservoir (volume equivalent of 43% of the barrel length)
Shot 2 = 14.5cc dump reservoir (volume equivalent of 100% of the barrel length)

DATA-
Shot 1, 2100 psi,  6.2 cc dump shot, .22 cal 10.2 gn, 1183 fps
  Calculated residual muzzle pressure 629 psi

Shot 2, 2100 psi, 14.5 cc dump shot, .22 cal 10.2 gn, 1300 fps 
  Calculated residual muzzle pressure 1050 psi

DISCUSSION-
As shown in the graph from a few posts before this one, another discussion had suggested that when the MV was in the 1200fps range, that additional HPA introduced into the barrel breech after the projectile was more that 15% of the distance down the barrel (3.5") would not increase the MV.  This "ineffectiveness of additional HPA" was extrapolated over a broad velocity range, and this is only an example of data from the 1200fps range.
These two example shots seem to show that additional HPA will increase the MV, no matter how late in the shot cycle it is introduced.  Both of the example shots are at or above the mentioned 1200fps MV, yet, the second shot, with the additional HPA, is 117fps faster (9.9%) than first shot.    Despite the diminishing returns (the first shot used only 43% as much HPA as the 2nd shot),  the increase in velocity of almost 10% is significant, and certainly beyond the margin of error for the experiment.   
Therefore, I suggest that as of yet, any upper limits on velocity increases to be gained from increased valve dwell have yet to be proven.

BACKUP INFORMATION-
Shot 1 is the average velocity of 4 shots (1203, 1175, 1182, 1171), all taken at 2100 psi, 10.3 gn, 23.3" barrel, .22 cal, 6.2cc dump.
Shot 2 is normalized to 2100 psi and 10.3 gn from these 2 shots:
shot 2a- 1800 psi, 10.2gn, 23.3" barrel, 14.5cc dump, 1254 fps.
shot 2b- 2500 psi, 10.2gn, 23.3" barrel,  14.5cc dump, 1373 fps.

Lloyd-ss
 

[Buldawg76]

Lloyd
It all makes very clear and common sense to me as I said I am not a numbers cruncher but understand how air and fluids work and essentially air is a fluid in the sense that it moves and react to forces applied to it like a fluid does just not as fast or dramatic as a solid fluid such as oil does but it does react none the less.

So for me it only makes sense if you add more air ( fluid ) behind an object in a closed system before it leaves the end of the barrel ( the closed system ) it will continue to accelerate to the end of the barrel albeit maybe only marginal for the added air but none the less it will continue to increase until the added force is removed or the pellet exits the barrel out of the closed system.

The key and I believe the main point of this post is where is that point is where added air is no longer any benefit to increasing the pellets velocity so as to reach the point of maximum velocity with as little air as possible thereby gaining the maximum efficiency possible from our PCP guns with the highest power for the air used per shot.

Mike 

[Buldawg76]

Lloyd
It all makes very clear and common sense to me as I said I am not a numbers cruncher but understand how air and fluids work and essentially air is a fluid in the sense that it moves and reacts to forces applied to it like a fluid does just not as fast or dramatic as a solid fluid such as oil does but it does react none the less.

So for me it only makes sense if you add more air ( fluid ) behind an object in a closed system before it leaves the end of the barrel ( the closed system ) it will continue to accelerate to the end of the barrel albeit maybe only marginally for the added air but none the less it will continue to increase in velocity until the added force is removed or the pellet exits the barrel out of the closed system.

The key and I believe the main point of this post is where is that point where added air is no longer of any benefit to increasing the pellets velocity so as to reach the point of maximum velocity with as little air as possible thereby gaining the maximum efficiency possible from our PCP guns with the highest power for the air used per shot.

Mike

[Buldawg76]

Sorry for the dual post and the first one can be deleted

Mike

[lloyd-ss]

................................
The key and I believe the main point of this post is where is that point is where added air is no longer any benefit to increasing the pellets velocity so as to reach the point of maximum velocity with as little air as possible thereby gaining the maximum efficiency possible from our PCP guns with the highest power for the air used per shot.

Mike 

Mike,
I agree 100% that that is the real goal of all this effort, but at the same time we are going to have a whole lot of fun wasting air to find out what the upper limits are, too! 
Lloyd

[rsterne]

There is no question that closing the valve earlier in the cycle is more efficient.... and that smaller benefits occur the later you close the valve.... In many PCPs, closing the valve when the pellet is 50% of the way to the muzzle only loses about 3% of the maximum velocity you can get with that PCP setup with a dump shot.... yet is uses roughly half the air....

BTW, Lloyd, who you callin' a big chunk?.... 

Bob

[lloyd-ss]

Bob, ha, ha.  Its good to be among friends where we can keep it fun.  Heck, maybe the big chunks are ideas, LOL.   
We certainly have one specific goal of making more efficient airguns, but also, and I think this fits most of us, just learning more about how these PCPs work is enough reward in itself.
   
Also, I want to add about Steve's sonic horizon, even though I don't agree with him on that topic, he is incredibly smart when it comes to airgun theory and practical application, and I will never dismiss offhand what he has to offer.  I have learned quite a bit from the wealth of information that he has shared over the years.
Lloyd-ss

[rsterne]

^X2....

Bob

[Buldawg76]

................................
The key and I believe the main point of this post is where is that point is where added air is no longer any benefit to increasing the pellets velocity so as to reach the point of maximum velocity with as little air as possible thereby gaining the maximum efficiency possible from our PCP guns with the highest power for the air used per shot.

Mike 

Mike,
I agree 100% that that is the real goal of all this effort, but at the same time we are going to have a whole lot of fun wasting air to find out what the upper limits are, too! 
Lloyd

Lloyd and Bob
I agree that if there is no fun in the efforts put forth in this post then it would soon lose it appeal and following as I know I have contributed little to nothing in the way of knowledge or insight and actually only gained by leaps and bounds from it due to both of you with your dedication and passion for the hobby and for that I am grateful.

I also am not discounting any of the very little I have seen or read about Steves contributions to this hobby but just that his theory about sonic horizon sounded more like a scene out the new start wars movie than any dealings with air guns and still seems to not hold up to common practice or theory IMO.

Big chunks are not bad especially if they are dark chocolate with bunny ears. LOL  Yes the whole expanding our knowledge and hobby together is by far the most rewarding and enjoyable part of the hobby/sport , well next to actually shooting the guns I guess.

Mike 

[rsterne]

I just ran across this post by Steve in NC, and while it is not precisely on topic, it certainly is interesting....  http://www.network54.com/Forum/79537/message/1211671778

If you read through, you will see that the absolute maximum energy in a given quantity of air (at 1 bar) is 3.875 FPE/CI.... To achieve that, the temperature of the air at the muzzle would be absolute zero, and with no molecular motion, there would be no pressure, and hence no report.... 

Just out of curiosity, I ran the numbers for the 5% barrel distance from Steve's graph above for 1600 fps, for Lloyd's 1605 fps test shot.... The bore is .22 cal, and 5% of the barrel length is (just under) 1.2".... so the volume is 0.0456 CI.... The reservoir was 14.5 cc (0.8847 CI), and the pressure was 4500 psi so when the 10.2 gr. bullet has moved that far, the pressure has dropped to 4500 x (0.8847/(0.8847+0.0456)) = 4500 x 0.8847/0.9303 = 4279 psi / 14.5 = 295 bar x 0.0456 = 13.46 CI at 1 bar.... If we multiply that by 3.875 we get a maximum energy of 52.1 FPE.... Since according to the Sonic Horizon Theory, nothing that happens at the valve after the bullet has moved 5% of the barrel length can generate more FPE in the bullet, it would follow that is the LIMIT for the energy possible in Lloyd's test gun....

However, Lloyd achieved 1605 fps with the 10.2 gr. bullet, which is 58.4 FPE.... a theoretical impossiblilty if the Sonic Horizon Theory (and Steve's calculation of the maximum energy in one CI of air) are correct.... Could someone please check my numbers and see if/where I made an error?.... Perhaps the graph is just drawn in error, as I don't have the math/logic Steve used to develop it.... Still, it is an interesting anomaly....

Bob

[Scotchmo]

The difference between 52.1fpe theoretical (one theory anyway) and a single 58.4fpe test result is not enough prove/disprove anything.

Weighing errors, velocity reading errors, rounding errors, atmospheric conditions, etc.

Empirical "proof" from testing would require the ability to consistently exceed the "theoretical" limits.

A repeatable test that would regularly produce over 1700fps would definitely swing me. As it is, I'm not committing either way to the "sonic" limitations.

However, knowing that a mach 2.5 wind tunnel is possible using a dump chamber makes me believe that it should be possible to drive a pellet at higher velocities than 1640fps.

Can a "pellet" comprised of a single lead molecule riding the mach 2.5 peak of the velocity gradient exceed 1700fps? What about a pellet composed of a reasonable number of molecules?

[rsterne]

You're right, of course.... I just thought it interesting that the theoretical maximum yielded "solid air" out the muzzle at zero pressure and zero report.... so the achieveable FPE would be of course much less than 52.1 FPE.... On the other hand, any transfer of heat from the barrel to the expanding air would rule out 100% Adiabatic expansion, which was assumed in the calculations.... and it would be pretty hard for that not to occur (even in the short time frame of a shot).... which would raise the maximum FPE, wouldn't it?.... Too many conflicting things happening for the 3.875 FPE/CI maximum to really mean anything, IMO....

A while back I used an online calculator for the energy contained in a given volume of compressed air that is expanded to 1 bar to create this graph....



There are two interesting things about this chart, compared to the 3.875 FPE/CI number.... First, the maximum FPE/CI depends on pressure, it isn't constant.... Secondly, that 3.875 number lies in between the two curves.... Here is the calculator I used.... http://www.tribology-abc.com/abc/thermodynamics.htm

Bob

[lloyd-ss]

Here are two more shots in the series of shots that I have been taking with my max vel dump shot gun.
Again, .22 cal, 23.3" barrel, 4500 psi, dump reservoir increased from 14.5cc to 26cc.
Two matched projectiles of 7.5 gn each.  Size 004 o-ring stretched over a .115 dia centerpost.  A little more attention to detail this time.  The fits were good, and the projectiles were just strong enough to hold up for the shots. The barrel was cleaned and lightly lubed with silicone chamber oil.

Here's a pic of the 2 matched projectiles being weighed together.  7.5 grains each:
 

Here is the set-up. Same as before, but this time with a heftier lever for the release cam, and a leather lanyard to trip it. The chrony is only a foot and a half away. The HPA is coming in thru the black microbore tube at the middle center.


Here is the chrony after the first shot.  1745 FPS


And here is the chrony after the second shot.  1714 FPS


Based on my previous fastest shot of 1605fps with a 10.2 gn projectile and 14.5cc dump reservoir, the calculator showed that the velocity for this set-up, using the same system loss percentages, and with the 7.5 gn and 26cc reservoir, would be about 1736 fps.  The two shots of 1745fps and 1714 are right there!
I think it is important to note that there is nothing extraordinary about these two shots.  They fall right in line with the usual efficiencies of HPA shots.  No magic, just light weight projectiles, plenty of HPA, and an efficient air delivery system.

I think there is a need for someone to come to my place and serve as a witness to a repeat performance of these shots.

Lloyd-ss

[rsterne]

So much for a speed limit.... *grin*.... My only comment is....

WOO HOO !!!

Bob

[lloyd-ss]

Thanks Bob!
Attention to detail is what did it.  My previous attempts were a bit hurried, but this time the shots got the effort they deserved.  And thanks to you guys for prodding me along on this.
Lloyd

[Scotchmo]

Thanks Bob!
Attention to detail is what did it.  My previous attempts were a bit hurried, but this time the shots got the effort they deserved.  And thanks to you guys for prodding me along on this.
Lloyd

Great work Lloyd!!!!

My spreadsheet shows a predicted velocity of 1724fps, ignoring pellet friction.

With one pound of dynamic friction, it predicts 1716fps.

My spreadsheet currently makes no account of "sonic choke" or "sonic horizon".

I can't mathematically disprove a sonic horizon theory as I don't really understand what it's affect might be. Our spreadsheet models use proven engineering principals and no hypothetical theories. Results jive with their predictions so far. Even when exceeding 1640fps by a significant amount.

One more thing to check next time - the temperature of the dump chamber before firing. Just to make sure. Hopefully close to ambient so as not to introduce another variable.

At this point, I'm leaning HEAVILY toward: 1640fps limit = MYTH = sonic joke

I was hoping that our prodding would work . Thanks for all the work. Your test should be an episode of Mythbusters.

BTW: The spreadsheet I'm using is from my last iteration and includes an empirically derived fluid friction of the form:
Xfactor = 100000
lbf = (PSI x FPS)/Xfactor

[lloyd-ss]

Scott,
I think it is important, and reassuring, that at least 3 of us have independently, and using different methodologies, come up spreadsheets/calculators/models, that all predict very, very similar results.  The GTA has provided a great meeting place for us to explore this hobby and freely exchange ideas with each other.  Personally, this is very exciting and I am having a great time!  Of course, I am not doing some of the things that I really need to be doing, but I will get back to them shortly.  Those activities happen during my silent periods, LOL.

I was able to retrieve the two projectiles, one is in good condition, the other is in fair, so I can hopefully get 2 more shots, but probably not until the first of next week.  I have a good temperature probe that I can tape to the dump reservoir as you suggested.   The reservoir is just about exactly one pound, almost entirely aluminum, with a few stainless parts, so it has significant heat capacity.  I have a CF fill tank and a shoe box mainifolded to the gun, and I first fill to about 3700 from the tank and then close that off and finish the fill with the shoebox, using the CF tank gauge to read the dump reservoir pressure..  The shoebox takes about 2 minutes to top it up to a solid 4500, it clicks off, and I fire.  I never have checked the temp of the aluminum before, but do not remember it feeling particularly warm or cold.  I will record some readings with the next shots.  I will also see if I can check the final barrel temp after I am satisfied with validity of the dump reservoir readings.

Regarding your fluid friction Xfactor, I am assuming that you use that to reduce the force in the F=ma calculation of each iteration, correct?

Lloyd

[Buldawg76]

Bob
I read thru the last link you posted to Steve's 54 post on velocities and all the ensuing comments from others but found his comment in his original post somewhat contradictory to his Sonic horizon theory in that it states the expanding air is now trapped between the valve and pellet as shown below between the quotation marks, so what happened to that invisible wall of the sonic horizon in his theory. He seems to not stay true to his beliefs except only when it suits his purposes.

"" In phase 2, the valve is closed. So the fixed charge of air now " trapped between the valve and pellet expands as it continues to accelerate the pellet ", causing both its pressure and temperature to drop. Because the expansion is so rapid that no significant heat can transfer from the barrel to the air, the only energy available to push the pellet is the initial thermal energy of the molecules in the charge. This means that the sum of the energy of the pellet and the molecules is constant, as one speeds up at the expense of the other slowing down. ""

Lloyd
I second Bobs " WOO HOO" as in now we are moving on up and getting into the extremes of just what is actually capable with the proper equipment and can only say I want/feel that you can get it even faster with just a bit more tinkering like say over 2000 fps or close to Mach 2 with a lighter projectile and even more volume of air. Just a thought on my end as I cannot crunch any numbers but just base it on land speed records at Bonneville where it has been stated many times you cannot go any faster and yet they continue to do so quite often.

Keep your head down and nose to the grind stone and anything is possible IMO.

Mike

[Airsnipe]

Lloyd, In the two shots you had a 31fps difference. This setup is meant to eliminate variables and make things consistent and predictable. So is this 31 fps seem within expectations? I guess it's only like 1.8% variance.

I'm not criticizing your setup or your efforts in any way. With all things considered, it seems your setup should offer closer shot variance? Am I just picking at stupid details or is this 31fps good for the speed of the projectile and I'm really just seeing nothing out of the ordinary?