Taming the bark of an Avenger

[stylnchris]

Peter's Latest Design.






Seems to Tame the Bark -  Which is what I thought this post was about.

Thanks again.

[Outer_Rondacker]

In thinking about the airstripper and sound moderation functions, I went back to this video (best played at .25 speed).
I don't know what gun that is and what caliber (though I guessed .177)
I then took a few screen shots and overlayed a scale with pellet diameters (estimated) as the units. If it is .177, then 6 units is a little over an inch.
To me it looks like the air behind the pellet has structure for maybe the first 10 or 12 units and suggests that if you have baffle openings at the 1 and 2 inch points, then perhaps after that you just want volume until you get to the exit.
Unfortunately, highspeed cameras are hard ($$$) to come by, so getting videos for an Avenger for example is not likely.

Interesting. With this in mind you would think venting the barrel while the pellet is still in it would be the best solution. We do this in PBers to not have this issue on ultra mag rounds. Its kinda like a muzzle brake but will stabilizes the projectile whiles the gases are expelled. We might want to look into this more.

On another note I had a chance to run four five shot groups at 30 yards with both the V4 and the v2 prints from Peter and was not able to notice any difference in grouping. Point of impact did move from v4 to v2 and back for repeatability. I will play with this more once the days warm up. Fun stuff.

[subscriber]

Seems to Tame the Bark -  Which is what I thought this post was about.

Thanks again.

Thank you, Chris.  It looks good.

[subscriber]

Than you, Stan, for capturing and calibrating those video images.

A few things:  Understanding what airgun airflow does from the muzzle in open air is useful.  However, it is not the same as when constraining that air in a tube.  Especially in a narrow tube, where the air reaches the inner wall and wants to follow it on forwards.  The air that flows forwards passes through the bore of the LDC, and that focuses it again into a narrow stream, to partially expand in the next chamber.

We don't know how potent the airgun in the video is.  The projectile looks like a Sheridan 5 mm.  If so, how many pumps were used.  A more potent PCP is likely to have a longer, wider "cloud".

Many different approaches to baffling seem to work.  The Weihrauch approach is three large uneven chambers.  The first chamber is larger than the second; and the third larger than the first.   The space usually has felt covered "hair curlers" in it.  Contrast this against the Marauder shroud.  That has an air stripper with a flat wall, feeding the rear of the shroud.  Then simple 7 conical baffles, all about at a 3/4" stacked spacing.  Both the Weihrauch LDC and Marauder are known to be quiet.  This suggests there are a few different overlapping dynamics at play.  I think the large chamber hair curler approach reduces sound waves that reflect off the hard tube walls.  The multiple conical baffle approach reduces bulk pressure flux at each chamber in small "bites" that ring less per step, and at higher frequency due to the short length (probably with a lot of ultrasonic content).

I think of air flow from the muzzle as a shot pattern that spread with distance.   This is discrete from the pellet, that follows a more constrained path.  If the first air stripper wall or cone opening is too close to the muzzle, the "shot spread" of air is less, and more air simply follows the pellet down the bore of the baffle.  If the first wall or cone lip is further away, then the "shot spread" of air is more, and a much larger proportion of that air runs into the wall or cone, than passes through the hole.

Now, the "shot pattern" air spread model is a bit more complex.  The closer to the barrel muzzle, the higher the air pressure directly behind the projectile.  The higher the pressure the more radial the air spreads from the bore axis (until it meets the LDC tube ID).  As the pressure decays rapidly further from the muzzle, the more of a narrow jet it forms.  So, the early baffle dealing with higher pressure may be closer together, while those further forward should be placed further apart; else air takes a shortcut from one baffle bore to the next.  It is a matter of how much air volume we are dealing with at what pressure.   

All that said, it is common to start with a large expansion chamber to lower the bulk pressure in a large initial increment.  Then restrict and slow the expansion after that with more closely spaced baffles.  With a shrouded PCP, that initial "blast chamber" is the rear of the shroud.  Else, with a removable LDC, the first chamber from the barrel muzzle may be larger than the rest, often with another large chamber just before the LDC exit.

If air passes the pellet in a narrow tube section, then runs into a restriction, that air is likely turbulent ahead of the pellet.  If that turbulence is asymmetrical where the pellet is, the projectile may be shifted off course.   Turbulence behind the projectile can't affect the projectile path.   Such churning air flow trailing the projectile helps delay the exit of that air from the LDC, to reduce the peak intensity of the slap when the still partially pressurized moving air crashes into the ambient air.

I have a family of LDC designs that utilize baffles shaped like toroidal spirals.  These use the velocity of the expanding air to keep it in the spiral.   I will see if I can package such a design in the Avenger "tube", and make it printable as a single part.  The latter is a huge challenge due to part of the structure "floating in air".    As supports can't be removed from a closed structure, I will have to devise workable permanent internal support structures that don't take up too much expansion volume...

[subscriber]

With this in mind you would think venting the barrel while the pellet is still in it would be the best solution.

An air stripper that is comprised of a series of holes cut directly through the barrel wall,  perhaps an inch or two back from the barrel muzzle would be the preferred embodiment. 

The holes would need to be small enough so that the projectile support is not compromised.  A process that does not raise a burr on the hole edge would need to be used - or deburring very carefully would be required.

The hole pattern would need to be such that it does not remove too much stiffness on the barrel, or the muzzle section would "wave" back and forth at the "soft spring" the vents create. 

The vents would need be made ahead of the choke; or in a parallel section of choked barrel.  Vents that are placed in a convergent cone will by their nature tend to shave the projectile on the forward hole edges.  Breaking those edges so they do not catch on the projectile will add significant cost.  And added cost is the reason why vented barrels in airguns are unlikely to be used commercially.  If such venting seems easy, consider partially cutting some lands with the holes, or deliberately placing the vents between lands in a 12-groove .177 barrel.

An ideal process for barrel venting is electrical discharge machining (EDM) because it vaporizes metal and does not leave a burr.  Drilling (with a stiff endmill), reaming and deburring appropriate size vents become easier as the caliber gets larger.

[WhatUPSbox?]

It was interesting in the sequence that the initial expansion travels with the pellet and stays within about a 1 in diameter volume. The air that follows does not expand, at least for the first couple of inches.

[subscriber]

Stan,

If the forwards moving air column does not expand further, that indicates that while it has forwards momentum, the bulk pressure in the column has reached ambient air pressure.   
 The diffraction "chatter" (that look a bit like Mach diamonds) seen in that airgun stream are pressure oscillations that represent sound waves, or generate sound waves.   

Contrast that to this slow motion .45 pistol propellant cloud, expanding into a ball behind the bullet, due to a muzzle pressure of several thousand PSI:  .   Note how the first gas behind the projectile leaves the muzzle radially due to the high pressure differential against atmospheric pressure. 

Do not confuse the gas behind the bullet, with the gas that escaped in front of the bullet, leading it at much lower pressure - and a much shallower cone angle from the muzzle.  Then, as the projectile moves out of the way and the gas pressure in the barrel decays, the jet of gas from the barrel become narrower and longer.

The ball of spent gas that moves forwards without expanding further, again indicates that the bulk pressure in the ball has decayed to meet atmospheric pressure.

[Outer_Rondacker]

Stan,

If the forwards moving air column does not expand further, that indicates that while it has forwards momentum, the bulk pressure in the column has reached ambient air pressure.   
 The diffraction "chatter" (that look a bit like Mach diamonds) seen in that airgun stream are pressure oscillations that represent sound waves, or generate sound waves.   

Contrast that to this slow motion .45 pistol propellant cloud, expanding into a ball behind the bullet, due to a muzzle pressure of several thousand PSI:  .   Note how the first gas behind the projectile leaves the muzzle radially due to the high pressure differential against atmospheric pressure. 

Do not confuse the gas behind the bullet, with the gas that escaped in front of the bullet, leading it at much lower pressure - and a much shallower cone angle from the muzzle.  Then, as the projectile moves out of the way and the gas pressure in the barrel decays, the jet of gas from the barrel become narrower and longer.

The ball of spent gas that moves forwards without expanding further, again indicates that the bulk pressure in the ball has decayed to meet atmospheric pressure.

Well said.

[Spin]

Published test results on the in shroud moderators I have in my 2 Avengers showed close to a 7db reduction in sound.They add zero lenght to the gun and just about Aer additional weight.
     To my ear there is both a notable reduction in both volume and pitch ( with in shroud the tone is lower ) The Donny on my FX .25 Classic Dreamline is Really impressive particularly after firing the FX sans moderator, was quite louder than I expected, shrouded barrel not withstanding.
    I hope folks keep on mind that periodic cleaning of shrouds and moderators is necessary as very fine lead dust does a real Champion Job
of gunking up up things and will make disassembly a real PIA if let go too long.

[Outer_Rondacker]

Published test results on the in shroud moderators I have in my 2 Avengers showed close to a 7db reduction in sound.They add zero lenght to the gun and just about Aer additional weight.
     To my ear there is both a notable reduction in both volume and pitch ( with in shroud the tone is lower ) The Donny on my FX .25 Classic Dreamline is Really impressive particularly after firing the FX sans moderator, was quite louder than I expected, shrouded barrel not withstanding.
    I hope folks keep on mind that periodic cleaning of shrouds and moderators is necessary as very fine lead dust does a real Champion Job
of gunking up up things and will make disassembly a real PIA if let go too long.

I have had a Mikes TKO for years not and it does not come apart. It is a PIA to clean.

[JimD]

Does lead dust also make the moderator less effective?  I measured the sound from my P35 louder recently than right after I did the hair curler modification of it's shroud.  I haven't done it yet but I've been thinking I need to at least change (or clean) the felt. 

I also found that packing the baffles and hair curlers in tight increased the sound a few db versus letting them be a bit looser.  That seems consistent to me with lead dust also increasing sound.

[subscriber]

I have a family of LDC designs that utilize baffles shaped like toroidal spirals.  These use the velocity of the expanding air to keep it from leaving the spiral, via centripetal force.   

I will see if I can package such a design in the Avenger "tube", and make it printable as a single part.  The latter is a huge challenge due to part of the structure "floating in air".    As supports can't be removed from a closed structure, I will have to devise workable permanent internal support structures that don't take up too much expansion volume...

It took me awhile, but I have a design that incorporates what I believe will be an effective 3D printing support structure.    The supports do not need to be removed, and should work, providing the LDC is printed standing on the printer platen, on its front cap.

Image number 12 below shows the "cathedral arches" used to support the "floating" ring structure that would otherwise be impossible to print.  At least as part of a complete monolithic LDC assembly (can be printed as individual baffles without supports, for use in a stack placed in a separate outer tube).

My design is a variant on one of Maxim's original patents - image attached directly below: https://patents.google.com/patent/US916885A/en?oq=916885

With an explanation of how effective this Maxim device was, way back then:  https://books.google.com/books?id=RX2Z7bJz4xwC&pg=PA234&dq=hiram+maxim+centrifugal+silencer&hl=en&sa=X&ei=piqFVNf6D4WsogTgroLoAQ#v=onepage&q=hiram%20maxim%20centrifugal%20silencer&f=false
(click "no thanks" to sign up; then view the pages of that old publication)

I will email the STL to Chris, and anyone else who requests it via PM, providing their email address.

I have tested in-shroud toroidal spiral baffles similar to this, machined from delrin.  By ear, they were twice as effective as their external volume would suggest.  It seems that they convert a large fraction of the report into inaudible ultrasound.   So, this LDC may sound quieter by ear than it measures, when using a broad spectrum meter.


Yes; this design also uses my two stage reflex air stripper, and Thingyverse threads...

[subscriber]

I tried to emailed the STL for the above "Maxim LDC" to Chris and two other members.  It is almost 30 mB and was too large to send directly.  So, I used Wetransfer to send it instead.   Expect email from them.   

The Wetransfer link in that email should be valid for 7 days:  https://we.tl/t-q4p6fO6AIz

I get the impression that anyone with the link can download it...

[Outer_Rondacker]

Peter this is a work of art.

[subscriber]

Peter this is a work of art.

Thanks.  It took about 12 hours to design.

[Spin]

With this in mind you would think venting the barrel while the pellet is still in it would be the best solution.

An air stripper that is comprised of a series of holes cut directly through the barrel wall,  perhaps an inch or two back from the barrel muzzle would be the preferred embodiment. 

The holes would need to be small enough so that the projectile support is not compromised.  A process that does not raise a burr on the hole edge would need to be used - or deburring very carefully would be required.

The hole pattern would need to be such that it does not remove too much stiffness on the barrel, or the muzzle section would "wave" back and forth at the "soft spring" the vents create. 

The vents would need be made ahead of the choke; or in a parallel section of choked barrel.  Vents that are placed in a convergent cone will by their nature tend to shave the projectile on the forward hole edges.  Breaking those edges so they do not catch on the projectile will add significant cost.  And added cost is the reason why vented barrels in airguns are unlikely to be used commercially.  If such venting seems easy, consider partially cutting some lands with the holes, or deliberately placing the vents between lands in a 12-groove .177 barrel.

An ideal process for barrel venting is electrical discharge machining (EDM) because it vaporizes metal and does not leave a burr.  Drilling (with a stiff endmill), reaming and deburring appropriate size vents become easier as the caliber gets larger.

For very fine, high precision metal parts a vapor hone ( a pressurized air system that employs a lapping/polishing compound in water injected into the airstream)  proves very adequate, fast and economical. I have serious doubts it would work on plastics however at least any relatively soft ones.
   

[WhatUPSbox?]

Peter, Do you have a slicer program like CURA (free)? You can preview all the printing supports it thinks it will need for the overhangs and decide which type best fits if they are to be left in.

[subscriber]

For very fine, high precision metal parts a vapor hone ( a pressurized air system that employs a lapping/polishing compound in water injected into the airstream)  proves very adequate, fast and economical. I have serious doubts it would work on plastics however at least any relatively soft ones.
   

This may not be at all related to your suggestion, Spin:

You have just given me an idea:  Use waterjet to cut air stripper vents into the steel barrel.  One would need to add an emulation or something to prevent rusting, or use an oil to carry the garnet perhaps.  Minimum size holes may be better suited to larger calibers, but waterjet cuts amazingly fast.

[subscriber]

Peter, Do you have a slicer program like CURA (free)? You can preview all the printing supports it thinks it will need for the overhangs and decide which type best fits if they are to be left in.

I don't have Cura, but suggesting I use it to vet designs is a great idea.  Thanks; I will look into it.

I use generalizations for maximum slope angles and max width of unsupported bridges.  I am actually surprised at how well the prints posted in this thread do at the overhanging sections of my air strippers.  Images below. 

So, I may be overly careful.  I just don't want to waste people's time with clearly impossible overhangs.  Or ones so messy that they cause massive clipping.  Or droopy prints that change the shape of the part to the point that deliberate air flow no longer works.

[subscriber]

Stan,

I downloaded Cura and ran my "Maxim" file for a Ender 3.  I am impressed with how fast it slices - about 20 seconds.  It also predicts the print will take 5 1/2 hours.

What I don't know is what to look for, if there is trouble with a lack of support.  I take it the color codes I see are for extrusion mode?

I get the impression that the default mode is not at 100 % fill in the outer walls of the part?  EDIT: Found the "prepare" tab that shows risk in red - image below.

How does one zoom in with Cura, without the bottom of the image going out of frame - or to move to the field of interest after zooming?

Thanks