Author Topic: Hard Air Magazine - Technical Editor (Read 31656 times))

rsterne · « **Reply #40 on:** December 17, 2018, 08:17:52 PM »

Not in precisely the form they are on HAM.... It wouldn't be fair to copy/paste my threads over there and then get paid (nominally) for them.... HAM hold the copyright on the articles posted there, of course.... That is why I have linked to them here.... There are, after all, free to view....

The information I have posted here, over the years, has all the same content (and more)…. just not organized in the same way....

Bob

Tater · « **Reply #41 on:** December 18, 2018, 07:40:55 AM »

Maybe there could be a spot in the library for a list of links to all of your articles. Or a pinned thread at the top of your forum section with all the links.

rsterne · « **Reply #42 on:** December 18, 2018, 10:50:50 PM »

That is the purpose of this thread.... Links to all my HAM articles are here....

Bob

rsterne · « **Reply #43 on:** January 15, 2019, 01:31:43 PM »

Article 8 in my series on PCPs is now online.... https://hardairmagazine.com/ham-columns/bobs-guide-to-tuning-unregulated-pcps/

This one is on Tuning Unregulated PCPs....

Bob

rsterne · « **Reply #44 on:** February 17, 2019, 10:02:58 PM »

Article #9 on PCPs is now online at HAM.... https://hardairmagazine.com/ham-columns/introduction-to-the-pcp-regulator-dont-ignore-the-plenum/

This time it is on setting up regulated PCPs....

Bob

rsterne · « **Reply #45 on:** March 21, 2019, 12:14:45 AM »

Part 10 of my series on PCPs is now online.... https://hardairmagazine.com/ham-columns/tuning-regulated-pcp-airguns/

Bob

rsterne · « **Reply #46 on:** April 25, 2019, 11:24:51 AM »

Part 11 of my series of articles, and the last one on PCPs is online....

https://hardairmagazine.com/ham-columns/pcp-efficiency-what-is-it-what-changes-it/

Next month I begin a series on Ballistics, Internal, External and Terminal....

Bob

WhatUPSbox? · « **Reply #47 on:** April 25, 2019, 01:54:01 PM »

Bob. great article as always.
Towards the end there is a line "Insert “Dual Power Hammer” here." looks like a graphic didn't make it in.

Thanks for your work.

rsterne · « **Reply #48 on:** April 25, 2019, 07:05:03 PM »

Yeah, I know.... I sent it but they didn't put it in.... I let them know but they are on holidays....

We'll get that inserted.... eventually....

Bob

Back_Roads · « **Reply #49 on:** April 25, 2019, 08:06:16 PM »

Wow didn't notice the error I mentally inserted the image

I see Bob will be going ballistic next

rsterne · « **Reply #50 on:** May 28, 2019, 11:30:59 AM »

My first article on Ballistics is now online at HAM.... https://hardairmagazine.com/ham-columns/pcp-airgun-internal-ballistics/

This covers the Internal Ballistics of PCPs, so is basically a transition from the PCP Series to the Ballistics Series....

Bob

WhatUPSbox? · « **Reply #51 on:** May 28, 2019, 08:19:39 PM »

Bob,
Nice write up as always. Is there a description somewhere of the model you used and what assumptions it makes.

Thank you again

rsterne · « **Reply #52 on:** May 28, 2019, 08:34:43 PM »

Lloyd's Internal Ballistics Spreadsheet uses Newtonian Mechanics (F=ma) as the primary calculation.... where the force is the pressure in the barrel times the base area of the pellet/bullet.... The mass starts out as the mass of the projectile, but as air is added to the barrel, that is included in the mass to be accelerated.... That (declining) force is applied to the (increasing) mass until the pellet/bullet leaves the muzzle.... so the acceleration is constantly decreasing during the shot....

Other correction factors (losses) included in the calculation are the starting and sliding resistance of the pellet (guesstimated), the pressure drop due to the (wasted) transfer port volume, the volume of the reservoir or plenum (the smaller it is, the faster the pressure drops during the shot), and the mass of the gas used (eg. Helium is lighter than air, CO2 is heavier)…. Other input variables are the valve dwell and an "efficiency factor" which looks after other losses such as port size, etc.etc.etc…. The spreadsheet makes a calculation for every 0.00002 sec. until the projectile leaves the muzzle.... and so "integrates" the result (each increment uses the end point of the previous one for its starting conditions)….

It turns out that for the calculator to match real world results, there is only one combination of dwell and the efficiency factor that will produce both the test velocity and the volume of air used in the empirical data you are trying to match.... While there are undoubtedly things not accounted for, the model "works", and can be used to predict quite closely what happens when you change different inputs, such as pressure, caliber, pellet weight, etc.etc.etc…. Simply put, it is the "best" tool we currently have to model PCP Internal Ballistics (also CO2 guns and pumpers)….

Bob

WhatUPSbox? · « **Reply #53 on:** May 28, 2019, 08:46:15 PM »

Bob, thank you for the description.
It looks like from the charts that the pellet starts with full pressure behind it. Is that assumed or is the starting friction input at high value to allow the pressure rise in the breach to complete its rise time?

rsterne · « **Reply #54 on:** May 28, 2019, 09:51:51 PM »

When the valve opens even a few thou, the transfer port volume fills with air at a very high speed, probably close to the 1650 fps molecular velocity, ie it takes about .05 mSec for the air molecules to reach the base of the pellet, but at that instant the only volume of air that has flowed through the valve is enough to fill that volume.... With such high pressure available in such a short period of time, the pellet then starts a very rapid acceleration.... The starting force is only a few lbs. for a pellet, somewhat more for a bullet, and a lot more if the bullet is too tight a fit in the bore....

There is a small initial pressure drop to fill the ports, and then a larger, more gradual drop as the pellet moves down the barrel during the "valve open" phase.... Once the valve shuts, the pressure starts a much more rapid decline as the air between the valve and pellet then expands as it accelerates the pellet down the bore....

It there is a small error in the assumptions, that is compensated for by the overall "efficiency" factor used to balance the spreadsheet and make it agree with your test results.... Once that "fudge factor" is known for a given gun, you can change pellet weight, pressure, valve dwell, even barrel length or caliber, and get a close estimate of what to expect....

Bob

WhatUPSbox? · « **Reply #55 on:** May 28, 2019, 10:28:25 PM »

I was thinking back to the breech pressure transient test data that George presented in his long testing thread in the engineering gate. There the pressure rise took about .5 msec and never reached the full pressure you would expect for CO2. The pellet started at about .2 msec and 200 psi. Granted the Disco is running at higher pressure but I wonder how that time affects the results. Perhaps it just looks like additional dead volume.

Thanks for the explanation

rsterne · « **Reply #56 on:** May 28, 2019, 10:54:49 PM »

There are definitely "time lags" not taken into account in Lloyd's spreadsheet.... how long it takes between the hammer strike and the valve actually cracking as the poppet head decompresses.... the time it takes for the random molecular motion in the valve to "organize" and flow into the ports.... when the pellet actually sees enough pressure to start moving.... and probably some others.... Think of the start of Lloyd's graph as the instant the pellet start to move and it might be easier to understand.... ie you have to add all the "lag times" to the "valve open dwell" to get the "total dwell" from hammer strike to valve close.... All those lag times are to the left of "zero"....

No model is perfect, and when somebody comes up with a better one, I'll only be too happy to use it.... Lloyd has been working on his for many years, ever since I "met" him in fact, and probably before that.... We still don't know if the expansion is closer to Adiabatic (as theory says) or Isothermal (which it isn't as the exhaust plume of a PCP definitely cools)…. I am 99.9% sure it isn't purely Adiabatic, because that would put a maximum on the efficiency (eg. ~ 2.34 FPE/CI at 3000 psi) which we are closely approaching and sometimes exceeding.... Again, in practical terms, all that changes if you use Adiabatic, Isothermal, or something in between (Lloyd's spreadsheet can do that, and separately for the valve open and valve closed stages)…. is that you end up using a different combination of valve dwell and efficiency to make the spreadsheet agree with empirical data.... The cool thing is that regardless of the assumptions and inputs, there is only ONE combination of those two inputs that will work to match your data....

Bob

WhatUPSbox? · « **Reply #57 on:** May 28, 2019, 11:26:50 PM »

Thank you Bob.
I'm not questioning the validity or the effort that you and Lloyd put into building the model. I am always interested in how modeling compares to test data. It usually helps me understand what is going on. I understood the chart to be from pellet start. Is there a chart of that shot cycle plotted against time?

rsterne · « **Reply #58 on:** May 29, 2019, 12:21:43 AM »

There is one available in the spreadsheet, but I have never used it.... I am pretty sure this is the same inputs as I used to generate the first chart in the HAM article....

Bob

rsterne · « **Reply #59 on:** July 08, 2019, 11:35:00 PM »

External Ballistics of Pellets at HAM....

https://hardairmagazine.com/ham-columns/the-external-ballistics-of-diabolo-pellets/

Bob

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Hard Air Magazine - Technical Editor

Author Topic: Hard Air Magazine - Technical Editor (Read 31656 times))