Quote from: HectorMedina on April 14, 2021, 11:52:43 AMThanks for your kind words Marty!Our aims are a bit different from the Titan guys.We start by showing everyone that wants to read in detail, how to put together their OWN testbench.The whole thing costs under $250 with a pocket digital oscilloscope we found that works reasonably well. We also show how to CALIBRATE the instrument.These guys just purchase stuff that is way more expensive that the guns themselves. So, while interesting, it is still in the realm of the "specialist" or "professional".I've done that already, and my main objection is that it does not empower shooters to find out for themselves, and does not provide a general platform for improvement.I also think they complicate things needlessly by insisting on measuring forces directly. We START with velocity, then integrate for displacement and calibration, and derive for acceleration.Once you know accelerations, forces are easy to calculate because masses are constant.Anyway, let's not get ahead of ourselves.Tomorrow we publish the first part and show how to build "the apparatus".;-)Keep well and shoot straight!HMVery noble goals in enabling the DIYer with the knowledge and tools to do the same testing at home, Hector. I think we all benefit from a better understanding of what is going on inside and outside of a rifle. That said, I’m sure there will still remain some mysteries that can be tackled by the GTA community.-Marty
Thanks for your kind words Marty!Our aims are a bit different from the Titan guys.We start by showing everyone that wants to read in detail, how to put together their OWN testbench.The whole thing costs under $250 with a pocket digital oscilloscope we found that works reasonably well. We also show how to CALIBRATE the instrument.These guys just purchase stuff that is way more expensive that the guns themselves. So, while interesting, it is still in the realm of the "specialist" or "professional".I've done that already, and my main objection is that it does not empower shooters to find out for themselves, and does not provide a general platform for improvement.I also think they complicate things needlessly by insisting on measuring forces directly. We START with velocity, then integrate for displacement and calibration, and derive for acceleration.Once you know accelerations, forces are easy to calculate because masses are constant.Anyway, let's not get ahead of ourselves.Tomorrow we publish the first part and show how to build "the apparatus".;-)Keep well and shoot straight!HM
Very interesting read. Looking forward to the next segment. As to gain more information on where the piston bounces, aside from cutting a view port in the compression tube or making a transparent compression tube (both highly unlikely), can the rear of the piston be viewed through the cocking slot? If so, camera lenses have become very small and could be added to the sled to view the piston skirt during its movement. Possibly modify the stocks or remove and make a custom one of same weight just to get an idea of movement. I'm sure a sticker marked in millimeters could be added next to the slot, to gain measurement of the piston doing it's oscillations from the back of the piston. Don't know if it's even possible, but just an idea. It would be very interesting to see a gas piston/ram on the dynamograph to compare to a spring piston. I know there's a lot more coming, but the possibilities of tests that could be done are almost endless, very neat stuff. Thanks for sharing with us.
Quote from: MartyMcFly on April 14, 2021, 01:22:45 PMQuote from: HectorMedina on April 14, 2021, 11:52:43 AMThanks for your kind words Marty!Our aims are a bit different from the Titan guys.We start by showing everyone that wants to read in detail, how to put together their OWN testbench.The whole thing costs under $250 with a pocket digital oscilloscope we found that works reasonably well. We also show how to CALIBRATE the instrument.These guys just purchase stuff that is way more expensive that the guns themselves. So, while interesting, it is still in the realm of the "specialist" or "professional".I've done that already, and my main objection is that it does not empower shooters to find out for themselves, and does not provide a general platform for improvement.I also think they complicate things needlessly by insisting on measuring forces directly. We START with velocity, then integrate for displacement and calibration, and derive for acceleration.Once you know accelerations, forces are easy to calculate because masses are constant.Anyway, let's not get ahead of ourselves.Tomorrow we publish the first part and show how to build "the apparatus".;-)Keep well and shoot straight!HMVery noble goals in enabling the DIYer with the knowledge and tools to do the same testing at home, Hector. I think we all benefit from a better understanding of what is going on inside and outside of a rifle. That said, I’m sure there will still remain some mysteries that can be tackled by the GTA community.-MartyMarty,What still perplexes you?-Y
Looking good! AC coupling is certainly reasonable (as your testing shows). Having good initial and final conditions might help the integration problem, if the data acquisition will run long enough to get data at the final resting position. I am not used to working with coils as generators, but normally a relatively longer solenoid gets one away from the end effects of the coils or of the magnet for that matter. The hard part is creating a uniformly wrapped coil. I can't help you there unless you want to make a coil winding machine....Weirdo coil geometries like Helmholtz coils get a theoretically better magnetic field, but how it would act in this situation (moving magnet) is not something I know. It makes sense, but wrapping one's head around a generator with no coils over the magnet isn't easy...Fortunately, they aren't hard to make, and might even be more tolerant of hand winding. The only downsides are size and overall field strength (or sensitivity, in this case).It is easy to find student assignments online looking at both (but no data!), but here's a link that actually discusses the field properties of each configuration (i.e. slides 13, 24 and 25): https://courses.physics.illinois.edu/phys401/sp2012/Files/Hall%20Probe/Hall%20probe%20experiment2.pdfTheorizing aside, I can say I have experience with a possible solution to the limits of readability of your millimeter scale: print a vernier scale to mount on the sled instead of that single hash line. The fixed camera creates some parallax issues, but it may be better than nothing. I've done them on transparencies (for laser printers) and paper, which I then laminated. Very easy to make in CAD or even Microsoft Word. You just make an ruler scale in whatever units and nominal divisions, grab a copy and shrink that to whatever one division would result in. For example, try an inch scale in tenths. Grab a copy of a 1 inch section and shrink it to 90 percent (i.e. nine 10ths) to make the vernier. Print both your main scale and the vernier to compensate for small printer errors and bam! 0.01" readability, just like that! 0.01mm is a little trickier, but only just under half of what 0.01" requires. And you don't have to limit yourself to 10ths. Humidity is not your friend for paper, but lamination helps that. I've done them for student labs and even have a 2D one (horizontal and vertical) taped over a crack in the wall of my daughter's bedroom, watching it open and close and shift over the season, just for kicks.Good luck with further work!
Yogi, I don't want to hijack this thread so I'll keep it brief and PM you the details but in a nutshell I'm curious about the relationship between efficiency and recoil in light piston/small compression setups versus heavy piston/large compression setups.PS. I'm enjoying the first chapter - thumbs up to you guys!-Marty
I started reading through the work shown to date. In the discussion on the actual firing process and recoil physics a few things stood out thus far.1. I hadn't considered the spring's return to stop as impacting the rifle's recoil before It suggests that a nitrogen filled piston system shouldn't have the same recoil dynamics and hence may have quantitatively different performance to a conventional springer. 2. Assuming that interpreted the data correctly, the pellet leaves the barrel as the piston is moving forward and the rifle is moving rearward - as in a powder gun. That says the pellet leaves the barrel before all the shaking and rattling around in the piston area occurs. That should mean that it should have little to no impact on the external ballistics of the pellet and its path. That suggests that all the care and effort put into developing the artillery hold and the other esoteric things we do to make our rifles shoot better are not really relevant. It also suggests that a tight hold as used in powder arms should be the best approach. 3. Group size statistics are the best way to evaluate the performance of a rifle. I was interested to see that the C-T-C group sizes and standard deviations are quite close to those that I see with my two 54s - both set up to shoot at about 850f/s. I also shoot at just about 20 yards (a nominal 50' pistol range) from a very solid bench rest. I also shoot a minimum of 10 shot groups and usually shoot 10 or more groups to develop my statistics. Whiie I'm not going to suggest that the authors do another tranche of experiments a couple things do come to mind.1. Lots of people expend great effort in measuring and weighing their pellets to improve accuracy. My experiments suggest that it may be a waste of time with generally available scales and calipers/micrometers. Most use powder scales and balances for such weighing. A few years ago, I did this and found that both the electronic (load cell) and balance-based powder measuring devices could not reliably detect a 0.1 grain differences in two pellets or even repeat the same number on the same pellet. I found and bought a gem scale that purports to weigh to the nearest 0.001gram or approximately 0.01gr. A good experiment would be to separate pellets Doing that, I could separate batches in a 0.02 batches fairly reliably. The shooting results showed that the all stayed within the average group size for the rifle. There was detectable vertical string but not nnough to justify the effort. While I actually own a two pan analytical balance capable of weighing to 0.0001gm accurately and repeatability. I didn't think it worth the considerable effort to set it up. 2. Following on the comment in #2 above, it seems that the barrel vibration and harmonics may be the real cause of hold sensitivity as they impact the pellet's path while it is still in the barrel. That follows from the separate discussions of the value of, and need for, harmonic stabilizers on the barrel as Hector has been promoting in his "tuned" 54s and the new 54 Pro. Anyway, these are interesting analyses and I look forward to seeing more of them!!!
"Figures lie and liars figure"?Not really.A look into how to use statistics to really look into the accuracy and precision of your airgunChapter 3 is here: https://www.ctcustomairguns.com/hectors-airgun-blog/shot-cycle-dynamics-in-3-spring-piston-airguns-chap-3 Hope you enjoy, and if not, hope you get some useful insights.Keep well and shoot straight!HM