GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on July 02, 2015, 02:36:26 AM
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Something for you to ponder and comment on....
A Ballistics Coefficient is a measure of how the drag of a projectile measures up to a standard projectile.... The most commonly used standard, the G1 projectile, looks like this....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/g1_zps09zm9zd2.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/g1_zps09zm9zd2.jpg.html)
It is 1" in diameter, and weighs 1 lb.... That, by definition, gives it a Sectional Density of 1.000, a Ballistics Coefficient of 1.000, and a Form Factor of 1.000.... It's drag over the range of velocities up to Mach 5 is well established, and the Drag Coefficient (Cd) is nowhere near a constant, it varies with velocity like this from 300 to 1500 fps, which covers the range of velocities we are interested in....
(http://i378.photobucket.com/albums/oo221/rsterne/Ballistics/G1%20Drag%20Model%20Net_zpsopno7nwx.jpg) (http://s378.photobucket.com/user/rsterne/media/Ballistics/G1%20Drag%20Model%20Net_zpsopno7nwx.jpg.html)
If a projectile matches that drag curve precisely, it would have an FF of 1.000, so it would have a constant BC that equals the SD.... If it had a similar drag profile, but a higher FF (let's say 1.5), then the Cd would be 50% greater at every velocity.... The BC would still be a constant over the entire range of velocities, but the BC would only be 2/3rds of the SD, because BC = SD / FF.... That is how the BC is supposed to work.... However, almost no projectiles match the drag curve, because they have a shape different to the G1 model.... Obviously, our diabolo pellets are completely different in shape, so we would expect the drag curve to be a completely different shape as well.... Let's say that in the subsonic range (<Mach 0.8 = 900 fps) the Cd matched the G1 model, but by the time the pellet was going supersonic (>Mach 1.2= 1350 fps) it had twice the drag of the G1 model.... In the transonic range (Mach 0.8-1.2 = 900-1350 fps) the slope of the drag curve would be much steeper.... This is typical for our pellets.... The result is that if you measure the BC at, say 800 fps, you would get one value, but if you do it at 1200 fps you will get a much lower value.... Using either value at a different velocity than where it was measured will give you false results for trajectory, velocity and energy retention, and wind drift....
I understand that the Military gave up on the idea of using the BC for artillery over 50 years ago.... They now have a drag curve for each projectile, and using a computer and a known muzzle velocity, they can predict accurately the trajectory, drift, etc.etc.... PB bullet manufacturers, for the most part, still use the antiquated concept of a single BC.... While that may work acceptably when you are starting at 3000 fps and still going 2000 at the target, for our purposes, where we spend nearly all the time shooting right in the range of velocities where our results are very wrong, it just no longer makes sense....
Coming very soon, we will have the ability to measure the downrange velocity, and watch the way it decays, with a Doppler Radar that is (barely) within the reach of hobbyists.... The LabRadar should be available in August, priced at about $560.00 US.... It has a range of about 100 yards in .30 cal, less in smaller calibers, more if the bullet has a larger base.... It is set up beside the shooter, aimed roughly at the target, and acquires the bullet about 6-10 feet out, triggered by the muzzle blast.... From there until the bullet is out of range, it returns the velocity, to within 0.1% accuracy, every 0.002 seconds, or about twice per yard for our use.... By downloading the results into a spreadsheet like Excel, and smoothing them, you can calculate the Cd at each velocity, and plot a graph for YOUR individual gun and pellet, that looks like the drag curve above.... With just a few shots, at a few different velocities, you can build yourself a drag model.... You can then input that drag model into a program like ChairGun and voila, the trajectory, retained velocity and energy, and wind drift will match your setup....
I have a dream.... I would like to build up a database of Cd curves for every pellet and bullet we can test.... over the widest possible range of velocities.... I don't know how practical that will be, but I do know that even the attempt will give us huge amounts of knowledge about ballistics as it applies to airguns.... Imagine if it turned out to be feasible, and ended up being adopted by a program like ChairGun.... When you select the JSB King, it selects the appropriate drag profile, you put in your muzzle velocity, and it crunches all the numbers for you.... The result is a much more accurate representation of what to expect downrange.... IMO, it can and will be done eventually.... The only question, really, is now the time?....
Bob
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Bob, I think that is an awesome dream to have! I say go for it! There is no better time than now! There are not many people I know that even have the ability to do something like that..... Wanting to use your talent, wisdom, and knowledge to help other people and contributing to something that you are passionate about is a great thing and can be very rewarding. I admire your ambition and want to thank you for all you do.
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There's a guy on Airgun Nation who have had the Labradar for some months. He's posted some ballistics. Here is a comparison of the JSB Kings and Heavy pellets
(http://img.photobucket.com/albums/v392/Kyogle/Labradar%20Superimposed%20graphs%20for%20Heavy%20and%20light%20JSB%20Kings%20Cd_zpski6bkrwx.jpg) (http://smg.photobucket.com/user/Kyogle/media/Labradar%20Superimposed%20graphs%20for%20Heavy%20and%20light%20JSB%20Kings%20Cd_zpski6bkrwx.jpg.html)
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That is what I'm talking about, but over a much wider velocity range....and for a large variety of pellets....
Bob
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Bob........You, of all people, NEED this Labdar. When is it available for order/purchase?
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Supposed to be for general release in August.... I am on a list to get an email when they are....
Bob
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That will be one of the best worst days ever! Best because that is an awesome tool and will really revolutionize data collection for shooters. Especially in the hands of a guy like you. Worst because all activities other than Lab-dar will seem to be a distraction. ;D
Bill G
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Bob;
I think you are right about this as well as the others. This data can give us the information we need to build a better pellet.
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Barnes has invested heavily in this modern approach. Our Doppler radar system can track bullets out to 1500 meters, recording the velocity and time of flight of that bullet every few feet along the flight path. Consider the graph below showing a bullet specific drag curve referenced to the more common G1 and G7 curves:
Ballistics Barnes Bullets Doppler Radar G1 G7 curve model drop chart DOF 6 degree of freedom
Neither of the standard curves is a particularly good match to our test bullet. In the legacy approach to generating a downrange trajectory table, the BC value is in effect a multiplier or a fudge factor that’s used to shift the drag curve of the test bullet to try and approximate one of the standard curves. This leads to heated arguments as to which of the standardized drag curves is a better fit, or if multiple BC values should be used to better approximate the standard curve (e.g., use one BC value when the velocity is between Mach 1 and Mach 2, and a different BC value when the velocity is between Mach 2 and Mach 3.) Barnes’ approach to creating trajectory tables is to generate bullet-specific drag curves, and use that data directly in a modern, state-of-the-art, 6 DOF ballistics program called Prodas to generate the firing solution.
Cut and paste from "Accurate Shooter" web sight in the recent "tech section".
I tried to move the picture (graph) as well, but it didn't take.
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I am on their list as well to receive updates. I think this is a great tool that will provide much needed data. Because I live in the mountains I shoot in wooded areas that are heavily shaded. This machine does not need light to work and should increase my usability. Love that I will be able to get down range velocities to calculate real world BC data on all my swaged slugs.
I noticed .177 caliber will only get about 30 yards so not sure if that will be great - I just ordered Corbin swaging dies to make .177 caliber slugs to add to my list of slugs.
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Interesting. In which sport will the added predictability of ballistics be profitable?
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Interesting. In which sport will the added predictability of ballistics be profitable?
Long range shooting of all types will profit from more accurate drop and drift data.
10 meter or Triathalon? not so much. ;-)
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most important, a LabRadar owner needs access to air rifles capable of driving the projectiles quite above the usual pellet velocities...somebody like Bob...:)
We could CloudFund you Bob...make the account and I'd donate...:)
cheers,
Douglas
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I'd think that Chair gun would be interested in assisting with the funding as well. After all, it makes there job a bit easier in the long run.
I'd be willing to toss in a couple bucks myself, I'm sure Hawke would like to get in on this action. Anyone know someone on staff that they could talk to, and put a bug in their ear???
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Although I am flattered by the offers of funding, I have a problem with the basic idea of asking for money for such a project.... It just doesn't sit right with me.... :(
I think that rather than start in on a new PCP project gun, this will have to take priority.... I'm already working on a Helium booster to allow a few shots that start Supersonic to analyze and confirm that part of the velocity range as the pellet/bullet drops back through the Transonic range.... The upper end of that is considered to be Mach 1.2 (1340 fps), so if I can get the muzzle velocity up into the 1400s with even a few pellets I should be able to cover all of the usable Subsonic and Transonic velocities....
Bob
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I'd think that Chair gun would be interested in assisting with the funding as well. After all, it makes there job a bit easier in the long run.
I'd be willing to toss in a couple bucks myself, I'm sure Hawke would like to get in on this action. Anyone know someone on staff that they could talk to, and put a bug in their ear???
You might be making assumptions about people, entities, who may not agree with you? I'm going with maybe we just let the dust settle and give people whom you do not speak for an opportunity to speak for themselves?
Please don't take offense.
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I would agree, we can't make assumptions about any of the Commercial companies (Hawke now owns ChairGun, or at least the rights to it, BTW) and what they would or would not find a good "investment".... In addition, I have no interest in hitching my wagon to any particular horse for compensation....
Bob
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Although I am flattered by the offers of funding, I have a problem with the basic idea of asking for money for such a project.... It just doesn't sit right with me.... :(
I think that rather than start in on a new PCP project gun, this will have to take priority.... I'm already working on a Helium booster to allow a few shots that start Supersonic to analyze and confirm that part of the velocity range as the pellet/bullet drops back through the Transonic range.... The upper end of that is considered to be Mach 1.2 (1340 fps), so if I can get the muzzle velocity up into the 1400s with even a few pellets I should be able to cover all of the usable Subsonic and Transonic velocities....
Bob
;D so from this statement, would it be safe to say that you are all in for one of the new LAbdar systems Bob? ;D ;D
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I can only hope that by the time they are released I can afford one.... but that is certainly the plan....
Bob
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Bob, I have great respect for your work, and agree that someday this will happen. I hope it is you who accomplishes it. Yes ,it would be a lot of work, but just imagine the possibilities! I will be watching this one closely! 8)
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For starters, I can't state enough how much I enjoy and learn from each of these jewels that you post, Bob! Really shakes the foundations of what I thought that I knew. I really appreciate the time and effort you put into these posts.
Now did I miss the post about what became of the Labradar?
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Preproduction sold out, as did the first production run, delivering currently.... Next run will be early in 2016, but I doubt I will have the funds lined up yet.... Maybe by the time I do, they will have caught up to demand and they will be in stock.... I have recently been contacted by NOE Bullet Molds to supply them with drawings for all my Bob's Boattails.... This covers .217, .250, and .300 (airgun) calibers, plus .224, .257, .284, .308, .357, .408, and .458 bullets for airguns as well.... There are 2-5 weights per caliber, so they now have my designs for 37 different bullets.... I doubt that many of them will ever make production, but because there are only 8 basic designs that are scaled up and down for the different calibers, it should be possible to develop Cd curves for all of them, once I have a LabRadar....
Bob
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Still say we "pass the hat", and take up donations here in the name of "continued learning" to ensure you have the funds ready, when they become available once again.
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Harry in Australia recently recieved one of the devices. His comments indicate that tracking pellets is difficult.
Perhaps it is the hollow base.
Just a comment.
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Harry produced some excellent results already with his, on the old and new JSB Kings.... I never saw mention of any tracking problems out to about 90 yards, IIRC.... The larger the caliber, the further away it can be tracked, however....
Bob
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So..... At the risk of this being a REALLY stupid question. Might I ask?
Without something like the LabRadar, is there a way to determine and/or discover the correct BC and Cd curves for our little old pellet guns and pellets???
Especially in the smaller calibers like .22 and the .177 since the device might have trouble with that size pellet anyways.
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A pair of chronometers
one near and one far. Just were near is, and were far is, is up to the investigator.
To evaluate the ballistics at some desired velocity it may be required to setup at 80 and 90 meters etc.
or......some other increments
good shooting... ;-)
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You can determine the BC at ONE velocity by using a pair of Chronys.... Unfortunately, our pellets are a poor match for any of the Drag Models used to calculate the BC, so it will only be correct at that one velocity.... Use a bunch of different velocities, and you can plot your own drag curve.... The LabRadar allows that in fewer steps, and for one particular pellet and velocity, just one step, out to the acquisition range of the radar....
Bob
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I have a Labradar on order but it seems they will not be out until December or January because of a back ordered part. I think the smaller the projectile the harder for it to pick up and hollow bases will be even harder. For my slugs it should, hopefully work good for.
Here is a question, would love to hears Bob's take on. Is there a standard we could do for testing across all pellets, slugs, etc like a BC at 50 yards?
Trying to come up with a value that would be useful for air gunners to relate to and set some type of standard. Hard to test at too many velocities to be feasible.
Would reporting a set yardage, say 50 yards for now, for all testing along with the velocity at that yardage listed be a representative value for people to use and determine what pellet or slug to use? Is there some standard we could use to at least have a comparison to evaluate different pellets and slugs to each other?
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I wish that was the case, but there are two basic problems.... First, the muzzle velocity affects the drag coefficient between the muzzle and 50 yards, so you would also have to specify the velocity to be used.... Secondly, the BC you determine depends on the model you use (G1, G7, GA, RA4, etc.).... We can solve the latter by using the most widely accepted and known G1, but it is a very poor match for pellets, although much better for most bullets.... The G1 drag model, and the shape of the projectile used, is shown in the first post in this thread....
Even if you used a standard muzzle velocity, such as 850 fps, and distance, such as 50 yards.... The BC you determine would likely not even be close if you used 600 fps, or 1000 fps, and even further off the faster you go past that.... Even with JSB Exacts, I have seen the BC (G1) drop off from about 0.040 at around 800 fps to about 0.010 at 1200 fps.... They literally lose 100 fps in the first few yards if driven supersonic.... Testing them at high speed over 50 yards won't show this properly because of that drastic initial drag....
Bob
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Bob, I appreciate your feed back, like many on here, I truly value it.
Can you see anyway to offer a reasonable value. I would like to come up with something but for the same reasons you mentioned, I know it only works for that scenario.
As a reloader for many years I purchased bullets from major manufactures like Hornady or Barnes, etc who would list a BC on the box. I know those BC numbers are more for advertising and they can do testing at different speeds, ranges, etc and use the one with the best BC. There seems to be no standard and is more of a gimmick the way they are advertised. Bryan Litz has a lot of information about this online.
I want, if possible, to offer a real BC that is meaningful but can not see how. Mostly what I see is guys posting muzzle velocities which to me tells me nothing other than the power of their gun.
The other part of measuring BC is the actual gun. Since in this scenario we are using velocities at two ranges one gun could fire it faster or slower than another even if the muzzle velocity is the same. If rifle A fires in a very straight line with no spiral, the bullet would be traveling faster through the second chrony than rifle B if the bullet has a spiral out of that gun and therefore travels further to get to the second chronograph. I guess if the twist rate is a good match for the bullet we would get a better BC than if the twist rate is not optimal.
I may be chasing something that is not possible/feasible.
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For those inclined
http://throwinglead.com/index.php?page=ballistics_external (http://throwinglead.com/index.php?page=ballistics_external)
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The difference in distance due to spiralling would be unmeasureable.... but if the twist rate is wrong and the bullet yaws, then the BC increases.... If your Stability Factor is 1.5 or greater, that shouldn't be the case.... For your purpose, I would use a standard muzzle velocity of about 850 fps, first Chrony at 1 yard and second at 50 yards, and use the G1 model to calculate the BC.... Don't forget to record your atmospheric conditions and correct to NTP.... That would give you a direct comparision between your bullets, and by specifying the test conditions would be as realistic as possible.... The BC below that would be relatively constant, but may be somewhat lower above that, but by specifying the conditions at least you are comparing apples to apples.... If somebody pushes the bullet faster, the lower BC won't apply for very long anyway before the velocity is down to 850 again.... Besides, that will give you the highest possible BC (or nearly so) for bragging rights.... *LOL*....
The Cd of the G1 Model is within 10% of the value at 850 fps from 300-900 fps, and it's only 22% higher at 950 fps.... Since the better ballistics program use the G1 model as their baseline for calculating drop and drift, those changes are already taken into account.... So, you only have to deal with how much of a mismatch there is between YOUR bullet shape and the G1 model.... Compared to a pellet, most of your bullets should be close enough at 950 fps and below....
Bob
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That sounds reasonable to me. I guess the best we do.
Thanks for taking the time, and great advice.
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Now I'm starting to feel like I'm the "King of dumb questions", but here is another one :)
Using a .22 cal JSB pellet as an example.
Knowing that the LabRadar will have a short range with such a small pellet, would it possible to fire the first string of pellets (5 shots per string) and determine the max range that the LabRadar can read, then move the LabRadar down range at or near that distance in order to be able to get the rest of the distance. Let's say for example that the LabRadar can read the .22cal pellet out to 50 yards. Could you then move the LabRadar out to 50 yards to get the remainder of the readings out to 100 yards? Of course you would need to stitch both pieces of data together at the mid point. That is also "assuming" that you can achieve the same muzzle velocity for both strings fired. It might not be perfect, but maybe it could get you really close.
I hope that made sense cause I'm not sure I said that correctly :)
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Now I'm starting to feel like I'm the "King of dumb questions", but here is another one :)
Using a .22 cal JSB pellet as an example.
Knowing that the LabRadar will have a short range with such a small pellet, would it possible to fire the first string of pellets (5 shots per string) and determine the max range that the LabRadar can read, then move the LabRadar down range at or near that distance in order to be able to get the rest of the distance. Let's say for example that the LabRadar can read the .22cal pellet out to 50 yards. Could you then move the LabRadar out to 50 yards to get the remainder of the readings out to 100 yards? Of course you would need to stitch both pieces of data together at the mid point. That is also "assuming" that you can achieve the same muzzle velocity for both strings fired. It might not be perfect, but maybe it could get you really close.
I hope that made sense cause I'm not sure I said that correctly :)
sounds logical to me...
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The LabRadar is triggered by the muzzle blast, I doubt it would trigger downrange very far from an airgun.... I plan to adjust the velocity for subsequent shots and "stitch" the data together in Excel.... The datastream you can download is about 2 readings per yard, IIRC.... It needs smoothing, but should be possible to just use sequential blocks of data to get the whole drag curve.... With pellets, the first part of the curve (supersonic) will all occur well within the range of the LabRadar because the pellet slows so quickly....
Bob
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Oooooh,
I didn't know that the LabRadar was triggered by muzzle blast. That would make it tough to pickup the blast of a fully shrouded air gun downrange for sure. I suppose one could remove any LDC or shrouds for the test though. I just assumed (incorrectly of course) that the radar was an active radar, and that it picked up what ever passed within its "area of detection".
That LabRadar sounds really high-speed :)....... Pun intended :) Hehehe...
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Bob,
I am buying one of these. It's going to be my next shooting purchase guaranteed, even before a 4500-psi carbon tank.
$560 is doable.
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Very interesting indeed... looks like a very valuable tool thats for sure. Time to throw the old Chrono to curb lol
It also has more than one way to trigger it... I guess it is an extended Microphone.
You will need a Air Gun Trigger Adapter http://www.buymylabradar.com/product.sc;jsessionid=4EDDD3CFCA4447F0C45D3325BDEA405E.m1plqscsfapp02?productId=14 (http://www.buymylabradar.com/product.sc;jsessionid=4EDDD3CFCA4447F0C45D3325BDEA405E.m1plqscsfapp02?productId=14)
http://www.mylabradar.com/ (http://www.mylabradar.com/)
What range performance can one expect with LabRadar?
In general LabRadar will typically measure the velocity of a 7.62 mm projectile from muzzle up to 100 yards. After extensive testing of a variety of calibers you can expect to obtain velocities at these distances when in the Standard Power Mode. Low Power setting will have about 30% less tracking*.
.177 Pellet - 30 yards;
.177 BB - 30 yards;
22 LR - 60 yards;
223 - 60 yards;
270 - 70 yards;
308 - 80 to 100 yards;
9mm - 130 yards;
40 S&W - 130 yards;
45 ACP - 130 yards;
500 S&W - 130 yards;
12 gauge Slug - 90 yards;
Paint Ball - 50 yards;
Arrows - 50 yards;
http://www.midwayusa.com/product/523157/labradar-ballistic-velocity-doppler-radar-chronograph (http://www.midwayusa.com/product/523157/labradar-ballistic-velocity-doppler-radar-chronograph)
(http://media.midwayusa.com/productimages/880x660/Primary/523/523157.jpg)
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Looks like a more sensitive microphone for guns with a quiet report (airguns, rimfire, and supressed are mentioned).... still triggered by the muzzle blast....
Bob
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Just splice into the cable and make it 50 yards long. Problem solved. :)
Of course, it might need a battery packed added to boost the signal over that distance. :) Hehehe. How bout if we just add one of them "clap on, clap off" sensors like they used to show on TV infomercials. Hehehe. :)
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Since we have to "stitch" the data together, it's just as easy (for me) to just dial down the velocity to about what it is at the longest range the LabRadar can pick up (say 60 yards) and shoot another set.... By repeating that a few times I should be able to get the whole drag curve.... and not have to move the LabRadar downrange several times....
Bob
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But as close as that might be it is an approximation and introduces a variable. Maybe put a parabolic reflector on the microphone.
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The data, while very good, still requires running through a smoothing (curve fitting) program, as each individual pair of measurements are so close together (about 18" apart) the BC jumps around due to the (extremely tight) tolerances on each measurement.... I don't see a problem with stitching together the data.... as I said whether you do it by moving the LabRadar, the microphone, or changing the muzzle velocity, you will still only get, say, 65 yard "chunks" of data anyway.... There is no way you will be able to measure one shot from 1100 fps down to 500.... which might require 300 yds. for a pellet, much more than that for a heavy bullet with a good BC....
Bob
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I was looking for some information on how to calculate the Drag Coefficient (not the BC) if you know the starting velocity and time to target last night.... and I never did find an equation to do that.... However, I did run across this EXCELLENT article explaining in layman's terms exactly the point of this thread....
http://www.airgunmagazine.co.uk/features/pellets-in-drag/ (http://www.airgunmagazine.co.uk/features/pellets-in-drag/)
There is a graph there that explains how the drag of the drag model is "moved up" to compensate for the Ballistics Coefficient, and it shows at a glance how that helps, but since our pellets don't match the drag model, it will only be correct at one velocity.... It's worth a read IMO....
BTW, if anyone has the formula I am looking for, I would really appreciate it....
Bob
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Hey what ever happened with that "LabRadar" thingy??? Did it pan out?
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Couldn't afford one yet.... but it appears the order backlog is getting smaller....
Bob
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Midway got them in last week and already out of stock on them.
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I still say we pass the hat on this. I would not mind at all to toss in a few bucks for a VERY worthy cause.
Besides you've been such a great help to so many on this forum, and have provided awesome educational info and ideas to further Air rifle shooting. You've shared your time and vast amount of knowledge with us all freely.
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Could pellet/bullet coefficients of drag be measured in a wind tunnel? A small wind tunnel and a strain sensor shouldn't be too hard to setup, I would think. But I don't know since I've never done anything like that.
Taso
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The Air Heads show , built large mock up of pellets and put them in a makeshift wind tunnel .
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The Drag Coefficient changes with velocity, so you would need a wind tunnel capable of handling all the velocities you want to examine.... Since the Reynolds Number is critical, the models would have to be full size (ie pretty darn small), which means the instrumentation would have to be very sensitive.... All in all, probably a LOT more expensive and less accurate than using a radar (or even two Chronographs)....
Bob
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James,
I had never heard of them till you just mentioned it. I watched the wind tunnel episode but it only seems they observed air flow.
I was thinking more in the lines of like an electric radio controlled plane engine connected to a 1 or 2 inch pvc pipe. I'm sure air speed can easily be adjusted and measured. A pressure or strain sensor, I don't know the right term, could then be rigged to measure the push of the air on the pellet? I just guessing here.
Bob,
I do understand the LabRadar is proven and a wind tunnel would be cool to build but I don't know how the data collected would apply. So you're probably right. I haven't been able to even figure out how to size a motor to create the flow equivalent to 1200 fps of bullet flight.
Taso
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Wind tunnels for low subsonic velocities can use scale models and variable air density to model what happens full size.... However, when you get into the velocities we are interested in, particularly over Mach 0.8 (900 fps) you enter "compressible flow", and the best way is to use full size and actual velocities....
Bob
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While it is true that the BC can be calculated from the drag function, and also calculated using two chronos, it is also true that the BC can be calculated from two drop points along the trajectory, and that the drag function can be calculated from the BC table.
Going to radars and wind tunnels, IMHO is overcomplicating stuff.
And if you add that the BC of each pellet depends on the barrel it was shot from, you will realize that trying to get the drag function, while a commendable intellectual exercise, has no practical value.
What we need, as airgunners is what OUR pellets, shot from OUR barrels, under OUR conditions, are doing out in the air. That is what will define if we hit our target, or not.
It is the USE of a calculated BC obtained from the MV and the drops at different ranges what allows to calculate precise wind drifts for each section of the trajectory.
JMHO
Héctor Medina
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I wish it were that easy, but the Cd changes with velocity over a very wide range.... It may not be much of a problem if you stay below 900 fps at the muzzle, because by the time the pellet has travelled just a short distance, the velocity has declined to the point where the drag is tending to plateau, and a single value of BC can be made to work quite well.... However, if you push the velocity closer to the speed of sound, the drag goes up so quickly that the pellet slows down drastically within the first few yards.... What I hope to accomplish is a better understanding of how the drag changes over a wider range of velocity.... Then using any of the available methods, radar, two velocities, or two drop distances (along with the starting velocity) could be used with that improved drag model to predict pellet behaviour over a wider range of velocities, and hence over greater distances....
I agree with everything you say, that the BC can vary from gun to gun, and certainly with velocity.... I would like to improve the model, so that we can gain knowledge of, and make better use of, how the velocity decays over longer distances.... If I can accomplish increasing the accuracy of predictions to the point that it will duplicate a drop table over many distances from zero to 100+ yards, for various muzzle velocities, without having to shoot every combination.... I will feel that I have added to the body of knowledge....
You probably remember when it was common to use a constant Cd to calculate the BC of pellets.... That method is so glaringly wrong, when used to compare a velocity of 500 fps to one of 1000 fps, as to be a joke.... The G1 model is better, and the GA model slightly improved again (for pellets).... but from the measurements I have made, neither account for the huge increase in drag as a pellet crosses Mach 1.... Perhaps that doesn't interest anyone, but that doesn't mean we should settle for using a drag model that is in error.... when the technology now exists to measure and quantify drag in the transonic region.... If exploring that region does nothing more than discouraging the use of velocities over 1100 fps with pellets, by showing graphically how fast they decelerate.... it will focus people on doing what DOES work....
Ultimately, if this whole exercise turns out to be a monumental waste of time.... then at least it will be my time that is wasted doing it.... For me, the journey will be worth it.... ::)
Bob
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Bob,
No research is time wasted. Like you said, the data will help us understand what is going on in the transonic region. I can't speak for others but I'd definitely like to know.
I am going to oversimplify for a bit here: If shooting a projectile at 1000 - 1100 fps has no downrange benefit than from starting at 900 fps then we can save on air in our tanks.
Thank you Bob,
Taso
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That is pretty much the case.... a pellet starting at 1100 fps ends up below 1000 in about 5 yds.... I think a few shots starting at 1200-1300 with the LabRadar set to show velocities at 10 and 25 yards will shock a lot of guys....
Bob
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Bob,
No research is time wasted. Like you said, the data will help us understand what is going on in the transonic region. I can't speak for others but I'd definitely like to know.
I am going to oversimplify for a bit here: If shooting a projectile at 1000 - 1100 fps has no downrange benefit than from starting at 900 fps then we can save on air in our tanks.
Thank you Bob,
Taso
x2
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I wish it were that easy, but the Cd changes with velocity over a very wide range.... It may not be much of a problem if you stay below 900 fps at the muzzle, because by the time the pellet has travelled just a short distance, the velocity has declined to the point where the drag is tending to plateau, and a single value of BC can be made to work quite well.... However, if you push the velocity closer to the speed of sound, the drag goes up so quickly that the pellet slows down drastically within the first few yards.... What I hope to accomplish is a better understanding of how the drag changes over a wider range of velocity.... Then using any of the available methods, radar, two velocities, or two drop distances (along with the starting velocity) could be used with that improved drag model to predict pellet behaviour over a wider range of velocities, and hence over greater distances....
I agree with everything you say, that the BC can vary from gun to gun, and certainly with velocity.... I would like to improve the model, so that we can gain knowledge of, and make better use of, how the velocity decays over longer distances.... If I can accomplish increasing the accuracy of predictions to the point that it will duplicate a drop table over many distances from zero to 100+ yards, for various muzzle velocities, without having to shoot every combination.... I will feel that I have added to the body of knowledge....
You probably remember when it was common to use a constant Cd to calculate the BC of pellets.... That method is so glaringly wrong, when used to compare a velocity of 500 fps to one of 1000 fps, as to be a joke.... The G1 model is better, and the GA model slightly improved again (for pellets).... but from the measurements I have made, neither account for the huge increase in drag as a pellet crosses Mach 1.... Perhaps that doesn't interest anyone, but that doesn't mean we should settle for using a drag model that is in error.... when the technology now exists to measure and quantify drag in the transonic region.... If exploring that region does nothing more than discouraging the use of velocities over 1100 fps with pellets, by showing graphically how fast they decelerate.... it will focus people on doing what DOES work....
Ultimately, if this whole exercise turns out to be a monumental waste of time.... then at least it will be my time that is wasted doing it.... For me, the journey will be worth it.... ::)
Bob
Interesting point of view Bob, but IF we want to add to the body of knowledge, shouldn't we be exploring OTHER things than velocities? The drag function for common bullet-shaped projectiles is pretty well known by now after 200 years of experimenting, and as you know, really serious ballistics always goes to real data gathered with the specific projectile used , consider that all other ballistics software offer some sort of "calibration" routine to make your results closer to what the computer tells you WILL happen next time you pull the trigger. Research into different SHAPES would be much more productive, coupled with MATERIALS to stay within the energy levels that different airguns can produce.
I've been involved in ballistics (seriously), since 1987, when I bought my first Chrono. Measures 2 ft long and still works. Windows are tiny by modern standards, weight and battery usage, really poor and archaic, but it opened a whole world of REALITIES to me. I started researching into UHV projectiles, mostly of the sabotted (heavy and light) architecture, achieved speeds in excess of 6,000 fps using steelhead cases, primer tubes, solid brass turned bullets, slow twist barrels, and all sorts of specifics to augment the energy available for the true payload.
I started swageing my own bullets and had a ball making "H Mantle" jackets out of thick copper tubing (a good alternative to turning).
Yes it was an interesting journey, but in the end, it convinced me that the real future was not even there.
So, yes I remember the tables of nose and tail shapes that would then estimate (very poorly) a BC based on SD and FF.
Anyway, the REAL difference between our points of view is that you want to find the "law" that regulates the phenomenon in order to extrapolate results, I, on the other hand, lost faith in extrapolation a long time ago.
But no post should go without a positive proposal, and since several here have expressed the intention to help, let me propose something:
Let us form a research group.
Every person that wants to help subscribes with Bob
Bob will set "areas" of research he wants explored: Pellet/bullet styles and speeds, mainly.
People will volunteer to take whatever experiments Bob sets out for these "areas".
Bob condenses all the info and sends it to me.
I will run PP Calc for every four sets of data (one "zero range" and three other ranges) and return to Bob the BC for EACH section between the ranges, together with the matching velocity degradation curve.
With a large enough amount of data, shooter errors should be statistically minimized and the differences between barrels should even out to an "average" (albeit always inexistent) barrel.
With all the BC's then the Drag function can be calculated.
If someone was suggesting "crowd funding", this is suggesting "crowd data gathering". There is strength in numbers and this could be a good test of that.
No need for money, all it takes is some serious commitment to do things properly and a little of everyone's time.
Just a humble proposal.
Héctor Medina
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Compared with you, Hector, I am but a neophyte, trying to coax airgunning into the next century.... I think we both feel the same, however, that airguns, particularly PCPs, have far outstripped the humble diabolo pellet, which is why a mere 2 years ago I suggested that boattails may be appropriate as the next leap in downrange performance in airgunning....
Your idea of "crowd gathering data" has much merit, but sadly my organizational skills are sorely lacking to undertake such a complex project.... I can only imagine the complexity of trying to gather consistent data from hundreds (hopefully) of participants, let alone trying to make sense of it.... A worthy project, yes, just beyond my abilities, and likely lifespan as well....
In the short term, I hope to plod along and develop a better drag model for pellets in the transonic region.... Longer term, I hope to get some hard data on the improvements that can be made using boattails over flat-based in the subsonic and transonic regimes.... Like you, I have a basic suspicion of much of the data we have now, even though I have a deep belief in the science of ballistics.... I just think that the small corner of OUR world is poorly understood, and even less well documented.... In some small way, I hope to improve on that situation....
Bob
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bump :)
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ttt
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As far as I can tell a gun firing an energy pulse , would not be considered a fire arm , there is the future , set your cell phone to stun ! LOL
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As far as I can tell a gun firing an energy pulse , would not be considered a fire arm , there is the future , set your cell phone to stun ! LOL
In the "Dune" series of books, one interesting aspect is that in the far future (or far past, of humanity) weapons divide themselves into (IIRC) three classes:
Energy/pulse when humans use them against "thinking machines"
Kinetics when you want to destroy humans or property
Blades for fighting with shielded enemies (shields prevent high speed objects from reaching the target, but a blade is a low velocity action).
I do think we'll be headed that way.
BTW, have you seen the fireball out of electromagnetic launchers? If that is not a firearm, then we need to revise the term, LOL!
Keep well and shoot straight!
HM
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I haven't forgotten about this idea.... but life keeps getting in the way.... Our Motel keeps me busy in the summer (shooting season), and therefore it's difficult to justify the expense of a LabRadar at the present time.... We will be closing the Motel in February of 2021, so in 4 more years time I should have tons of time to devote to shooting and research.... Perhaps by then there will be a selection of low-cost Doppler Radars available, I have heard rumours of other Chrony manufacturers looking into it....
I told you this would be a long term project.... ::)
Bob
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Something for you to ponder and comment on....
I have a dream.... I would like to build up a database of Cd curves for every pellet and bullet we can test.... over the widest possible range of velocities.... I don't know how practical that will be, but I do know that even the attempt will give us huge amounts of knowledge about ballistics as it applies to airguns.... Imagine if it turned out to be feasible, and ended up being adopted by a program like ChairGun.... When you select the JSB King, it selects the appropriate drag profile, you put in your muzzle velocity, and it crunches all the numbers for you.... The result is a much more accurate representation of what to expect downrange.... IMO, it can and will be done eventually.... The only question, really, is now the time?....Bob
Hey Bob,
I would think this information sure would help airgun tuners optimally tune a gun for best shooting pellets and specific shooting distances.
Do you think this could make a noticeable difference in accuracy or is this just marginal, theoretical puffery?
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Having been an avid hunter for some 20 years (in the 60's and 70's) and a shooter now for almost 50 years I personally go with marginally useful. I believe that the same amount of energy spent actually shooting one's guns to ID the best projectile(s) for your particular gun/need/varying conditions is much more productive for the vast majority of shooters. But I do have to applaud Bob for the effort he puts into the research and useful information could well come from such theoretical efforts, especially considering the potential changes in pellet design that seems to be occurring.
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The current GA Model is better for Diabolo pellets than the G1 Model used by PB.... and FAR superior to the constant Cd of 0.2 which was used previously.... It was so far out of whack at velocities over 800 fps it was pretty much a joke.... I still think that there is a much larger increase in drag in the Transonic region than currently modelled by the GA Model, and hope to prove that conclusively....
While the improvements in the model may be marginal at most of the velocities we use.... I think that once we have a better understanding of how quickly the drag increases above 1000 fps it will put to rest once and for all the suggestions of shooting at Supersonic velocities that keep popping up....
Bob
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So, Bob, are you thinking that the issue with drag across the limited velocities at which we can shoot airguns will overcome and negate any advantages that may be gained through more "efficient" projectile design for airguns?
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Not at all.... a lower Form Factor is always a good thing, for the same SD you will get a higher BC.... Bullets have higher BCs for two reasons, they have a lower Form Factor (drag) and usually a higher Sectional Density (weight vs. cross section).... Just as a rough guide, Diabolo pellets (good ones) have a FF about the same as a roundball.... ie not very good, about 1.5.... Flat-Based bullets with a Meplat run around 1.0-1.2.... Boattail bullets can have up to 30% less drag than a FB of the same general shape....
Each type of bullet has its own relationship between drag and velocity.... The problem is that we are comparing things to drag models that are not really close in many cases.... More information will allow us to have a better picture of that relationship for various shapes.... but one model will never work for all shapes.... If you use the right drag model, the BC will be constant throughout the range of velocities, instead of only being right at one speed....
Bob
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Thanks Bob. I do hope you manage to acquire the equipment you want and do the research. Interesting concept.
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Awesome idea about constructing an empirical Drag Coefficient curve for each pellet!
Bob, are you using some sort of a regression model to construct the curve from test data?
I imagine the test data would probably be quite noisy.
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Yes, the data is "noisy", particularly if you use a Chrony, not a LabRadar.... The best way is to use the radar data (which has tons of data points, every foot or so) and then do curve matching with Excel, and use the smoothed curve for your data points.... I have also had reasonable success using just the "screen data" from the LabRadar, with the distances set to 10, 25, 50, 75 and 100 yards.... I wrote an Excel spreadsheet where you can input those numbers for numerous (say 10) shots, and it averages them and then calculates the Cd over each increment.... This comes up with a Cd for each average velocity within each increment, and then using the G1 model can compute the BC (G1) for each increment.... You get results like this....
(http://i378.photobucket.com/albums/oo221/rsterne/LabRadar%20Data/16%20gr%20JSB_zpsdoq7a1rp.jpg) (http://s378.photobucket.com/user/rsterne/media/LabRadar%20Data/16%20gr%20JSB_zpsdoq7a1rp.jpg.html)
Those were actually done using 5-shot average velocities from a Chrony, so you don't need a LabRadar to do this.... Note the big increase in drag (reduction in BC) of the first data point (0-10 yards).... The program also calculates the average BC (G1) over longer increments, 0-50, 50-100 and 0-100 yards as well.... For that pellet, the results were....
0-50 yards.... 0.039
50-100 yards.... 0.037
0-100 yards.... 0.039
There is a basic problem with BC.... that you are no longer using the raw Cd data, you are comparing it to a "standard" drag model that doesn't match your pellet.... hence why the BC changes with velocity.... If the model was an exact match, then the BC would be constant.... That is why using the raw Cd data would be better, but then we have another problem.... All the ballistics programs I am aware of don't compute drop and drift from the raw Cd, they use the BC as an input.... Chairgun gives you the ability to create your own custom drag model, but there are a limited number of points you can input.... It's kind of like Bryan Litz's tables where he gives different BCs for bullets for different velocity ranges.... That is better than a single BC, but neither is likely correct for velocities right where you change from one range to the next....
AFAIK, Lapua is the only company that has ballistics software that uses actual Cd curves for their own bullets to calculate drop and drift.... but I have never had the opportunity to use it, nor would there be any point, because they basically use a drag model for each of their own bullets....Still, wouldn't it be great if you could just use actual, measured Cd data to calculate drop and drift?....
Bob
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Thanks, these BC and Drag Coefficient curves look really great - nice and clean.
Looking at the rate of change of the Drag Coefficient and BC, I think that probably 5-6 points might be sufficient to define the curve and between those 5-6 points it could be interpolated. It would indeed be great if pellet manufacturers started specifying at least 5-6 data points instead of a single BC. One could then simulate in software the optimal shot for different conditions.
So, it seems with the currently available types of pellets it doesn't make much sense going above 900 fps.
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Specifying 5 or 6 data points (like Bryan Litz does for PB bullets) would help.... but you then have to blend from one to the next as the bullet goes downrange and slows.... Doing it like Lapua, with a complete Cd curve is the ideal.... It is my hope that after I retire in 2021 I will have enough time to shoot, and a LabRadar (or who knows what is available by then).... to come up with an improved "GA" drag model that more closely resembles the MAJORITY of pellets.... so that the BC is nearly constant over the entire range of velocities, including all of the Transonic range (up to Mach 1.2 = 1350 fps).... My guess is that I will have to come up with at least three drag profiles, one for Wadcutters, one for Round Nose, and one in between.... but that remains to be seen....
Bob
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I still say we "PASS THE HAT" in order to make this happen more quickly. I'm sure many of us wouldn't mind tossing in a few bucks to further the air rifle shooting sports for all. I myself would love to see this mystery finally solved completely. Heck there are so many active members lurking here that if each one put in one dollar you'd have enough to buy 2 or 3 of those things. I would think that the big time pellet manufactures would have already jumped on this sort of thing.
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That has been suggested before.... but in reality I won't have the time to devote to this project until 2021 after we close the Motel.... Patience is a virtue....
Bob
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Patience is a virtue....
Bob
Patience my @@@!!! I want to kill something!!!! Hehehehehe...... LOL. :)
I just couldn't resist. ;D
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Pellgunfun
"...nuke it from orbit. It's the only way to be sure." Ripley
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I just seen a magazine article that tests pellets BC via Lab Radar,
17,22,25,30 calibers.
I don't think I can name the mag here........... ;)
But have posted the info in the Airgun Guild site.
Tia,
Don