Ballistics Coefficients Discussion

[rsterne]

I hate stealing other guys data, but this supports what I have been trying to achieve, and gives an idea just how much information can be found using the Doppler Radar....

http://www.network54.com/Forum/79537/message/1433753419/Here+is+a+Drag+Coefficient+comparison+from+some+data+I+collected+at+the+farm.

The graphs clearly shows the trend I have been finding with the Cd going through a minimum at about 700-750 fps, and a large increase in drag at 900 fps.... They haven't examined the transonic region yet, hopefully that will come.... I'm really wishing I had a LabRadar setup....

Bob

[Scott Endler]

You have shown us how BC falls  rises and falls again as it goes through the peak from 750 to 830 fps but it is very surprising to see CD actually getting worse as you get slower than that. There must be a different skin effect or changing reynolds number or a standing wave effect filling in the skirt at the minimum CD.

[QVTom]

I hate stealing other guys data, but this supports what I have been trying to achieve, and gives an idea just how much information can be found using the Doppler Radar....

http://www.network54.com/Forum/79537/message/1433753419/Here+is+a+Drag+Coefficient+comparison+from+some+data+I+collected+at+the+farm.

The graphs clearly shows the trend I have been finding with the Cd going through a minimum at about 700-750 fps, and a large increase in drag at 900 fps.... They haven't examined the transonic region yet, hopefully that will come.... I'm really wishing I had a LabRadar setup....

Bob

Nice to see your (Bob's) findings supported by another group with a completely different method of measurement   Yes we are all waiting for the tests to move to the transonic.

You have shown us how BC falls  rises and falls again as it goes through the peak from 750 to 830 fps but it is very surprising to see CD actually getting worse as you get slower than that. There must be a different skin effect or changing reynolds number or a standing wave effect filling in the skirt at the minimum CD.

Having some experience with Doppler systems in the past, I wonder how the data is being integrated and if the last bit a data just before the return is lost is reliable.  I'm not saying it isn't but a weak (sporadic) return could explain the rise in cD at the end of contact. 

Tom

[rsterne]

The data measurements are about two per yard, and part of the datastream is the Signal to Noise Ratio, you can see it deterorating in the last few measurements.... However, with that many data points (150ish), so what if you have to throw out the last couple.... Unfortunately the range of the LabRadar is about 100 yards on a .30 cal, and less for smaller calibers.... so the only way to get a large range of velocities is to do like I am doing an launch them at a series of velocities that overlap and then try and blend the results.... at least that would be my approach....

I don't think it's a Reynolds number effect causing the rise in Cd at low velocities.... Even the G1 Model shows increasing Cd below 600 fps (although not as drastic), and it's 1" caliber, over 3" long, and weighs a pound....

Bob

[rjorge]

What do you think about starting a funding pot to get Bob a new toy?!

 I'm not sure how we can get this going, but I have learned so much from him and would be very happy to chip in a few bucks. What do you say??

Btw, no strings attached. He can do whatever/whenever he wants to do with it.

[SpiralGroove]

Hey rjorge,
I'd be more than happy to help fund such a pot   -----> "The Cookie Man's R&D fund."
Definitely, No Strings Attached .
Kirk

[Rizen 1]

I would also generously donate to that cause!  Keep us updated on when and how ...

[Bwalton]

Thats very interesting seeing the lowest drag@ 715fps for a 34gr pellet? The whole idea of going to a much heavier pellet is to gain ftlb... The graph only shows to 900fps I am wondering does the drag began to drop back down at an higher velocity? further down the graph?

[rsterne]

While the lowest drag is in the 700s, the difference in BC is extremely small between say, 750 and 850 fps.... However, I would expect the drag to continue to increases with the King Heavies just as it does with the smaller caliber Exacts, showing about a four-fold increase in drag by the time you push the pellet to Mach 1.... It is understanding and recording that increase in order to develop a new Drag Model for pellets that this thread is all about....

One of the nice things about using the LabRadar is that we should be able to record the entire drag curve with much fewer shots required.... I would expect that a single shot at about 1300 fps would supply all the data needed for the transonic range, down to about 900 fps, possibly even lower.... because the pellet slows so quickly in that speed range....

Bob

[Bwalton]

Where can I buy a  LabRadar?

[Bwalton]

Seems that it does have some limitations, wondering how well will it work with pellets....

Different bullets can be read at different ranges. Preliminary testing has shown these approximate range limitations for some popular cartridges. I was told that these are conservative, and the Labradar may read them further out:

    22 LR: 50-60 yards
    223 Rem: 40-60 yards
    270 Win: 50-70 yards
    308 Win with boattail bullet: 60-80 yards
    308 Win with flat base bullet: 100 yards
    9mm Luger: 100+ yards
    40 S&W: 130 yards
    45 ACP: 140 yards

- See more at: http://www.alloutdoor.com/2015/02/02/chronograph-labradar-personal-doppler-radar/#sthash.d4dUVXkQ.dpuf


Great info on it:


http://www.alloutdoor.com/2015/02/02/chronograph-labradar-personal-doppler-radar/

[rsterne]

How far the LabRadar (or any radar) can take accurate measurements is dependent on the area of the base of the bullet available to reflect the radar waves back to the unit.... A .177 pellet may only work out to 30-40 yards.... They were getting measurements on the .25 cal pellets out to 75-95 yards.... One of the things measured and recorded is the SNR (signal to noise ratio) and once that drops below a set value, the unit no longer records data.... eliminating any worthless data....

Bob

[Bwalton]

I do not see any for sell?

[rsterne]

They hope by August, according to the email I got from them....

Bob

[Bwalton]

They hope by August, according to the email I got from them....

Bob

Thats a must have for me

[PakProtector]

While the lowest drag is in the 700s, the difference in BC is extremely small between say, 750 and 850 fps.... However, I would expect the drag to continue to increases with the King Heavies just as it does with the smaller caliber Exacts, showing about a four-fold increase in drag by the time you push the pellet to Mach 1.... It is understanding and recording that increase in order to develop a new Drag Model for pellets that this thread is all about....

One of the nice things about using the LabRadar is that we should be able to record the entire drag curve with much fewer shots required.... I would expect that a single shot at about 1300 fps would supply all the data needed for the transonic range, down to about 900 fps, possibly even lower.... because the pellet slows so quickly in that speed range....

Bob

So for the time being, all we need to do is supply Harry a properly potent pellet shooter.... My first .25 Marauder will do the trick, and the .30 cal I built would take care of it nicely...heh-heh-heh Or one of Doug's condors.
cheers,
Douglas

[PakProtector]

Hey rjorge,
I'd be more than happy to help fund such a pot   -----> "The Cookie Man's R&D fund."
Definitely, No Strings Attached .
Kirk

Check out the 'cloud funding'...ought to be a pretty easy set up and drop it into a PayPal account to supply the goodies.

[rsterne]

Yes I'm sure Harry would find out things change rapidly at supersonic and transonic velocities.... It would take a pretty potent .25 cal to push the 34 gr. at 1300 fps, which would then give you the entire flight down to 900 or so, I would think, in one shot.... Perhaps using Helium for a few high velocity shots would be a good idea....

I've been working on a program (spreadsheet) to calculate the Cd for every data point returned from the LabRadar.... I saw a similar graph in one of Harry's posts that was a nice smooth curve.... I have NO idea how he did that, because some of the LabRadar datapoints (they are every 2 milliseconds, which is about 2 per yard) are faster than the preceeding ones, which calculates out to a negative Cd (impossible, of course).... The reason for the velocity swings is not hard to explain.... LabRadar claim a 0.1% accuracy, which at 750 fps is 0.75 fps.... Since in the datastring Harry published for the 34 gr. King Heavy, the pellet lost about 100 fps in 65 yards, which is about 130 data points, that is a drop of only 0.77 fps per datapoint.... Therefore if one velocity is 0.75 fps low, and the next is 0.75 fps high, the velocity would increase by ~0.73 fps.... In Harry's data, the drop per reading varied from 2.46 fps decrease to a 1.38 fps increase (should have been relatively constant at 0.77 fps decrease).... When I plotted out the Cd vs Velocity, I got a line that zig-zagged all over the place, from negative Cd values to ridiculously high.... Even if I used a moving average over 5 to 9 datapoints (the velocity swings were greater as the pellet travelled downrange), I still got a zig-zag plot as below....



The dotted black line is a polynomial trendline, compliments of Excel.... Both the shape of that trendline, and the Cd values I calculated, are wayyyyyyyyyy different than the results Harry published on the Yellow, so I've definitely got some work to do on analyzing the data.... I think there may be some spurious returns that I should throw out, but have no idea where to draw the line on doing that.... Certainly when the velocity increases from one datapoint to the next there is a problem, but which one should you throw out, or both?.... There is going to be a lot of headscratching before I come up with a way to analyze the output of the LabRadar and turn it into a smooth and accurate depiction of the Cd as the pellet travels downrange.... I'd really like to know how Harry did that.... It is interesting how smooth a line the actual velocities are, when plotted against the downrange distance....



Hard to imagine that those tiny wiggles in the measured velocity cause such wild swings in the calculated Cd, isn't it?.... The practical solution may be to only take data every 10 yards or so, but even then I wouldn't trust one datapoint, I'd want to use an average of 5 or so.... maybe average the one you want plus 2 either side of it?....

Bob

[rsterne]

I think I figured out how to smooth the Cd curve.... RonB suggested that I plot Velocity vs Distance from the LabRadar output, and that didn't work because the distances wander around slightly, so I plotted Velocity vs Time, which is in 0.002 second increments.... I then used the Trendline feature in Excel to generate an equation of the line that fits best through all the points.... The initial attempt was linear, but I got a better fit with a 2nd order Polynomial.... Here is that chart....



The blue dots are the datapoints from LabRadar.... The red line is the linear approximation, you can see that it is too low at the beginning and end and too high in the middle.... The black line is the 2nd order Polynomial, and the equations are shown on the graph.... If you click on the graph you should get a larger version that is easier to see.... Once I had the equation, I created a new column in the spreadsheet with velocities taken from that, and then used those velocities to calculate the Drag Coefficient.... The results are below.... They incorporate the correction to standard atmosphere, which I didn't do in the previous (zig-zag) graph above....



So, we now have a smooth curve showing how the Cd varies with velocity for the 34 gr. JSB Heavies, from 683-784 fps.... What is still somewhat confusing to me is why the values I have calculated are quite a bit lower than what Harry got on the yellow, and why the curve is inverted compared to his (albeit the Cd is nearly constant in practice).... He found a low point in the drag at about 740 fps, whereas I have a high point at about 725 fps....

Bob

[rgb1]

I thought I made it reasonably clear but apparently not, so lets try this
again........smooth the values for velocity and distance by plotting them
vs time or, since the time increment is uniform, they are just as easily
plotted vs point number (1,2,3,etc). Then use the smoothed values (those
calculated from the equation) to compute Cd.
BTW, what equation is being used for Cd ?

                                                                                       Ron