📈 How to Calculate a Custom Drag Model (CDR)

[JungleShooter]

How to Calculate
a Custom Drag Model​

I need some help.
Because I'm going far off the reservation.

▪ "The reservation" being current standard drag models, like GA and G1.

▪ "Going far off" meaning I am shooting projectiles far off the "standard" domed pellet that the GA was designed for. And farther off yet of the century old(!) G1 "standard" projectile.
And I am shooting these projectiles at velocities so high where this difference in shape really makes A DIFFERENCE!

➠ Just look at the shape of the Polymag, the Crow Magnum, or the Ultra Shock Heavy!

➠ As anti-aerodynamic as they are shaped, shooting them at 1000fps really must put the BC miles away from what the GA model would predict....



I have therefore decided it would be helpful to derive and calculate a custom drag model (CDM) for these projectiles.
I could then enter that drag model into Strelok Pro or the pirated Strelok (named "Chairgun Elite" by the thieves).
And share the model with the rest of the AG world.



➊ Basically, I measure velocities at different ranges.
And between 10 and 20 data points for each range will give a pretty good reliability of the results, unless the data varies wildly.
Preferrable over average velocities are velocity pairs that measure two or more velocities of the same shot.
For that have 3 chronies (which I have already secured — you see I've been dreaming this custom drag model dream for a good while now....).

➋ It would be good to check chronies to see if there is any discrepancy between their velocity measures.

➌ Atmospheric conditions and distances between chronies must be precise.

➍ The less wind the better.



இ But once I have the velocity data, converted into mach, how do I transform that into a drag value (cd)
To me this is about as confusing as these lines: இ


Thanks for any advice!!

Matthias




The advantage of custom drag models (CDM):

G1 BC vs G7 BC vs Bullet-Specific Drag Models

“In the past 4-5 years we’ve made quantum leaps when it comes to predicting bullet trajectories.” –...

precisionrifleblog.com

Our resident ballistician, Miles Morris, has been mentioning the need for better drag models for a long time.

 

[dgeesaman]

Does the software let you begin with a standard drag model?

If so you can compare your velocity and drop data to the calculated velocity and drop data, and where your data is different, begin increasing or decreasing drag coefficient at that velocity.

 

[Ballisticboy]

How to Calculate
a Custom Drag Model​

I need some help.
Because I'm going far off the reservation.

▪ "The reservation" being current standard drag models, like GA and G1.

▪ "Going far off" meaning I am shooting projectiles far off the "standard" domed pellet that the GA was designed for. And farther off yet of the century old(!) G1 "standard" projectile.
And I am shooting these projectiles at velocities so high where this difference in shape really makes A DIFFERENCE!

➠ Just look at the shape of the Polymag, the Crow Magnum, or the Ultra Shock Heavy!

➠ As anti-aerodynamic as they are shaped, shooting them at 1000fps really must put the BC miles away from what the GA model would predict....



I have therefore decided it would be helpful to derive and calculate a custom drag model (CDM) for these projectiles.
I could then enter that drag model into Strelok Pro or the pirated Strelok (named "Chairgun Elite" by the thieves).
And share the model with the rest of the AG world.



➊ Basically, I measure velocities at different ranges.
And between 10 and 20 data points for each range will give a pretty good reliability of the results, unless the data varies wildly.
Preferrable over average velocities are velocity pairs that measure two or more velocities of the same shot.
For that have 3 chronies (which I have already secured — you see I've been dreaming this custom drag model dream for a good while now....).

➋ It would be good to check chronies to see if there is any discrepancy between their velocity measures.

➌ Atmospheric conditions and distances between chronies must be precise.

➍ The less wind the better.



இ But once I have the velocity data, converted into mach, how do I transform that into a drag value (cd)
To me this is about as confusing as these lines: இ


Thanks for any advice!!

Matthias




The advantage of custom drag models (CDM):

G1 BC vs G7 BC vs Bullet-Specific Drag Models

“In the past 4-5 years we’ve made quantum leaps when it comes to predicting bullet trajectories.” –...

precisionrifleblog.com

Our resident ballistician, Miles Morris, has been mentioning the need for better drag models for a long time.

I just looked at the article referenced above. The main thing which comes out of it is how out of touch with the ballistics world the small arms industry has been. Purpose drag laws replaced BC's in the 1960s in the large calibre world with the widespread use of computers. I had not even heard of BC's until I had to work with the small arms industries in the UK in the early 1980s, when I had to look in the old historic ballistics books to find out about them. So to say there have been great advances in the last 5 years is a bit of a stretch, more like the small arms industry has finally woken up to modern ballistics.

I would also say that the fixed head Doppler radar they are talking about is useless for long range tracking. We always used Doppler tracking radars to follow bullets for their entire trajectory with their tracking movable heads to obtain complete drag laws over the entire velocity range. The only problem there is that they cost millions of dollars, not $100,000. They can be rented from the manufacturers, so may be usable by the larger companies.

Sorry, rant over.

Back to the object of the original post.

Without a radar, you are not likely to get a custom drag law directly from three chronies. You really need hundreds of data points. What you can do is create an estimated purpose drag law and then use the firing data to calibrate it. LabRadar can be used to produce purpose drag laws for airgun projectiles as the range is short, so trajectory angles are very small. To use the LabRadar you need to use the detailed data, not the data displayed at fixed ranges on the screen. The problem with the Labradar is that the range of velocities will be relatively small.

Of course, the tricky bit is creating an estimated drag law, for which you need reference charts and a few years experience on what works and what doesn't. However, you can cheat a bit, and for the very blunt projectiles mentioned try using the wadcutter reference drag law as the starting point. You will then have to model each short distance between measured velocities one at a time, adjusting the drag law up or down to get a match. Start with the highest velocities. Once you have the first pair of velocities matched, move on to the second pair and repeat, but do not change the drag coefficient you got for the first pair. Do this along your trajectory until you have drag coefficient values for each part of the trajectory. It is quite likely that you will end up with values varying wildly from one speed to the next, so what you will have to do then is plot all the values you have calculated and put a best fit line through the data by eye, do not try to use a curve fitting routine from the spreadsheet.

When you have finished, try out your result and tweak to get the closest you can to the measured velocities. Hopefully this will get you somewhere close to what you want, but it may need a bit more work.

If you need more help, PM me.

Good luck.

 

[JungleShooter]

Does the software let you begin with a standard drag model? If so you can compare your velocity to the calculated velocity, and where your data is different, begin increasing or decreasing drag coefficient at that velocity.

For the very blunt projectiles mentioned try using the wadcutter reference drag law as the starting point. You will then have to model each short distance between measured velocities one at a time, adjusting the drag law up or down to get a match. Start with the highest velocities. Once you have the first pair of velocities matched, move on to the second pair and repeat, but do not change the drag coefficient you got for the first pair. Do this along your trajectory until you have drag coefficient values for each part of the trajectory. It is quite likely that you will end up with values varying wildly from one speed to the next, so what you will have to do then is plot all the values you have calculated and put a best fit line through the data by eye, do not try to use a curve fitting routine from the spreadsheet.

When you have finished, try out your result and tweak to get the closest you can to the measured velocities. Hopefully this will get you somewhere close to what you want, but it may need a bit more work.

If you need more help, PM me.

Good luck.

THANK YOU, David and Miles, thanks a lot!!

Yes, this looks like a doable plan.
I already imported the WC₀ drag model into Strelok Pro.

Matthias

 

[Ballisticboy]

When you have some velocities for one of the projectiles, send them to me and I will use them to derive a drag law which we can compare to your estimates.

 

[JungleShooter]

When you have some velocities for one of the projectiles, send them to me and I will use them to derive a drag law which we can compare to your estimates.

I will!
Thank you for offering that!

Matthias

 

[DesertSilver]

Following.