Projectile Drag Coefficient versus Velocity at Trans-Sonic

[Centercut]

(Posted the following in the other thread last night but it is equally applicable to this thread so...)

You’re right. I misunderstood when I watched it the first time. I thought he was extrapolating between two different states of tune. My apologies for adding to the confusion already swirling, and thank you for the polite correction.

Looking back at Ted's video again just now, I notice in his Chairgun screenshots that he is using a custom drag profile...which means there is a very simple but not so satisfying answer: the drag profile is wrong. If it were correct, the BC would not vary with velocity. That's the whole point of a drag profile.

Elsewhere Bob Sterne pointed out that ballistician Miles Morris found the commonly used G1 profile is wildly off in the transonic region for typical slugs, so if Ted used it as the starting point for his custom profile, that would represent a significant source of the discrepancy.

The good news here is that drag doesn't mount up nearly as quickly as we approach the speed of sound as we might have expected based on the "best we got right now" G1 profile.

Any of you guys use the RA4 profile instead of the G1?

 

[lbc_PSI]

@nervoustrig made the observation on the other thread that Ted's using a custom drag profile.

@Centercut may be on to something with the 50yd zero being too close. Here's an approximate snapshot..given we don't know what the real drag model in use..38yds to 58yds and the trajectory is +-0.17" which begs the question..how does one know if they're actually sighted in at 50yds when you have a 20yd window that is less than a slug diameter and the scope click resolution may only be between 0.125"(MOA) and 0.18"(MIL)? He could have a scope with 1/8 instead of 1/4 MOA clicks..IDK.

and here's a screen shot of all the drag models in Chairgun..curious they're all doing something interesting at around 1100fps..

 

[lbc_PSI]

Really random thoughts.

At around 4:13 into the video describes flow drag.

Flow separation in a dome shape vs hollow-point “dimple”..turbulence causes a delay in flow separation.

If you can delay “flow separation” from 80 degrees to 120 degrees, this decreases pressure drag significantly.

“Less drag allows dimpled balls to have greater lift and ultimately travel further.”

The Role of Physics in Golf

What if the “hollow point cavity” in the slug..or maybe the unique size and shape the Nielsen 31.2gr slug..acts like a golf ball dimple when the slug reaches the ~1100fps zone?

Queue the music…Too Much Time on My Hands..by Styx

 

[SkeeterHawk]

What if the “hollow point cavity” in the slug..or maybe the unique size and shape the Nielsen 31.2gr slug..acts like a golf ball dimple when the slug reaches the ~1100fps zone?

Now THAT is a thought! So essentially you can (possibly) achieve lower pressure due to the hollow-point, so this may mean that there be a unique ballistics curve for each hollow point size/depth, etc.

 

[JungleShooter]

OK, you mentioned the new drag models that ballistician Miles Morris calculated specifically for airgun slugs.

They are published on GTA and also here:

Some New Airgun Slug Drag Models by Miles Morris - Hard Air Magazine

HAM Technical Editor Bob Sterne cis working on slugs. In this latest post he summarizes new airgun slug drag models proposed by Miles Morris.

hardairmagazine.com

If you want to use them, this new ballistic calculator offers it, along with:
GA | GA² (domed pellet drag profile by Miles)
G1 | RA4 | GS | G7

The ballistic calculator — GPC Ballistics Applications — is a full suite, for Windows, Android, iOS, Linux.
Free.
Link:

GPC Ballistics Applications

For shooting solutions I still prefer Strelok Pro, but for any other calculations in the field — BC calculations, optimal PBR, fills and shots in tank, validating zero, maximum range, etc. — the GPC is my go-to.

Matthias