Author Topic: Transonic Drag (Read 8364 times))

rsterne · « **on:** August 20, 2012, 01:07:09 PM »

I found an interesting graph on Transonic Drag that I thought was worth sharing with you....

Note that the drag starts to rapidly increase between Mach 0.80-0.85 (900-950 fps) and has doubled by Mach 0.9 (1000 fps) and tripled by Mach 0.95 (1050 fps).... This means that the extra energy it takes to get a pellet above 950 fps will quickly be shed in the first few yards anyway.... That ties in exactly with the work I have been doing on how the Ballistics Coefficient changes as you push the velocity close to the speed of sound....

Since 950 fps gives you an energy of 2 FPE per grain of pellet weight (an easy number to remember).... this is just one more reason not to push the velocity past that....

Bob

gmupatriot2010 · « **Reply #1 on:** August 20, 2012, 01:53:16 PM »

Thank you for sharing. Further goes to show that we should not be worrying about pushing pellets to their limits or guns for this matter...accuracy is more important!!! and that does not happen near the transonic range

pappa · « **Reply #2 on:** August 20, 2012, 03:36:38 PM »

rsterne,

Thanks for the info. We all needed that! Without any real proof, I have believed this for a long time and I know it will stick with me for the rest of my life.

cncjerry · « **Reply #3 on:** August 20, 2012, 03:57:27 PM »

Somebody should tell the manufacturers to spend time on precision and consistencey and give up in speed.

aom22 · « **Reply #4 on:** August 20, 2012, 05:40:20 PM »

I like springers, however, I can't help but to take notice of PCP air rifles.
The next major step in PCP development has to be with ammunition.

With the advances in PCP technology and power-plant control, perhaps, it is time for PCP manufactures
to to convince pellets manufactures to produce projectiles that compliment all the power PCP systems
are capable of generating.
To the contrary, it is time to move forward with pellet development ... into the supersonic region.

And to concentrate on developing pellets that can buck the effects of the penetrating the transconic zone
and are stable at supersonic speed.
While maintaing a high level of accuracy at very high velocity.

QVTom · « **Reply #5 on:** August 20, 2012, 06:35:51 PM »

Bob, A question about BCs. Does the BC really change or are we not calculating the transonic region properly with programs like chair gun that appear to use linear functions. Maybe it's just a matter of semantics? In my reading on BCs they are almost always treated as a constant. Is plotting the BC across FPS just a patch for over simplified calculations in the programs we love?

Tom

sghr220 · « **Reply #6 on:** August 20, 2012, 09:49:41 PM »

Ballistic coefficient is a function not a number. It varies as the mass, diameter and the drag coefficient of the bullet (pellet) changes and since the drag coefficient (C) itself is a function in speed or FPS then the BC does vary with the speed at which the projectile travels. At low to medium velocities (AG velocity ranges) the BC becomes even more complicated and sensitive to measure and a much more important factor at evaluating the performance of an AG's ammo.

rsterne · « **Reply #7 on:** August 21, 2012, 01:39:11 AM »

The BC does, indeed, vary with velocity.... and not in a linear manner.... I've done enough measuring to establish that.... In fact, it does some very strange things, the JSB Exact Series, for example, actually increasing with velocity, peaking (in the 650-950 fps range, depending on weight), and then decreasing.... Others (wadcutters) seem to just get worse as the velocity increases, but that may be because I didn't test them slow enough to see a peak.... The BC is directly proportional to the Sectional Density (SD), and the rest comes from the CD, which is dependant on the shape and velocity.... I have measured BCs dropping by nearly a factor of four at 1100 fps compared to 800ish.... which seems to be supported by the graph....

Bob

QVTom · « **Reply #8 on:** August 21, 2012, 01:51:02 AM »

I guess what I'm getting at is the BC really is not a coefficient but a function. Do we need to convince the ChairGun developers to allow a spreadsheet or simple array to describe the curve? Or should generic curves be applied in ChairGun to adjust for the transonic? I think some pellet shape info would be required in the library. Maybe too much rambling.

Tom

sghr220 · « **Reply #9 on:** August 21, 2012, 02:11:18 AM »

Quote from: rsterne on August 21, 2012, 01:39:11 AM

The BC does, indeed, vary with velocity.... and not in a linear manner.... I've done enough measuring to establish that.... In fact, it does some very strange things, the JSB Exact Series, for example, actually increasing with velocity, peaking (in the 650-950 fps range, depending on weight), and then decreasing.... Others (wadcutters) seem to just get worse as the velocity increases, but that may be because I didn't test them slow enough to see a peak.... The BC is directly proportional to the Sectional Density (SD), and the rest comes from the CD, which is dependant on the shape and velocity.... I have measured BCs dropping by nearly a factor of four at 1100 fps compared to 800ish.... which seems to be supported by the graph....

Bob

The BC as a function is velocity-range specific (hence the strange things

) which i believe is called the transient nature of the BC. I myself doubt very much that renowned AG's pellet makers use the precise Doppler radar method in measuring the BC of their pellets and i believe many discussion like this concluded that the current pellets design (drag stabilized) needs more development to cope with modern AG's velocities.

rsterne · « **Reply #10 on:** August 21, 2012, 01:31:45 PM »

Quote

should generic curves be applied in ChairGun to adjust for the transonic?

I don't think you will find anything "generic" about how different shaped pellets behave in terms of how the BC varies at different velocities.... Here is an example of measured BCs for a few .25 cal pellets....

Unfortunately, I didn't have a powerful enough gun to push the velocities very far into the transonic region.... but you get the idea....

Quote

current pellets design (drag stabilized) needs more development to cope with modern AG's velocities.

Absolutely correct.... Airguns have far outstripped the traditional ammunition we have, IMO.... That is why the Europeans are now using 41.5 gr. RWS (rimfire) bullets in .224 cal for long range benchrest work....

Bob

sghr220 · « **Reply #11 on:** August 21, 2012, 10:00:38 PM »

Quote from: rsterne on August 21, 2012, 01:31:45 PM

That is why the Europeans are now using 41.5 gr. RWS (rimfire) bullets in .224 cal for long range benchrest work....

Bob

I very much appreciate your work and experimentation with this ammo and especially the barrel twist rate bits and pieces of valuable information

.

RedFeather · « **Reply #12 on:** August 21, 2012, 10:26:30 PM »

Something to consider in your quest for an aerodynamic PCP bullet. Once you've successfully developed a bullet that breaks the sound barrier and is accurate, you are in the realm of firearms. Sooner or later, someone is going to take notice that such air guns are on a par with some rifles and, dollars to donuts, you will be seeing legislation to lump them in. Guns such as the Rogue already have ammo such as you described.

thekid · « **Reply #13 on:** August 21, 2012, 10:53:20 PM »

Redfeather...
For us it is already too late.
Anything over 500fps needs fire arm permit...P.A.L (purchase acquisition license).

Great work Rsterne

rsterne · « **Reply #14 on:** August 22, 2012, 01:52:57 AM »

I'm not on any particular quest to go supersonic, in fact most of my posts argue not to go over ~950 fps.... but like thekid says, it's too late in Canada anyway, anything over 500 fps is considered a firearm and you have to have a license to own one....

Bob

aom22 · « **Reply #15 on:** August 30, 2012, 03:47:42 PM »

Quote from: rsterne on August 20, 2012, 01:07:09 PM

I thought I seen might have seen chart before ... just don't recall when and where.

rsterne · « **Reply #16 on:** August 30, 2012, 04:13:13 PM »

Yes, that's a quote of my post.... did you have a point or question?....

Bob

aom22 · « **Reply #17 on:** August 30, 2012, 04:23:29 PM »

Quote from: rsterne on August 30, 2012, 04:13:13 PM

Yes, that's a quote of my post.... did you have a point or question?....

Seems that you've done a lot of research on pellet dynamics.
Trying to find the source(s) of your information ... to read the entire article(s) for myself - that is all.

I was able to find a source thread for the chart .25 cal. Ballistics Coefficients and I've bookmarked it.
Excellent work ... very, very interesting stuff.

But the other chart ... Qualitative Variation of Cd with Mach Number ... had me stumped.
For some reason, I thought I had recognized or had seen this specific chart from sometime ago ... just can't recall for certain.

I guess ... my point is: If you have a link to some good information ... please post it ... others may like to see the source.

rsterne · « **Reply #18 on:** August 30, 2012, 04:56:56 PM »

IIRC, it may have been on a wiki site.... I just googled "transonic drag coefficient" and got this wiki page.... http://en.wikipedia.org/wiki/Drag_(physics)

It is on that page, but I don't know if that is the source article or not....

Bob

aom22 · « **Reply #19 on:** August 30, 2012, 05:28:34 PM »

Quote from: rsterne on August 30, 2012, 04:56:56 PM

I just googled "transonic drag coefficient" and got this wiki page.... http://en.wikipedia.org/wiki/Drag_(physics)
It is on that page, but I don't know if that is the source article or not....

Thank you.
Don't laugh, but, I've spent at least an hour ... probably more ... trying to locate the full article.

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Transonic Drag

Author Topic: Transonic Drag (Read 8364 times))

sghr220

sghr220

sghr220