Do pellets tumble ?

[Troy Bakel]

Well, I do not fully agree with you when you say pellets are "not drag stabilized". Yes, drag is not the only form of stabilization on a pellets but a large percentage of the stability, and the ability to "self correct//stabilize", on a pellet comes from the fact that the center of gravity (GoC) of a pellet in flight is in front of the center of drag (CoD), change that by giving a pellet less drag at the back by giving it more of a boat tail and you will see the stability disappearing VERY quickly.

Influence of air rifle pellet geometry on aerodynamic drag - Nicos Ladommatos- Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology ~

The research for this paper established that free shear layer separation and reattachment determined the distribution of drag forces along the pellets length.

The net drag along the pellets waist and side slopes being nearly zero.

Pellet head geometry NOT tail geometry determined when the shear layer re-attached and thus influenced drag at the base of the pellet.

Pellet designs with the best geometry (and lowest drag coefficient) had 65% of the total drag at the face of the pellet and only 35% at the base.

@Rudix which research data are you quoting from?

Please advise!

Troy

 

[dgeesaman]

The drag produced by the flare is there all the time, but it becomes more pronounced, and is directional when the pellet is disturbed (in yaw)

I made this mistake for too long also.

Read Miles thread on aerodynamics. Drag force could mean a few things in common thinking but in formal aerodynamics its definition is the force component that is axial to the overall direction of travel. In other words the drag force direction does not follow yaw. There is a stabilizing force coming from yaw but it’s a different component.

If you want to talk and think about a force component aligned with the center axis of the projectile it needs to be called something else like axial force.

 

[DesertSilver]

@dgeesaman tomorrow I should be free. Between the Aerodynamics of Ping-Pong balls, shuttle-cocks, dimpled spheres Freezbies, Canterbury Tails, AGN, and Emily Dickinson, it has been a 6 week LSD trip.

Long Slow Day?

 

[Troy Bakel]

I made this mistake for too long also.

Read Miles thread on aerodynamics. Drag force could mean a few things in common thinking but in formal aerodynamics its definition is the force component that is axial to the overall direction of travel. In other words drag the force direction does not follow yaw. There is a stabilizing force coming from yaw but it’s a different component.

If you want to talk and think about a force component aligned with the center axis of the projectile it needs to be called something else like axial force.

The other issue is that "drag" at the front of the pellet is a positive pressure and drag increases as pressure increases, whereas drag at the base of the pellet is a vacuum and drag increases as the vacuum increases.

As such, the last thing you want is to increase pressure on the front of the skirt as that maintains air flow separation and increases the vacuum at the back of the pellet.

The formation and change in vacuum at the back of pellet does not form in line with the pellet's trajectory but at various angles to that trajectory (turbulence) which may induce or reduce pitch & yaw.

So if "stabilized" means keeping the pellet's axis aligned with its trajectory with no pitch or yaw..... it appears drag at the skirt is not the solution.

 

[Ballisticboy]

The drag produced by the flare is there all the time, but it becomes more pronounced, and is directional when the pellet is disturbed (in yaw) and that is what causes the pellet to try and auto correct. That is the beauty of a traditional pellet and why it will even work to some extend from a smooth-bore barrel. The same "drag cone" principal was used to stabilize and auto correct other larger projectiles fired from barrels and dropped from aircraft though these days other "active" correction methods are preferred. It is a field I was involved in on large calibers (mostly 150mm and air to ground weapons) in the mid 1980's.

The base of a slug is VERY different from that of a traditional pellet so not sure why you bring that into the discussion. The main differences between slugs and pellets are the relative positions of the CoG, CoP and CoD thus a slug needs to be spin stabilized, a different discussion altogether.

Edit: I want to add, using something you wrote as a reference to prove your point is a principal really frowned upon in academic circles so sorry your link carries no weight.....

If you want to learn how flares stabilize projectiles, look up flare stabilization in any reputable projectile aerodynamics text book. I was calculating and predicting the stabilization of flare stabilized projectiles for 40 years as a gun projectile aerodynamicist, and they all flew as predicted using slender body theory, which tells you how flares produce stabilizing moments about the CG. At pellet yaw angles, the change in drag due to yaw is negligible, and at anything above about 1 degree of yaw the pellet is simply unusable due to group size problems.

 

[Peashooter]

When people write tumble, my Danish brain perceive that as tumbling end over end.
Spiraling or even corkscrewing i feel are more in line with that i have observed myself, but of course as the shooter i am observing from a less than ideal vantage point, ideally i should be shooting past a very much slow motion camera.


Someone should enlist the Solmo guys on youtube to investigate / nail this down once and for all.

 

[Troy Bakel]

On the first of these references, it is unfortunate that the tests were all carried out on relatively high nose drag pellet designs, all with recognized poor BC values. This led them to make the wrong conclusion on where the majority of the drag is generated. The wind tunnel tests on pellet shapes I initiated when working also gave some spurious results from pressure measurements (though they were flare stabilized bomblets, not pellets, honest guv)

As for using CFD for pellets, if you were to make a list of projectile characteristics which cause CFD accuracy problems, pellets have them all. It does produce pretty pictures, which look impressive, though.

Good point.

The papers I referenced are all apart of research conducted for the Journal of Sports Engineering and Technology.

It appears the research was focused on improving 10 Meter competition pellet performance and thus limited the range of pellets studied to those used in the sport or could be.

The boundaries used were .177 competition pellets. Air velocity of Mach 0.58 ~ 200 m/s ~ 656.2 ft/sec and a Reynolds number of 54,000.

Some tested various pellets to Mach 0.95 but the error spread in determing Reynolds number made the results much less accurate which was noted in the data table.

One paper included testing BBs and the results above Mach 0.7 were all over the place.

It would be nice if someone made a large scale mockup of a MEISTERKLUGEN .177 pistol pellet and wind tunnel tested it so we could have accurate Reynolds numbers, center of pressure data and pictures of airflow around the pellet at various degrees of pitch and yaw, various spin rates and various wind speeds.

 

[dgeesaman]

It would be nice if someone made a large scale mockup of a MEISTERKLUGEN .177 pistol pellet and wind tunnel tested it so we could have accurate Reynolds numbers, center of pressure data and pictures of airflow around the pellet at various degrees of pitch and yaw, various spin rates and various wind speeds.

Agreed on the papers - I haven't made much progress on them yet but they are consistent with match rifle performance or at least, sporting cases with very low kinetic energy limits.

Troy, our club would benefit from a good 50m indoor ammo testing tunnel. Maybe it could double as a wind tunnel? What's the top speed on your van?

Anyway get me access to a wind tunnel and I will figure out the overscale projectiles.

Which makes me wonder, is aerodynamic stabilization scalable behavior? I know with the fluids testing we do in my work, not all phenomena are.

 

[Ballisticboy]

Which makes me wonder, is aerodynamic stabilization scalable behavior? I know with the fluids testing we do in my work, not all phenomena are.

This is why we want the matching Reynolds number where possible. Many tunnels work by using accelerating or slowing air speeds to measure over a range of Mach numbers, trouble is there is something called aerodynamic hysteresis which means you get different results depending on if you are accelerating or decelerating so you have to stop for a short while on each Mach number to get consistent results. Getting good results for a large scale model over a range of speeds would be a very expensive hobby.

 

[JEFFPPC]

I would like to hear more about .223 rounds tumbling. Or did you mean yaw?

 

[Troy Bakel]

This is why we want the matching Reynolds number where possible. Many tunnels work by using accelerating or slowing air speeds to measure over a range of Mach numbers, trouble is there is something called aerodynamic hysteresis which means you get different results depending on if you are accelerating or decelerating so you have to stop for a short while on each Mach number to get consistent results. Getting good results for a large scale model over a range of speeds would be a very expensive hobby.

Reynolds Number?!!?
Scalable Behavior?!!?
Aerodynamic Hysteresis ?!!?

You two make my head hurt!!!

Keep it up and I am going to go full Cartman on you two and declare - " SCREW YOU, DIABOLO PELLETS ARE DRAG STABILIZED, GOING HOME, I'm not listening, screw you, going home."

 

[3Crows]

That is all nice but more likely you do not have a backer on your paper targets. If that is true, the targets are tearing, I see this often with lower velocity bullets. It is not tumbling. The paper is tearing. Even with high velocity I sometimes see paper tearing with no backer.

 

[Troy Bakel]

That is all nice but more likely you do not have a backer on your paper targets. If that is true, the targets are tearing, I see this often with lower velocity bullets. It is not tumbling. The paper is tearing. Even with high velocity I sometimes see paper twearing with no backer.

If I did all this research because of a target impact over a hole in the backer or because of no backer........................Screw you all, I am going home!

 

[3Crows]

If I did all this research because of a target impact over a hole in the backer or because of no backer........................Screw you all, I am going home!

LOL, if your paper target has no backing, you will often get really strange looking holes and torn out holes. I nearly had a panic attack years ago when I bought an expensive custom 1911 pistol. I was freaking. The old guy shooting next to me, laughed. The targets had no backers and when I looked at his, it was the same torn out, often triangle shaped holes. A worn out backer full of holes will be almost as bad if the pellets hits over a torn out area.

Put a piece of cardboard or better yet foam board stapled/tapped tightly to your paper targets and shoot again. Fingers crossed that is your issue but I bet it might be. I have seen this with my air rifles.

Indoor shooting ranges where the target is just held by paper binder clips with no backing and no tension, key hole city! That is the worse case.

Back to the OP, I have seen .223/5.56 NATO bullets tumble with the slower twist rifling and 60+ grain bullets.

 

[JDGriz]

I've always sort of intuitively felt that if CG and CP are too far apart, then aerodynamic stabilization can cause or increase precession, proportionately to the RPM. This is why you can't use high twist rates with pellets; at some point the one type of stabilization goes against the other (gyroscopic and aerodynamic) - they don't stack up cumulatively .

 

[dgeesaman]

This is why we want the matching Reynolds number where possible.

That would have been my first guess. At my work we have a lab where we perform testing and our fluids always involve liquid so sometimes it’s scalable on Reynolds, sometimes Froude, sometimes nothing. Its the compressibility that gave me pause on just using Re.

Getting good results for a large scale model over a range of speeds would be a very expensive hobby.

It would be cheaper by far to figure out how to machine pellets of almost any shape for testing purposes. Ain’t no wind tunnel as good as real wind.

 

[Troy Bakel]

That would have been my first guess. At my work we have a lab where we perform testing and our fluids always involve liquid so sometimes it’s scalable on Reynolds, sometimes Froude, sometimes nothing. Its the compressibility that gave me pause on just using Re.

It would be cheaper by far to figure out how to machine pellets of almost any shape for testing purposes. Ain’t no wind tunnel as good as real wind.

Froude, Ludwig Freebase Froude.

In my day, at Naval Nuclear Power Shool, we were real pocket protector nerds and calculated Froude numbers using slide rules.

Lesson learned and rembered: In a Nuclear Steam Supply System a Froude less than one is a leak, 1 or greater is a rupture. You find leaks, ruptures find you.

 

[dgeesaman]

Froude, Ludwig Freebase Froude.

In my day, at Naval Nuclear Power Shool, we were real pocket protector nerds and calculated Froude numbers using slide rules.

Lesson learned and rembered: In a Nuclear Steam Supply System a Froude less than one is a leak, 1 or greater is a rupture. You find leaks, ruptures find you.

In my brief time doing inflatable products for aerospace, we always said “fill” or “inflate” instead of “blow up”.

At least I never had to say “unscheduled rapid disassembly”. I think Musk has that technology patented these days.

Back to the OP, airgun forum threads can definitely tumble. It only takes a moment.