Transsonic/ subsonic stability

[Bryan Litz Ballistics]

The overturning moment experienced by a bullet in flight is well understood and documented. It's the reason why bullets need to be spun; in order to not 'overturn' or tumble. The aerodynamic force applied at the center of pressure, which is forward of the center of gravity for a bullet, will 'overturn' the bullet if it's not spun fast enough.

The magnitude of the overturning moment depends on many things including several aerodynamic properties which have severe changes near the speed of sound (transonic speed). This is also well documented, and manifests for some bullets as tumbling near Mach 1.

It can be confusing because, in supersonic flight, the bullet's forward velocity decays much faster than it's rotational velocity. Therefore, aerodynamic overturning moment is diminishing faster than the bullets spin rate, and stability does improve as the bullet goes downrange; to a point.
So one might wonder how stability becomes 'challenged' at transonic speed. The reason is because the aero properties (in laymans terms) just go crazy near Mach 1. So even though the bullet's forward speed has slowed more than it's rotation, the dramatic transient in aero coefficients near Mach 1 can still challenge the stability of the bullet.

This is not just theory, it's been measured and published by numerous credible sources.

The BS stops here.

Catshooter,
This forum was established as a place for shooters to ask ballistics questions and have educational and productive discussions. Your posts on this topic have contained a mixture of false information and berating of forum members. This type of conduct will not be tolerated.

-Bryan

 

[dcali]

To put a little more context into what Bryan just wrote, think about what a drag function looks like - flat below the speed of sound, then a huge jump at mach 1, followed by a slow decline as the mach number increases from there. The drag coefficient at mach 1.1 vs mach 0.9 is substantially different. We're not talking a few percent - we're talking potentially a few hundred percent.

To fully describe the motion of a spinning bullet, there are several such coefficients, and they all go similarly crazy near mach 1. Some more than others, but you can pick out mach 1 on pretty much any chart just by looking for the squiggly part of the line no matter what coefficient you're looking at.

The stability of a bullet depends on something like 4 or 5 of these coefficients (I can't recall exactly), all of which potentially change dramatically at mach 1. It's no surprise that we might see dramatic changes in stability at mach 1 as well. In fact, it's sort of surprising (to me, at least) that we don't see more.

Further, these coefficients are hard to measure precisely and are not really linear in real life to begin with. As far as I know (I'm not a fluids guy), it's tough to calculate them accurately even with sophisticated computer software. As a result, we tend to resort to trial and error guided by our theoretical knowledge and past mistakes when designing bullets.

 

[Laurie]

Very interesting information on the processes involved. The problem for many shooters of course, especially newcomers to very L-R shooting, is which bullets cope better or worse than others. I faced this conundrum when I first joined a club with 900 and 1,000 yards matches, and was shooting 80gn Noslers and Sierras out of an AR 223 with very modest MVs (probably ~2,550-2,600 fps but I never knew as I didn't own a chronograph at that time!)

I knew that transonic flight was bad news and read everything that I could find on the subject (not a lot back then) which basically said 'avoid it like the plague', but if you couldn't avoid it, well some bullets seemed to work, and others didn't. The one thing that reasonably clued-up 30-cal shooters knew was that none of the older 168s would work, although nobody knew why at the time. (I still wasted a lot of valuable match time with our two or three to a mound method of shooting with trying to spot shots for 308 Win owners who turned up with handnloads using these bullets for their first - and sometimes last - long-range match.) Ironically, when I was the only shooter on the national GB circuit shooting the 168gn .30 Hybrid in late 2013 and through last year, I was regarded as absolutely mad such is the aversion to 168gn weight these days. After a good outing in the Scottish Long-Range Meeting last June where I collected a few medals and the overall FTR trophy shooting the 168, views changed at an amazing rate and several GB team members loaded them for last year's European Championships - I should have kept my big mouth shut! (Only joking )

I still think the 'avoid it like the plague' advice is very sound, and certainly 308 bullets' BCs and propellant developments have moved on so far that I'm still frequently taken aback as to how fast Bryan's ballistic solvers predict my loads' bullets will be travelling at 1,000. The problem with 308 at any rate now if much more the tyro and/or those trying to shoot well at these distances with 26-inch or even shorter barrelled factory rifles. My usual advice is to try the 185 Juggernaut and I have advised people to try the 175 OTM tactical - have 25 loaded up with a hot IMR load for a 22-inch Howa Varminter to try at long distance, but never got around to it yet. (Carefully load-developed Bryan to achieve the same general rate of spin rotation in a 12-inch twist as 2,600-2,650 fps in 1-11.25" that you developed the bullet for. It certainly shoots well at shorter distances!)

Our Match Rifle people have a lot of good stuff they could tell us - this issue of how VLDs cope with transonic speeds that's raised in the US Rifle Teams forum post would be interesting - the hint about 'Jenny's performance' in the topic suggested not too well. Unfortunately, they are a small select band and with one or two notable exceptions tend to stay very close-lipped about their findings.

 

[243Mendoza]

...
So one might wonder how stability becomes 'challenged' at transonic speed. The reason is because the aero properties (in laymans terms) just go crazy near Mach 1. So even though the bullet's forward speed has slowed more than it's rotation, the dramatic transient in aero coefficients near Mach 1 can still challenge the stability of the bullet....

Adding to Bryan comments: when the bullet is below the speed of sound but near enough to it, the flow over portions of the bullet may be supersonic so there will be shockwave(s) on the bullet and if there is some wind turbulence this shock wave will move back and forth on the bullet.

 

[Bellatrix]

Very interesting subject! I have a question that pretty much made me register, although actively having read the forums for years.

I hear all this talk about bullets being transonic stable or not. But lets take a transonic stable bullet, say the 175 Berger OTM. When this bullet is through the transonic zone and fully subsonic (<0.8M), is it possible to predict if it will stay stable until it hits the ground at max range (~5000m)? Or will it most likely tumble after 10 seconds of flight time or so? Maybe there is no way to tell other than just trying. Find a way to compensatse for 1000+ MOA and send a few into a still lake from an elevated firing position... ???

 

[Bellatrix]

Very interesting subject! I have a question that pretty much made me register, although actively having read the forums for years.

I hear all this talk about bullets being transonic stable or not. But lets take a transonic stable bullet, say the 175 Berger OTM. When this bullet is through the transonic zone and fully subsonic (<0.8M), is it possible to predict if it will stay stable until it hits the ground at max range (~5000m)? Or will it most likely tumble after 10 seconds of flight time or so? Maybe there is no way to tell other than just trying. Find a way to compensatse for 1000+ MOA and send a few into a still lake from an elevated firing position... ???

Someone with any inputs on this?

 

[Joe Salt]

Bryan I've shoot 1000 yard Comp. for over 40 Years, and I think I know what you were saying about some bullets at Mach1 that are still fast enough but tumble in flight. Sometimes I've been in the pits and had what sounded like the bullet just breaking the speed of sound, but still putting a perfect hole in the paper. Then others coming in with such a crack and tumbling. So my thought on all of this is you need good bullet design. With the right twist. Yes and I hope that your new OTM bullets work for me the way I think they will!

Joe Salt

 

[Syncrowave]

There is no such thing as the "over turning force"... no where in physics is there mention of such a force.

Then why do we need rifling in the first place? Why not just use unrifled muskets?