Wind drift anomalies?

[jlmurphy]

While discussing the ballistics of .45-70 long range silhouette bullets, my friend mentioned the importance of staying below the speed of sound. I assumed it was for stability, but he said wind drift was his main concern. I thought time of flight was the major component, but after playing with an online ballistics program, it it was obvious he was right, the faster the projectile traveled, the greater the wind drift, with a large bump at the speed of sound. My guess is that a shock wave forms in front of the object, causing greater drag, although the time of flight remains proportional. Any thoughts?

 

[tim]

Your bud is commenting on the issue of sub sonic/wind issues but it relates to rimfire not center fire. I can't imagine anybody shoots subsonic big bore silhouette.

 

[Larry Elliott]

They do in Black Powder Cartridge Rifle silhouette. It's like rimfire, but at longer range and a lot smokier.

 

[alinwa]

While discussing the ballistics of ....................................


Any thoughts?

Yup,

your friend is right.

And there are no "anomolies," it's a well understood subject.

re your 'guesses'........ ................ if you're really interested you'd maybe enjoy some books on the subject.

Time of flight has little or nothing to do with it.

al

 

[Gene Beggs]

Alinwa is certainly right; it has little or nothing to do with time of flight.

Wind and how it affects bullets in flight is far more complex than it first appears to be.

I have studied the subject for many years and each year I learn something new.

Al and I and many others have had some very enlightening and lively discussions here on the forum about wind drift. I encourage you to search the archives and review those posts. It's fascinating!

Gene Beggs

 

[tim]

Alwina,Really? You might wander over to BPCR.net, there's literally 100's of loads listed over there, most of them seem to average about 1200 fps or so. The rimfire assertion has more to do with the issue of slugs that start out supersonic and tend to destabilize as they pass back to subsonic, usually just before 50yds where most .22 BR targets are.

 

[WSnyder]

Here's an explanation from the NRA Firearms Fact book. The same info was published in an article in Precision Shooting or one of their specials or annuals years ago.

"Those unfamiliar with smallbore rifle competition are often surprised to learn that target shooters almost universally use standard velocity ammunition in preference to the many varieties of high and hyper-velocity rounds available. The reason given is the low-velocity ammunitions resistance to wind deflection.

Despite what might seem at first to be the case, wind deflection is not proportional to the time of flight. Instead, it is proportional to the amount of delay in the flight caused by air resistance. The 1145 f.p.s. standard velocity .22 long rifle round takes .287 seconds to go 100 yds., but would take only .262 seconds to cover the same distance in a vacuum. The latter figure is easily found by dividing 300 ft. by the speed of the bullet (1145 f.p.s.), which would remain the same throughout its flight if it were in a vacuum. Thus the delay caused by air resistance is .025 second with the standard velocity ammunition.

The 1335 f.p.s. high velocity ammunition, which will take .259 second to cover 100 yds., would take only .225 second in a vacuum. Thus, the delay for this bullet is .035 second or 37% greater than that of the standard velocity round .22.

The high speed round, then, suffers about 37% more wind deflection than the standard velocity.

This remarkable result is due to the very rapid rate at which air resistance increases with increase in bullet speed in the region near the speed of sound. The .22 rimfires rimfires are the only important rifle cartridges that occupy this speed range, and they are the only ones that show more wind deflection as velocity is increased.

While air resistance always increases when the bullet is shot faster, the rate of this increase is less steep at supersonic velocities. Rifle bullets in general, contrary to the case of rimfires, are made less sensitive to the wind by raising their velocities."

NRA Firearms Fact Book

 

[alinwa]

Alwina,Really? You might wander over to BPCR.net, there's literally 100's of loads listed over there, most of them seem to average about 1200 fps or so. The rimfire assertion has more to do with the issue of slugs that start out supersonic and tend to destabilize as they pass back to subsonic, usually just before 50yds where most .22 BR targets are.

mit, Really? I could "learn" something there?

You're 'wayyy over my head here man. I have no idea what you're trying to tell me. I'm guessing it's sarcasm? I don't even know what you're referring to....

Maybe just spit it out instead of asking me to read between the lines?? I'm really not good at that whole "interpretation" thing (as my wife well knows!)

al

 

[Vern]

Since I dont know,... this means this is a real question.
If subsonic speeds are more accurate then we should be loading down the 6ppc?

 

[John Kielly]

It's just the window round sonic velocities where it hits the fan.

Why not run a few velocity ranges thru the ballistics table of your choice & see what the relative gains & losses there are?

 

[jo191145]

Now you guys got me thinkin bout using the ol 1911 for F-Class

 

[Charles E]

Grouper,

Drag is drag,at whatever speed. That we cannot predict, but only empirically measure it in the transonic region is not a physical property, just a human one. You will not be "all right" just by ignoring it.

For fun sometime, read the Lyman book on pistol reloading. Some bullets "pick up BC" in the transonic region, some lose it. AFAIK, no one yet has been able to model just which shapes go one way, which the other. Modeling (predicting) does work pretty well as you get above 1400 fps.

 

[tim]

mit, Really? I could "learn" something there?

You're 'wayyy over my head here man. I have no idea what you're trying to tell me. I'm guessing it's sarcasm? I don't even know what you're referring to....

Maybe just spit it out instead of asking me to read between the lines?? I'm really not good at that whole "interpretation" thing (as my wife well knows!)

al

Well, let me try again. First, as to BP cartridge rifles it would seem that since the BPCR site lists so many loads well above the speed of sound, that most in fact, do not load subsonic. [ I also happen to cast/shoot a 32-40 and an 8.15x46 and most loads seem to be between 1200-1300fps].
Second, it seems the biggest incidence of subsonic in recent years has been rimfire. A lot of the faster match ammo used to be slightly supersonic and it was proven that when it passed back to subsonic it would breifly destabilize...usually just before it hit the target. I'm guessing the whole deal gets translated into "subsonic for everything" except most loads for heavier cast slugs would seem not to be, silhouette, scheutzen, BR or otherwise, you gave me an opinion, I gave you a fairly widely used source...even the lightest loads for some of these guns are supersonic.
The wind drift issue I'd suspect, impacts a slug only more so in it's pass back to subsonic, given it's blowing, etc. If one was inherintdly less wind sensitive I'd be shooting my 6PPC @ 1050fps.

 

[alinwa]

Well, let me try again. First, as to BP cartridge rifles it would seem that since the BPCR site lists so many loads well above the speed of sound, that most in fact, do not load subsonic. [ I also happen to cast/shoot a 32-40 and an 8.15x46 and most loads seem to be between 1200-1300fps].
Second, it seems the biggest incidence of subsonic in recent years has been rimfire. A lot of the faster match ammo used to be slightly supersonic and it was proven that when it passed back to subsonic it would breifly destabilize...usually just before it hit the target. I'm guessing the whole deal gets translated into "subsonic for everything" except most loads for heavier cast slugs would seem not to be, silhouette, scheutzen, BR or otherwise, you gave me an opinion, I gave you a fairly widely used source...even the lightest loads for some of these guns are supersonic.
The wind drift issue I'd suspect, impacts a slug only more so in it's pass back to subsonic, given it's blowing, etc. If one was inherintdly less wind sensitive I'd be shooting my 6PPC @ 1050fps.

hmmmm

Well, I'm no expert on subsonic flight characteristics but I do shoot over a chronograph, a lot. A while back I lined up three of my chrony's on a board and noted that at 10ft off the muzzle I was losing about 2-4fps/ft with supersonic projectiles. (empirical, an estimate, rough) But they shed vel very quickly.

I submit that IF a projectile is shedding even 2fps/ft then these projectiles that're loaded to 1200-1300 may well be going subsonic in 50-100ft. That they stabilize from blast effects, lateral jump and inbore cant effects AS they are going subsonic, not clear out by the target board. And that this combination, this lumping together of several effects into the same frame of flight is advantageous....

Threads like this illustrate for me why the REAL ballisticians (HBC et al) refrain from trying to illustrate/explain such a broad subject here on the board.


al

 

[Greg Culpepper]

While discussing the ballistics of .45-70 long range silhouette bullets, my friend mentioned the importance of staying below the speed of sound. I assumed it was for stability, but he said wind drift was his main concern.

Mornin' Mr. Murphy,

Out of my experience and observation, staying below the speed of sound is not a crucial issue for being competitive. As in most pursuits, a balanced program that embraces the best set of compromises wins. The least wind drift alone doesn't guarantee a win.

I thought time of flight was the major component,

Time of flight has nothing to do with drift. "Wind drift" is the product of time lag only. How fast a projectile slows down is the predictor of its drift. How fast it slows down is determined by its bc and air density.

but after playing with an online ballistics program, it it was obvious he was right, the faster the projectile traveled, the greater the wind drift, with a large bump at the speed of sound.
My guess is that a shock wave forms in front of the object, causing greater drag,

Right.

although the time of flight remains proportional.

I'm not following you here. Proportional to what?

Any thoughts?

When shot with black powder, big old roomy cartridges shoot with the least velocity extreme spread with some compression of the powder charge. The lowest velocity extreme spread will give the least vertical shot variation at the long range target. Vertical is one important issue in field and at target. To a large extent, achieving that optimum powder compression for velocity consistency influences the resulting bullet velocity. Long, heavy for diameter, high bc bullets that drift less than shorter, lighter bullets help suppress velocity but most frequently not to below the speed of sound in the cartridges that we shoot. So there is, as always, a compromise involving trade offs between vertical spread at the target and drift at the target. The compromise that typically appears seems to be a little less than 1200 fps which as you note is a faster than the speed of sound where drag (and drift) take a big jump. Maybe the best performance is available just sub-sonic. I don't know. My BPCR experience is with 40-90 (about 1500 fps) and 40-65 (about 1200 fps), both with the same 416 gr 0.410 bullet at about 0.50 bc. Predictably, the slower 40-65 cartridge drifts much less and (edit here) the 40-90 is not at all competitive for its greater velocity.

I shoot them both exclusively with black powder. I get chronographed velocity consistency with black powder that will break the needle off most folk's BS meter so I'll leave it at that. I expect much better results on target than if I used smokeless. As Steve Garbe is given to say, "Smokeless powder is a passing fad".

Books have been and will be written on this subject. A great one is by Bryan Litz who also also walks the walk. One (sort of) them developed on this board a few years back and contains the work of many authors. Gene references it above. It's a beautiful thing to see Al come to lose his miss-perceptions about "wind blowing bullets over" and come to a full appreciation that they are actually "backing down the wind" along their own axis due to vectoring into the apparent wind and moving across the ground due to drag alone. Al is a great student, experimenter (and teacher) and shares voluminously to the gratitude of many, myself included. Gene is a fantastic teacher of many subjects. Like Al, he is a Renaissance man. I hope you will take the time to do as Gene suggested and search "What exactly does a bullet do in the wind?" or something like that. It's a good (although long) read.

Greg

 

[pacecil]

I put this in another thread - I'll try it here and see what happens.

Nothing happens to a bullet when it's velocity changes from sonic to subsonic, or subsonic to sonic. There is no "shock wave" effect. All that takes place is a change in the slope of the curve that describes the effect of wind. It's like you were climbing a hill, when you come to a place that the hill becomes steeper, or less steep, it simply becomes harder ,or easier to climb the hill. I'll go out on a limb here and say bullets might be described as "going very gently" into or out of the transonic region!

I'm sure there will be many who think they could almost hear their bullet hitting a wall in this terrible region called "transonic". Truth is it's difficult to pin point exactly where this region is. Probably it can only be done mathematically and somewhat arbitrarily.

 

[alinwa]

I put this in another thread - I'll try it here and see what happens.

Nothing happens to a bullet when it's velocity changes from sonic to subsonic, or subsonic to sonic. There is no "shock wave" effect. All that takes place is a change in the slope of the curve that describes the effect of wind. It's like you were climbing a hill, when you come to a place that the hill becomes steeper, or less steep, it simply becomes harder ,or easier to climb the hill. I'll go out on a limb here and say bullets might be described as "going very gently" into or out of the transonic region!

I'm sure there will be many who think they could almost hear their bullet hitting a wall in this terrible region called "transonic". Truth is it's difficult to pin point exactly where this region is. Probably it can only be done mathematically and somewhat arbitrarily.

OK pacecil..... I'm only conversant with supersonic flight phenomena but I CAN read, and I DO have real books written by real ballisticians at my disposal so's I refuse to just let'cher allegation lie

As you no doubt know, it's more than just a "steeper slope," the transonic area. And why you're presenting it a "gentle" I dunno but tests agree with theory here, it's not.

Quote for me ONE real ballistics text, show me ONE opinion other than your own or otherwise substantiate your position else'twise I'll have to give it the same credence as your last exposition wherein you asserted that "smoke rings don't exist because you pointed your airhose around the shop and nothing untoward happened."

According to (Rinker/McCoy/Litz/Vaughn et al) there most certainly IS a shockwave, this shockwave is illustrated/described (verbally/dramatically/photographically) in EVERY ballistics text with which I'm conversant. And all texts agree that in that region where the bullet is "crossing over" it's strongly "buffeted" (this is the most common descriptor) due to the fact that as it ascends/descends through this region there's a period of "back-and-forth" action where parts of the system are protruding or mixing between the two states of flight.

To quote Robt Rinker, 'Understanding Firearm Ballistics, Basic To Advanced Ballistics Simplified & Explained,' "a lot of phenomena occur during this transonic range. Most of the difficulties are associated with shock wave induced flow and pressure changes."

He goes on for pages (with pixtures) explaining/showing how airflow transitions from laminar to "behind the shock wave" with a little "NOTE: Shock waves are different than airflow" accompanying the pixtures.......

In fact Rinker goes so far as to say that "for the study of aerodynamics and airflow at velocities below the speed of sound, air is termed incompressible. At the higher speeds there are large and important changes in air density and compressibility."

Much reference to "turbulence" and "pressure changes moving in all directions...."


sooooo, more input? Something more than just "nothing!"

I've got no answers, no real understanding of laminar airflow flight characteristics, but I hesitate to let your assertion go unchallenged. Wouldn't be prudent.

Good luck

al

(BTW, the (nonexistent) "shock wave effect" as you so indelicately put it is rawther well and easily explained in ALL of the ballistic tomes on my bookshelf....)

 

[Boyd Allen]

I have gotten to the point that I am amused by the various statements that come from discussions of wind drift. For example, I have never heard a less apt description than the contention that bullets "back up". Find another way of saying what you mean. I have never seen a fired bullet "back up", and neither has anyone else. They are positioned by the sum of the forces acting on them. None of this involves reversing direction. which is the common understanding of "back up". As to the need for a bullet to tilt if it is to be caused to drift by it interaction with the wind, round balls drift the most, and they cannot turn into the wind. Correlation does not prove causation. Also, owning and reading a number of books on a subject does not guarantee mastery of a subject, it makes it more likely, but there is no guarantee. Understand, I am not trying to contradict ballistic theory that has proven reliable in predicting how a bullet will behave, it is just that some of the "explanations" do more to obscure than illuminate. One of my favorites is the one where the author comments that if there was no lag, there would be no wind drift. Please, the only way that there would be no lag, would be in a vacuum , where there can be no wind...so what is the point, yet this nonsense has developed a life of its own, being repeated over and over, as if it meant something.

 

[MilGunsmith]

As was explained to me by Chuck Yeager; ''The bullets at subsonic are like being in a boat moving along with the throttle just cracked open a little, the boat moves along displacing the water and is very controlable. The supersonic stage is the same as the boat wide open up on plane, controlable. The trans-sonic range is the stage were the throttle is partialy open and the boat is trying to get up on plane, there is water built up in front of the bow trying to get around and the boat is uncontrolable"" Most of us ave been in a boat so we can understand tis example.
At work we have tracked bullets out to 2000 meters using Weible Radar and the .300Win Mag with 190gr SMK bullets, we watched a normal ballistic curve until the bullets went trans-sonic at around 1200 to 1400 meters depending on the tempratures. In the trans-sonic range, nothing was predictable until the bullets went subsonic and then stabled out sometimes.

 

[Charles E]

In the trans-sonic range, nothing was predictable until the bullets went subsonic and then stabled out sometimes.

I like that. It shows -- to me anyway -- that what Lyman found by the empirical testing of their pistol bullets in the 1200 to 1500 fps region -- is still valid. They found that you couldn't predict what would happen. Apparently, all the variables still aren't known, only empirical testing of each bullet gives useful information.

Happens a lot in shooting. We don't know all the causes of in-bore yaw, and the resulting shot dispersion, but from the dispersion, we can calculate what the in-bore yaw was -- we just don't know all the causes. Or didn't at the time of McCoys book . . .

We don't know the exact cause of dynamic instability. We know the result -- don't use Sierra 168 MKs if you can't stay above the transsonic region. But not all bullets behave that way. Is it the boattail angle of the 168? Maybe, but not yet proven.

As an aside, what is all this talk of a vacuum? The difference between a vacuum and life on Earth is air. The primary effect of air on projectiles is "drag." There are a few others, but they can be ignored like anything else down in the <1% effect region. The complexities of drag are best covered in books on exterior ballistics. The descriptive/predictive model is mathematically complex.

People try to find shortcuts to avoid the math. Me too. At the level of an individual bullet, that's called empirical testing, and is valid if done properly. If extending the result of that test to a general rule proves false because another (say bullet) doesn't behave that way, you don't have a good model. So use the test for what it is worth-- applicable to one situation.