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Dissimilar Wind Drift Question for Bryan

I was reading David Tubb's reticle spec sheet where he describes dissimilar wind drift......left wind having less effect than right wind in a right twist barrel. Bryan, have you done any testing to validate this? Tubb says he has done careful testing and verified this effect.

Thanks,
scott
 
There is no known mechanism in the field of ballistics which explains 'dissimilar wind drift' the way that Tubbs describes it. It sounds a whole lot like spin drift, but he insists that it's not. He and I spoke about it years ago, and it's been in the back of my mind ever since. David seems to be the only one who observes dissimilar wind effects, as I've not heard of anyone else talking about it.
-Bryan
 
He said it was tested by shooting right and left twist barrels simultaneously and measuring the difference in drift during right wind and left wind scenarios. It seems like a pretty easy idea to prove or disprove....you have any left twist barrels? :D
 
I do have a pair of identical barrels with exception that one is right twist and the other is left. I got them to fire two rifles simultaneously to measure spin drift. I was able to measure the expected amount of spin drift, but there wasn't much wind to see any 'dissimilar' effects.
-Bryan
 
IMHO the only possible explanation is spin drift effect according wind direction. As engineer I cannot see any other phenomena.
 
http://www.zediker.com/DTR_PDF_links/DTR_DWD_article_testing_2013.pdf

The Tubb article is linked above. Interesting. I don't really know what to make of that. But if there is a difference in wind drift between left and right twist barrels, it seems to me that the effect should be bigger for faster twist rates.
 
It makes perfect sense to me, but then again I know just enough to be dangerous.

If spin drift is cause by gyroscopic precession on a spinning falling projectile, the responsible force being the difference in pressure between the upper and lower sides caused by gravity, then any force applied by a crosswind will either accelerate or decelerate the spin drift according to whether the force is applied pre or post gravity moment. Is that what he is talking about or am I a kindergartener in physics lab?
 
I don't think it's reasonable to think that a difference as large as he claims would not have been noticed decades ago. Combine that with the fact that there is no theoretical justification for it, and I remain deeply skeptical. Tubb's article references Bob McCoy and "lateral drifting" - I would love some more specifics there. I've not read every word McCoy ever wrote, but I've not seen this mentioned in what I have read.
 
From "the arms guide":

when it exits the barrel, the bullet wobbles because of gyroscopic precession. When the wobbling is dumped out, and the bullet is dynamically stabilized, the bullet longitudinal axis no longer points in the direction it’s travelling, but instead has a yaw angle, called yaw of repose (or equilibrium yaw), toward the direction of the spin. The bullet is actually skidding along the trajectory, with the center of gravity following the trajectory and the tip pointed to the right (for righteous spin bullets) and slightly upward. The incoming air pushing on the left side of the bullet causes it to drift.
 
That's spin drift, though. What is this "lateral drift", which according to Tubb, is different?
 
I won't pretend to be smart enough to understand the theoretical side of the discussion.

But the experimental question seems pretty clear and straightforward to test, and Tubb seems to have done a decent test. Of course, improvements in the experiment are possible, and confidence can be gained by having results repeated by independent parties.
 
I have shot with Tubb's out at his place. We shot the left twist and right twist guns simultaneously in 9 oclock 12mph wind at what I recall was a 50% ipsc. We held the exact same wind hold and the right hand twist barrel hit just right of center and the left twist barrel missed the target. We did it multiple times. Then I went and shot on the 270degree range with my gun(rh twist). I shot targets at 12 oclock,9,and 3. The wind was again about 12mph. When shooting the 12 oclock target we were shooting into a head wind. So when I shot at the 3 oclock target I had to hold less wind than when I shot the 9 oclock target.

Since I am a little skeptical by nature I also shot the 270 degree range with Davids left hand twist barrel and immediately shot it right after mine and sure enough it was the complete opposite of my RH twist gun. I continued to shoot just to see if this was consistent and I can tell you it was very consistent.

Since I have seen this with my own eyes I believe it. Can I tell you why this phenomenon exists? Nope, but I do think it has something to do with what jrm850 mentioned above.
 
Bryan Litz said:
There is no known mechanism in the field of ballistics which explains 'dissimilar wind drift' the way that Tubbs describes it. It sounds a whole lot like spin drift, but he insists that it's not.
-Bryan

Bryan,
I originally thought the same thing,"it just sounds like spin drift". However, if that were true wouldn't a bullet from a right hand twist barrel have more wind drift in a left to right wind than a left hand twist barrel?



Thanks
Scott
 
That's what I learnt and it's easy to verify at the range. At 600 yds, early morning, if I have a 1.5/2 mph 3 o' clock wind (right to left), I have no windage correction.
 
The problem I have with this is that if it is real, then it almost certainly invalidates decades of engineering that has been widely researched, measured, and used to accurately model projectiles from small arms to artillery to rockets. The current engineering thought is known to be very accurate in a wide range of application, and that cannot be the case if such an effect is being overlooked.

That's not to say that there isn't more to learn. But the days of discovering major effects are over. We are in an age where fine tuning of available data and perhaps some subtle modeling of tricky stuff like transonic flow are the remaining frontiers in ballistics. There is still more to be done in enhancing bullet design capability. Not so much in discovering new physics.

In other words, extraordinary claims require extraordinary proof.
 
You guys need to get out and shoot some short range BR. The effect is there. But don't take my word for it. When Tony Boyer, who has about triple the Hall of Fame points of the #2 guy says there is more drift in a L-R wind vs. a R-L wind, it is worth taking notice. Reference page 270 of Tony's book (The Book of Rifle Accuracy). This may not be documented by current ballistic theory or calculations, and Tony will not have conducted blind back to back controlled studies, but the proof is in the pudding so to speak. Ignore the effect at your risk!
 
I am trying to comprehend that wind effect myself, and the theory in my mind as to the effect of (whilst travelling forward) rotating against a wind vs. rotating away from the direction of wind. The projectile rotation becomes involved with an opposing force as compared to devolved with an supportive force assisting rotation. So does that have an affect on velocity as well?

Therefore, in a right twist barrel and wind Right to Left is the POI low left because of a resistance in rotation and reduction of velocity, and the opposite (high Right) for a Left to Right wind?
 
A simplistic diagram of how I was thinking the phenomenon could be related to spin drift. I'm not sure about the question in blue.

Looking at the arse end of a bullet.
 

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There is no left and right wind force, only the drag force and the (sideways) lift force due to the yaw of repose. I think what is being witnessed here is just classic spin drift.

The yaw of repose is created by the force of gravity, a downward force acting on the CG of the bullet. A downward gravity force tends to try to tip the nose up, but the gyroscopic spin rotates that 90 degrees - so you get the nose pointed in the direction of the spin. That yaw of repose is what generates the force that causes spin drift. (The air flow being slightly off axis causes a sideways force.)

So in order for a similar force to be related to the wind and spin, you would need the direction of the wind to somehow generate a force that tries to tip the bullet up or down. I don't see how that is possible.
 

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