mechanism of wind deflection

[rgchristensen]

The mechanism of wind deflection is well-understood. When a bullet is fired into air moving across its path, it turns "into" the wind,(like an arrow or wind-vane) so the bullet feels no wind pressure along its side(s). The deflection is caused by the drag on the bullet, and occurs because of the sideward component of the drag vector. Since the bullet is a gyroscope, when its point steers left into a wind from the left, the point of a right-twist bullet is directed downward. This is the cause of the bullet deflected to the right being also deflected in a downward direction. The Magnus effect on rotating bullets is negligible.

CHRIS

[hepburnman]

So, extending on this, a right-hand or clockwise, spinning bullet, as I guess most of us have right-hand twist barrels, this bullet, experiencing a 9-o'clock wind will tend to drift to the right and down, while a 3 o'clock wind, will tend to drift this bullet to the left and up slightly?

[rgchristensen]

Yes. It depends on a lot of factors, but with the usual sorts of rifles and loads for BP long-range shooting, it amounts to requiring about 1 MOA elevation change for each 5 MOA of windage required.

CHRIS

[bruce m]

chris,
you are raising some interesting topics.
for a right hand twist, a bullet deflected to the left by a rh wind will shoot high, and of course the opposite.
in my experience, with smokeless and black, the bullet will shoot high in a rh wind about 1 moa per 10 of deflection.
this could well be wrong due to other variables, not the least of which is my own ability to hold and squeeze.
for about a 2 moa change this is not noticeable, but a 20 moa change you need to do something about it.
if as you say it is 1 moa per 5, you will need to do more.
bruce.

[John Bly]

While shooting in MI several years ago SSShooter and I observed this same thing while shooting rams at 500 Meters. With a 9 o'clock mirage our hits were a minute low and with a 3 o'clock mirage our hits were a minute high.

[bruce m]

john,
how much wind did you have on the sight?
bruce.

[Woody]

Friendship experiences fishtailing tail wind with great frequency. Because the conditions are normally very mild, requiring 1 to three minutes of windage with a complete reversal, the elevation component is negligible for most of our situations. But!!! it is there and if you "cone of fire" is multiple MOA, you better pay attention.

Woody

[mdeland]

In just about all of our shooting there is multiple direction deflection that the air movement is imposing on the projectile,each wind vector is an accumulative drag burden for the bullet to over come while it tries to stay on the original trajectory path. The vector that imparts the largest burden is the the one that produces the most amount of deflection. Which ever vector by itself or in tandem with another slows the flight time to which ever direction is manifesting, gives it the predominate influence over the amount it is deflected. The longer that influence is being applied the greater the deflection.

[John Bly]

I don't recall how much wind there was, not a great amount. We just noted that we shot high and low depending on the wind direction from left and right.

[rgchristensen]

How much vertical/horizontal? Getting to be an interesting discussion. I must confess that I came up with the 1:5 ratio by hearing someone call it the "ten-to-four" effect, referring to the slope of the line on the target. This angle will be 15 degrees, implying a ratio of about 1:4. 10 degrees a ratio of about 1:6. As 1:5 is easy to calculate with, and is also the ratio of effect of head wind vs. side wind that Halford/Metford came up with, I decided to go with fewer numbers to remember. It has served me well for nearly 20 years, but I have NOT got any hard experimental data to support it.

The slope can possibly depend on a lot of things. Some that come to mind are BC of the bullet, velocity of the bullet, the stability coefficient of the bullet, and, since these values are not actually constants, also on the range.

I'd really like to get out to the range, preferably with a straight-across wind blowing at variable speeds and shoot scope/bench at say, 300 yd. Plot the slope of the bullet impacts on the targets and see what I SHOULD be using. Hard to find such a range here in the East, but easy on the High Plains.

CHRIS

[desert deuce]

John Bly wrote:

I don't recall how much wind there was, not a great amount. We just noted that we shot high and low depending on the wind direction from left and right

.

From experience with Mariah the Goddess of The Wind, it seems she is fickle. She can, and will, tease the distant wind flags to appear at from 3 O'Clock when in fact they are from 2 O'Clock and then makes the mirage look less that the intensity shown in the flag. It seems that finding a way to average the effect of the combined conditions is the key to success in both silhouette and target with head and tail winds the most challenging.

The Wyoming Windage Wheel may help in conceptualizing how this influences bullet strike and even this is an approximation based on the user's ability to determine the influences of the existing and ever changing conditions in consulting the chart.

That said, normally when shooting long range target I add 1.5 min elevation from a right wind to a left wind and take 1.5 min off going back to right wind. May not be dead nuts but averaging seems a better solution rather than agonizing over perfection with limited time constraints when the score is added up.

In silhouette both pigs and rams in particular it seems the head and tail winds that require elevation corrections can be score killers while head winds impede and tail winds accelerate and in which direction and intensity can influence windage drift.

So, we are back to once the basic science is generally understood the art of wind calling increases with experience. JMHO

[mdeland]

In the end that is exactly right but getting rid of incorrect ideas that some how get stuck in our thinking eventually will be and incremental advantage.

[mdeland]

If one sees a condition near the muzzle and then through the spotting scope or down range flag near the target or at lease mid range of the trajectory, picks up a simultaneously occurring reversal ,what would be the call? Quarter value into the reversal? Oh, and no time to wait it out!

[bruce m]

don't shoot

[mdeland]

No time to wait it out. We have to take the shot or the clock gives us a miss.