Significance of wind and Magnus Effect?

[RonAKA]

I'm trying to learn to deal with the wind beyond using 4 letter words... I gather that wind from right to left can cause the bullet to drop due to the Magnus Effect described here:

http://en.wikipedia.org/wiki/Magnus_effect

and illustrated like this:

And wind from left to right causes the bullet to rise. I think I understand the theory, but I'm a believer in the 80-20 rule. Is this really a significant effect? Is it more an issue with centerfire HV, or LV rimfires?

Can you point me to any links or information that would help me determine how to compensate for this effect? My current interest is rimfires and I know from ballistic software depending on the bullet design a 10 mph wind moves the bullet 1-1.5" sideways at 50 yards, but what about up and down?

Any and all help appreciated,

Ron

 

[Bryan Litz Ballistics]

Ron,
Good questions! I think I can help.
The illustration you posted of the Magnus effect is not relevent for a bullet in a crosswind. The reason is because a bullet will point it's nose into a crosswind, like a weather vane, and the wind doesn't actually blow on the side of the bullet as suggested in the illustration.

There is a vertical component of wind deflection, but it's not caused by the Magnus effect.

When the bullet shifts it's axis of rotation to align with the on-coming air,when it 'weather vanes'), this sets up a series of precession cyles which quickly dampen out. The net effect of this precession is called 'aerodynamic jump'. The effect is upward for a left right wind, and down for a left right wind. This 10 O'clock to 4 O'clock diagonal is very familiar to short range benchrest shooters who shoot rifles with enough precision to see the effect clearly. How much vertical deflection you get is related to the gyroscopic stability of the bullet. The higher the stability factor, the more vertical deflection.

The Magnus effect you asked about is real, and has an effect at long range. The effect is called 'spin drift', and acts to the right for right twist barrels. At very long range, the bullet can fly with a slight nose up orientation, which will cause the situation shown in your illustration. Spin drift can be around 6-10" at 1000 yards depending on bullet stability, time of flight, Magnus moment coefficient,which is highly sensitive to Mach number).

I hope this helps sort it out for you.

You can read more about gyroscopic drift and coreolis drift in my website:
http://bryanlitz.bravehost.com/
Click on 'Gyroscopic and coreolis drift'.

Take care,
-Bryan

 

[RonAKA]

Bryan, thanks for the great response. I had a quick read through the article on your website and it was very helpful. I was aware of the weather vane effect of a bullet in the wind, and also of the coreolis acceleration. While I do have significant training in math, I'm very visual in my thought process, and things never seem to click unless I can "see" them. As a kid we had a playground that had a large rotating platform. You ran around it to get it up to speed and then jumped on. If you try to walk a straight line from the center to the rim you get a quick feel for it.

On the vane effect, I always visualize the classic airplane landing in a wind video, where the plane points off to the side of the runway rather than straight down it. I appreciate this is slightly different than a bullet as bullets don't have rudders. However, when I go through the simple math, I'm still not sure that a side wind does not go over the bullet like the illustration I posted. My ballistics program tells me it takes about 0.147 seconds for my .22 target load to get to 50 yards. In a 10 mph,14.7 fps) wind, something floating wind neutral would drift a little over 2 feet in the 0.147 seconds. However, the target bullet only drifts about 1" in a 10 mph wind. So, while it might be pointing at a very slight angle,2'x150', or 1"x150'?), it still must have a wind vector passing over it like in the illustration, only very slightly reduced by the vane angle. Your thoughts on this?

To put things in perspective, what I'm trying to do is improve my score on the 50 meter UK rimfire target. It requires touching a 0.275" ring to score a 10 and obliterating a center .050" ring to get a 10X. So far I have been unable to crack a score of 240 with four 10X's. But perhaps this Magnus Effect is not that significant at this short distance? Sounds like it would be very difficult to calculate... And I'm thinking similar about the gyroscopic jump due to the bullet weather vane move into the wind?

Again thanks for the thoughtful response. Very nice website and I will take some time to read and digest.

Edit:

I think you had a bit of a typo in your response. "The net effect of this precession is called 'aerodynamic jump'. The effect is upward for a left right wind, and down for a left right wind." I suspect on one of these you mean right left wind? In other words does it compliment or fight the Magnus Effect,assuming there is one)?

Ron

 

[blacknwhite]

Very illuminating conversation guys...I like!

 

[Tcon260]

Brian,

Since I rarely get the opportunity to shoot at 1000 yards in "dead" conditions, I don't know how much my bullet drifts due the Magnus Effect. So not knowing anything else, for a 6.5 mm Berger 140 VLD at 2850 fps, how much should my 100 yard be "off" to have a 1000 yard zero that is correct?

Or is his effect so small that I would never see it shooting an F-class rifle?

 

[Bryan Litz Ballistics]

You'll have about 6-8" of spin drift at 1000 yards with your rifle/bullet.
But to put it into perspective, even in a dead calm it would be hard to observe this.
If your scope is canted by just 1 degree,arguably imperceptible even with a bubble level), that will cause about 5" of horizontal displacement at 1000 yards.
Also, if your scope is perfectly leveled but the elevation turret isn't perfectly vertical, you can have a similar 5" of horizontal deflection if the turret is tilted 1 degree.

So you could try zeroing 3/4" to the left at 100 yards and see if your 1000 yard POI is any closer to center but I suspect you won't be able to tell.

-Bryan

 

[Tcon260]

Bryan,

Thanks for the reply.

My observations match your reply. I have set my wind zero right on at 100 yards,leveled rifle, calm, yada, yada, yada) and in a seemingly dead calm did not have a 1000 yard zero. Done it the other way as well. Zeroed the rifle to impact 1/2 MOA left at 100 yards, and did not have a perfect zero at 1000 yards......the spin drift is lost in the noise,canted rifle, imperfect scope, imperfect shooter)