Cross-Wind & Bullet Vertical deflection

[DaveT]

Yesterday was the first time that I had shot a 'modest' calibre in strong winds and i feel that I learned some significant lessons..... hopefully I have extracted the right analysis but would appreciate confirmation or otherwise from the more experienced shooters on this forum (Ones I am aware of are Baldie Dave and Vince the Gun-Pimp but for those I have missed please forgive my ignorance....no 'slight' intended!)

 

I shot a 260 Rem at 200 yards (1:8 right hand twist) and Lapua Scenar 123g going at @3000 fps using 42g of N550.

 

In calm conditions at 200 yards I usually need @1.25 MOA come-up from a 100 yard zero.... 10 clicks on a 1/8th moa Schmidt & Bender PM11 scope.

 

In the early part of the day there was a moderate wind blowing Left to Right ...didn't measure it but guessed at 5+ MPH...... I found that I had to increase my come-up to 12 clicks (+2 over usual setting) at 200 yards.

 

later on it really got up and changed to a full-value Right-to-Left wind at between 12 - 20 MPH (measured with a Kestrel meter) and I had to TAKE OFF 4 clicks for 200 yards to get back on centre as well as allow for windage.

 

As far as I am aware a cross-wind will deflect DOWN and RIGHT (to 4 O'Clock) for a L-to-R wind (Right twist barrel) and UP and LEFT (to 10 O'clock) for a R-to-L wind BUT I didn't realise that the effect would be so pronounced.

 

Later on I looked up my Bryan Litz ballistics book and found a (complex) formula to predict the vertical displacement fairly reliably (P78 or thereabouts) using the Gyro Stability factor of the bullet being used along with its length and calibre plus other factors....... and the calculations do work out close to field-based reality...... the man is a God!

 

Anyone see any holes in my findings?

 

Cheers

 

Dave T

[terryh]

Dave T

 

no holes, just nice to see a 'result'.

 

What you are reffering to is 'Magnus effect' which is a known fluid dynmic 'consideration', a reaction at right angles to the fluid flow, in your case the wind 'blowing' over the cylindrical body of the bullet.

 

I've only come personnaly experianced it in shooting using larger bullets travelling at slow speed e.g. 45-70's where they are in the air longer. My local range sems to always have a L-R wind off the sea, as all the local leaning trees can testify

 

Mr. Litz's book is a good read, cannot remeber what he calls this effect as someones borrowed my book (BD )

 

Brgds Terry

[DaveT]

Thanks Terry.... I was glad to be able to explain the effect as it had me head-scratching initially until I remembered that this can happen. Guess the trick now is in predicting it rather than correcting it after unexpected 'flyers'!!!!

[Ronin]

You didnt describe the area you were shooting , but another factor may have been the air flowing downward / upwards over obstacles which will induce lift and depression of the bullet.

 

 

Something I try and imagine is the air behaving like water.....

[DaveT]

You didnt describe the area you were shooting , but another factor may have been the air flowing downward / upwards over obstacles which will induce lift and depression of the bullet.

 

 

Something I try and imagine is the air behaving like water.....

 

See what you mean...... generally its open ground next to the Severn estuary but the range is in a depression so there could be some additional 'swirl' effect I guess as the wind crests the surrounding ridges and blows across the range..... not easy to quantify / correct for predictably!

[terryh]

Ronin,

 

If you want a good read about this type of thing I'd recommend 'Fluid Dynamnic Drag' by Hoerner, it is the bible for this sort of stuff, all imperical data and information, just about everything that flies or is fired etc. thru' fluids (air being such).

 

Looking at the price now you can borrow mine, I'm not doing anything related at the moment.

 

Terry

[taylor]

it does happen at long range

right hand twist with a left to right wind knockes you down and to the right ,opposite for a right to left knocking you up ,thats why you get unexpected elevation when you gauge the wind but forget the elevation correction due to wind direction

and the lie of the land makes it even worse depends on the slopes if they go with the wind direction

hope this makes sense ? it does to me lol

[DaveT]

it does happen at long range

right hand twist with a left to right wind knockes you down and to the right ,opposite for a right to left knocking you up ,thats why you get unexpected elevation when you gauge the wind but forget the elevation correction due to wind direction

and the lie of the land makes it even worse depends on the slopes if they go with the wind direction

hope this makes sense ? it does to me lol

 

 

As I said... a valuable lesson learned...... wind-reading is a dark art all of its own!

 

I can't recommend Brian Litz book enough....sooooo much in there!

[eldon]

Hi Guys a quick search revealed this text Terry is it the right one?

 

http://www.scribd.com/doc/35866801/Hoerner-Fluid-Dynamic-Drag-1985

 

[terryh]

Eldon,

 

That's the one, basically a German engineer that 'did it' for real.

 

The download certainly saves some beer tokens!

 

It is the 'Bible' if you are doing anything with fluids as the information contained was derived by experimental result(like most of Litz's stuff), not theory.

 

Brgds Terry

[Guest richness]

Makes sense. Very interesting. Reliably factoring in spin drift (as i've simplistically taken to calling it) is one of my goals for 2012. It seems to me to be the biggest extra factor that one might overlook, using normal bullet flight thinking.

Cheers and good luck moving forward in 2012 Richard

[brown dog]

Yesterday was the first time that I had shot a 'modest' calibre in strong winds and i feel that I learned some significant lessons..... hopefully I have extracted the right analysis but would appreciate confirmation or otherwise from the more experienced shooters on this forum (Ones I am aware of are Baldie Dave and Vince the Gun-Pimp but for those I have missed please forgive my ignorance....no 'slight' intended!)

 

I shot a 260 Rem at 200 yards (1:8 right hand twist) and Lapua Scenar 123g going at @3000 fps using 42g of N550.

 

In calm conditions at 200 yards I usually need @1.25 MOA come-up from a 100 yard zero.... 10 clicks on a 1/8th moa Schmidt & Bender PM11 scope.

 

In the early part of the day there was a moderate wind blowing Left to Right ...didn't measure it but guessed at 5+ MPH...... I found that I had to increase my come-up to 12 clicks (+2 over usual setting) at 200 yards.

 

later on it really got up and changed to a full-value Right-to-Left wind at between 12 - 20 MPH (measured with a Kestrel meter) and I had to TAKE OFF 4 clicks for 200 yards to get back on centre as well as allow for windage.

 

As far as I am aware a cross-wind will deflect DOWN and RIGHT (to 4 O'Clock) for a L-to-R wind (Right twist barrel) and UP and LEFT (to 10 O'clock) for a R-to-L wind BUT I didn't realise that the effect would be so pronounced.

 

Later on I looked up my Bryan Litz ballistics book and found a (complex) formula to predict the vertical displacement fairly reliably (P78 or thereabouts) using the Gyro Stability factor of the bullet being used along with its length and calibre plus other factors....... and the calculations do work out close to field-based reality...... the man is a God!

 

Anyone see any holes in my findings?

 

Cheers

 

Dave T

 

I've just been sad enough to work all this out using Bryan's formulae - for your 1:8 3000fps 0.264 123grscenar

 

5mph left wind would require an additional 0.2MOA to be applied so about one and a half 1/8MOA clicks

 

and a

 

16mph right wind would require 0.6MOA to be taken off so about five 1/8MOA clicks

 

 

So your field observation is pretty bloomingly stunningly spot on

[Si-Snipe]

Very interesting read guys. Rich and I experience huge elevation differences as we change our shot angle in relation to the wind. Especially beyond 400 yards. There are some serious winds up on the exposed moors as well as undulating ground so it's something we both need to analyse very carefully.

[DaveT]

I've just been sad enough to work all this out using Bryan's formulae - for your 1:8 3000fps 0.264 123grscenar

 

5mph left wind would require an additional 0.2MOA to be applied so about one and a half 1/8MOA clicks

 

and a

 

16mph right wind would require 0.6MOA to be taken off so about five 1/8MOA clicks

 

 

So your field observation is pretty bloomingly stunningly spot on

 

I am a sad sod but have now sat down and worked out windage tables for horizontals and verticals....... if only it blew bsolutely constantly!!!!