WIND DRIFT: START OF RANGE VS END OF RANGE

[DaveMc]

We have all heard the range talk/arguments about wind at start of flight versus end of flight and which is more important.

I decided to have a go at some modelling to work it out. Forgive me for any errors as it is 20 years since I did anything like this. but here goes.

Assumption 1: if the wind blows for a while and then stops, the bullet will keep travelling on that angle to the target (gradient of the windage curve at point where wind stops).

Assumption 2: wind is constant at various heights above ground and it is a flat open range with no valleys, hills, trees etc. (I know this is incorrect but for now will make this assumption and get to modelling this in later if interest is there.)

OK to start: I will use a 308 HBC projectile doing 3000fps. Using JBM this gives a total wind drfit at 1000 yards of 80.3 inches.

Results so far:

10 mph wind for first 100 yards only will result in 11.13 inches on target at 1000 yards.
10mph wind for 2nd 100 only = 10.61
3rd = 10.51, 4th = 10.27, 5th = 9.63, 6th = 8.70, 7th = 7.52, 8th =6.05, 9th = 4.17, and final 100 of only 1.61 inches (total 80.3 inches)

I have a spreadsheet with all the workings on it for those who would like a copy but found it difficult to upload here. Basics of workings are 1) fit a high order polynomial to the curve, 2) take the derivative to get gradient and 3) transpose that line across to the 1000 yard target mathematically.

I haven't modelled in the exact wind versus height curve yet but some early estimates result in a basic rule of thumb of 1 minute deflection at 1000 yards for every 100 yards of 10mph wind up to 6-700 yards and then tapers off rapidly as projectile falls back down and last 100 is almost negligible (0.15 minutes).

[Ovenpaa]

Yes I would like a copy please as this is something we have discussed at home many times. I am used to shooting at ranges that often only have a flag over the butts so other than obvious indicators like trees and the back of my neck it is all I have to work with. This week the range I am shooting at (600m) has no flags at all, is in a valley and the prevailing wind is on the back of my neck. Yesterday with gusts to 25mph I was never more than two clicks of wind and had to contend with slight shifts in verticals.

I will PM you my mail addy.

[DaveMc]

sure thing - on the way.

Please note: do not take this as gospel until it has been reviewed properly and passes some scrutiny. I may have made a mistake somewhere but think it is generally on the right track.

Have just read how to link uploads of spreadsheets so will give it a go.

http://www.mediafire.com/?796dpvse99zxekb

[alocky]

Happy to argue this out when you come to Perth in a couple of weeks!
It's my understanding that the lateral movement of the projectile due to wind is a function of the lag time, ballistic coefficient and wind strength across the line of flight. The lag time is the difference between the vacuum time of flight and the real time of flight. This means that the wind drift over a given interval will be larger when the projectile is traveling slower. It also leads to the counter-intuitive prediction that a projectile going faster than the vacuum trajectory (ie - a rocket) will move upwind. Apparently they do!
Of course, this applies to point mass, flat fire trajectories only. If you want to take the aerodynamic jump, Magnus effect, and subsequent precession of the bullet into account it's a lot more work, but I suspect these are second order effects only.
cheers,
Andrew L.

[DaveMc]

Yes Andrew you are absolutely right and that is the argument used for bullet moving more at end of flight. I have no issue with anything you have said. The reverse argument often touted is the bullet might not move as much in first 100 but then it continues on at an angle resulting in more at the end. I thought I would try and qualify both ends of this speculation. I have ignored spin drfit, magnus effect, Gyroscopic moments, coriolis effect etc.

In fact it does move more for last 100 than first 100 (1.6 inches compared to 0.6 inches) if the bullets stopped there. BUT if wind is only for the first 100 then the bullet will continue on at the tangent (gradient) of the curve if the wind was to stop. By simply projecting those tangential lines through to 1000 we can get an accumulative effect from each 100 that adds up to the final total.

So if the wind blows for he first 100 yards and stops it would move the bullet 0.6 inches at the 100 yard mark but then continue on at an angle that will result in a further 10.53 inches for total 11.13 at 1000 yard line.

[alocky]

Aha - but if the wind stops blowing, the lateral velocity of the bullet will also start to fall off again due to the wind resistance. I thinks it's a function of the lateral speed of the projectile at the start of each 'segment' as well as the lag time, not the angle, since we've resolved the problem into two orthogonal vectors.
Like I said - hours of fun ahead arguing this one out!
In my practical experience on the range, I space my windicators evenly along the range and just sum the effects equally. I think the uncertainty in the angle and speed estimate are far more significant than the relative lag time influence.
cheers,
Andrew.

[DaveMc]

Andrew,
I am enjoying this discussion but after this might indeed save the rest for Perth as it sounds like we could go on for hours and would distract from the thread.

One thing I am not is a rocket scientist or ballistician and don't claim to be but I think I can argue this one out using someone elses data.

Mathematically if what you said is true then the resultant wind drift at 1000 yards would be far smaller than it is. For example get JBM to do the sums.
Add up the first 100 drift (0.6 inches) then start again at 900 and do it again with a velocity of 2831 (0.7 inches) then start at 200 with velocity of 2649 etc etc. the resultant wind drift would be in the order of 10-12 inches not 80.

I have uploaded a page (http://www.mediafire.com/i/?9k3aazegb4o2ry3) from Harold Vaughns book 9Rifle Accuracy Facts) and I believe most ballisticians concur. The bullet quickly realigns itself with new trajectory once the wind stops and hence there is no horizontal drag vector to slow it down in the horizontal plane. It then contnues on its path.

[DaveMc]

ps what you see practically is basically what I am saying. The wind effect is almost consistent all the way down the course. Especially if you factor extra wind effect into top of flight path in middle of course. The only time this will drop off is right near the target end when flight path returns to lower altitudes and there is less room to act on target last few hundred yards but especially last 100 (albeit with increased effect due to low velocity).

What I think is interesting from an F class point of view is we tend to use mirage at the target line for one of our wind indicators. There has been much discussion over the benefits of focusing a spotting scope halfway down the course (as do TR sooters) and this adds some weight to that arguement.

[M12LRPV]

he he.

The hardest things about ballistics discussions is the conversion to lay terms. That's where the arguments start

As pointed out the lag time is where the real drift comes from. A bullet that did not shed velocity would not drift at all due to wind.
But bullets do shed velocity so drift will occur and it's effect is like compound interest. It builds upon the drift and velocity loss of the previous flight segment.

[bruce moulds]

speaking of definitions, drift has nothing to do with wind.
what wind does is causes deflection.
for example, '"the deflection due to wind was 3 moa".
drift is caused by the rifling imparted spin.
for example, "my deadwind zero at 300 is different to my deadwind zero at 1000 due to spindrift.
bruce.

[AlanF]

Just moved this from the General forum.

Sounds like something that would be useful to know, because there's conflicting opinions. Many (including me) tend to give more creedence to the closer (longer range) flags. However I read some tips by David Tubbs recently where he quite clearly stated that he prefers the ones slightly closer to the targets. For those who don't know about Tubbs, he's probably the top US High Power shooter of all time.

I reckon it would save a lot of time to pose the question on longrange.com and ask Bryan Litz to give his opinion. He would certainly be able to give a good theoretical answer at least.

Alan

[DaveMc]

In Bryans book he glosses over it. Stating that it theoretically is the wind closer to mound BUT range conditions such as valleys, contours and also exta wind when the bullet is at its highest part of trajectory should be as important or certainly confound the data. After reading this chapter in his book was one of the reasons I decided to have a crack. I would be a bit embarassed taking this to Bryan just yet as I took the easy way out and let excel work out an approximate (although quite accurate) fit to the JBM data with a 6th order polynomial. Before taking it to him I should probably go back and do it from first principles with correct formula.

[RDavies]

Jeeze, Dave, just ask your neighbor Marty, he used to have this sort of thing plotted, printed, mapped, disproven, modified, proven and applied, according to barrometric pressure, humidity, leap year, chinese calender, moon phase, ambient temp in shade, in the open, in the jacuzi full of hosties, compensating for spin drift, sun flares, geosyncronicity, Bernoulies theorum, accomodating feng shui all in one folder.
Then he held center, shut his eyes and yanked trigger for another X.

But then,



He only calculated it to the 5th order polynomial which explains why he dropped the shots when the flags over near the trees on the left side of the range switched from 3 oclock to 1 oclock.

Gotta get me one of those polynomial checkers from ProCal.

[DaveMc]

Yeah I thought the forum could do with a distraction from some of the current events. This was all I had in the bag. Sorry!

ps those 5th order polynomials NEVER fit! make sure you get the latest version from Procal and not old stock!

[M12LRPV]

Brian is right.

Wind speed increases relative to the height above ground. A 10mph wind 1m above the ground could be 12mph at the flags and 15mph a meter or so above the flags. (note arbitrary figures to provide an example but the phenomenon is real).

With that in mind in a situation where the mid range flags indicate the same as the near flags the midrange wind effect could equal the initial range wind effect.

What the mid range wind effect gives up from it's deflection only effecting half the flight of the projectile it can make up due to increased wind strength through that portion of the flight.

This increased wind strength issue is also covered in the old sweets coaching manual.

It's not as big an issue on flat ranges but some ranges are very sloping and you can really get the projectile high up above the flags at mid flight.