Coriolis Effect - practical considerations

Norm · Post by **Norm** » Mon Feb 24, 2014 2:21 pm

Thanks Dave.

IanP · Post by **IanP** » Mon Feb 24, 2014 3:32 pm

Norm wrote:Dave,
Do you have any idea of what magnitude of effect we are talking about?
At 1000 yards would it be more than or less than 1 MOA for a 7mm bullet in a full value 3 O'clock right angle side wind?

I had this on my ballistic spreadsheet when Litz first released his book a few years ago and he provided a simple formula for approximating vertical deflection. It starts on page 75 of his current second addition.

Long story short for 7mm 180gr VLD

10mph at 3 o'clock gives 0.34 moa vertical up deflection.
10mph at 9 o'clock gives 0.34 moa vertical down deflection.

* Deflection is the same regardless of range distance *

Vertical Wind Deflection Caused Thru Cross Winds

Note: Assume RH twist barrel!
1. Cross wind from left causes vertical downward deflection
2. Cross wind from the right causes vertical upward delection

Formula for Litz approximation: Y = 0.01 * SG - 0.0024 * L + 0.032

Y = Vertical deflection in MOA for 1 mph crosswind
SG = Gyroscopic stability
L = Bullet length in calibres

You guys need to look more closely at his book, he provides the long range shooter answers and formulas that the average Joe, (or Ian) can understand and use!

Download my spreadsheet: http://fclassdu.com/origin/wp-content/u ... ection.xls

Ian

Brad Y · Post by **Brad Y** » Mon Feb 24, 2014 3:38 pm

So much for the individual shooter to think of while watching wind, mirage, concentrating on pulling off a good shot, breathing etc etc.

Norm · Post by **Norm** » Mon Feb 24, 2014 4:15 pm

Thanks Ian,

I didn't remember that section. I will look it up tonight and have a good read.
I look to make vertical correction into account for my field shooting but only with regard to the slope of the surrounding land mass, wind strength and direction.
This is something that might help as well.
Cheers.

bruce moulds · Post by **bruce moulds** » Mon Feb 24, 2014 4:21 pm

ian,
match rifle shooters have said to allow 1 moa of elevation for every 10 on the wind arm.
how does that tie in with litz.
(I am too lazy to work it out.)
keep safe,
bruce.

Norm · Post by **Norm** » Mon Feb 24, 2014 4:25 pm

Ian,
Its interesting to read from your spread sheet that the 155gn HBC and 230gn Hybrid are not that far apart with their correction. Amazing considering the difference in size and shape of the projectiles.

DaveMc · Post by **DaveMc** » Mon Feb 24, 2014 4:30 pm

Ian - I have his first book but not the second.

Your quotation of Litz's approximation is very similar to the McCoy formula (which gave 0.37 inches per for 100 yards for 308*168 grain.

Both had the vertical component as linear over range whereas the horizontal component was exponential (This means the slope will decrease over range). ie for 10mph a vertical component of 3.7 (10*0.37) inches and horizontal of maybe around 80 inches (guesstimate) at 1000 yards.

IanP · Post by **IanP** » Mon Feb 24, 2014 4:35 pm

Norm, its easy just to allow 0.33 moa, (1/3) for every 10 mph for the calibers I shoot.

Bruce, I already answered your question in the post as well as provided a spreadsheet. Its easy just to do what I have shown above (1/3 moa) but best to check using the spreadsheet in case you are using a different bullet and stability factor.

Ian

bruce moulds · Post by **bruce moulds** » Mon Feb 24, 2014 4:38 pm

my bpcr rifles have about 3.6 moa of spindrift at 1000 yd.
if not allowed for, this can get you well out on the target using a deadwind at 100 zero.
it can be allowed for by putting a slight cant on the rear sight staff, such that the higher the sight goes, the more to the left it goes.
of course experimental testing is not without its difficulties.
keep safe,
bruce.

DaveMc · Post by **DaveMc** » Mon Feb 24, 2014 4:38 pm

Bruce yes we use 1 click for a minute but this sis slightly too much for 1000 yards and beyond so Match rifle specs seem spot on.

sin 5 = 0.087 = 1 in 12
sin 6 = 0.104 = 1 in 10

at shorter ranges it is steeper.

using Ians 0.35 moa of vertical over say 5.5 minutes of horizontal is 0.0636 and slightly less than 5 degrees

DaveMc · Post by **DaveMc** » Mon Feb 24, 2014 4:43 pm

By the way Ian - the fact that this vertical component is linear with range also adds to the argument that it is not Magnus force (as this would act to accelerate throughout the range) but more an initial vertical velocity component that is enacted when the projectile first kicks around into the wind..

EDIT - Oh and I see Bryan has used gyroscopic stability factor in his formula YAY - This also means he agrees with the logic this is a gyroscopic force not Magnus.

bruce moulds · Post by **bruce moulds** » Mon Feb 24, 2014 4:48 pm

dave,
1 click is very close to 0.1 moa, but a bit more.
I have tended not to worry about this for small adjustmens, but know that for bigger adjustments,, say 3 moa, you had better keep it in the back of your mind.
I find just thinking in minutes and minutes easier than mph and minutes, when possible, i.e. 1 per 10. just remembering up or down is easy now, but when under pressure I am known to fail with religious fervour.
keep safe,
bruce.

DaveMc · Post by **DaveMc** » Mon Feb 24, 2014 4:50 pm

Bruce - My trick is to wind the elevation the opposite direction to the windage. ie 10 minutes left (clockwise) will mean 1 minute up (counterclockwise).

IanP · Post by **IanP** » Mon Feb 24, 2014 4:56 pm

DaveMc wrote:By the way Ian - the fact that this vertical component is linear with range also adds to the argument that it is not Magnus force (as this would act to accelerate throughout the range) but more an initial vertical velocity component that is enacted when the projectile first kicks around into the wind..

EDIT - Oh and I see Bryan has used gyroscopic stability factor in his formula YAY - This also means he agrees with the logic this is a gyroscopic force not Magnus.

Dave, I'm yet to research Magnus force in relation to ballistics but I'm looking forward to learning something new!
Litz is using stability and I agree its probably more applicable than Magnus otherwise I'm sure he would have discussed this force as well.

Litz's approximation formula in comparison to running a 6-DOF simulation never gives more than 0.05 moa when tested on a number of bullets with varying SG's. Thats a possible max error of 0.5" at 1000 yards!

Ian

williada · Post by **williada** » Mon Feb 24, 2014 10:42 pm

Dave and Ian are on the right track and the gyroscopic force is part of the mix. While projectiles lose velocity quickly, their r.p.m. is very slow to decay. So drift induced by mechanical forces, other than environmental conditions such as wind, terrain, light, humidity, barometric pressure and the nut behind the butt needs to be understood to maximise score. It calls for new skills to work out the net effect of all the variables. The sport may be demanding of this as the target shrinks as gear and projectiles get better and small variables begin to show up in score. The best individuals may meet the challenge in a competitive sport. Team shooting is another can of worms because you as an individual are reliant on a partner i.e. your coach.

In a practical sense, in terms of team shooting (and I apologize to the original poster, for drifting off topic), the coaches will be in a superior position if they know where the relative zero’s of shooters are at each distance.

For lateral zeroing, may I suggest that shooters establish zeros at 300, then have a squad shoot at the same time (simultaneously - under 8 seconds on command) at 500, on the same target with groups nominally separated by a minute of angle as to distinguish shooters from each other. In other words, shooter “A” shoots at centre, shooter “B” shoots a minute right, Shooter “C” shoots a minute left and Shooter “D” shoots 2 minutes left and shooter “E” shoots 2 minutes right. Have shooters fire 4 shots on command from the coach. Use as many shooters as practical and rotate squads and their composition for comparative purposes. Do not patch out the shots. On completion check the target. It should take less than a minute to shoot. Repeat a few times for statistical relevance.

It is important that scopes use the same magnification for the exercise, as this can vary the amount of windage used and complicate the estimate of drift. We also assume there are no problems with gear and instructions are given to shooters to wind on the appropriate “clicks” to separate groups by the coach. Better still put small aiming marks on the target for each shooter a minute of angle apart. Another trick is to use different colours for each shooter marked on the projectile using a marker pen, and all fire at the centre at the same time. The colour wipes off as the projectile penetrates the target and reveals who shot where. Group size is not important but group centre will enable comparisons of zero. This practice is then repeated at longer distances. Any differences in drift in the lateral plane can be seen.

If you want to take this further, elevation differences can be logged over various distances hopefully on a fishtail day with a long string of 10 shots with all shooters firing on command. The number used for this exercise can be reduced to a pair and shots plotted after each pair is patched out and the next command to shoot is awaited. The command to shoot is given on a wind change without making adjustment or aiming off but aiming at the designated spot. This process is arduous and annoying for some waiting for the wind to change both direction and strength significantly. Notes on both are taken. The process is designed to train the coaches to make elevation changes and not merely respond to group centering. Anyway, probably too much verbal diarrhea.