Range Report Are these Lapua numbers wrong or am I losing it?

[Greg308]

Hi All,

I've been playing with Lapua's Quicktarget Unlimited ballistic software and, running some theoretical shots through it to see how it handles certain effects (spin drift, coriolis, eotwos, etc), I'm getting some numbers I just can't figure out.

Essentially I am having it plot 4 1000m shots in 4 different directions (0, 90, 180, 270 compass degrees) which seems to show eastward shots showing less drop and westward shots showing more, which is in line with the eotwos effect. So far, great.

What I can't figure out is that the more south I go (shooting position), the less windage correction (Spin drift compensation) I need to put in. For example I need to dial in about 2.5clicks (0.1mil) at 50deg latitude, 2clicks at the equator (or with coriolis effects switched off), and 1.5clicks at -50deg lat. And these windage adjustments are identical no matter whether the shot is shot at 0, 90, 180, or 270 degrees (compass) which makes no sense to me. Any effect should change depending on the direction, and any change related to latitude should be reflected on the other side of the equator, yet this doesn't. Am I missing something or is the software's math off?

Thanks!

 

[Ledzep]

Re: Are these Lapua numbers wrong or am I losing it?

Coriolis and spin drift are separate. Your spin drift stays the same.

The effect of corolis north of the equator makes the shot drift right (regardless of direction), more so the farther north you are. At the equator, it zeroes out. South of the equator it drifts left (again, regardless of direction).

What you're seeing is that north of the equator, Coriolis and spin drift add together (both go right), but south of the equator they try to cancel each other out.

http://www.youtube.com/watch?v=aeY9tY9vKgs

 

[Greg308]

Re: Are these Lapua numbers wrong or am I losing it?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Ledzep</div><div class="ubbcode-body">Coriolis and spin drift are separate. Your spin drift stays the same.

The effect of corolis north of the equator makes the shot drift right (regardless of direction), more so the farther north you are. At the equator, it zeroes out. South of the equator it drifts left (again, regardless of direction).

What you're seeing is that north of the equator, Coriolis and spin drift add together (both go right), but south of the equator they try to cancel each other out.

http://www.youtube.com/watch?v=aeY9tY9vKgs </div></div>

I know, and that makes sense, but I don't understand why there's no change regardless of which direction I'm shooting in. If shooting in one direction from a certain point counteracts the SD, then shooting in the 180degree opposite directions shouldn't also counteract the SD. The earth isn't spinning in both directions! Obviously in shooting in a direction where coriolis shifts the bullet left (or rather the earth right) like south africa towards the equator would cancel out some of the SD, but it shouldn't if I shoot from there towards the south pole, as the effect should be reversed (Coriolis, not SD obviously).

Is their software just whack? I mean the numbers are different even when shooting along a longitudinal axis (zero coriolis effect), depending on what latitude you're at. That I don't get. Sure drop is affected due to gravity effects, but there's no other sideways (as far as the bullet's travel) component involved.

So is it me or is there something wrong with the program? Again, no difference whether shooting in north, south, west, or east directions.

 

[Greg308]

Re: Are these Lapua numbers wrong or am I losing it?

Wait, I just reread and see that you're saying direction isn't a factor. This I don't get. My whole understanding of coriolis is the moving reference plane. If I shoot towards the equator where things are moving faster due to the bigger radius around the axis of rotation, how can shooting in any other direction be the same?

 

[Greg308]

Re: Are these Lapua numbers wrong or am I losing it?

This is turning into a monologue thread but after your input LedZep I've come to a bit of an epiphany on coriolis. I've been thinking of it in terms of differences in speed between target and gunsite according to their latitude since things at the equator move faster than anything else (eastward) due to the earth's rotation and varying circumference from poles to equator. Obviously what direction you shoot in is affected in this case.

It seems the term is used more in reference to the fact that at the poles you're spinning around your own axis, albeit in a different direction depending on the pole obviously. That that I've visualized that it makes perfect sense.

I'm going to do some math on this. What term is used for the rotating spherical reference plane issue of the target moving slower or faster eastward than us then?

Thanks!

 

[tobias]

Re: Are these Lapua numbers wrong or am I losing it?

The Coriolis force is F = 2 m v x w.
v x w is the cross product of the velocity of your bullet v and the angular velocity of earth w (both are vectors). Calculating the horizontal component you get 2 m v w sin(alpha) where alpha is the latitude of the firing site. The horizontal deflection can be calculated with the horizontal acceleration 2 v w sin(alpha) and is independent of the direction in which you fire!
The same can be done for the vertical component and you get 2 v w cos(alpha) sin(beta) for the acceleration, where beta is the azimuth of fire measured from the north. Thus the vertical deflection is dependent on the direction in which you fire. Firing to the north (beta = 0, sin(0) = 0) you don't get any elevation. Firing to the east (beta = 90°, Sin(90°) = 1) you get maximum elevation, Coriolis counteracts gravity and the bullet strikes slightly higher.

 

[Ledzep]

Re: Are these Lapua numbers wrong or am I losing it?

Here's a little picture I made (mildly hung over, hopefully nothing is too jacked up) showing why coriolis has more effect the further north/south you go, and why it's very minimal at the equator shooting north/south, and non-existent shooting east/west at the equator.

Initially this concept is rough to catch on to. It took a few days of deep thought for me. I still don't try to touch the math of it. I'm happy knowing enough to know I don't know.