Vertical Deflection For Pellets In Crosswind
This month, HAM Technical Editor Bob Sterne reveals some surprising information about what happens to airgun pellets in crosswind. You may need to read this more than once…
It would be nice if when the wind blew it was always from one direction at one speed. Unfortunately, that is seldom the case…
As I mentioned in last month’s article, when the wind is in your face (12 o-clock) it tends to slow the pellet more, so it hits slightly low. When at your back (6 o’clock) the pellet slows less and hits a bit high.
The slower the pellet (or stronger the wind), the greater the effect.
In addition, a low BC, like with a wadcutter, will also increase the effect. Here is a diagram that represents what should happen to the POI of pellets in crosswind with different wind directions:
As the wind clocks around, you should get a more or less elliptical pattern for where your pellet lands.
The proportions of the ellipse will depend on how much the pellet slows into a headwind. The less it slows, the closer the pattern will be to a horizontal line. (I have exaggerated the height of the ellipse in the above diagram for clarity).
For slugs, the vertical POI change could be very small because of their high BC. In fact, you might only see the sideways shift, in almost a straight line.
There is a piece of information here that is very useful. Note what happens when the wind direction swings around from 2 to 4 o’clock or from 8 to 10 o’clock. The sideways drift hardly changes at all.
However, note what happens when the wind shifts from 11 to 1 o’clock or from 5 to 7 o’clock. The POI shifts sideways a huge amount!
This means it is actually harder to know where to aim in shifting winds if the wind is head on or behind you than if it is from the right or left!
Spin Causes Vertical Deflection
It would be nice if the above diagram was the whole story about pellets in crosswind, but it is not, for a rifled barrel.
It would apply to a smoothbore, but once we spin the projectile something strange happens. Here is what you might expect to happen in a wind from 3 o’clock that varies in strength from 0-8 mph:
The horizontal shift increases as the wind velocity increases. However, if you have an accurate enough rifle, you won’t see your pellets forming a nice horizontal line as the wind gusts. You will instead see the group take on a slope like this:
This is caused by what we call “Aerodynamic Jump” (aka jump). It is caused by the effect that happens when you push on a gyroscope from one direction, it reacts by the axis moving at 90 degrees to the force.
With a spinning projectile, the center of gravity (CG) and the center of pressure (CP) seldom line up. In fact the CP moves as the velocity changes.
Diabolo pellets are designed with the CG forward of the CP to give them “shuttlecock stability”. When the pellet exits the muzzle, and runs into a crosswind, it tends to turn into that crosswind.
However, pellets in crosswind react to the side force by pitching up or down, depending on which way it is spinning and which side the wind comes from.
Real World Proof
With a right hand (RH) twist and a wind from the right, the pellet pitches nose down, and hits lower on the target. With a wind from the left, it pitches up and hits high.
Here is a diagram called a “wind rose” drawn by Mike Niksch of Thomas Rifles, showing this effect at only 25 metres in a 3 mph wind:
Now bear in mind that without an accurate rifle and shooter (shooting 1 shot per target), all these shots would be in one ragged hole at 25 metres.
Here is a larger generic diagram, spreading out the effect, so that you can see what is happening:
Once again, this diagram is for a RH twist barrel shooting pellets. If your barrel is a LH twist, the results would be reversed.
In other words, wind from the right would cause the pellet to hit high. Wind from the left would cause it to hit low.
Note the large shift in POI between a wind shift from 11 to 1 o’clock, or from 5 to 7 o’clock, not only horizontally as expected, but vertically as well.
Many shooters (including me, until recently) mistakenly blame this vertical POI shift on the “Magnus effect”. However, since the pellet always turns into what it sees as the “relative wind”, as far as it is concerned there is no “side wind”, and hence no continuing lift or downforce on the pellet due to Magnus effect.
It is the gyroscopic reaction to the side force of the crosswind, while the bullet is turning into it, that causes the jump which we see as a vertical POI change.
Slugs Act the Opposite
Anyone with extensive experience shooting slugs will be now be telling me I have this backwards. Those who shoot an accurate powder burner can tell you that their groups in a gusty crosswind slope from 10 to 4 o’clock.
Well, we are both right!
Bullets tend to have the CP ahead of the CG, particularly if they are flying Supersonic. This causes the opposite vertical reaction to a crosswind. Here is a diagram I made up from a typical .22LR wind rose used by benchrest shooters:
Once again, this is for a RH twist, for a LH twist the vertical jump is in the opposite direction.
Both this diagram, and the previous one for pellets, are generic. The vertical deflection may be more, or less, depending on the slug and the twist rate of the barrel. A faster twist will cause more jump. Long thin slugs tend to have more jump, and short fat slugs less jump.
Slope Changes With Range
OK, so you think you’ve got that? I wish it was that simple…
The initial aerodynamic jump occurs shortly after the pellet leaves the muzzle, as the side force from the crosswind yaws the pellet and the spin causes it to pitch up or down.
If the wind is constant all the way to the target, since the pellet is now flying into the “relative wind” (a combination of the crosswind and its own velocity), it experiences a further yaw change as it slows down and that relative wind changes. This results in a slight further vertical deflection from additional jump.
This is again a situation where pellets in crosswind react differently to Supersonic bullets in a powder burner.
With them, their MV is much greater than the wind speed, and their BC is high, so they don’t slow down as much, and the relative wind is nearly constant.
If the bullet is deflected 1 MOA up, it hits 1 MOA high (relative to the bore), regardless of the range to target. However, the additional drag on the bullet from the crosswind continues to push it off course.
As you double the flight distance, you get twice the sideways drift (in MOA, and actually a bit more than that). This means that if the slope of your group is 30 degrees at 50 yards, it will only be about 15 degrees at 100 yards.
Diabolo pellets, however, slow much more quickly. As they do, even in a constant crosswind, the relative wind moves further to the side, and the pellet continues to pitch up or down. This means that the vertical deflection (jump) continues to increase, though at a slower rate than the sideways drift.
The slope will decrease, but at a slower rate than with bullets in a powder burner. Slugs in an airgun, at Subsonic velocities, will likely react in between the two.
Eventually, the initial vertical jump (particularly with slugs) may become lost in the average group dispersion. This is an effect that is easier to see at close range.
That is why it is of greatest concern to those shooting Field Target or 25-50M Benchrest with pellets. It may also be significant for those shooting slugs from a benchrest, at 100-200 yards.
Pellets in crosswind also react to changes in the crosswind that occur part way to the target.
The new side force will cause the pellet to realign with the new “relative wind”. The spin then causes a small change in pitch, and a different vertical POI. However, this is usually so small compared to the change in side drift you may never notice it, particularly if the wind change is near the target.
The crosswind at the muzzle has the greatest effect on the jump, since the trajectory change that occurs there is magnified as the pellet goes downrange.
Next month I plan to talk about slug stability, both Gyroscopic and Dynamic.