Rimfire Wind Charts are unreliable for diabolo air rifle pellets.

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Yrrah

Yrrah

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Apr 1, 2015
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NSW, Australia
Rimfire Wind Charts are unreliable for diabolo air rifle pellets.
Many times you may have seen rimfire wind charts like the ones presented in the "Tailwind" thread further down the page:

However - the "rise" or "fall" of POI in 9:00 and 3:00 o'clock winds - actually seems to be the exact opposite for diabolo pellets compared to that of rimfire or other bullets.
The diabolo pellets, which we shoot, impact higher in a 9:00 wind (than in still air) and lower in a 3:00 wind. Bullets impact lower in 9:00 winds and higher in 3:00 o'clock winds.

Below is a target shot at 100 yards; firstly in a variable strength 9:00 wind with no attempt to adjust sighting for the wind - aim point was always at the target center with the sight zeroed for 100 yards.
Green group 9:00 o'clock wind:The harder the wind blew the higher and further to the right was the POI.
Red group 3:00 o"clock wind: Was later shot on the same target when the wind was strong but from 3:00 o'clock. But in this case the wind was "read" and the shots were only fired when the wind was deemed to be of the same strength for each shot. The group so formed is to the left (of course) and quite low of the central aim on the disc.


The reason for this difference, to that often presented in bullet charts, is complex. Here is an excellent readable explanation of the reasons for the difference between bullets and our diabolo pellets:
Quote Ron Burnett

"YFVertical deflectionAugust 8 2010, 8:22 PM The typical 10:00 - 4:00 response to a wind from the left is well known to rifle shooters. Aero ballisticians use the phrase aerodynamic jump when referring to this component of the general equations of motion, the governing mathematics being applied in the '40's to firing from the side of aircraft. In very simplistic terms, it happens as a result of a gyro's response to a disturbing force applied anywhere but the CG, and not much else is involved. For a RH twist barrel (angular momentum vector M pointing forward) the modifying angular impulse due to the force from a L-R wind acting thru the CP rotates the M vector (bullet) about the CG slightly downward. This small negative angle of attack essentially alters the initial line of departure producing the downward deflection. It's the classic reaction of a gyro moving at a right angle to the applied force. The horizontal drift due to gravity is the same fundamental response, the frame of reference being rotated 90 degrees.But as we make the transition from "powder burner" types of projectiles to one that is more of a pellet shape, a change occurs in the relative positions of the CG and CP. The removal of material inside the base serves to move the CG forward. The additional removal of material to form a waist also helps move the CG and has the effect of disrupting the airflow around the base resulting in a lower base pressure. This adds to the overall level of drag but more importantly moves the CP toward the rear. If we carry these alterations far enough, then at some point the CP will end up aft of the CG (...statically stable). When this happens, the situation is completely reversed as compared to centerfire slugs. Upon looking at the effect of the same crosswind we now find that the upsetting torque is in the opposite direction and the resultant vector cross product produces a "nose up" response. But we must bear in mind that CP position changes with V and angle of attack so that what we have at MV=600 may not be the same at 800. McCoy covers the topic with examples in Chapter 11 and 12."
Unquote.

This 60 yd target which I shot at my farm clearly demonstrates the difference that a 9:00 wind of variable strength shows between the more "bullet like" JSB Monster (lower wind- spread group angling down) and the higher wind-spread group of the JSB Exact diabolo pellet projectile (angling up) under controlled 9:00 o'clock wind condition: Shots were alternated between the pellet types to improve the experimental design.

Some may find it also of interest that the superior ballistic coefficient of the 25.4 gr Monsters did not drift as far in the wind even though they started at over 170 fps slower from the BSA Hornet .22.

You may find the original thread's discussion of interest.
http://www.network54.com/Forum/79537/thread/1281230781/Wind+deflection+diabolo+pellet+versus+bullet++group+pics+for+Don+Matzeder+and+others+---

Next time you see the rimfire charts like this presented in the context of airgun shooting, perhaps read them in the context of the above original research for air rifle pellet behaviour in wind.
http://www.thewindisnotyourfriend.com/read/bernoulli.html

.... Best regards to all readers who have hung in there with me ..... Harry in OZ.
 
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I am so happy to learn that the change in direction is due to gyroscopic effect, not air pressure related. 
I tried to make the deflection work with the Magnus effect (same effect that gives aa backspinning golf ball lift) but that turns out to be of negligible effect on both bullets and pellets. 

Thanks Yrrah for bringing both theory and experiment together.
 
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Very interesting. Not that I understood any of the technical stuff mentioned above. :) But now I have an explanation for why my shots at 130 + yards "sometimes" hit 1/2 or 3/4 mil dot higher or lower when the wind was blowing from 9:00 or 3:00 o' clock. Thanks for this. I always wondered why this happened. 

However i I have also faced scenarios where the wind blowing from 3:00 o'clock or 9:00 o'clock moved the pellet to the right or left in line with the centre of the target. Neither up nor down. Just perfectly in line. Like in the pic below (150 yards). In the pic below the aim point was the first hole on the extreme right. The wind was blowing from 3:00 clock and was getting stronger. The shot string is perfectly horizontal and not moving downwards even though the wind was getting stronger. 

GROUP 2- 12 shots (That's about 3.5 inche @150 yards)
 

My question is "Given the above explanation for Diablo pellets--- why would the shot string wary? If the wind is blowing from 3:00 clock and getting stronger, It should shoot to the left and lower all the time and not give shot strings like the one in the pic above. Any explanation on this would be very helpful. Non technical explanation please. :) :) . Never understood calculus or vectors. 

 
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The trend does seem somewhat downward .
However, there are a couple of reasons why a flat line can also happen.
It only takes a touch of wind bias from 4:00 O'clock, somewhere down range on a long shot like that, to change the incline - especially late in flight when the pellets are falling and velocity has dropped off; and the pellets' angles of attack relative to the trajectory path may be diverging due to pellet spin stability. 
To quote Ron Burnett above, "But we must bear in mind that CP position changes with V and angle of attack ..." Unquote ... CP= Centre of wind Pressure.
Then, not all pellets have the same BC even when shot from the same tin and rifle. Those with higher BC will generally impact higher than their mates.
Today I did a BC test using radar tracking that showed a range of BCs from 0.0263 to 0.0306 for 8 shots between 5 and 20 yards. The variation at 150 yards could be much greater.
I do have some wind-spread groups that appear flat, but for every one of those I would have half a dozen that show this trend as being highlighted.

There is still much to be done. But my main thrust here was to not trust rimfire charts to be automatically accepted in the airgun scene even though many shooters themselves see the patterns I have presented. 

Did you notice in the green and red groups, on the disc, that a trend line through the green (or is it blue) 9:00 group, when extrapolated, would pass through the red 3:00 group?

Another phenomenon associated with the angle of the wind affected groups is that the projectile's Stability Factor can pretty much be calculated from the ratio of the value of the x,y coordinates! But that is another story for another time perhaps. ....... Best regards, Harry.



 
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No Greg, not in relation to the vertical component highlighted here.
.It is the one of the great challenges at the range. In some 530+ targets shot in Brisbane, at the recent World Benchrest Championships in the two air rifle divisions LV and HV, , only 3 targets scored 250/250. The wind was bad. I heard many shooters from the 15 countries commenting about unread vertical jump combined with lateral shift.

But if you have interest in the background to wind drift per se, then this discussion is probably the best yet publicly entertained in airgun circles.
Harry's Lad is Dave Eades who wrote and continues to develop the "Chairgun " program for us all. Miles works in a ballistic laboratory in UK; and Steve Woodward has some electronic gadgetry on one of the Space probes I understand. He wrote the Airgun Calculators that can be found in the directory of the YF.

Read at your leisure:

file:///C:/Users/Owner/Downloads/Wind%20deflection%20Airgun%20forum%20%20BC%20and%20wind%20drift%20again.%20A%20bit%20long%20with%20some%20maths%20and%20graphs%20and%20stuff.htm

Kind regards, Harry.

 
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I am a bit confused here, it has to vary from rifle to rifle, My rifle with a right to left wind @ 10mph shoots to the left and up... no if ands or buts... here is live details, one with a target @ 100yards and one with a ground squirrels @ 103yards both are consistent to my findings..


In this video it is as we all ways do practice before the hunt....this is a right to left wind that takes my pellet to the left and up.

If you listen close you can hear us discussing the shot and what the pellet will do in a right to left wind...same as in practice that correlated into the field.
and just in case if you are wondering if my rifle was indeed zeroed at 50 dead center here is my zero group...
 
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Another question, Harry. What barrel were those shot from, or rather, what was the twist rate, do you know?

It roughly looks like there is 1 unit of elevation for every 3 or 4 units of windage shift. So, for a lower twist rate barrels (aka FX ST) the vertical component should be proportionately less. E.g. 1 to 15 or even 30 in case of typical ST made in the past few years. At that point, do you think vertical component shift due to horizontal wind is becoming lost in the noise from other factors?
Not that I ever can hope to get to this level of proficiency to care about those in practice (heck, I'm having hard time controlling my breathing) but I am very interested in understanding the mechanics behind this.
 
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"WCT_Editor"So what happens with head or tailwind and diabolo pellets? Also, is there a difference then between the rimfire charts and hollow point, semi-wadcutter, and specialty pellets like the Polymags and cylindrical (i.e., piledriver) pellets?

Eric
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Tailwind the pellet hits slightly higher. Headwind it hits slightly lower. I only use the JSB Diablo pellets with my gun. Haven't tried the other kind of pellets.
 
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"Bwalton"I am a bit confused here, it has to vary from rifle to rifle, My rifle with a right to left wind @ 10mph shoots to the left and up... no if ands or buts... here is live details, one with a target @ 100yards and one with a ground squirrels @ 103yards both are consistent to my findings..


In this video it is as we all ways do practice before the hunt....this is a right to left wind that takes my pellet to the left and up.

If you listen close you can hear us discussing the shot and what the pellet will do in a right to left wind...same as in practice that correlated into the field.

and just in case if you are wondering if my rifle was indeed zeroed at 50 dead center here is my zero group...





Click to expand...
"my rifle was indeed zeroed at 50 dead center here is my zero group.."






No. It was zeroed correctly for R to L (providing that was dead calm.
But it was zeroed high at 50 yards:

So, when you took the 75 yrd squirrel, POI shows the pellet taking out the top of its head, say 3/4" above presumed POA, which is pretty much reflecting the high 50 yd zero subtended to 75 yards:


Your 100 yards target group could also be reflecting the high 50 yd zero subtended in trajectory to 100 yards.:

To test the point of this post one must zero in still air at the range, say 100 yards, then shoot in a 90 degree wind as shown by wind indicators and so determine a lift or drop by comparison. 
If we have not done our own BC tests with our rifle and pellets it may be worthwhile because the generic BC values given in tables are just a guide. I have two .25 cal rifles that give me BCs of 0.043 and 0.036 with the same pellets. This can be reflected in high or low shots if tables are used for holdover/under. 50 yd velocities from same MV for those two are 760 fps for one and 740 fps for the other. 

Just a note on CTC group measure. If one is to measure the outside to outside group size, one cannot simply subtract the nominal pellet calibre. One must actually measure one pellet hole outside edge to outside edge and subtract that figure. The reason is that, as we all know, the apparent pellet holes are always smaller than calibre, sometimes much smaller. Just using the nominal calibre always makes the group seem smaller.

I did enjoy your well presented videos B Walton which is why I have spent a good hour with them to frame a considered reply. ... Best regards, Harry.


 
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"greg"Another question, Harry. What barrel were those shot from, or rather, what was the twist rate, do you know?

It roughly looks like there is 1 unit of elevation for every 3 or 4 units of windage shift. So, for a lower twist rate barrels (aka FX ST) the vertical component should be proportionately less. E.g. 1 to 15 or even 30 in case of typical ST made in the past few years. At that point, do you think vertical component shift due to horizontal wind is becoming lost in the noise from other factors?
Not that I ever can hope to get to this level of proficiency to care about those in practice (heck, I'm having hard time controlling my breathing) but I am very interested in understanding the mechanics behind this.
Click to expand...
1. BSA Hornet with 1:17" RH nominal twist but closer to 1:18 inches from my own check shooting marked pellets through two or three screens and computing the actual rotation.

2. The x,y coordinate ratio does, I think, vary according to the form factor and stability factor too. In any case I get different results from different rifles/pellets. The slopes do vary but in "pure" 90 degree winds varying in strength, each shows a linear slope when enough pellets are shot.

3. And, yes, I think other factors will obscure the vertical shift phenomenon. Pellet-to-pellet weight and particularly pellet-to-pellet BC/ Cd/ SF variability (same tin) can have a noticeable affect on vertical spread at anything beyond 50 metres. ... Of course fishtailing wind from say 2:00 to 4:00 O'c or 11:00 to 2:00 or similar, can be very misleading in the context of this post.
Thanks for your interest ... Best regards, Harry. 
 
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"Bwalton"When I get home I am going to look to see do I have a right hand twist or a left hand twist inside the barrel...Never thought about that. Now that more and more ballrels are being designed maybe from a PB view could be some differences.
Click to expand...
Left or right hand twist affects the normal windage drift (in still air) but is not that relevant in the context of this post - its affect is pretty much lost in the other factors explained in the references above.
But it may come as a surprise to some that the drift due to rifle twist direction is also reversed for our best stable diabolo pellets. Not my research, but Miles Martin points out that it is to the left for a right twist barrel for pellets whereas the classical right drift for right twist relates to bullets.
It would be a most difficult phenomenon to research in the airgun context. ... Regards, Harry.
 
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The only problem that I have is that I am at the range that the National BR is being held at this weekend, 5mins form my home..So when I zeroed at 50yards At 7:30am with the wind was like 1mph or zero thats when I shot the 50yard group to get a zero, Have have shot at that same distance of 50 yards using the many wind flags that we have there at the range. Here I another 50yard group, where the wind is blowing left to right, previously I had shot a few sighters to see what my windage holdover would be, which turn out that the pellet would go right and down, as you can see when the wind did stop for a second sending the pellet back to my cross hair point of aim which was up an 1mil left. OH my rifle Barrel has a right hand twist. Also this is the chart that I use in the field for a right hand twist.










 
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"WCT_Editor"So what happens with head or tailwind and diabolo pellets? Also, is there a difference then between the rimfire charts and hollow point, semi-wadcutter, and specialty pellets like the Polymags and cylindrical (i.e., piledriver) pellets?

Eric
Click to expand...
WCT_Editor:

1. Horizontal head and tail winds lower and raise the POI respectively as others imply. But it is never so simple. Consider an extreme range shot into a nominally "horizontal" head wind shot at an elevated target at relatively close range with the pellet climbing all the way. Perhaps that wind will lift the little pellet still higher. Now shoot at a target on the level but to 200 yards. Early in the trajectory as the pellet is climbing it MAY get some lift to offset the more rapid reduction in velocity. But then when it is descending from its apogee and has lost a lot of velocity PERHAPS it will get driven down moreso. ... Coming up with a research design to sort that out would be a worry!
My experience with my slow motion video tracking pellets to long range indicates that the wadcutters, most hollow points I have had, and pointeds (with the exception of JSB Straton .22s) spiral somewhere past their trajectory apogees - for reasons given in the references above. That makes grouping very chancy and so it is difficult to draw many valid conclusions based upon shot fall..

2. The specialty pellets and odds you mention will react according to their centres of wind pressure as discussed in the reference I gave. Predicting the behaviour of rubbish pellets is not something I spend time on.
But I can say from my own studies that the .22 Piledrivers and .22 JSB Monsters act as bullets not as diabolos.
Best vregards, Harry.



 
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