Spin Drift

Discussion in 'General Airgun Chat' started by Ballisticboy, Jan 11, 2019.

  1. Ballisticboy

    Ballisticboy Active Member

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    The recent thread on crossover suggested that some people are not aware of how a pellet moves in the air when fired from a rifled barrel. Everyone knows that as soon as a pellet leaves the barrel it starts to accelerate towards the ground giving a curved trajectory. However, any spinning projectile also flies in a curve to the right or the left of the barrel line of fire depending on the barrel twist rate, twist direction and on the pellet aerodynamic design. This curvature is much less pronounced on a pellet out to 50 yards but it still exists whereas you are looking in a straight line through your sights. This means that as you look through your sights the pellet will only truly be in the position you are viewing at one point which is your zero range. Figures 1 and 2 will hopefully help to explain.

    spindrr.jpg
    The horizontal numbers are the drift in inches and the vertical numbers are the range in yards. The values are for a .22 pellet but, do not take them as being exact as the value will change depending on the calibre and what design of pellet you are using.


    In figure 1 the red line represents the pellet trajectory relative to the barrel and the line along which the barrel is pointing (vertical axis). The blue line represents the sight line which is not looking straight down the barrel as the pellet follows a curved trajectory and your sight is angled off to intersect the pellet at the zero range. The zero range is approximately 30 yards. You can see how the pellet is offset from your sight line at all other ranges. Figure 2 shows how much the pellet is offset depending on the range. In this figure the vertical axis is your sight line.


    The first thing that strikes you is that the offset values in figure 2 are relatively small. Even at 50 yards you are only looking at an error less than the pellet diameter. When this is combined with all the other errors such as wind speed etc then it is a small part of the total error.


    If you take a more severe case for a different pellet design then the error at 50 yards is approximately doubled, but, it is still less than the error given by a 1mph cross wind on the same pellet at the same range.


    Now I have no wish at all to start a calibre debate, but, the curve on .177 pellets is less than it is on the identical design .22 pellets due to the lower gyroscopic stability of the smaller calibre. Hence the errors at different ranges will be less but again it all depends on the individual pellet designs since the .177 designs are seldom exactly the same as their .22 relatives.


    The effects are relatively small and will go un-noticed in most cases. Where you may see an effect is in extreme range shooting, but, as you presumably won’t be using a 30 yard zero for such shooting, the effect will be reduced as far as you the shooter is concerned since you will be automatically compensating with the longer zero range. You may see a change in the POI when you go from one pellet make to another, but again, it should be small unless you change from a stable to an unstable pellet design.


    One time you may notice something is when the wind drift on your pellet appears to be worse if the wind comes from say right to left than it is if the wind comes the left to right. This is due to the difference between the sight line and the pellet drift. Suppose the pellet is in a 1mph cross wind. The pellet trajectory will move by about half an inch at 50 yards left or right depending on the wind direction. So, if the wind is from right to left the POI will be around 0.7 inches from the sight line, but if the wind is from left to right the POI will only be 0.3 inches away from your sight line causing you to think the wind has less effect on the pellet trajectory. It doesn’t, it just appears to.
     
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  2. C.Eaton

    C.Eaton Confirmed Anschutz Nut...

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    Great explanation, Holly should be reading this to really fry his brain over what's happening with his various rifle set ups.
    This has implications more for mini-kills, i.e. 15mm's at say 15yds where your zero is 30yds, just a slight wobble and you're toast.
     
  3. RobF

    RobF Administrator Staff Member

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    Very interesting...

    So (in theory) this could account for seeing 20mm of 'drift' at 50m for a zero that's sub mm perfect at 10m... and it's better to say ensure you're on at say 30m if you're shooting 10-50m because it means the error is about the same either side of that range, and minimised...?
     
  4. Brian.Samson

    Brian.Samson Allowed in Sales Staff Member

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    That's interesting, thanks for the post Miles.

    Are those diagrams approximately representative of typical amounts of drift? (5mm drift at 50 yards when zeroed at 30)
     
  5. DeanB

    DeanB Active Member

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    Many thanks for that information, it is really thought provoking. At 1 mph, I'm surprised the pellet could move 18mm at 50 yards. I now have another excuse for missing!
     
  6. Dan Smith

    Dan Smith Active Member

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    Can this be a sticky?

    Really great to have your expertise on board !!
     
  7. Tone

    Tone Active Member

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    Interesting thanks Miles, something to be mindful of in the future.
     
  8. simona

    simona Active Member

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    Those figures are for a .22 and .177 drifts less, how much less are we talking for .177?

    I have to say that I can't resolve this error in shooting my .177 rifles indoors but the drift must be so small and so small relative to group size that I just can't weed it out. I tend to set my windage at 40 yards. I'm not sure just how much attention I've paid to whether I'm a couple of millimetres out at say 15 yards. I'd never see the error at 50/55.
     
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  9. Waldo

    Waldo Active Member

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    Excellent reading ,doesn't get bogged down with maths , my old grey matter cant cope with sums ! I soon run out of fingers .:(:D
     
  10. tillygti6

    tillygti6 Tilly's gun stocks

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    You mention the barrel twist rate, what sort of differences could we potentially expect between a faster and slower twist rate?
     
  11. Jesim1

    Jesim1 Active Member

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    I'm firstly assuming from the text on BBs post that the scale on the bottom is a decimal fraction of an inch? Please correct me if I'm wrong.

    Bear with me on this, I'm trying to get my head round it myself, but to my thinking you would not see a 20mm drift at 50m as it would be a drift based on your line of sight, and at a 10m zero your line of sight would be around 0.1 of an inch off at 50m based on BBs graph? This would then give the difference of drift as 0.1 to 0.45 = 0.35 inch = roughly 9 mm.

    Or I could be talking large testicles? :eek:

    Add this to the recent temperature and "cant" threads and I'm going to take up rocket science just to relax :D

    James
     
  12. Paul_T

    Paul_T Active Member

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    Thanks Miles. This is very informative and interesting. You certainly seem to have a nack for explaining these things in plain English. Cheers.
     
  13. SpeckleD

    SpeckleD Active Member

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    I didn’t for one second think that spin drift would apply to projectiles at such a short range.
     
  14. Evo

    Evo SCOUSER and top wheel nicker

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    many thanks for that miles, looks like i,ll have to come out the kill at 50mtr, now i know why i miss lol,brilliant read
     
  15. Welsh Wizard

    Welsh Wizard FT: You Give, You take! Staff Member

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    I'm with the Professor, if you want to make AA in ft, forget about Spin Drift.
     
  16. RobF

    RobF Administrator Staff Member

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    Yep, you're right. My thinking was off.

    I'll go back to it being cant :)
     
  17. subaru swift

    subaru swift I don't have a solution, I just love the problem.

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    Miles as always a cracking read. Even I can understand 'some' of it,,,, :D
     
  18. Jesim1

    Jesim1 Active Member

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    I've really enjoyed some of the past weeks threads on power/cant/spin rotation etc. It kind of brings it back to me just how much there is to it and just how much we gloss over and ignore through complacency, but if you add it all up it makes the difference between a good shot and a great shot - I'm the former by the way :D

    James
     
  19. Ballisticboy

    Ballisticboy Active Member

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    First of all thanks for all the responses I appeciate all the comments.

    The amount of drift is a good question. Measuring the drift accurately without serious equipment and facilities is very difficult. I have a possible method which I need to try sometime involving firing at an indoor 50 yard range with the gun fired from a stand holding it sideways at 90 degrees to the vertical. This is obviously somewhat difficult but it takes away the need to know exactly where the barrel is pointing. When I was working we used survey equipment to measure the exact line of fire but even that is not certain. The drifts are computed using coefficients derived from other methods and matched to observed behaviour such as yaw wave lengths as seen in videos etc. The drift uses the same coefficients and inertia terms as the yaw equations so if they are close then the drift should be in the right ballpark, hopefully. The computed firgures are small enough for no one to have noticed them in practice which is encouraging. If they had been much larger then I would assume something is wrong if no one had ever commented on the effect.
    The drift for a .177 should be less but the amount will depend on the pellet and the barrel. I would expect only a few mm diference at normal ranges. Of course at long ranges (say 75 yards or above) the effects and differences will increase but it will also depend on your zero range as to how much you will notice it. If you zero at 40 yards and only fire out to 50 yards you are unlikely to notice anything.
    In theory drift is directly proportional to the ratio of sin rate devided by speed and thus will vary directly with barrel twist rate.
    Rob, James is right, you cannot blame all the miss distance on spin drift unless you have a different pellet shape.
    The point of the thread was to make people aware of the existence of spin drift, so don't get too hung up on the actual numbers. A small change in pellet design will give a change in drift, possibly even making it drift to the right rather than the left if your pellets are aerodynamically unstable rather than stable as some are. Most conventional round head diabolo pellets are aerodynamically stable and thus will drift to the left.
     
  20. simona

    simona Active Member

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    If you're saying that some pellets might drift left to right and others right to left is it also feasible that some designs might not drift at all, or have drift which is cancelled by other factors?
     

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