Magnus Effect & Airgun External Ballistics

Discussion in 'General Airgun Chat' started by PeteM, Sep 17, 2010.

  1. PeteM

    PeteM New Member

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    Does anyone have any experience of the Magnus Effect in relation to airgun ammunition (pellets).
    http://en.wikipedia.org/wiki/Magnus_effect

    i.e wind from the right pellet POI high & left
    wind from the left pellet POI low & right

    I have made many inconclusive experiments,
    generally I get a horizontal line of impact points(POI's)

    My conclusion is that airgun ammunition (pellets) are not affected by the Magnus Effect, or the effect is very small.
    unlike firearm ammunition, which is effected by the Magnus Effect.

    Any thoughts on this or experience of this effect. :)
     
  2. Jon

    Jon Member

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    Wrong way round when it does happen and thats very rare.
     
  3. PeteM

    PeteM New Member

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    what conditions doe it occur

    What conditions does it happen in, in your experience?
     
  4. greyskullnz

    greyskullnz Non member

    Generally the effect can be taken to be a 10:1 ratio for rimfire bullets.

    Wind from right, shot should (theoretically) be in the order of 1/2 in high and 5 in left.

    In practise, I have never seen it occur even under extreme wind conditions with any airgun pellet. Even with pellets being blown off the mildots and into the thick part of the crosshair, they stay on a horizontal line.

    Even though it was tested off a stable platform, it was only to 55 yards however. There is more potential for your own body movement to cause far more drastic imprecision.

    GS
     
  5. PeteM

    PeteM New Member

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    Interesting!

    The question arose from another thread I started - 100 yard Target Airgun. Target photos here.
    http://www.shooting-the-breeze.com/forums/showthread.php?t=5229

    I showed the results of 100 yards test to the coach of my shooting club, he asked me why or how the POI's were horizontal rather than oblique & quoted the Magnus Effect to me.
    The wind was quite strong that day.

    I had heard of the effect before but had no experience of it with airguns.
     
  6. RobF

    RobF Administrator Staff Member

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    My personal thoughts on it is that on really strong days, there's more to do with the buffet on the body than the effect on the pellet. Also, cant comes more into play with more windage. And the windier it is, I think there's more chance of a shot being snatched, which gives rise to vertical movement.

    I'd like to compare two really good left and right handed shooters, perhaps even using smallbore to explore that theory. Or you could use barrels with different twist directions.
     
  7. greyskullnz

    greyskullnz Non member

    Jon,

    It has just been proven to me, that you are in fact correct. It is in the other direction for Diabolo pellets. A 9 o'clock wind deflects a diabolo pellet up, 3 o'clock wind deflects a diabolo pellet down. Mind you, it was proved with .25 calibre.

    I still believe from my own observation of .177 cal diabolo pellets, that for this calibre, the magnus effect is either non existent, or so insignificant as not to be taken into account.

    The reverse applies to cylindrical bullets/pellets.

    GS
     
  8. PeteM

    PeteM New Member

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    This is getting interesting

    That might explain this - see attached photo -
    Shot at 100 yards with 9 o'clock i.e. full value left to right wind
    .177 JSB Exact Express 7.9gr
    Steyr LG110 FT

    Any explanation as to why diabolo pellets are opposite to cylindrical bullets/pellets ?
     

    Attached Files:

  9. greyskullnz

    greyskullnz Non member

    Mate!

    I'm just coming to grips with the busting of a commonly held myth. Other than " ït just does",......I cannot illuminate you.

    The explanation for the Magnus moment obviously still applies, but affects the different design accordingly.

    Its definitely counter to accepted firearms ballistic theory, and I bet more than a few will be scratching their heads until the theory has been re-examined. Until that time we are stuck with "it just does"......which I personally hate.

    GS
     
  10. PeteM

    PeteM New Member

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    Fair Enough

    Fair Enough.

    I've lived the conventional explanation for many years.
    Jon has put the proverbial cat among the pigeons with that one.

    This is explaining something I have experienced from time to time,
    that I have put down to poor marksmanship on my part.
     
  11. greyskullnz

    greyskullnz Non member

    I see you have shown it in that picture you posted, and it has occurred at 100 yards.

    What magnitude wind do you think you were shooting in, and do you think you could replicate it at 50m/55 yards?

    I'm still unconvinced that it is relevant to us in Field Target in the .177 calibre at the typical FT distances.

    I am still open to being converted however, and even though I have never seen anything discernable in my own shooting in wind, that is not a definitive proof to me that it is not occurring. It just means I have not observed it, or it is insignificant.

    In the field of long range air rifle shooting this may well be significant, and if 100 yard shooting does take off, it will be important to know.:)

    cheers
    GS
     
  12. PeteM

    PeteM New Member

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    The wind speed was fluctuating between 2 to 6mph. I was using a portable anemometer.
    I have from time to time experienced the Magnus Effect a 55 yards but because POI was going the wrong way to conventional thought, I assumed it was some technique error on my part.

    As to.177, 16 joule/12ft lb, 100 yard Target Competition.
    I think it could be a very interesting.
    I think it is the equivalent of 1000 yards with .308/7.62 without the cost.
    A bit like F Class for air rifles. :)
     
  13. greyskullnz

    greyskullnz Non member

    Yes, I have often thought FT was a bit like "F" class or long range shooting, given at 12 ft/lbs we are out in the " back curve" of the trajectory already at 55 yards. 100 yards is "out there" well and truly at that energy level. But the BC of the pellets improves dramatically as they settle down at longer range.........it may be that specialist long range barrels may have to be used with slower or almost non existent twist rate. (such as Ben Taylors smooth twist bore)

    Some good stuff done by Harry Fuller, and some USA personages. If I have been informed correctly, along with many sub MOA groups being shot with .22 calibre and .25 calbre now, there has been ONE recorded sub MOA group shot with .177 Steyr 24 J rifle in South Africa. (thats sub MOA at 100 yards)

    I have been meaning to turn the Steyr back up to 24J to do some 100 yard groups myself this year after the Spring Equinox winds subside a bit.

    cheers
    GS
     
    Last edited by a moderator: Sep 20, 2010
  14. PeteM

    PeteM New Member

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    100 meter in the Netherlands

    These may be of interest you.

    http://www.100mairgun.nl

    http://www.schulzdiabolo.cz/en/news/



    How many FT Clubs do you have in NZ ?

    Do you need a licence for air rifles there ?

    Are the clubs mainly on North Island or on both Islands?

    Do you know anything about FT in Australia ?

    A lot of question, but I am interested.

    I'm Australian, but have lived in the UK for the last 20 years.
    I've been shooting FT on and off for 15 years.
     
  15. Gibbs

    Gibbs New Member

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    I have a theory that the Magnus effect was magnified by the old fashioned pellets with ridged skirts.
    The effect would also depend on which way the rifling turns in your gun.(Or does all rifling turn the same way?)
    Now pellets are smooth, I think the effect is lost.
     
  16. PeteM

    PeteM New Member

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    Sounds like an experiment to make

    Sounds like an experiment to make.

    I think most rifling is clockwise but there are different rates of twist.
     
  17. PeteM

    PeteM New Member

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    Interesting extracts's from external site's

    EXTRACT from EXTERNAL SITE
    http://www.nennstiel-ruprecht.de/bullfly/fomo.htm#header_forcesmoments

    BOLD ITALIC UNDERLINED section may explain

    Magnus force and Magnus moment

    Generally, the wind force is the dominant aerodynamic force. However, there are numerous other smaller forces but we want to consider only the Magnus force, which turns out to be very important for bullet stability.
    With respect to the figure , we are looking at a bullet from the rear. Suppose that the bullet has right-handed twist, as indicated by the two arrows. We additionally assume the presence of an angle of yaw d. The bullet's longitudinal axis should be inclined to the left, just as indicated in the previous drawings.

    Due to this inclination, the flowfield velocity has a component perpendicular to the bullet's axis of symmetry, which we call vn.

    However, because of the bullet's spin, the flowfield turns out to become asymmetric. Molecules of the air stream adhere to the bullet's surface. Air stream velocity and the rotational velocity of the body add at point B and subtract at point A. Thus one can observe a lower flowfield velocity at A and a higher streaming velocity at B. However, according to Bernoulli's rule (see elementary physics textbook), a higher streaming velocity corresponds with a lower pressure and a lower velocity with a higher pressure. Thus, there is a pressure difference, which results in a downward (only in this diagram!) directed force, which is called theMagnus force FM (Heinrich Gustav Magnus, *1802, died 1870; German physicist).

    This explains, why the Magnus force, as far as flying bullets are concerned, requires spin as well as an angle of yaw, otherwise this force vanishes.

    If one considers the whole surface of a bullet, one finds a total Magnus force, which applies at its instantaneous center of pressure CPM (see figure ). The center of pressure of the Magnus force varies as a function of the flowfield structure and can be located behind, as well as in front of the CG. The magnitude of the Magnus force is considerably smaller than the magnitude of the wind force. However, the associated moment, the discussion of which follows, is of considerable importance for bullet stability.

    You can repeat the steps that were followed after the discussion of the wind force. Again, you can substitute the Magnus force applying at its CP by an equivalent force, applying at the CG, plus a moment, which is said to be the Magnus moment MM. This moment tends to turn the body about an axis perpendicular to its axis of symmetry, just as shown in the figure .

    However, the gyroscopic effect also applies for the Magnus force. Remember that due to the gyroscopic effect, the bullet's nose moves into the direction of the associated moment. With respect to the conditions shown in the figure , the Magnus force thus would have a stabilizing effect, as it tends to decrease the yaw angle, because the bullet's axis will be moved opposite to the direction of the yaw angle.

    A similar examination shows that the Magnus force has a destabilizing effect and increases the yaw angle, if its center of pressure is located in front of the CG. Later, this observation will become very important, as we will meet a dynamically unstable bullet, the instability of which is caused by this effect.


    Another Extract from another site
    http://www.worldlingo.com/ma/enwiki/en/Magnus_effect#The_Magnus_effect_in_external_ballistics.2C_also_known_as_.27spin_drift.27

    The Magnus effect in external ballistics, also known as 'spin drift'

    The Magnus effect can be found in advanced external ballistics. A spinning bullet in flight is often subject to a sideways wind. In the simple case of horizontal wind, the Magnus effect causes an upward or downward force that depends on the direction of the wind which affects the projectiles point of impact. Even in completely calm air, a bullet will experience a small sideways wind component. This is because bullets have a yaw motion that causes the nose of the bullet to point in a slightly different direction from the direction in which the bullet is actually traveling. This means that the bullet is "skidding" sideways at any given moment, and thus experiences a small sideways wind component. (yaw of repose) All in all, the effect of the Magnus force on a bullet is not significant when compared to other forces such as drag. However, the Magnus effect has a significant role in bullet stability because the Magnus force does not act upon the bullet's center of gravity, but the center of pressure. This means that the Magnus force affects the yaw of the bullet. The Magnus effect will act as a destabilizing force on any bullet with a center of pressure located ahead of the center of gravity, while conversely acting as a stabilizing force on any bullet with the center of pressure located behind the center of gravity. The location of the center of pressure depends on the flowfield structure, in other words, it depends on whether the bullet is in super-sonic or sub-sonic flight. What this means in practice depends on the shape and other attributes of the bullet. In any case the Magnus force greatly affects stability because it tries to "twist" the bullet along its flight path, twisting it either towards the axis of flight (stabilizing) or away from the axis of flight (destabilizing).

    :):cool::D
     
  18. greyskullnz

    greyskullnz Non member

    Consider the statement that for the Magnus Effect to be present, a cylindrical projectile spinning about its axis MUST be in the" Yaw of Repose".

    The Yaw of repose is typical of bullets/projectiles which are NOT drag stabilised.

    Pellets ARE drag stabilised, like an arrow, or bomb with fins, or shuttlcock.

    I think the key difference can be found in whether the " centre of pressure" lies before or after the "centre of gravity". Namely, is it drag stabilised or spin stabilised?

    Comments?
     
  19. PeteM

    PeteM New Member

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    Deeper & deeper

    Are pellets not both drag stabilised or spin stabilised then ?

    Still doesn't explain why the Magnus Effect has an opposite effect on Diablo pellets.

    Possibly because diablo pellets are both drag stabilised & spin stabilised is the reason the Magnus Effect rarely affects them and when it does it affects them the opposite way.

    I think what is needed is some experiments in some sort of wind tunnel,
    so the wind can be eliminated as an unknown variable.
    With the rifle action in a test bed to eliminate operator error.

    This is probably getting a bit OTT though.
    Still it would be interesting.
     
  20. greyskullnz

    greyskullnz Non member

    Yes, for the moment "it just does"........:mad:

    GS
     

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