swirl behind a bullet

Discussion in 'The Training Wing' started by McMee, Sep 10, 2005.

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  1. i am in desperate need of a textbook discription of the swirl that is caaused by a round as it travels through the air. usually only seen on good range days ( i know not very common as bad weather is usually booked!!! zzzzzz) something to do with where the round is falling. please help tonight!
  2. We used to call it the votrex, on rapid fire the spotter could see this and correct the firers aim if needed. IIRC it happened mostly on days that a mirage could be seen. This vortex was affected by the wind.
  3. Best seen on a warm bright day, position yourself behind firer so that you can just see over his/her head (this puts you as close to the line of flight as poss) use bino's, target needs to be 200 metres plus to give you time to see it. It is a disturbance pattern caused by the passage of the spinning round through the air.
    Its wicked! Until you see it its like a myth, you think its a wind up,
    It can be used as a coaching aid as you can watch the flight of the round.
  4. Primarily a concern for snipers and cadre marksmen, air density is a factor that affects all bullets. As soon as a bullet leaves the muzzle it starts to slow down in velocity and also begins to drop. The reason for the velocity loss is of course drag caused by the air and the drop, by gravity. Gravity is constant, but air drag varies with air density.

    The density of the air into which we fire our bullets has a similar but opposite effect on aeroplanes, i.e an aeroplane performs more efficiently in terms of lift in dense air. If it has a carburetor engine, it produces more horsepower in denser air. Bullets, on the other hand perform better in less dense air. The performance of both is predictable if the density of the air is known and the performance characteristics of the bullet or aeroplane are known under standard conditions.

    The performance indicator of a bullet is its ballistic coefficient. Ballistic coefficient describes a bullet's ability to resist drag caused by the air. If the ballistic coefficient and muzzle velocity, are known, the downrange bullet performance can be predicted by modifying the ballistic coefficient from standard air density conditions to those of the present air density. Clearly, we don't get involved in this as a rule - indeed, tampering with bullets is prohibited by International Law!

    The ballistic coefficient of a bullet for standard conditions is available from the bullet manufacturer. It can also be determined from actual firing tests using two chronographs. Ballistic tables (based on G1 or Ingall's tables) use as standard conditions, 29.5275" of pressure, 59 degrees F., and 78 percent humidity at sea level altitude, often referred to as Standard Metro or Metric. These are the standard conditions used by Sierra and Hornady bullets, but other manufacturers (notably Speer Bullets and Nosler Bullets) have used the International Civil Aviation Organization, (ICAO), standards of 29.921" of mercury, the same temperature of 59 degrees F., and zero percent humidity at sea level. The difference results in a minor change in ballistic coefficient of less than two percent. On courses at IBS, you are taught the ICAO figures, but you might with to throw these into your brief to sound well informed!

    In effect, the 'swirl' that you see is affected by this air density. What you see is a compressed wave of air building up in front of the round in flight, interacting with the similar wave being produced by the tailend of the round - the dragwave. This is why it appears to 'shimmer' as your round hurtles downrange. It is a truism that you will only tend to witness this phenomenon on humid days, as the air density is greater, thus the build up of air in front of the round is greater, and is visible.
  5. Yeah, Darth's explanation is a good one.
    The shock wave compresses the air along the leading edge of a bullet causing water vapor in the air to momentary condense and become visible. To the observer, located to the rear of the sniper, the 'swirl' appears as a rapidly moving V-shaped vortex in the air following the trajectory of the bullet. This can best be seen if the observer's optics are
    directly in line with the axis of the rifle barrel.
  6. Yes - a very good point Tenty - I should have mentioned condensation - clearly water doesn't just 'materialise' into being.

    Unless you're Jesus, of course...

    Cheers mate.

    McMee - add his post to mine and I think you've got what you need! :D
  7. For the benefit of infantrymen:

    Air is full of very fine water droplets. These droplets are so fine that you can't usually see them. Humidity is the term used to describe just how full of fine water droplets the air is. 0% humidity happens in the Sahara desert on a sunny afternoon. 100% humidity happens in a valley at Sennybridge at 5am in November or April (except if it's raining - rain doesn't count as humidity).

    When a bullet flies through the air, it pushes the air to the sides, causing the droplets to crush against one another. This causes the fine droplets to join up into bigger droplets. If the air is sufficiently humid, a group of these droplets may be big enough to be seen. What you get is a white fluffy line that seems to follow the bullet much like the vapour trails that form behind the wing tips of aircraft.

    Because a rifle bullet spins, the air very close the bullet will be thrown off in a spiral, though only for a short time. In this spiral the larger droplets will be closer together, making the vapour trail more visible. The bullet will also cause a vacuum behind it, making the larger droplets collide with each other to form into even bigger ones.

    If there is no wind, the vapour trail will hang in the air and remain visible for a long time. If there is a gentle breeze, the vapor trail will widen quickly, separating the groups of droplets and make it difficult to see.

    Note that if the air is very humid, the bullet will cause large drops to form that fall quickly, so you won't get the vapor trail effect.
  10. Lairdx,

    I wasn't trying to be patronising, but trying to explain the effect in terms that the average soldier can understand. Darth's explanation sounds good, but I suspect that it's beyond the average squaddie - after all, where do IC engines come into it?

    I pick on the infantry (having been one), because they in particular don't get involved in technical stuff - hence the difference in pay between a MFC and a FOO - exactly the same job, but the blokes on the other end of the radio (on the arty net) are more used to the long words.

    Take the various explanations to your local Inf unit, give them to a Cpl instructor and ask him to include ONE of his choice in a lesson. Which do you think he'll pick? Of course, if you do the same with the HAC, you may get a different response...

  11. The basic measure of the amount of water in air is called humidity or absolute humidity. This is defined as the weight ratio of water to air:
    Humidity (kg/kg), H = (mass of water) / (mass of dry air)

    If air at pressure P and temperature T reaches thermodynamic equilibrium with water it is said to be saturated, and the usual equilibrium relationships will apply.
    The vapour pressure of water at T can be determined and the mol fraction of water in the saturated air, yo determined:

    P yo = P*(T)

    From yo we can estimate Ho the saturation humidity by multiplying by the appropriate ratio of molecular weights (mol).

    At saturation the humidity of the air is Ho. Its precentage humidity is said to be 100%. Precentage humidity is defined as:

    Percentage humidity = H / Ho x 100%

    If the air contains less than the thermodynamic maximum then its percentage humidity is less than 100%.

    The relative humidity of air which is less than saturated is also expressed as a percentage, but this is not the same (confusingly!) the above percentage humidity, as relative humidity is defined in mol fraction terms:

    Relative humidity (%) =
    100 x (mol fraction of water in air) / (mol fraction of water in saturated air)

    Both percentage humidity and relative humidity obviously depend on temperature as well as the amount of water in the air. Fully saturated air is 100% humidity by both measures, but otherwise these differ somewhat. It can be shown that the realtionship between them is:

    percent humidity = relative humidity x (1-yo) / (1 - y)

    Since the mol fractions are relatively small the measures are quite similar, and approach each other at towards 100%

    Another way of expressing the amount of water in air is the dew point temperature.

    If the gas is saturated then it is by definition at its dew point, i.e. it is in equilibrium with water which can be thought of as just starting to condense out of the gas.
    Thus at temperature T the dew point of saturated air, i.e. at 100% humidity or relative humidity is just T.

    However, if the gas is less than saturated it would have to be cooled until condensation started. The temperature to which it would be cooled is called the dew point temperature Td and can be seen to depend only on the water concentration.

    At 1 atm:
    y = P*(Td)

    The dew point temperature is sometines called the saturation temperature.

    These relationships can all be shown graphically on a psychometric chart as can be seen here http://en.wikipedia.org/wiki/Relative_Humidity.

    A number of changes are brought about when a round (Arty or Small Arms) displaces air. As was previously stated molecules of water may be pushed together and because of increased adhesive intermolecular forces known as Van der Waals forces are made to adhere to the round.

    However, other changes are also at play because the effect of the round is to increase the local air pressure (See P in the above equations) at the leading end and reduce them at the trailing end. Also the passage of the round causes friction and heat, which in addition to the latent heat of the round, cause localised temperature changes to the air.

    I know all this despite them putting the thick pills in the tea bombs during my days as an infrantryman and even managing to get a science degree after years of taking them, so perhaps I am something of a failure in grunt terms.

    However, gleaned from wikipedia (I didn't kno this until I went hunting for a psychometric chart...
    "The statement that relative humidity can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson cloud chamber which uses, in nuclear physics experiments, an extremely brief state of "supersaturation" to accomplish its function." which I confess not to knowing previously, but then nuclear physics isn't really my thing.

    I can only hang my head in shame and beg the forgiveness of my fellow infantrymen. Some of you were deeply suspicious when I failed to drag my knuckles on the ground when marching, but still you took me into your hearts. I know you will feel let down, but I felt it was time to come out of the closet and tell the world I have a brain.

    I shall be organising a grunt pride march in London next summer and welcome any other infanteers who are ready to openly admit to any academic ability.
  12. But, in the absence of a thermometer and whirling hygrometer, is there any way to determine whether the wetness on the inside of your basha is due to body heat evaporating the dampness from your wet maggot or merely because the fabric temperature is below dew-point?
  13. I love education.....................stuff like this makes some threads worthwile finding. Just to let you guys know I have saved this in order to be able to assist in my eldests homework. Biology ....no probs! Physics shit!
  14. Your eldest's at Welbeck????
  15. No.......................but she is bright as are many on these boards........I am DAD but I don't know everything ..............yet! Smilie face cos mines not working

    Edited too say Genuine post not a pishtake...............
    same as any other Dad I will know the answers!!!