Jetting behind the pellet, strippers and silencers and power...

Discussion in 'General Airgun Chat' started by RobF, Apr 16, 2010.

  1. RobF

    RobF Administrator Staff Member

    Joined:
    Mar 4, 2010
    Messages:
    11,773
    Location:
    Poole, Dorset
    Club:
    Parkstone Gun Club, South Dorset FTC, Southampton Buccaneers
    I've always thought it was bollox as to a pellet accelerating after it's left the barrel, all my instincts said so, and i reckoned the physics did, but wasn't good enough in that dept to prove it.

    About a year ago i saw a strange phenomenon following a slow moving jet aircraft in direct sunlight, the camera captured what looked like a split wing. It wasn't a camera or shutter problem, and the "split" moved up the wing quite quickly. Posting it on a photographt forum revealed a much more severe version with an sea vixen with it's rear wing litterally severed by the effect. It can be seen here...

    http://shooting-the-breeze.com/external/_J4F9935.jpg

    and the other example is attached on this thread.

    http://photography-on-the.net/forum/showthread.php?t=741900

    it now seems as if that's a vortex of different pressure air created by the aircraft, which are moving much slower than supersonic... and this causes an index of refraction shift... (index of refraction being the amount light is bent in a material... the reason why the straw seems to bend when you look at it sitting in a glass of water at the surface)

    i know from shadowgraph photos of subsonic ammo, that low speed projectiles also have a pressure wave in front, caused by the sound at the muzzle... this travels at the speed of sound, and is the noise you hear.

    as this is sound wave is an intense change in pressure, i theorised it would cause an index of refraction change that could be severe enough to appear solid to a sensor on a chrono... or different enough from still air to appear so.

    it would appear pyramid air know about this, or have come to the same conclusion... (found this on http://www.airrifle.co.za/)

    So, by moving the chrono further away, this pressure wave disperses and you get a truer but higher reading... same with silencers or strippers that either disturb or divert this pressure wave away... although strippers/flippers and brakes don't remove the sound, a lot of this is blasted back or away from the pellet... thus removing a lot of the pressure wave, and thus seemingly giving a power increase.

    So, i think the jetting theory is even more bollox now.

    While im on it, i'm convinced that strippers etc only apparently improve accuracy by reducing recoil and vibration and allowing better follow through, but that the removal of this turbulent air front and behind does nothing for the actual accuracy of the pellet in flight... the wave in front is gone, as it travels 300 fps faster, and the one behind will probably always be there due to the low pressure formed by the pellet moving forward.

    Of course, when someone says something that agrees with your own theory, it doesn't always make it right... could just mean there's two nutters in the room... :D
     
  2. Mog

    Mog New Member

    Joined:
    Apr 17, 2009
    Messages:
    251
    Location:
    Lower Normandy
    Club:
    Truttemer Tir Club
    Was with you all the way up to 'I've always thought it was bollox', can you run the rest past me again? :rolleyes::D
     
  3. JAGXKRS

    JAGXKRS Sparky's Bodyguard

    Joined:
    Apr 16, 2009
    Messages:
    2,038
    Location:
    Lyneham
    Anyone know what the hell he's on about???:D:D:D
     
  4. NJR 100

    NJR 100 Because I`m AWESIME !!

    Joined:
    Aug 10, 2009
    Messages:
    5,577
    Location:
    Skewen
    Club:
    Avon Hawks, Tondu, Oaktree
    He is trying to explain why ripped jeans make you a better shot?
     
  5. steg

    steg Member

    Joined:
    Aug 11, 2009
    Messages:
    371
    Location:
    Mirfield
    Club:
    Emley Moor
    BARMAN...I'll have have what he's had...
     
  6. steg

    steg Member

    Joined:
    Aug 11, 2009
    Messages:
    371
    Location:
    Mirfield
    Club:
    Emley Moor
    BARMAN...I'll have have what he's had...:D:D:D
     
  7. Andy006

    Andy006 Member

    Joined:
    Apr 16, 2009
    Messages:
    291
    Location:
    Ruskington Lincolnshire.
    I think I understand Rob's theory.
    A pellet starts from rest and accelerates at +1.87 m/s2 for 6.50 s. At the end of this time, the pellet continues for an additional 6.43 s with an acceleration of +0.656 m/s2. Following this, the pellet accelerates at -1.43 m/s2 for 5.34 s. (a) What is the velocity of the pellet at t = 18.27 s? (b) Find the total displacement of the pellet.

    I got the answer to
    a)
    v=ta
    v=6.5*1.87=12.155
    v=6.43*.656=4.22
    v=5.34*(-1.43)=-7.64
    v=8.78m/s @ 18.27sec

    b)
    i cant seem to figure this out. i am trying displacement =vt, but this is getting me only 160, the answer is 198, but i dont know how to get 198
    i just tried using this equation, and its not working out:
    LaTeX Code: x = x_0 + v_0 t + (1/2) a t^2
    for my first segment, my LaTeX Code: X_0 will be 0 right? but for my second segment the LaTeX Code: X_0 will be my answer to my first segment, correct?
    I will do some more maths this afternoon, but I think Rob's on to something very special.

    Quite easy really.

    regards,

    Andy the physicist:)
     
  8. Scoch

    Scoch HOW!!!

    Joined:
    Sep 22, 2009
    Messages:
    466
    Location:
    Dundee, Scotland
    Club:
    Tayside Airgun Club

    Yep thats what i thought too:eek:
     
  9. Mog

    Mog New Member

    Joined:
    Apr 17, 2009
    Messages:
    251
    Location:
    Lower Normandy
    Club:
    Truttemer Tir Club
    Much clearer, why couldn't Rob have explained it like that in the first place? ;)
     
  10. RobF

    RobF Administrator Staff Member

    Joined:
    Mar 4, 2010
    Messages:
    11,773
    Location:
    Poole, Dorset
    Club:
    Parkstone Gun Club, South Dorset FTC, Southampton Buccaneers
    :D :p

    ok, simple terms... my theory... the pellet doesn't accelerate after leaving the barrel, strippers and silencers dont make guns more efficient... what a further gap between chrono and gun, or having a stripper/silencer does, is remove a pressure wave which distorts light which makes the chrono see the pellet in the wrong place when it's measured, which gives a lower speed reading than it should if this pressure wave is still present.

    the higher power the gun, the stronger the bang and thus sound pressure wave leaving in front of the pellet (assuming pellet is subsonic) and the more this upsets the chrono, requiring a greater distance between gun and chrono to allow this wave to dissapate.

    In short, the pellet isnt going faster, the chrono is just seeing it in the wrong place and measuring it wrong.

    this is the effect of a pressure wave below, subsonic... see how the boats and houses above and below the words US navy are distorted and not in the exact right place. It is more apparent in direct sunlight (another reason why direct sunlight might screw up chronos). However the effect still takes place even in diffuse light. Whether infra red is affected as much I don't know. Notice how further away, the wave has lost it's energy and the picture distortion is far less. Just below the fuselage, to the right hand edge of the picture, close to the plane, the distortion is massive. Although these aren't sound waves per se, they show what pressure waves can do to light.

    The pressure wave we hear as sound... it's not the compressed air behind the shot, that travels slower and within this pressure wave, which is like an expanding ball. Supersonic items overtake and puncture the surface of this pressure wave, leaving it behind, which is why supersonic rounds get to target before the muzzle report is heard, although you will hear the sonic crack a split second after the round has passed, the muzzle report could be some time behind.

    [​IMG]

    or this... albeit a rather big bang... the pressure/sound wave can be seen distorting the view of the shoreline about a 1/4 in and 3/4 across the image, putting it in the wrong place.

    [​IMG]
     
    Last edited: Apr 17, 2010
  11. Numb Nut

    Numb Nut New Member

    Joined:
    Apr 17, 2009
    Messages:
    41
    Location:
    West Lothian
    Could be why the instructions for the skan pro 1 series 3 chrono suggests to keep the muzzle some distance from the chrono for accurate readings. Although they put it down to pulse of dirty air, it could just be caused by refraction as per your observation. Relevant bits of the chrono manual attached below:
     

    Attached Files:

  12. Mog

    Mog New Member

    Joined:
    Apr 17, 2009
    Messages:
    251
    Location:
    Lower Normandy
    Club:
    Truttemer Tir Club
    If all this is correct about distance from the chrono how come my Combro is so consistant and accurate when measured against other chronos?
     
  13. RobF

    RobF Administrator Staff Member

    Joined:
    Mar 4, 2010
    Messages:
    11,773
    Location:
    Poole, Dorset
    Club:
    Parkstone Gun Club, South Dorset FTC, Southampton Buccaneers
    might be that it's so close to the muzzle that the wave hasn't moved that far away from the pellet at that point...
     
  14. HotShot

    HotShot Active Member

    Joined:
    Dec 1, 2008
    Messages:
    1,938
    Location:
    Barnsley, Yorkshire
    Club:
    Redfearns FTC
    Can someone condense it down into about 7 words so I can add it to my "reasons for missing a target" excuses list - thanks in advance ;)
     
  15. rick

    rick Member

    Joined:
    Dec 4, 2009
    Messages:
    220
    Location:
    Barnsley
    The pressure wave made my pellet tumble.
     
  16. MojoCrow

    MojoCrow New Member

    Joined:
    Apr 17, 2009
    Messages:
    100
    Location:
    Hailsham, East Sussex
    Allowing for how close to sea level you are, you can add air density to the excuse equation. "I missed because my pellet hit the air equivalent of a brick wall.":)
     
  17. HotShot

    HotShot Active Member

    Joined:
    Dec 1, 2008
    Messages:
    1,938
    Location:
    Barnsley, Yorkshire
    Club:
    Redfearns FTC
    Can you believe it - this happened to me 5 times today ;)
     
  18. Scoch

    Scoch HOW!!!

    Joined:
    Sep 22, 2009
    Messages:
    466
    Location:
    Dundee, Scotland
    Club:
    Tayside Airgun Club
    Couldn't hit nowt, air's too thick today:D
     
  19. Andy006

    Andy006 Member

    Joined:
    Apr 16, 2009
    Messages:
    291
    Location:
    Ruskington Lincolnshire.
    I missed a few sitters today. I think it was air density mixed with volcanic ash.;)
     
  20. RobF

    RobF Administrator Staff Member

    Joined:
    Mar 4, 2010
    Messages:
    11,773
    Location:
    Poole, Dorset
    Club:
    Parkstone Gun Club, South Dorset FTC, Southampton Buccaneers
    it definitely happened to me on the standers, but any stander over 40yds was fine...

    clucking bell.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice