Center of gravity position

Discussion in 'ELR, Ballistics & Bullets Board' started by sparker, May 21, 2018.

  1. sparker

    sparker

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    Is there a detrimental effect on precision if the center of gravity of a bullet is ahead of the bearing length into the ogive?
     
  2. alinwa

    alinwa

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  3. damoncali

    damoncali Gold $$ Contributor

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    It definitely impacts stability, which impacts required twist, which impacts accuracy. None of that has anything to do with the location relative to the bearing surface end. Beyond that, I'm not sure. I'd love to hear some reasoning as to why that would be an important boundary.
     
  4. geo.ulrich

    geo.ulrich beware of owner Gold $$ Contributor

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    If center of pressure goes behind c.g. bullet will tumble if c.g. goes in front of center of pressure bullet will tumble. I don't know how you would get c.g. in FRONT of c.o.p. but the closer they get the less stable bullet becomes. weight to the rear is best
     
  5. Ballisticboy

    Ballisticboy

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    If the center of pressure is behind the C.G. the projectile is aerodynamically stable and will not tumble. This configuration is the basis for the stability of all low or zero spin projectiles and most aircraft. As stated the worst configuration is when the C.G. and the centre of pressure are very close or in the same position as there is insufficient aerodynamic moment to correct yawing behaviour for aerodynamically stable rounds and spin stabilised rounds are grossly over gyroscopically stable.
    If the C.G. is infront of the bearing length then any effects of an offset C.G. are probably going to be magnified both on shot exit and in the barrel for a high spin rate. Dynamic stability is also going to be more of a problem with increased Magnus moments. Of course, if the C.G. is in front of the center of pressure then you do not need high spin rates and so can avoid most of the associated problems but you will need a smooth bore or very low twist rate barrel.
    Air gun pellets are a classic example of a projectile where the C.G. is slightly in front of the center of pressure. They appear to be aero gyro stabilised but do also appear to be marginally dynamically stable at best and thus can suffer problems at high speeds and long ranges. Airgun barrel twist rates are very low, around one turn in 16 inches in many cases with some at much lower twist rates.
     
  6. damoncali

    damoncali Gold $$ Contributor

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    I’m having a hard time picturing why it would matter where the bearing surface is. Can you expand on this? It makes sense that having the C.G. to the front could exacerbate jump (and therefore dispersion), but why does it matter where the C.G. is relative to the end of the bearing surface?
     
  7. Ballisticboy

    Ballisticboy

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    I am just thinking that the forward position of the C.G. would give a longer moment arm between the C.G. and the rear most end of the bearing surface which, as you say, would increase jump and possibly give increased balloting in bore, through increased wear on the surface, again leading to increased dispersion. I could be completely wrong though.
     
  8. damoncali

    damoncali Gold $$ Contributor

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    That makes sense. I heard one report of some turned solid bore-rider designs "humming" as they went down the barrel and producing unacceptable accuracy (basically a perceptible vibration at what I assumed was the frequency of the cg oscillation in the bore, which very roughly ought to be about 2-3000 hz). I assume the cg was pretty far forward just by eyeballing. We suspected balloting as the culprit, but who knows.
     
  9. geo.ulrich

    geo.ulrich beware of owner Gold $$ Contributor

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  10. Ballisticboy

    Ballisticboy

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    The base shape of the bullets you illustrate is about as bad as you can get for stability, except perhaps for a hemisherical shape, so it is not surprising that they are poor. As for the use of fins etc. I mentioned that only as an example of an aerodynamically stable projectile which any projectile is if the centre of pressure is behind the C.G. There are plenty of aerodynamically stable projectiles in existence for guns of all calibres and types, the classic airgun pellet being the most common. All these projectiles when using a combination of spin and aerodynamic stability are said to be aero/gyro stabilised.
    As for base drag providing stabilisation, base drag no matter how large provides little or no stabilising moments unless it is being produced by a flexible drag stabiliser such as a ribbon or parachute. Even the flared base on airgun pellets stabilises through lift not drag. It is the gyroscopic response to the total aerodynmic moment which tries to reduce the yaw angle on conventional bullets since the ogive center of pressure is a destabilising moment.
    There have been plenty of experimental aerodynamically stable bullets produced with high spin rates, the use of a flat front face will give a centre of pressure behind the C.G. The problem is that, apart from the high drag, the aerodynamic moment arm is usually very short giving very high gyroscopic stability, particularly as the bullet slows down. I have seen projectiles behave in very unpredictable ways due to this problem at longer ranges. Of course, as soon as any degree of rounding is applied to the bullet nose the aerodynamic centre will move forward making the projectile aerodynamically unstable and thus dependent on spin for stability.
    As for computer simulations, as someone who has spent his entire working life studying, designing and testing gun launched projectiles of all types, from particle size to 1 metre diameter, at speeds from a few tens of m/s up to orbital velocities, I have only used simulation in the initial design stages, before the manufacture and testing under carefully controlled test conditions. Everything I have said is based on solid proven aeroballistic law and testing, not theory.
     
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  11. geo.ulrich

    geo.ulrich beware of owner Gold $$ Contributor

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    Sorry had work to ship. I agree with part of your statement. The drag on a flat nosed projectile is to high for transonic or supersonic speeds and yes it will stabilize with enough twist. so I ran numbers on bullet design program I use .25 cylinder 1.00 long according to program centre line is centre of gravity pretty much a no bainer its symmetrical. should be around 8.9 twist for g.s.# of 1.5 at mach 2.5 . so if I shift c.g. to the rear and add a parabolic nose for .5 we have shifted c.o.p. forward of c.o.g and voila twist decreases to 11.2 at mach 2.5. I guess Bill Davis and Robert McCoy pretty well had it figured 40+ yrs. ago other than their disagreement on g.s. #'s fyi program used is an updated version of McCoys from 2010 and not from jbm thanks for your input
     
  12. Ballisticboy

    Ballisticboy

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    I was working on aerodynamic prediction programs at the same time as Bob McCoy. We used to exchange and use each other's software and information. Some of my software forms the basis of the routines found in different sources on the internet just like Bob's does.
    It was all a long time ago.:(
     
  13. geo.ulrich

    geo.ulrich beware of owner Gold $$ Contributor

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    Yeah that should be in the 1978 ish area I think, after Bill Davis moved on to Tioga. I just found some papers he had written on testing the p.p.c for Doc Palmisano awhile back while cleaning out basement a few weeks ago. It seems the more that things change the more they stay the same.
     

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