Airsoft Canada - View Single Post - Complicated thinking: Why match cylinder porting and barrel length?

MadMax · November 20th, 2005, 19:51

Your analogy between a hammer striking a floating object doesn't hold because the interaction time between the hammer and the object is so short (collision really). In the case of an AEG, the piston is pneumatically coupled to the bb for a significant time. In a sense, the piston is connected via a nonlinear spring to the pellet because air is compressible.

The point of making the piston area so much larger than the barrel area is to acheive a good pneumatic coupling between the bb and the piston. Because the bb is a small diameter compared to the piston, you have a force division, a kind of pneumatic leverage if you will. It's related to the ratio of the square of the cross sections involved.

Consider this: if you put a 6mm piston in the barrel and it was well sealed to the piston, a 5kg force on the piston could be stopped with a dinky 0.288kg force on the little piston.

In the case of the bb, the little piston is allowed to move, the applied 0.288kgf force causes an acceleration of the piston.

As to hop up consideration, I only take velocity measurements with the hop up turned off. Hop up affects muzzle velocity in ways which may be difficult to recreate so I remove it from the list of sources of error. In any case, hop up reduces muzzle velocity, possibly because of increased barrel friction.

How about this for a closed system energy discussion:

If a 330fps spring exerts an average force of 5kgf (49.05N)

A piston travels roughly 46mm = 0.046m

Work done on the piston cranking it back and stored as potential energy:

force * distance = 49.05N * 0.046m =2.26J

Kinetic energy in a 0.2g bb moving 330fps (100m/s)

KE = 1/2mV^2 = 1/2 (0.0002kg) (100m/s)^2 = 1J

So the AEG in question would only be 44% efficient which doesn't really agree with my last post. I think I see a hole in my earlier reasoning. I did work out the piston force correctly, but there is no consideration of piston travel which means that there could be a significant amount of over compression to compensate for leakages, friction, and a reliability factor to assure against barrel suck.

In any case, 1J is not negligible in comparison to a total spring energy of 2.26J.

November 20th, 2005, 19:51	#19
MadMax Delierious Designer of Dastardly Detonations Trader rating Join Date: Dec 2001 Location: in the dark recesses of some metal chip filled machine shop	Your analogy between a hammer striking a floating object doesn't hold because the interaction time between the hammer and the object is so short (collision really). In the case of an AEG, the piston is pneumatically coupled to the bb for a significant time. In a sense, the piston is connected via a nonlinear spring to the pellet because air is compressible. The point of making the piston area so much larger than the barrel area is to acheive a good pneumatic coupling between the bb and the piston. Because the bb is a small diameter compared to the piston, you have a force division, a kind of pneumatic leverage if you will. It's related to the ratio of the square of the cross sections involved. Consider this: if you put a 6mm piston in the barrel and it was well sealed to the piston, a 5kg force on the piston could be stopped with a dinky 0.288kg force on the little piston. In the case of the bb, the little piston is allowed to move, the applied 0.288kgf force causes an acceleration of the piston. As to hop up consideration, I only take velocity measurements with the hop up turned off. Hop up affects muzzle velocity in ways which may be difficult to recreate so I remove it from the list of sources of error. In any case, hop up reduces muzzle velocity, possibly because of increased barrel friction. How about this for a closed system energy discussion: If a 330fps spring exerts an average force of 5kgf (49.05N) A piston travels roughly 46mm = 0.046m Work done on the piston cranking it back and stored as potential energy: force * distance = 49.05N * 0.046m =2.26J Kinetic energy in a 0.2g bb moving 330fps (100m/s) KE = 1/2mV^2 = 1/2 (0.0002kg) (100m/s)^2 = 1J So the AEG in question would only be 44% efficient which doesn't really agree with my last post. I think I see a hole in my earlier reasoning. I did work out the piston force correctly, but there is no consideration of piston travel which means that there could be a significant amount of over compression to compensate for leakages, friction, and a reliability factor to assure against barrel suck. In any case, 1J is not negligible in comparison to a total spring energy of 2.26J. __________________ Want nearly free GBB gas?