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Old April 23rd, 2008, 21:26   #58
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Join Date: Feb 2008
Location: Burnaby
Hmmm... was not as simple as I'd thought. Need to know som more things about airsoft springs, but as an approximation, probably slightly overestimated:

You need a ROF of approximately 28 rounds per second for a gun firing at 330 fps down a 450 mm barrel for the sector gear to cycle fast enough to begin retracting the piston before the bb has exited. Higher velocities and shorter barrels than this require higher rates of fire before the gear can make it round in time to engage the first teeth.

In general, long barrel, low fps, and high rof means the piston will begin withdrawing before the bb has exited.

If anyone wants to check my work: (if you hate equations and stuff don't read)
Assumptions: 1) that the sector gear releases the piston teeth at an angle of 45 degrees to the vertical and engages again at 45 degrees, passing through 1/4 of a rotation while the piston gets forward and the bb is exiting.
2) the acceleration is near uniform
3) The time taken for the spring to push forward is negligible before the bb begins its acceleration. That is, the bb begins accelerating before the piston head has reached the cylider head. The time for the piston to slam fully forward overlaps with the acceleration of the bb. This may be a source of error in the calculation. The true result could be a lower rof, if there is a significant "compression time" delay. Anyone know how long this could be?

Knowing the spring constant k for a typical airsoft spring (I know some are progressive, but k can be approximated) we can get the force of the spring on the air mass, and minusing a bit for friction and the compressibility of the air, we can get the acceleration, and thus the time taken, and thus can add some of that time on. But I have no idea what range we are talking about.

the math:

using d=1/2 (Vi + vf)t

time taken for bb to accelerate from 0 and exit 0.45 m barrel at 330 fps (100 m/s) is 0.009 seconds.

1/4 of a cycle in 0.009 seconds implies 27.78 cycles/s.

EDIT: true result could be lower still if the sector gear completes the "unengaged" portion of its circuit faster than the engaged portion. Very likely.

Thus, I would lower the estimate still. Perhaps on stock TM AKs and similar barreled guns with low fps the piston does indeed begin cycling back before the bb has exited, especially with a battery upgrade. Porting and nozzle retraction takes care of any potential suck.

Last edited by Bontic; April 23rd, 2008 at 21:33..
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