About overtune on VVD.
Here there is an old but very revealing article, where a transient gas-dynamic calculation of a direct-current VVD with ball acceleration curves is performed.
Instead of intro:
This is not scientific work. It is full of unscientific terms and something is indicated very conditionally, for understanding the process, and not understanding the nuances of gas dynamics.
In connection with the spread of VVD systems among the masses, statistics on injuries and negative cases at airsoft games have increased quite sharply. For a long time they did not pay attention to this, attributing the problem to the fact that VVDs themselves usually strive for the ceiling of the rules, and hits from them should be painful.
But, either the cries of the advanced public, or a couple of cases when pieces of the ball were taken out of a person with tools, or the phase of the moon … drew wide organizational attention to the problem of overtune of the VVD barrels.
So: There is a problem of overtune VVD.
Description: Using the current rules for measuring the maximum allowable speed of the ball, a person tunes the system to 170m / s 0.2bb and receives 3J. Allowed? Allowed!
But shooting goes to the game with 0.45 balls, getting 143m/s 0.43bb, which is 4.4J.
3<4.4 - This is how the owner of the VVD gets 1.4 Joules of advantage and injury. This effect is called the Joule Loophole.
Let’s first compare AEG and VVD. I have shown them schematically below.
Both systems fire using the energy stored in compressed air.
Both systems use pressure, one just before the shot, for the other we stock up on compressed air in advance.
What is the difference? – In the volume of air that is spent on 1 shot. And in how we manage this volume.
In AEG, we have a strictly fixed volume of air equal to the volume of the cylinder.
In VVD, we have a “conditionally” infinite volume of air, and we cut it off with a solenoid or mechanical valve.
What does it mean at the time of the shot? Yes, in different acceleration curves.
In AEG, the air pressure under the ball constantly drops while the ball moves along the barrel, ideally dropping to zero when the ball leaves the barrel muzzle.
In VVD, the boost pressure can be considered constant (if the valve and reducer are pumped), since the cylinder continues to supply air while the ball flies along the barrel, constantly increasing the volume of gas behind the ball and trying to maintain pressure.
So where does the extra joules on heavy balls come from in VVD?
A heavier ball (for example, 0.40) due to inertia, the same barrel passes more slowly than a 0.2 ball. A longer travel time allows you to supply more air from the balloon, pushing the ball “in the back.” More air –> more force that accelerates the ball. And at the output we have a greater speed and, as a correspondence, a greater energy.
This effect is called “joulecreep” over the hill. It is characteristic to varying degrees, primarily for VVD, GBB, and to a lesser extent for AEG and excess cylinder volume.
The most striking representative of this effect was the VVD rifle with the stock balloon on the SMP. Which showed 173m/s with 0.2 ball (3J) and then 166m/s with 0.43 ball (5.9J). “Loophole” gave an increase of 2.9 J, which is generally beyond all reasonable.
It is worth mentioning that the effect of the “Joule Loophole” increases with:
A) Increasing the length of the inner stem.
B) Reducing the length of the gas line from the cylinder to the rifle (the maximum it appears for butts with built-in cylinders).
C) Increasing the equivalent flow area of the valve and system regulator (Yes, RedLineSFR has very non-acidic flow areas!)
D) Increase Dwell on the main valve.
For AEG, this effect is also present and exists , but I have never met AEG in practice, which would give more than 15% increase in energy when measured with 0.2 with a ball, and with actual shooting 0.4+. Borap, cylinder without a hole, long barrel and voila! – a little cheat, a little overtune.
I think the problem is clear. It’s not really a problem, but a physical effect. The problem is directly created by the current method of measuring the energy of a shot through measuring the speed of 0.2 with a ball.
Yes, this is an outdated and incorrect technique, which allows you to use this “loophole” to get a small advantage and increase injury risk.
It is worth once and for all to understand a simple rule:
Any recalculation of the output m / through Energy to a ball of a different weight will result in an error . And the price of this error can be from a few% to 90% (experimental)
About direct-current VVD and with a storage chamber:
Not all VVDs are strongly affected by the Joule Loophole. In particular, there are at least 2 systems, both manufactured by the Russian Federation: Pro-T and Proteus, in which there is a storage chamber, the volume of which is equal to the volume of the AEG cylinder. Moreover, at the time of the shot, the air supply from the cylinder to this chamber is limited or completely absent, and accordingly, only the air that was recruited in advance goes to the shot, which levels the effect of the “Joule loophole” to the level of a conventional AEG. In direct-current systems, such as PolarStar or SMP, the storage chamber is directly fed from the cylinder, without cutting off the excess flow, and of course, the “loophole” effect for them is much more pronounced.
Storage chamber system with strict air volume limitation
Direct-flow system with direct air supply from a cylinder