qualifying the d/4 rule
Posted: Sat Oct 27, 2007 2:59 am
As most will know, I have been a long time enthusiast of the idea that in a coaxial pneumatic, as the piston only needs to move back 1/4 of the inner diameter of the barrel for maximum flow (you can find the math done here in defiance of the consequences of excess indulgence 
), then this should be the limit of piston travel. The benefits or a smaller pilot volume are clear - for a given pilot valve, the pilot chamber will emtpyquicker with a correspondingly faster acceleration of the piston, and faster valve opening time -> more power.
However, this does not take into account the phenomenon of piston bounce. When the piston slams into the back of the pilot chamber, it will tend to bounce back, thereby once again reducing the flow. Therefore, there are circumstances where having the minimum piston travel might not be a good idea.
A case in point was my first attempt at a "velocity" themed 6mm launcher, the HVBB which was built with the "ideal" d/4 piston travel. I had a launcher with a similar valve and calibre but a 12" barrel as opposed to the HVBB's 36" that I could compare results with.
The data (using 0.12 gram airsoft BBs) was rather perplexing:
At 100 psi, I got 633, 637 and 635 fps for the 12" barrel, as opposed to 655, 657 and 661 fps for the 36" - a slight increase.
Increased to 400 psi, I got 994, 1002 and 998 fps for the 12" barrel and, rather annoyingly, 720, 711 qand 718 fps for the 36" barrel
To see the effects with heavier projectiles, I tried with 1 gram lead pellets for the following results:
At 400 psi, I got 515, 513 and 520 fps for the 12" barrel and 566, 567 and 561 fps for the 36" barrel.
I tried increasing the chamber size to double the original and got 652 feet per second for a 1 gram lead pellet, but the 0.12 gram BB however insists on being the anomaly. At 300 psi, I clocked it at 953 feet per second. Upping the pressure to around 375 psi, I got 888 fps.
I can contrast the results with those obtained with my 6mm burst disc pneumatic (similar barrel length to the HVBB and an equivalent chamber size to the original build) - 1104 feet per second for a 0.12 gram BB at less than 350 psi, at 400 psi they were off the scale. Of course you'd expect better velocites from a burst disc as opposed to a piston valve but what's important here is the trend. For the burst disc with the light BBs, more pressure = more power, which was not the case with the HVBB. The trend was not however replicated with heavier projectiles.
My interpretation of this data is that since light projectiles accelerate faster, the chamber pressure drops quicker meaning there's less pressure to mitigate piston bounce, which to some extent reseals the barrel causing a loss of performance.
I can therefore conclude that while in most cases it's a good idea, this rule cannot be universally applied.
...and now I'm strangely in the mood for some more high velocity testing, off to buy new o-rings for my shock pump
), then this should be the limit of piston travel. The benefits or a smaller pilot volume are clear - for a given pilot valve, the pilot chamber will emtpyquicker with a correspondingly faster acceleration of the piston, and faster valve opening time -> more power.However, this does not take into account the phenomenon of piston bounce. When the piston slams into the back of the pilot chamber, it will tend to bounce back, thereby once again reducing the flow. Therefore, there are circumstances where having the minimum piston travel might not be a good idea.
A case in point was my first attempt at a "velocity" themed 6mm launcher, the HVBB which was built with the "ideal" d/4 piston travel. I had a launcher with a similar valve and calibre but a 12" barrel as opposed to the HVBB's 36" that I could compare results with.
The data (using 0.12 gram airsoft BBs) was rather perplexing:
At 100 psi, I got 633, 637 and 635 fps for the 12" barrel, as opposed to 655, 657 and 661 fps for the 36" - a slight increase.
Increased to 400 psi, I got 994, 1002 and 998 fps for the 12" barrel and, rather annoyingly, 720, 711 qand 718 fps for the 36" barrel
To see the effects with heavier projectiles, I tried with 1 gram lead pellets for the following results:
At 400 psi, I got 515, 513 and 520 fps for the 12" barrel and 566, 567 and 561 fps for the 36" barrel.
I tried increasing the chamber size to double the original and got 652 feet per second for a 1 gram lead pellet, but the 0.12 gram BB however insists on being the anomaly. At 300 psi, I clocked it at 953 feet per second. Upping the pressure to around 375 psi, I got 888 fps.
I can contrast the results with those obtained with my 6mm burst disc pneumatic (similar barrel length to the HVBB and an equivalent chamber size to the original build) - 1104 feet per second for a 0.12 gram BB at less than 350 psi, at 400 psi they were off the scale. Of course you'd expect better velocites from a burst disc as opposed to a piston valve but what's important here is the trend. For the burst disc with the light BBs, more pressure = more power, which was not the case with the HVBB. The trend was not however replicated with heavier projectiles.
My interpretation of this data is that since light projectiles accelerate faster, the chamber pressure drops quicker meaning there's less pressure to mitigate piston bounce, which to some extent reseals the barrel causing a loss of performance.
I can therefore conclude that while in most cases it's a good idea, this rule cannot be universally applied.
...and now I'm strangely in the mood for some more high velocity testing, off to buy new o-rings for my shock pump
) and they have a special niche in the spudgunning world. They're next to useless for large chambers but if you're making a small calibre launcher, they will easily fill a small chamber ( a couple of cubic inches ) to the high pressures required for good performance.
