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Just a quick note on "Valve speed" There are two parts to valve speed.
One is the pre fire pressure drop in the pilot. If this is large the time to drop the pressure before the valve cracks open is longer. If the EQ hole is larger, then the amount of chamber pressure lost through the EQ hole is greater especially with smaller chambers. For this reason a small pilot is better.
The second valve speed is the rate the valve opens once it starts to move. This is affected by 3 items beside chamber pressure.
1 Valve seat to piston OD or diaphragm OD. The lower the pilot pressure is when the valve cracks the later it opens and the faster it opens.
2 Pilot area. As the valve opens and moves into the pilot volume, the remainder of the pilot volume is compressed. The amount of compression is less with larger pilot volume. Larger volume is better.. Obviously a compromise with with EQ leak rate and smaller is better is needed.
3 Rate the pilot valve vents. A large venting trigger can overcome larger pilot volumes and long pre fire delay, or pressure rise from a moving valve.
Last edited by Technician1002 on Thu Nov 04, 2010 7:56 pm, edited 1 time in total.
What I may be hearing is pilot tube volume hissing for a split second before it fires. Yes, it goes "BANG!" when it fires, but, it definately has a touch more sloppy a sound than my 1-1/4 inch cannon. That being said, I am overly perfectionistic on any project I build. So, I may just be oversensitive to every tiny detail.
The slow pilot valve and high pilot voume you have now are probably better than electric actuation.
Thanks Guys. I really appreciate the info.
One thing I have found is, there are so many variables in this that it is tough to predict an outcome based on a suggested or immagined modification (specification change).
I have done some experimentation and have noticed that if I pull the trigger very slowly, I can get it to just hiss. That is, obviously, due to the EQ orifices in my two valves keeping up with the amount of venting I am doing through the trigger pilot. If I continue advancing the trigger very slowly closer to full trigger pull, it will fire. However, there is a slight reduction (but noticeable) in recoil power if the trigger is pulled very slowly versus quickly. Of course, this may be my perception, more than anything else. But, this is why I was considering moving back to electric actuation. Honestly, I hate wiring, though. It requires batteries, wiring, a switch, etc. I am good at the electric side of things, but I do not like it. I understand much of what was said, however, I never considered the compression of the added pilot volume actually helping in some fassion. That was interesting to learn.
I want to thank you guys for all the information you have given me on my projects. It is truely helpful. I am a mechanical problem solver, not a physics professor. I like to think these things through, but it is good to hear from those who are more knowledgeable in a given area.
Hey, on a side note, I mounted both of my compressors together and timed my fill rate. I was pretty close on my guestimation. At 20 volts, I am seeing 100 psi in 30 seconds with my 2 gallon tank. So, 200 psi should occur in about 75 seconds or so. Not bad.
What's the porting on your pilot valve like? Not the threading, the actual diameter of the hole through which the air flows. If it's fairy limited you might want to consider moving to a 3/4" QEV for a faster pilot dump.
You should check with GGDT if upgrading will make a difference. I know on a few cannons I've modelled, changing the pilot valve port diameter did almost nothing.
Just playing around with GGDT and HGDT, even an "advanced combustion" comes pretty close to this cannon in performance (with modest specs). Not trying to discredit your cannon at all, it is a good build and better than many pneumatic cannons on the website. Perhaps I've forgotten now that hybrids have taken my focus, but why do people build 'large' bore pneumatics instead of combustions?
Back on topic, I see no performance increase when increasing pilot valve porting in GGDT for this cannon. Same goes for the weight reduction for the piston/diaphragm, seems to have no effect if it is 250g or 120g. Increasing chamber length by 20% has little effect, increasing barrel length by 20% gives an extra 12m/s or so and increasing pressure to 300PSI gives an extra 16m/s in muzzle velocity. Projectile was 2" in diameter, 50g in weight.
How accurate this is in the real world I cannot say but I imagine it gives you a pretty good idea what to expect from the cannon when changing a few variables.
How much change would a CV over 100 change things?
Extra 19m/s with a warning for barrel choking flow. Again, 50g projectile, 2" barrel. I estimated barrel length at 4' and chamber length at 24", it was hard to tell from the photos, I doubt it would make much difference if they were a bit off anyway.
The barrel length is, indeed, 4'. The chamber is 12 inches in the straight section. So, it is roughly 14 inches in length figuring in the end helispheres.
I agree about the diaphram weight. Considering the fact that the pilot volume is more important than diaphram weight, this is understandable.
Oh, the pilot valve is a 1/2 inch thread valve. The inside bore is .600".
As for combustion cannons, pneumatics typically out-perform combustions. That is why I went this route.
As was mentioned earlier in the thread, I think a heavy slug is where this cannon will shine. For one thing, valve speed becomes less of an issue as does CV when the slug gets heavy. Also, with the added weight and PSI available, it just screams for a really heavy slug.
So, I wonder how it would compare in the GGDT to a combustion cannon with a 200 or even a 300 gram slug?
I was thinking the 2 inch barrel was a good fit for that valve. It looks like a real winner in performance.
CV will make an impact if and when you go for larger barrels. You should get very good performance up to about a 3 inch barrel. That would make a killer t shirt launcher with a 30 inch long 3 inch barrel.
I really need to fire a heavy slug. I am sure that is where this thing will really shine. I mean, it performs with a light projectile. But, I bet it will be frightening with a really heavy slug.
:EDIT: I just did a simple test fire of the cannon with a super thick 2 inch dog tennis ball at 250 psi. I wrapped it in a folded paper towel for a nice tight fit into the barrel. The recoil was absolutely intense! Also, the sound was far louder. I did not get a reading from my Chrony, however, because the ball actually grazed one of the upright supports and gave me an error. But, the ball tied that support almost in a knot, and went through my back stop, through a support behind the back stop, and nearly through a 4 gallon plastic bin full of wooden blocks! The ball was torn almost perfectly in half and did a HUGE amount of damage. Sheesh, I think this thing is powerful enough!
I shudder to think of what it would be like with a heavy slug and this same 250 psi!
With a 250g projectile you will see about 130m/s and 2100 Joules of muzzle energy. It increase muzzle energy by 900 joules for a projectile that is 5x as heavy (50g compared to 250g projectile). 2100 joules is still what you would see from most handguns bar .50 AE. The combustion model I have been running for comparisons falls quite far behind with only 95m/s muzzle velocity and only 1200 Joules. I don't think combustions are suited to heavy projectiles, perhaps they accelerate too slow and the pressure spike drops too rapidly.
An advanced combustion will typically out-peform most pneumatics up to 2" porting piston valves, by quite a bit too. 90% of the pneumatics on this site are probably 2" porting and below and an advanced combustion will beat most of them, if not give them a run for their money like with your cannon. When I say 'advanced combustion' I mean a combustion with a fan, multiple ignition points, fuel metered, 2" porting and perhaps even a burst disk.
Personally...I think you chose the best route. Extremely nice build!
I think the issue of combustions with heavy projectiles is related to combustion speed. A heavy slug requires more consistant pressure over a longer duration than combustion affords. Whereas a pneumatic provides that consistant pressure. That would be my guess, though I could be wrong.
The beauty of combustions is the total lack of restriction from chamber to barrel. Pneumatics (those with a complex valve like mine) must overcome internal valve restriction on the way to passing the pressure to the barrel. In that regard, combustions are very attractive to me.
Hmm, I would think an advanced combustion using metal construction with pure oxygen metered along with the fuel would be sweet (yes, I know it has been done).
Anyway, I digress............
I will post more info as I have it along with any videos I take.
The issue with your comparison is the sheer difference in the peak chamber pressures. While the pneumatic is storing 250PSI in the chamber, the adiabatic combustion pressure of an air/propane mixture at STP is only about 130PSI. Throw a 2-3X hybrid mix in the combustion model, and it will outperform the pneumatic by a significant margin, even with heavier projectiles.
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