Complicated thinking: Why match cylinder porting and barrel length?

[CDN_Stalker]

Originally Posted by midgetspy

Interesting thread Max... but irregardless isn't a word, dammit! ;-P

Nic

So, you are saying it's non-irregardless? Lol

[MadMax]

Touche Midget. I hate it when ppl invent new words without regard for perfectly good words.

Originally Posted by mcguyver

easiest way to measure "barrel suck" would be to measure velocity on single shot and full auto. there would be zero barrel suck on semi as the piston will stop at the end of it's compression stroke and the velocity difference between semi and full would have to be due to barrel suck, all other things being equal.

I guess you don't accept my comparison of the amount of time it would take to accelerate a bb to a final muzzle velocity vs. they ROF that a mechbox achieves.

I showed how a 900shot per minute AEG would take roughly 5x more time to get the sector gear to come around to catch the piston again in comparison to the time it takes for a bb accelerated to 330fps to leave the barrel. I assert that semi or full auto mode does not have anything to do with "barrel suck" and that if such a phenomenon exists it is due to an insufficient amount of air compressed in a firing cycle. If the effective cylinder volume is less than the internal volume of the barrel + leakage losses then the bb cannot be pushed out the barrel without a reduction in pressure below atmospheric pressure.

you also brought up an interesting notion on dry firing. so what happens to guns where guys use low-caps and there is always dry firing at the end of each mag (10-15% depending on mag capacity). do these guys put undue stress on their guns when you couple this with the dy firing that occures during testing and weapons clearing. i seriously doubt it. and does the presence of the bb in the chamber, considering losses around the nozzle/cylinder head junction, nozzle/bb junction. bb/barrel gap and around the hop bucking and chamber apply any measurable resistance to the piston in it's stroke?

I can confirm this effect with a piezo impact transducer I've designed for the testing of a novel piston head I'm working on. I have observed a significant difference in impact level from dryfire with no barrel ass'y, dryfire in an assembled AEG, and firing a pellet. The notion of impact reduction when shooting a pellet isn't too hard to confirm from the basis of thought experiment. If there is no place for energy to be absorbed (no pellet) all of the potential energy stored into the spring must be transferred to moving parts (kinetic energy in the piston and moving spring), movement of the very non dense air from the cylinder (so light it can't really absorb much energy), or heating of the air (cylinder exhaust can be a bit warm, but isn't that hot). With the air unable to absorb much energy relative to the very heavy piston and spring, all of the energy is transferred to the moving parts and must be finally dissapated by the mechbox.

If you fire with a bb, you present a restriction in airflow which means that some work is done on the pellet (a good outlet for potential energy from the spring). If your AEG worked on a 0% efficiency factor, then the bb presents no sizable backpressure and you have a very poor energy transfer i.e. then the bb would have a negligible effect.

DRY FIRE FATIGUE:

I've looked up aluminium fatigue cycle behavior tables and found that a 25% reduction in peak load can roughly account for a 10x improvement in cycle life of cast aluminum parts. I can measure a very large difference betwen dryfire and non dryfire impact (60% higher when dryfiring). Exceptional loads initiate cracks which sharply reduce fatigue cycle life. I strongly suspect that dryfire is the primary cause for initiating V2 mechbox failure.

rest are so phenominally different not to mention the discharging air at higher pressure in the barrel "feeding back" energy to a larger, lower pressure area like the cylinder, the laws of physics just don't allow for that hypothesis. the only resistance to be offered by this system would be if the hop-up made a very good seal and the bb required significant force to push its way thru the hop bucking and travel down the barrel.

The hop bucking applies it's resistance for a very short distance so it can't absorb a significant amount of work. Work is defined as force x distance. Hop up detent force is not very high and the distance applied is only millimetres.

If a bb trip time in a barrel is 0.00685s (see earlier post), then an accelleration to 330fps in such a short time equates to:

330fps~100m/s (I don't think in imperial unitized forces very well)

Acceleration = 100m/s / 0.00685s = 14598 m/s^2

force = mA = 0.0002kg * 14598 m/s^2 = 2.919 kgm/s^2 [Newtons]

This correlates to roughly 0.297kgf (div by 9.81N/kgf)

If a constant acceleration persists during the bb trip time, you have roughly 300g of force pushing the bb down the barrel.

If the cylinder has a diameter of 25mm (roughly right I think) and the barrel has a dia of 6mm, you have a pressure reduction setup (kind of like reverse hydraulics) which has force divided by the ratio of the AREAS of the two bores. That means that the force on the piston is 25^2/6^2 of the force on the bb (neglecting oring friction or bb-barrel friction). That means that the force on the piston is 17.36x higher on the piston than the barrel.

F(piston) = 0.297kgf * 17.36 = 5.156kgf?!

Actually, this isn't too unreasonable for a steady force. I just crunched a SP120 spring on a bathroom scale to about fully cocked length. The scale reads around 12kgf. The force at preloaded length is about 3kgf. If the spring is linear (it really isn't when the tight winds start to stack up), you get a average force of 7.5kgf.

COOL!

Then I can deduce that an AEG is actually a pretty efficient device. An SP120 is supposed to blast out at 0.2g pellets at around 420fps with an average piston force around 7.5kgf. Then an AEG firing at 330fps should be using a spring about 330^2/330^2 less average force (1/1.6) which results in an average force of 5kgf. Pellet energy is related to the square of speed. Piston travel distance is the same so only the force can change to affect work done.

Lower than the 5.165kgf I calculated above, but still reasonably close considering the crudeness of my measuring equipment. I would surmise that an AEG is pretty efficient at transferring spring energy to a pellet then (nearly 100%!).

[mcguyver]

my comparison of "relative" force exerted would be a man with a 200 lb of force on a sledge hammer hitting a 1lb weight. would the energy of that impact have an effect on the hammer or would it nearly all go to the weight with litttle effect on the hammer. this is a simple suposition based on mass and energy differences. if a bb weighs 200 times less that a piston and we ignore pressure differences it just doesn't seem right to me. i still think that for the fraction of a second that the hop seal allows pressure to build up in the piston to bb volume may itself be enough to start the piston decelerating and the net energy from the expansion of this volume is what propels the bb, adding to it the expansion of remaining air in the cylinder as the piston impacts the cylinder head. your numbers bear out some things for sure. my understanding of what you're saying is that energy must go somewhere and if no bb is present then it must be going only into the mechbox. my thinking is that some energy goes to moving air down the barrel (bb present or not) and that energy dissipated by the mechbox is always going to be significant. also, did you test with hop on full to off to see if that makes a difference, just for intersts sake?

[MadMax]

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.

[mcguyver]

i'm just playing the devil's advocate here, but i can blow a .2g bb to about 200 fps down a stock tm m4 barrel. the maximum pressure my cheeks can hold is 2.10 psi. a quick burst from my mouth is much less than this about 1 psi. the volume of my burts is at best probably no more than 3, but probably 2 aeg full cylinder volumes. to reach 400 fps, my kinetic energy would need to be 4 times what 1 psi could create. there is alot of probablies here and no hard measurements, but you understand my line of reasoning here. i'm very certain that an aeg piston produces pressure orders of magnitude above 4 psi, far in excess of the volume difference between my mouth and an aeg cylinder. don't get me wrong, i'm not trying to poke holes in your reasoning or math, but something just doesn't sit right with me. if i provoke you to re-think something, which leads to another thought or idea on something, don't we all win?

[MadMax]

Well, you're poking holes with no data or quantitative reasoning. The value is kind of limited. I'm not using any rocket science in my analysis. It's mostly highschool level physics f=ma stuff that most airsofters should be able to particapate with.

What chrony are you using to spit bbs? I can only get 32.9m/s with my optoelectronic chrony (109.5fps).

1psi equates to only 0.0199kgf ~ 0.195N. For a 380mm barrel, that's only 0.07J which works out to 26.45m/s which isn't far off from my 32.9m/s chrony result.

[mcguyver]

it's one of those dual ir photocell jobs. cost 80 bucks from wgc. it belongs to one of our team members. the chrony doesn't lie. guns i've measured with it here were accurate in other cities with other chrony's. but psi at what volume? volume of air to expand = potential energy and said expansion = kinetic energy. more volume=more energy. basic premise behind gunpowder and bullet velocity, right. i do realize that i have no data to present nor do i care to spend time and energy to get any. my thinking is just on the simple mechanics of what occurs and you disregard things that i've brought up because it doesn't fit you're mathematical model. unless you have star trek sensors placed at every point in the aeg to measure stresses, your math is just a little more sophisticated supposition, but it is just that, supposition. the point is unless you employ high speed cameras to every point from spring guide to flashider and see absolutely everything that occurs, we don't really know do we? we only suppose and sometimes the math works and sometimes not (out of your keyboard from a previous post). but if we don't discuss like rational people then ideas are not exchanged and progress does not occur. i'm not saying i'm right and you're wrong. you may very well be correct and i may very well be wrong, but in the end it's not really that important of an issue for me so if i'm wrong i can live with it. all i'm saying is that from my "empircal only" standpoint it doesn't sit right with me.

[MadMax]

Well, I modified a mechbox so I can measure the impact against the cylinder head with a peizo transducer.

At 1psi, volume doesn't matter. You can only acheive a certain force (pressure x area) which corresponds to a certain amount of work done (force x distance) which provides an amount of energy imparted when you apply the force over the barrel length (I use 380mm in my calcs). From that, you can work out an fps with E = 1/2 mV^2

On the practical side, I did spit a bb thru a chrony with a brass barrel. I use a Guarder opto chrony which is pretty accurate. It agrees with my other opto chrony which I use for outdoor use (indoor fluo lamps mess it up). I got a speed of 32.9m/s.

I measured the force on the spring at fully compressed and standby length to determine avg force. From that I can determine spring energy. With a final bb velocity I can determine projectile energy.

It doesn't take sensors everywhere. It only takes careful observation and application of high school physics which actually does apply. Kind of like measuring the fuel intake of a vehicle and looking up the table value for the energy content of the fuel. Measure the torque and rpm of at the wheel with a dynomometer and you can determine the total mechanical efficiency of your vehicle which includes transmission losses.

An empirical standpoint is a supposition drawn from actual measurements. A theoretical hypothesis is a mathematical analysis. If you don't use either, you are using intuition.

[CDN_Stalker]

Hehe, speaking of spitting BBs, Foxtail and Test_Tube used a TM loading rod and the field Chrony one day, got upwards of 300fps using .2g BBs. I've always found that putting your tongue on the opening and rapidly removing it yields blistering velocities (made a blow gun two years ago, 5/8" tubing, made some darts out of maple dowel, burned the tip and sharpened, was able to punch them through 1/4" 3ply birch plywood from across the basement, talk about straw in a tornado, eh?).

Assume maybe the rapid compression from using the tongue is on par with what Carl has said about the sudden compression that happens with a ported cylinder. Instead of an increase in pressure, it is just a sudden "POP!"

[Ronin]

Admittedly Max, I haven’t done a great deal of Physics in a while. The one thing that I do recall though is that experimental error can be calculated by:

Error = (Experimental Value – True Value) / True Value X 100 = Percent Error

1psi equates to only 0.0199kgf ~ 0.195N. For a 380mm barrel, that's only 0.07J which works out to 26.45m/s which isn't far off from my 32.9m/s chrony result.

Using your calculated value of 26.45 m/s as your Calculated True Value and 32.9 m/s as your Experimental Value; your Experimental Error is almost 25%.
This is quite high for Experimental Error.

After you do your experimenting and collect your data you still need to determine your accuracy and precision in the experiment. Accuracy is how close you get to the target (True Value = Experimental Value = Bullseye); precision is how many times you hit the target in the same spot (repeatability).

If this value of 32.9 m/s isn't taken from just one reading of the chrono, I'd say your experiment has high precision but you should modify your experiment (which may not be possible) to increase your accuracy. Experimental error below 20% and in the range of 10 - 14 % is more acceptable.

Keep fightin' the good fight. :salute:

[Dirty Deeds]

I recenty took my SR16 (frankengun) and was doing some experimenting of my own.

1 - Full cylinder with a 509mm barrel was shooting at 392 FPS.
2 - I replaced the M16 barel with a stock M4 barrel and the FPS dropped to 375.
3- I recently bought a silencer to replace my homemade silencer that was covering the M16 barrel, it turns out the new silencer was about 22mm short and I hade to cut the barrel. After all was done the gun now shoots at 396 FPS. Which means a 487mm barrel is better matched to a full cylinder than a 509mm barrel, thus making better use of all of the air in the cylinder.

You could put a 300% spring in your gun, however if your cylinder and barrel are mis-matched (same volumetric space, cylinder and barrel) you loose alot of air pressure and may end up shooting 320 FPS and doing major damage to your mech box, When using a full cylinder and short barel (piston slamming with lots of force, due to no back pressure, for no reason).

The oposite would happen with a ported cylinder and a long barrel. Not enough air volume to maintain the push on the BB to the end of the barrel, would result in friction between the BB and barrel as it exits, slowing the BB before exiting the barel.

So in my opinion and tests, before you do any upgrades, make sure your barrel and cylinder are matched as closely as possible, as the spring only effects how much pressure is exerted (higher air velocity), not how much air is actualy expelled. Expelled air is determined by the cylinder size itself. Ideally the piston should strike the clinder head a nano-second before the BB exits the barrel.

Sorry if this was already covered.

[MadMax]

Originally Posted by Ronin

Admittedly Max, I haven’t done a great deal of Physics in a while. The one thing that I do recall though is that experimental error can be calculated by:

Error = (Experimental Value – True Value) / True Value X 100 = Percent Error

Using your calculated value of 26.45 m/s as your Calculated True Value and 32.9 m/s as your Experimental Value; your Experimental Error is almost 25%.
This is quite high for Experimental Error.

After you do your experimenting and collect your data you still need to determine your accuracy and precision in the experiment. Accuracy is how close you get to the target (True Value = Experimental Value = Bullseye); precision is how many times you hit the target in the same spot (repeatability).

If this value of 32.9 m/s isn't taken from just one reading of the chrono, I'd say your experiment has high precision but you should modify your experiment (which may not be possible) to increase your accuracy. Experimental error below 20% and in the range of 10 - 14 % is more acceptable.

Keep fightin' the good fight. :salute:

I wasn't actually trying to match my chrony result with a calculated result in that foray.

I just approached mcG's problem from two ends. Firstly by determining the theoretical velocity one could obtain from a 1psi set of lungs and zero barrel friction (assuming 380mm barrel).

The fps measurement I took was to see if 1psi lungs are in the ballpark of lung shot bbs. You can't really say there's any degree of experimental error between the two analysis since they do not use related inputs. I didn't measure the pressure I developed and calculate with that result so there really isn't any reason that the two results should agree. I used 1psi as quoted by mcG.

[Ronin]

I understand now what it is your demonstrating, Max. I glazed over the part where you were actually “spitting” the bb down the barrel. So I guess 25% accuracy on lungpower isn’t too bad at all, unless you feel you’ve been inhaling to many metal shavings on a daily basis. I really shouldn’t try to comprehend things before my 10th coffee of the day.

My questions for you are:

If the volume of a barrel/silencer combination is matched to the volume of the cylinder, am I limited to always using the silencer? If for some reason I remove the silencer will the cylinder head slam into the bottom of the cylinder at a greater velocity from the lack of backpressure because the bb leaves the end of the barrel before the end of the cylinder head stroke? Will this cause damage? Is this at all a consideration?

[ILLusion]

the silencer has nothing to do with it...

it's the inner barrel length you're taking into consideration.

If you're asking what happens what happens if you remove the silencer and shorten the inner barrel length, then I suggest reading the very first post again where it was stated that yes, the piston head will slam into the cylinder head with greater force.

interesting
for the longest time I wanted to put a 250mm silencer on my SR16 and drop in a PSG1 tightbore but was worried about the suction effect. There wasnt much info regarding this thus far. And judging for Dirty Deeds last post about the SR16 with the M16 barrel, its a good thing I didnt. Ill prolly hold off untill we come-up with a barrel-length to cylinder matching guide.
awsome info