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PostPosted: Sat Jan 17, 2015 9:49 pm 
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Location: Vancouver
Penage Guy wrote:
What kind of information would these sensors reveal?

It was discussed slightly earlier in this thread, measuring pressure ahead of the piston during the compression stroke. Such sensors could help speed up development of spring piston airgun tuning, offering exact figures after any given change to spring force, stroke length, various lubes, transfer port diameter changes, pellet weights and diameters, etc. Of course I'm not volunteering. More of a hands-on experimental sort of guy, intuition-based sort of thing.


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PostPosted: Sat Jan 17, 2015 11:27 pm 
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I have a Raven that I would be willing to "play" with. One idea I came up with is, to bore out the TP to about 3/8". A solid sleeve with a bore matching the alignment and I/D of the TP would be epoxied inside the tube. This would act as a backer plate for semi-rigid inserts (durometer of about 110). These would be inserted into the oversize TP and each insert would have a different size bore. The only other thing that would benefit this experiment would be threaded barrels, which is to say, I'd have 3 or 4 of various lengths. The test would consist of running 3 or 4 shots per insert per barrel.

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PostPosted: Sun Jan 18, 2015 12:36 am 
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Seems as the discussion is going on else where....
.http://www.network54.com/Forum/79537/th ... s.+SteveNC


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PostPosted: Sun Jan 18, 2015 4:03 am 
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Handy; I have another idea if you're interested, just an option: Drill/tap the port to say 5/16" (or 3/8), then use brass bolts to plug it. You could also use 1/8" NPT plugs. The best imo, but most work, would be smooth brass shafts with an o-ring for sealing at the piston side, one to three setscrews on the other end to lock the shaft in place. Bolts or shafts would obviously be cut to perfectly fill the area, then drill each one the diameter port you want to test. Plus you can also test other stuff like putting a radius on the port, tapering the port, roughening it, anything you want. I wouldn't use the Raven, but it's better than nothing. I would choose a typical gun of ~18ftlbs which the port is more critical. This project is on my list of things to do too, but all back burner right now, especially since I feel I already know the answer. I don't believe the perfect port will change with barrel length, but will change if the bore/stroke/spring etc change. That's why I wouldn't use a Raven, the results would only be good for other Ravens which are not popular and for good reason. Well, they're not popular outside Canada...


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PostPosted: Sun Jan 18, 2015 5:09 am 
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If I'm understanding Chevota correctly regarding the brass insert, I'd add that counter-boring may be helpful. Depends on the particular piston/cylinder arrangement perhaps. But in the case of the only springer I'm working on (really not interested in tweaking my Webley pistols as they're happily mediocre and it seems pointless), the QB57, there are visible markings from the end of the cylinder in the urethane seal. Any protrusion inside that face is likely to cause an unwanted impact on the seal bolt head or worse. And of course counter-boring the end of the cylinder weakens it a little, and is dependent upon the thickness of the steel in that face as well. If there isn't enough thickness you may not be able to use an insert without weakening the end cap excessively. Not that a brass insert isn't a good idea, which it seems to be, but I'd recommend keeping that end face flat as it will likely be impacted on every shot and an impact on your insert can't be a good thing.

I just tested chamfering the edges of the transfer port and metered the velocity. No significant change, though I reduced the flat edge of the port by more than 50%. Seems not to be relevant for this airgun. Adding a thicker, more durable plastic liner surrounding the spring inside the piston likewise made no change in velocity, but seemed to reduce noise slightly before I began adding the neoprene and perhaps also improved smoothness of the recoil.


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PostPosted: Sun Jan 18, 2015 7:43 am 
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Chevota wrote:
Handy; I have another idea if you're interested, just an option: Drill/tap the port to say 5/16" (or 3/8), then use brass bolts to plug it. You could also use 1/8" NPT plugs. The best imo, but most work, would be smooth brass shafts with an o-ring for sealing at the piston side, one to three setscrews on the other end to lock the shaft in place. Bolts or shafts would obviously be cut to perfectly fill the area, then drill each one the diameter port you want to test. Plus you can also test other stuff like putting a radius on the port, tapering the port, roughening it, anything you want. I wouldn't use the Raven, but it's better than nothing. I would choose a typical gun of ~18ftlbs which the port is more critical. This project is on my list of things to do too, but all back burner right now, especially since I feel I already know the answer. I don't believe the perfect port will change with barrel length, but will change if the bore/stroke/spring etc change. That's why I wouldn't use a Raven, the results would only be good for other Ravens which are not popular and for good reason. Well, they're not popular outside Canada...


Interesting. Brain overload...

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PostPosted: Sun Jan 18, 2015 1:05 pm 
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An interesting read to while away a cup or two of coffee. :)

Don't have much to add regarding springers...not really my cup'a.

I have played with waste oil burners over the years though.

Likely the first thing you learn is that liquids do not burn. :o

Need to convert liquids to vapors before you can ignite anything.

In say a diesel engine...you take a liquid, spray it under pressure to atomize the liquid (create SMALL droplets, but still in a liquid state), then add heat (compression, cylinder temp, glow plug) to first vaporize and then ignite those vapors. Gasoline engines work in similar fashion.

Wicks work by spreading the liquid super thin over a surface, reducing the amount of heat required to vaporize.

Takes a good deal of heat to vaporize a pool of liquid with a high flash point. Can't think of anything in a springer that would be capable of atomizing grease, so a good deal MORE heat required to go from a thin film attached to a surface to a vapor state.

Not going to buy into the Cardews combustion theory.

Just my .02

Al


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PostPosted: Sun Jan 18, 2015 1:43 pm 
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I have learned a lot in this thread. Excellent thought provoking opposing points of view. You guys rock.

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PostPosted: Sun Jan 18, 2015 3:24 pm 
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The compressing force is the inertia of the piston driven by the spring. What if we increased piston speed? Perhaps a vented compression tube that allowed the piston to accelerate more quickly, higher terminal speed, less cc's of air to compress would lead to a no-bounce soft landing? A lot of energy is wasted with piston bounces - three in a utube video. I would think that the most efficient springer would be calculated to near-zero bounce, and would probably require less spring for the same fps.


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PostPosted: Sun Jan 18, 2015 3:29 pm 
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Regarding this topic, I have a question that I cant seem to find a suitable answer to. That is, In a spring powered air rifle, which is more desireable or a better use of air, 'faster' moving air, or, 'more', air behind the pellet.

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PostPosted: Sun Jan 18, 2015 5:34 pm 
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The key to springer power is "more air", which is why the swept volume is the most important factor in the potential FPE of a springer.... In fact, it is so much the dominant factor, that doubling the force of the spring will do little to add velocity (assuming we're not talking a detuned gun).... Once you are talking about a given gun, with a given swept volume, then the next most important factor is the peak pressure reached during the firing cycle AT THE PELLET....

Individual air molecules are travelling at ~1650 fps (at 20*C) in random directions, colliding with everything, from the piston face to the walls of the compression tube, to the insides of the transfer port, the sides of the barrel, each other, and most importantly the base of the pellet.... The speed of these molecules, and therefore how "hard" or "fast" they impact the base of the pellet doesn't change much with the flow rate, but the NUMBER of them hitting the pellet increases directly with the pressure.... The velocity of the molecules also increases with temperature, causing a cascade effect, raising the effective pressure, which is why the added heat of compression (or indeed combustion, if it occurs) is so important to the efficiency of a springer....

Trying to get the most out of a springer, then, as I see it, is maximizing that pressure, and the length of time it is applied to the pellet.... If the transfer port is too small, or the pellet too heavy, the piston bounces back because the pressure peaks too soon.... If the transfer port is too big, or the pellet too light, the pellet slides off down the barrel before the pressure peaks at its optimum value and the piston crashes into the end of the cylinder.... Only when all these factors are in balance does the pressure at the pellet peak just right, for the longest possible time, and the piston come to rest on the cushion of air in the front of the cylinder, just as it all rushes through the transfer port to give every available bit of power....

Sounds easy, doesn't it?.... :mrgreen:

Bob

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PostPosted: Sun Jan 18, 2015 6:00 pm 
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The explanation of what is key to more springer power is timely for this discussion. A key thing to remember is that it hardly is a state secret. Airgun makers have the ability to experiment with different sizes of transfer ports, different piston weights, and different springs. It should not be doubted that they have done so and continue to do so.

One gets the impression from this thread that there are airgun enthusiasts who believe that little has been done to understand how spring piston technology works or how it can be improved.


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PostPosted: Sun Jan 18, 2015 6:12 pm 
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^X2....

Incremental improvement, or at least specialization for a given weight of pellet, may be possible.... but it should not be assumed to be easy to achieve without numerous negative outcomes along the way.... particularly on well proven, well established designs....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Jan 18, 2015 6:14 pm 
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Thanks Bob. That explanation was very easy to understand.

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PostPosted: Sun Jan 18, 2015 7:15 pm 
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Handy: Yeah, kinda hard to explain, plus I'm not the best at verbal descriptions. So I made one of my stick figure pix to help relieve brain stress. This is the thought using a smooth hole of 3/8" or so with a snug fitting brass or alum shaft that fills it. Of course adding an O-ring is a pita, but it would be sweet. Otherwise I suppose some silicone sealant or something that will seal but also be easily removable, unlike epoxy. The setscrews will/should hold it from popping out, but the force is pretty strong so I'd think three 1/4" setscrews with points and mating tapers in the rod. Again, this is just a thought that came to mind, I'm sure there are better options for you and your available tools, but it's better than nothing. The idea of a threaded rod/bolt would be the same minus the O-ring, and of course the receiver would be threaded to match. That actually seems like the better idea to me, but again just a thought to add. Yet another is to drill out your port little by little, then fix or toss the gun. The bennie with the removable plugs is you aren't getting metal shavings and grit into the gun, which ideally you'd need to remove, and if the gun is taken apart to remove said debris it may change the power a bit each time and upset measurements.
Another thing I always wanted to try is shortening the transfer port. The thought being would a shorter but bigger port of the same volume be better or? To do this you'd need to cut the receiver and weld it back together, maybe 3mm at a time? And test various port diameters at each length... A lot of work, and I imagine any gains found would be minimal, but it would be cool to see the results.


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