Author Topic: AirForce Airgun Design  (Read 237 times)

steveoh

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AirForce Airgun Design
« on: December 24, 2019, 07:42:40 AM »
Bob, can you talk about the differences between an AirForce design and normal PCP design in terms of valves, transfer ports, hammers etc? Trying to wrap my head around it.

Is the AirForce inline set up more efficient and capable of more power? Any deficiencies?

What other questions should I ask?



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rsterne

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Re: AirForce Airgun Design
« Reply #1 on: December 24, 2019, 09:41:40 AM »
Sure, Steve.... It's a pretty big subject, though, and the platform is new to me, but I'll try and summarize what I know so far....

The AirForce Condor, Talon and Texan use a "inline" or axial-flow valve, but that doesn't mean it is a "straight flow" design as it is often described.... The air passing through the valve itself still must flow around the poppet, which is almost identical to most conventional valve designs.... In a stock Condor valve, the valve stem is drilled through at 90 deg. to the airflow, so in fact the air, after passing the poppet must make two additional 90 deg. turns before it gets to the straight through part of the valve stem.... After that, the airflow is, indeed, a straight line through the valve stem and then through the sliding breech and into the barrel, which is in line with the valve stem.... The tank is, of course, located at the back of the gun, and forms the buttstock....

In most other PCPs, the reservoir is forward, with the valve at the back, so the air must turn through 180 deg. before it gets to the pellet.... This will lose some energy, but at the limit, with well designed, large ports, it isn't a huge amount, just a few percent.... I think the reason that the Condor has a reputation for being so powerful is that it is tuned that way, which is also what makes it an Air Hog.... Having said that, by redesigning the valve stem to straighten the flow, the Condor can be made to produce a LOT of power....

There is another significant difference between the Condor and other PCPs.... The hammer in the Condor fires backwards, towards the shooter, sliding over the barrel, with the spring in front of it.... When you push the sliding breech forward to load it, you are also cocking the hammer.... It has a power adjuster (the power wheel, or PW) on the left side of the gun, forward of the trigger, which is a simple hammer spring preload adjuster.... If a conventional PCP has a preload adjuster, it is usually at the back.... Increasing the preload will increase the velocity, up until you get to the maximum amount of air the gun will flow with the port size, barrel length and pressure it runs.... My experience with my Condor is that even with a .257 cal barrel installed, shooting a 63gr. slug, the velocity maxes out when the PW is only about halfway.... In fact, past PW = 9 the hammer (and the sliding breech it hits) bounces off the front of the valve, and the velocity actually drops.... There is NO point in increasing the PW past where the velocity peaks, you will only waste air, the same as with any other PCP....

The AirForce guns have one "quirk" that is, to say the least, unusual.... They can suffer a "tank dump", where all or part of the air in the tank dumps out the barrel during a shot.... For the life of me, I do not understand why owners would suffer through such an event, even if it only occurs rarely, and IMO this is a serious flaw in the gun design.... I did not have a clue why it happened, until I got my own, and then it didn't take me long to figure out what was happening.... Unfortunately, it is "designed in"....  ::)

In a conventional PCP, the outer end of the valve stem is at atmospheric pressure, and the pressure inside the throat of the valve, acting on the stem area, helps to close the valve, along with the spring, and flow through the valve itself.... In the Condor, the front of the valve stem is inside the sliding breech.... Therefore, during the shot, it is subjected to air pressure, just like the poppet end, so there is no pressure differential to help close the valve.... In fact, because the front of the "Top Hat", which slides into the breech, is larger than the OD of the valve stem, the opposite occurs.... During a shot, there is actually a force backwards on the valve stem, pushing the valve OPEN....  :o

This is good and bad.... The good thing is that it helps to open the valve, so you don't need as much hammer strike.... The bad thing is that if the pressure inside the breech is too great, that force can hold the valve open, and dump most or all of the air in the tank.... If is a fine balancing act, and is affected by many factors.... The smaller the caliber, and/or the heavier the pellet, the harder it is for the air inside the breech to escape out the barrel.... This raises the pressure inside the breech, and increases the chance of a tank dump.... To avoid the possibility of a tank dump, a very strong valve spring helps, and I suspect that is exactly what Air Force use, but I haven't investigated that yet.... With my .257 barrel, the wall of the barrel that slide inside the breech is quite thin, compared to a .22 cal or .20 cal barrel, which the stock valve I am running is supposed to work with.... So far, I have not experienced a tank dump.... I'm guessing that the .177 valve has a smaller Top Hat, or a stronger spring, to avoid tank dumps, but that is just a guess....

You can ask questions and I will do my best to answer them, as I gain more knowledge about my Condor....

Bob
« Last Edit: December 24, 2019, 08:34:21 PM by rsterne »
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steveoh

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Re: AirForce Airgun Design
« Reply #2 on: December 25, 2019, 08:59:17 AM »
Thank you Bob. This is well written and extremely clear!

I’ve read through twice and will re-read again when I have some peace and quiet. :)

I’m looking at images, diagrams, and exploded views of AirForce valves right now but am getting dragged away kicking and screaming to help with Christmas entertaining chores. Dang it.

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Alan

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Re: AirForce Airgun Design
« Reply #3 on: December 25, 2019, 10:57:51 AM »
This thread brings up a myriad of other issues related to (the/a) venturi effect or throttling of the airflow.

I admit it has been over 50 years since I was in college studying these types of problems. However today, it is possible to simulate airflow using nothing more that a fast computer. It would seem to me, that you could improve on most airgun designs, if enough R&D was applied to the issue. I say this, because just looking at the airflow path of some airguns—especially those with adjustable port restrictions—a lot of efficiency could be gained with proper design. Do you agree Bob??
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Re: AirForce Airgun Design
« Reply #4 on: December 25, 2019, 07:34:21 PM »
I don't know enough about CFD programs (computational flow dymanics) to know if they can really help us.... We are operating in quite an unusual set of circumstances.... We are dealing with flow that is starting and stopping, in a pipe that is capped at one end by a movable object that is accelerating as a result of the flow.... The air is at very high pressures, operating as a "supercritical fluid" any time the pressure is over 550 psi.... It no longer follows Boyle's law for ideal gasses, particularly once the pressure is over about 3000 psi.... At the end of the shot cycle, as the poppet approaches the seat, the flow eventually exceeds the local Speed of Sound, at which point the flow "chokes"....

While it is possible that a CFD program might accept those conditions and do the calculations properly.... it is also possible that the program may have to be rewritten to predict the flow properly.... I simply don't know....

I happen to believe that at the beginning of the shot cycle there is very little "flow", all that is happening is that the air molecules are moving into the ports and barrel chamber, and raising the pressure at the base of the pellet.... After that, as the pellet moves down the barrel and accelerates, the airflow behind it accelerates as well.... and restrictions to the flow become ever more important.... All this happens in 2 mSec. or less, and for very efficient PCPs, with a short dwell, a LOT less....

Bob
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Alan

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Re: AirForce Airgun Design
« Reply #5 on: December 26, 2019, 05:25:24 AM »
My assumption is also based on the ability to simulate airflow over the superstructure of an F35 at well over Mach 2. Obviously, it takes several gigaflops to do that. Whew!
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Alan

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rsterne

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Re: AirForce Airgun Design
« Reply #6 on: December 26, 2019, 08:07:13 AM »
That was pretty much my point.... The F35 is a large object that flies through air that has a pressure of 5-15 psi, not 1500-4500 psi flowing through very small ports.... The CFD  program that works for one may, or may not, work for the other.... without extensive modification to the operating conditions and their effect on the output....

Bob
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rsterne

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Re: AirForce Airgun Design
« Reply #7 on: December 26, 2019, 11:46:16 AM »
Alan, I just found this quote from an expert in External Ballistics in the UK (with 40 years experience), who just happens to also be an airgunner….

Quote
I am not sure that there is a lot of expertise goes into the little lumps of lead we all use. There is a lot of expertise in the manufacturing, but apparently not so much in the science and ballistics. Whenever I have been talking to anyone connected to the industry most of what I have said causes blank stares and confusion. I am not sure many of them would recognise an aerodynamic moment if they met one and probably less know how to go about calculating one. Some have been trying to use something like CFD (computational fluid dynamics) or PRODAS (a very expensive aerodynamic and ballistic prediction system) neither of which are suitable for pellets or very good on bullets.

Thought that might interest you....  ;)

Bob
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Alan

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Re: AirForce Airgun Design
« Reply #8 on: December 26, 2019, 12:25:38 PM »
Yes I am. I did a bit more research to find out that even the Navy has been having issues designing projectiles for their (to be) ship-mounted hyper gun. It is capable of exceeding Mach 8 (≈9,000 FPS). Imagine trying to simulate that!

As your quote alludes to, there is very little effort (so it seems) in redesigning pellets. The recent FX effort to design a bullet shootable in a "pellet gun" (essentially), hasn't proven to be all that accurate. I suspect they are with a faster twist, and we all know FX has barrels for about any projectile. We'll soon see, I suspect.

A related issue are the original design concepts of pellet, specially the overall length. You wouldn't have too much success selling a longer pellet, if it couldn't be shot in an existing pellet gun (like the above issue).

One reason I like our site as much as I do (has nothing to do with whose name is on ICAAN as owner), is we all have a level of expertise, and we're all willing to express out views, good, bad, or indifferent. In other words, an ideal learning platform, and you, Bob, are at the top of that pentacle—if you excuse the pun!
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

d_boom

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Re: AirForce Airgun Design
« Reply #9 on: January 12, 2020, 07:42:56 AM »
I also have experienced an air-dump once shooting my .22 cal. Condor.  With your explanation
I now understand what happened, THANKS.  Most of my shooting with the Airforce Condor is done
at 100 yards at steel targets, which can be hit regularly for about 15-20 shots.  Then an air tank
recharge to 3,000 psi. is need. My Condor likes shooting H&N's 29 gr. Piledriver pellets, which
are more like a slug than a pellet.  I have not shot for groups at 100 yards, or checked the velocity
of the Piledriver pellets.
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steveoh

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Re: AirForce Airgun Design
« Reply #10 on: January 12, 2020, 11:05:45 AM »
A couple days ago I took out my .177 QB-79 which has a crudely skeletonized valve.

Installed new o-rings on the tank block, and installed a paintball bottle that’s regulated somewhere around 900psi. I’m trying to figure out a way to actually test that pressure and then rearrange the washers to get it to 1k. 

Anyway, after first installing the tank I first filled to 1k because, well, I was Skeered.

Big grin on face as I realized I had no obvious leaks, and on first cock, I was rewarded with a resounding pop as I squeezed the trigger. Huzzah!

A couple more shots and tank dumped out of the barrel. Hmmm. Refilled tank to 3k and set up chrony and got pretty consistent 10 shot strings. Huzzah! The QB79 is only making about 13 fpe with 10.34 grain JSB Heavy Pellets.

I kept shooting and sometime, around the 600-700 fill pressure the tank dumped again.

That tank dumping is rather unsettling.

I don’t recall you mentioning a tank dump with your QB79 Ninja project. Surely it has something to do with my hacked valve.
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rsterne

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Re: AirForce Airgun Design
« Reply #11 on: January 12, 2020, 04:46:56 PM »
I have on occasion had a PCP dump the reservoir in a "machine-gun" fashion, with a wildly bouncing hammer.... but not on a QB.... IIRC, the easiest way to get one to do that is a very light valve spring and a slight amount of preload on the hammer spring.... That is a recipe for runaway hammer bounce....

Bob
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