Condor type build

[rsterne]

I'm still trying to understand where the valve seat is on the poppet?.... 

Bob

[darkcharisma]

The seat must be inside the outer shell of the valve. very interesting.

so when the valve opens, air flow through the 3 holes next to the valve stem
====
i take that back...i am in the same boat

[sb327]

It’s in the valve body.

5/16” throat with 1/8” stem. Stem is a sliding fit, no reason to try to hold it tight in this setup. Back of barrel/sliding breech capture top hat.

I’ve got a sketch I built it from, I’ll try to find.

Dave

[sb327]

Here it is. Some dimensions got changed but this is the idea.

Dave

[rsterne]

Ahh, OK, so the poppet seat is on the flat end of the poppet where the 3 holes are.... Got it!....

It looks like the O-ring on the stem and the one on the top hat are the same diameter?.... If so, then no net opening or closing force on the stem, right?.... The balance chamber offsets some of the cracking force, but once the pressure in the balance chamber equalizes (pretty quick with 3 vents) the only closing forces will be from flow resistance past the poppet head and through the 3 holes in the stem, plus the valve spring.... I'm guessing you have a pretty beefy spring to avoid a tank dump?.... Sorry, got that backwards for top hat and stem being the same diameter, very little valve spring is required... 

Bob

[sb327]

You are correct Bob. No net closing force besides the spring. I started with a beefy but couldn’t make power so went lighter. No tank dump.

Now, if someone can explain the closing ability of an Airforce, I’m all ears. I’m hitting this thing pretty hard. I left plenty of travel for dwell. So no bouncing off the backside. I’ve had no tank dump.

If you look at my tx2 valve on my 45, it has a net OPENING force and doesn’t experience tank dump till 23-2400.

Part of my reason for playing with this idea is to try to better understand what is going on in an af style valve.

Dave

[sb327]

Oh, and yes, I was only trying to eliminate cracking force on this initial try. I wanted as little  ‘blow open’ as possible, allowing the piston to float.

Dave

[Rob M]

seems like a good product on your hands , assuming it transaltes well to the larger bores

[sb327]

I’m not breaking any ground. I’m really just playing, learning a little. I did get the performance I wanted, but that’s been done. Doug Noble can do that with his eyes closed. I do think it is doing good efficiency wise.

Here is a tx2 valve for 45 for you fellas to look at. There are 5x .156” holes in the top hat.

Dave

[rsterne]

Here is a drawing of a stock AirForce valve showing the closing forces at work when the valve is open....



This may have been well understood before I drew this up, but I had never seen anything similar, either drawn or written about.... Of course when Doug Noble saw it he said "yep, you got it right".... 

If the top hat and stem are the same diameter, the valve does not blow open, all the work must be done by the hammer.... If the top hat is larger than the stem (D > d), then there is an opening force once the valve is cracked that assists the hammer strike.... The higher the Sectional Density of the slug, the slower it accelerates, so the higher the pressure on the front of the top hat, and the further it blows the valve open.... Carried to the limit, that can cause a tank dump....

In addition to the valve spring, the air rushing past the poppet provides some closing force.... and particularly in a stock valve (where the holes in the stem are drilled at 90 deg.) the air rushing though the throat and hitting the stem adds some closing force as well.... When you angle and streamline those ports, that closing force to a large degree disappears, and you can end up with a valve that does not respond well to changes in hammer strike.... The valve tends to either open wide or barely crack, causing a "cliff and plateau" response to the power wheel setting.... ie the valve either cycles properly or it doesn't....

When D = d, you need more hammer strike, so the valve once again begins to respond to the power wheel setting.... particularly if the valve spring is weaker.... Doug Noble uses a stronger hammer spring, lighter valve spring, and a heavier hammer to gain tuneablity in his high power valves.... 

Bob

[sb327]

That I do understand and I have seen your write up on it. I believe it relies heavily on the flow around the poppet (sail) coupled with whatever other forces (example: friction of flow through stem holes) AND a significant drop in pressure at the top hat OD for it to blow closed. It’s that significant drop in pressure I’d like to get around. That tx2 of mine has a relatively small hole for caliber. .325 vs .45. That would give it a pressure differential once Elvis has left, but I feel it wastes air and could be improved.

My valves efficiency is pretty good and performance is where I want it. Now I’d like to see if I can make it easier to cock. It’s not unbearable but would be nice if easier.

I’m sure mine has no ‘blow open’ advantage which I believe makes it less likely to dump but harder to get to dwell. I’m planning to try conventional poppet then start venting slowly.  I also will see what letting the poppet ‘float’ on the stem will do when the bv vent is tiny....may blow it right off and dump for sure

Dave

[sb327]

Well it is official, the bv is definitely making a difference. With a plain poppet, it only gets 511 FPS at 3000 psi.

My next test is to drill a small vent.

Question. What size hole and how far back to set piston?

Dave

[rsterne]

No idea where you are planning on venting.... Are you talking about through the poppet from the throat to the balance chamber?.... If you don't already have a vent there, I don't know how the valve is closing at all, frankly.... and how does the pressure in the balance chamber stay at atmospheric when the valve is closed to create a balance....

I am not familiar with the TX2 valve, nor have any ideas what the holes in the top hat do.... 

Bob

[sb327]

I just made a plain poppet. Bv style with no vent just to have a comparison to my vented one. I reported that difference above.

Now I want to start venting it a little at a time to find a sweet spot or see what the difference is between big vents (little blow open) and small vents. I know how it normally works in a non-af, just curious here.

I’ll see how small of a drill I have and go from there.

Dave

[sb327]

Well I have smaller but decided on a .029”.

Not very scientific testing here but I was able to move the bv piston further back for some comparison testing.

Initial setting 660fps
Initial +.062”—770fps
Initial +.125”—885fps

These were with a lighter hammer spring. I did not shoot a string. Just a couple shots each setting at 3000psi.

Keep in mind the poppet stem can slide easily in the top hat. So the top hat movement cannot close this poppet. Only open.

Dave

[darkcharisma]

your previous posts stated a 62 grain going at 725fps. but now its going 500fps. and i dont see the BV speed anywhere.

can you post all the numbers on one post for comparison please?

[sb327]

I’ve only shot 70 gr since the caliber change to .257.

My first heavily vented bv with heavy hammer spring

“I was quite pleased with the numbers it’s making as configured. Starting at 3250psi, down to 1800psi, .22liter tank, 13 shots average 130fpe. Shooting cast 70gr.  Fastest 958fps, at 2900psi. ” (from reply#15)

Then I tested same spring with conventional poppet (no balance vent). That is when I got 511fps.

Then I vented (making it balanced) to .029” and tested at various piston positions with a lighter hammer spring.

Dave

[rsterne]

With no vent, there is no guarantee that the balance chamber is coming back to atmospheric pressure between shots.... If not, then it may not be doing any "balancing" at all.... In fact, if the pressure inside creeps up to 3000 psi, it would act as an air spring (adding to the valve spring force).... IMO you need "some" venting or it's not a balanced valve, at least not reliably....

Bob

[sb327]

Bob,

It has a .029” vent, sorry for the confusion. I only tried it without a vent just for a reference of straight poppet design. Then I drilled the.029” vent and started moving the bv piston out a little at a time.
(Reply#33and34)


So I put the lightest hammer spring I had in. I set just enough tension on it to get the valve to crack and allow poppet to blow open.

So now the shot cycle is completely dependent on the balance valve.

I shot a string of 10 shots. As expected it has a declining shot string.

Starting at 3200psi down to 1900psi

1017
1007
1003
987
980
967
948
928
902
879

961.8 avg

It is pretty easy to cock now and I can tune it via position of my bv piston.

My next testing will be with a bigger tank to see how many accurate shots I can get and some tethered. I don’t have many  bullets left though. Have to order some more. I do plan on making a swaging die for this one, just don’t feel like it right now.

Dave

[JuryRigger]

All I have to say is 
Jesse