Hammer springs- 2 different approaches. Got Power?

[rsterne]

That would appear to be an excellent model.... If you make a major change, say a 50% heavier pellet, and only change that input in the model, how close are the results to real world then?.... or do you have to adjust several parameters to get it to "work" again?.... It's those kinds of tests that tell you how good your model is working....

Bob

[Bill G]

Sorry about the short answer, my wife was rushing out the door.  The begin and end psi are +/- 20psi.  The velocity is as recorded rounded in the decimal range.  Stroke and volume are derived.  Air used per shot are not discernable from calculated to real due to 2% accuracy of gauge.  Lots of other parameters, but what I posted is a breif synopsis. 

Sorry about any poor writing but I've gone mobile.  My wife is making me go to a KISS/Motley Cruel concert.  She just turned 40 and is regressing 

[Bill G]

If I change from a 25.4gr at 900 to a 31.02gr the velocity reduces within 4% of real world.

[lloyd-ss]

Yes calculated.  They match fps within 1fps and and psi is +/- 20psi

wow!
Lloyd-ss

[rsterne]

The velocity is as recorded rounded in the decimal range. 

So if I understand correctly, you are not using the stroke and dwell to calculate the velocity, but to try and explain how it happens?.... That would still be a good model, especially if the derived pressure drop over the string is within 20 psi.... or is the pressure drop measured as well?.... 

Bob

[Bill G]

 Yes, the whole reason for me to start this thing was to find out how they worked.  I input a string of shots, then I enter the volume required to achieve that velocity.  The sheet then calculates the stroke, psi used, and remaining psi in the reservoir.  There are a LOT of conditions that are plugged into the sheet that are used for the calculations.  Basicly the characteristics of the rifle in question.  With the new psi, I enter a volume to achieve the next recorded velocity. Repeat for the entire string.  With each new reservoir psi and new volume entry there is a new output for stroke,psi used, ect.  In the end, the remaining psi in the reservoir is extremely close to what is calculated.  The stroke is within. 005" of what is adjusted on the hammer if I remove it and measure it.  The potential volume flow is equal to what actually flowed at the last shot.  From there if I enter the availible volume as the shot volume, the calculated velocity is within 4% or closer to what is actually shot.  All the while the ending psi is accurate.  I've learned a ton but only made more questions.

[melloroadman]

I think that in the tests I ran tonight, if I had increased the valve travel by only .020", instead of .040", the results would have given almost the same power, but with much less air usage.  I have pretty much shotgunned the adjustments, so now it is mainly a tuning exercise.

Marvin, most of the tests I have done with the new valve with the .25 throat have been with the 60 gm hammer.  The valve just doesn't seem to require the heavy hit to be knocked open.

That is why I asked if you had tried more than the one shot with the 60 grain hammer with this dwell .And if a 80 grain might work better than the 100 . Marvin

[lloyd-ss]

Marvin,
I am sorting the data various ways to spot trends, but there are numerous anomalies. One trend is that as the velocity goes up, the efficiency goes down, but even that has exceptions. I need to ponder this for a while.   
The hammer weights and spring preloads for some of the shots are in my notes and not always in my spreadsheet, so that is slowing the analysis a little, too.
Lloyd-ss

[rsterne]

One trend is that as the velocity goes up, the efficiency goes down

If the pressure and pellet are a constant, that is probably true for the most part.... It particularly seems true when you start pushing the velocities near the speed of sound.... This may be due, in part, to compressibility factors in the air passing through the valve throat, ports, and the barrel itself.... One would think that the same drag factors that affect supersonic flight must occur in some form inside a tube when the flow approaches Mach 1.... Here is a graph showing how the CD acts in transonic flight conditions....



Note that the drag coefficient is about 4 times higher at Mach 1 than it is at Mach 0.85 (950 fps).... The drag also increases by the square of the velocity, even if the CD was a constant.... The actual drag at 1150 fps is about 6 times what it is at 950 fps.... If we have a narrowing of the ports,, it is quite possible that the local flow is supersonic even before the muzzle velocity reaches Mach 1.... Who knows what effects corners and edges have on the flow as we approach (or exceed) the speed of sound....

Every time I look at this, it just convinces me more and more to stay in the 900-950 fps range with an airgun....

Bob

[melloroadman]

Every time I look at this, it just convinces me more and more to stay in the 900-950 fps range with an airgun....

Bob

[/quote]
That depends on a lot of factors Bob . Marvin

[rsterne]

just my opinion....

Bob

[Bill G]

I think more than aerodynamic drag, atleast internally, the lack of laminar flow is the big limiter.  I believe it is responsible for harmonic issues.  Ill explain in a few.

[rsterne]

Could be.... You certainly can't get laminar flow at supersonic velocities, that's a given....

Bob

[Bill G]

That is the first.  I can't recall the rule for sure but you have to have 8 diameters prior to an 12 or 16 after a bend or joint.  With that, one can see how difficult it would be achieve ideal flow in the conventional airgun design.   Add the craziness of compressability and thermal dynamics, its a wonder we try.  But try we must.  I suspect that at certain pressure and passage combinations, the pressure resonates.   I first suspected this while observing shot strings.  The curve is relitively smooth but you'll notice a plateau in the string I posted.  I see a jump at 890 to 901, and it climbs to 908, then smoothly retreats.  That's not bad I know but it is a pattern.  If I take out the middle 5 or so shots out and use a quadratic regression, the curve would be much smoother.  Peak would be more like 902fps.  That would be a nicely improved ES.

[rsterne]

Yes, if you have that much porting length you might get better flow, but the pressure loss from all that extra volume wouldn't be worth it....

Bob

[Bill G]

Precisely.   The increase in dead volume always negatively influences velocity.  The balancing act becomes more evident with every new consideration.

[Bill G]

I agree with you Bob.  950fps is a solid max velocity target.  If one needs or wants more energy, increase mass and maintain velocity.  When it comes to taking game, if you are going subsonic, heavy and slow will out penetrate light and fast.  Hunting with an airgun is about shot placement and punching holes in vitals.  Supersonic is cool for the sake of study but impractical when applied to airguns. 

Lloyd,  thinking back on the subject of your poppet angles and engagement with the inlet.  what do you think the success of sealing would be with a radius into a tapper.  I'm going to make a new poppet for my Marauder and can't easily or accurately measure the lead in angle of the inlet.  I intend on giving it a bullet point leading end.  I have been thinking about the the benefit of a radius transitioning into a shallow tapper, similar to a boat tail bullet, visually.  I will be reassembling the Marauder in the morning testing it to see what kind of performance I'll get. With the increased port diameter along with the removed valve spring and much more open plenum, I expect to get maybe 100fps increase.  That's what I am estimating with my sheet.  If the results are as priedicted I think I will also drill some small holes longitudinally in the hammer.  Two points of reason here.
1.  Reduce compression between the hammer and valve.  I'm thinking this will allow full use of the hammer momentum.  It would seem to me that some of the momentum is absorbed by this possible compression.
2.  lighten the hammer to reduce the momentum thus allowing quicker reaction of the valve.  I'm thinking that the dwell or duration can be better modulates by controlling force input. 


Interested in your opinions
Bill   

It seems that sealing the inlet from inside as opposed to overlapping the inlet would make big differences and I think you have given solid evidence to that.  If I recall correctly, you indicated that it required less force to pop the valve and gave quicker reaction of the valve.   

[lloyd-ss]

Working on this .25 cal has been quite an experience.  I have learned a lot, which is what keeps me going.
Tried many different combinations of almost every variable that can be varied.  In the end, I had to step back and look at all the data from my admittedly "shotgun" approach and look for trends.  Excel is so nice for that!
After studying the data, I made some adjustments to the gun and focused in on a particular spring and hammer combination for the task at hand, which was to get lots of power while still keeping the gun reasonably efficient with its air usage.

I think I have accomplished that. There are no long, flat strings, but there are solid strings of high energy shots with efficiencies above 1.0 FPE/cuin of air. For a high powered airgun, that is pretty decent.

I have two charts below.  Both charts show different characteristics of the same five strings. The first shows FPE at various spring and hammer settings, and the second shows the FPE of the same strings. 
The spring force was varied with a power adjuster, finding coil bind (CB) and backing the spring off from there.  All strings used the same spring.
Hammers of 60gms and 100 gms were used.  The 100gm was just the 60 with a heavy knob on the cocking rod.

PLEASE NOTE that all shots were with 51.7 gn bullets EXCEPT for one string that used a 31gn and a 51.7gn both loaded to give a 82.7gn load.

The coloring of the lines on the charts is consistent so that you can easily find the FPE and Eff for a single string.

As always, this shows that you have to pay for the power, but in this series of high powered shots, as the power in the string increased, generally, so did the efficiency.

Lloyd-ss


Tank on the gun is 135cc. Top 5 lines are FPE (scale on the left),
Efficiency is the bottom 5 lines (scale on the right)   You should be able to click on the chart to enlarge it.



Top 5 lines are FPS (scale on the left), all strings 51.7gn bullets, except one with 82.7gns.
Efficiency is the bottom 5 lines (scale on the right)

[lloyd-ss]

Bill,
Regarding your valve shape ideas, they all sound good to me.  Putting the radius on the valve will most likely have to at least partially be a hand operation.  Holding some fine sandpaper to achieve the look you want.  But just a touch with the paper or you might loose the concentricity.

And yes, the air cushion in front of a hammer can be a problem.  Bob ran into that and it vexed him for a couple of days.  He drilled an exhaust hole in the tube near the valve.   The hammers are case hardened, so drilling them might be a challenge.
Lloyd

[rsterne]

Excellent results, Lloyd.... I emailed you as well....

Bob