Springer pressure at end of stroke

[subscriber]

A gas ram accelerates a given piston faster than a spring, at the same peak load.  One reason is because the average force provided by the gas ram is higher:  The  force of a steel spring drops off much more steeply. 

Also; the gas in a gas ram does not have the same inertia as a steel spring.  For that same reason, I think a gas ram has more of a tendency towards piston bounce.   The bounce may be quickly reversed because of the high force at the end of a gas spring stroke; but that would be too late; after the pellet is very near the muzzle. 

Hence; this is why Theoben added the secondary piston mass.  Mass resists changes more than the relatively low force (even of the gas spring).  The difference is like that, between the acceleration seen when a car with a powerful engine crashes into a concrete block.  No way any engine can match the instantaneous force that a moving (or stationary) mass can apply.

Gas rams should have more "area under the curve" when it comes to average force x stroke.  Yet, my impression is that they do not always deliver that energy efficiently to the pellet.  Perhaps a "stronger" effective spring is a detriment, unless you also increase the piston mass.  That combination in a light airgun can become harsh to shoot.  (although no springer recoil ever bothers me when I apply a light hold)

If you have too much "spring" and "too heavy a piston" for the swept volume, I think the piston just crashes into the front of the cylinder, rather than adding energy to the pellet.  This is not good for power or accuracy, as the crash surely produces massive vibration down the barrel.

Increasing the peak air pressure behind the pellet too much probably distorts it to the point of making it inaccurate.  A harder and heavier pellet would probably be better then.

If you want to emulate PCP pressure delivery, perhaps springer compression ratio should actually be reduced.  This reduces efficiency only if there is zero piston bounce, yet high compression with a light piston will cause more bounce.   In other words; sacrifice peak pressure to achieve more area under the pressure curve, before the piston starts reversing direction.   The reverse acceleration would also be reduced if the peak pressure were reduced.

What I am suggesting, is how small displacement turbocharged racing car engines make huge power:  The static compression ratios are made very low, so that the boost can be increased dramatically without causing knock.  This means that the engine flows a lot of air and burns a lot of fuel.  That makes huge power; even if very inefficiently compared to the engine intended for a commuter car.

What you want in your gas spring air rifle, is a screw adjustable counter piston; so that you can vary the volume of the compression space to achieve the best overall performance...  Such a compression chamber might have a volume that is adjustable from zero to 1 CC; assuming a swept volume of 100 CC. With it set to .5 CC, it should produce roughly 3000 PSI peak pressure (a good target value, I think).  More because of adiabatic heating; less due to some residual piston bounce.

If you produce such a variable compression springer; start testing with it set to "zero" and gradually increase the secondary volume.  If you start with too low a compression ratio, your piston will crash right into the end of the cylinder...

[Theoben HE]

so a gasram is maybe to fast to compressing the air to peak pressure, not give air the time to build and heat up when compression is done.

mine ram sits with the body inside the piston, piston weight 460 grams.
a spring looks to me its little faster in first part and when air is compressed it holds little back and so peak pressure holds up little longer than a gasram, that give no room to hold the peak pressure at little.

about the theoben gasrams, what I know people how have them theoben gasram system removed to spring there fpe out put drop a good amound to.

what I under stand that you can make 2 ways to make power:
1 high compression ratio high pressure wave  ( high effi )           
2 go for peak pressure with alot of swept volume behind it to compensate to low compression ratio. ( low effi )

[rsterne]

In Adiabatic compression, the heating is instantaneous.... The longer it takes to compress the air, the more heat can transfer out into the metal, lowering the peak pressure.... However, the faster the piston is moving, the more momentum it has, so the more likely it is to crash into the end of the compression chamber.... which is very bad for accuracy, and probably longevity as well.... not to mention hard on scopes.... A lighter piston would reduce the momentum, but increase the velocity even more, which is why oversprung Springers have such a harsh shot cycle....

Bob

[subscriber]

Bernard; zoals ze zeggen, de duivel is in de details...

If we consider peak cocking forces; it makes sense that replacing a gas spring with steel spring would generally drop the pellet velocity.  However, how do we know that in each instance where such a velocity drop was reported, that the most appropriate steel spring for that airgun had been used?

All I can suggest is that you explore the changes you want to make and measure what happens. 

Generally, higher compression means better efficiency; thus greater pellet energy.  Just to confound matters; consider this:

A lower compression ratio allows the piston to travel further down the cylinder.  Thus, the fact that the gas spring is still pushing hard near the end of the stroke adds more energy; that would not have been added, if the piston had bounced sooner off high pressure air...

What you are modeling is a person jumping off a 10 story building onto an airbag.  You are asking what effect the pressure inside the air bag has on how deep the person sinks into it.  Also, how far and how quickly the person might bounce up.   If the airbag is 5 foot deep, we tend to ignore that detail when calculating the gravitation potential energy, and multiply by 10 the floor to floor height. 

However, if the airbag is 9 stories high, then the gravitational energy released is only one story; or about 10 feet of free fall; rather than 100 feet.  An extreme example, but my point is that one has to consider the whole system in detail; otherwise our predictions are only educated guesses.

If we can't calculate or model exactly what happens inside an airgun, we can still test a system with known characteristic.  Provided such a system does not have a leaky piston seal, or excess friction it should provide valid results. 

Remember; just because you increase your airgun's swept volume by using a larger diameter piston (and cylinder), does not mean it will produce the increase in pellet energy you expected.  This may be because the piston also needs to weigh a lot more to offset the increased "bouncing force" the now higher air pressure applies to the larger frontal area...

[MartyMcFly]

As I twiddle my thumbs waiting for my packages to arrive, I’ve been trying to come up with objective ways to measure the changes in efficiency as well as recoil characteristics that may result from these experiments.

For testing energy transfer efficiency I will measure the before and after velocity of three different grain pellets (heavy to light) over an average of 30 shots per pellet type and then calculate FPE for each. My goal is to maintain the same piston weight from the unmodified to the modified versions. If the velocity improves I think we can say that efficiency has improved.

I have not come up with a good way to measure recoil or to estimate the actual piston bounce that might be already present in the rifle. The first thing that came to mind was using a camera with graphing paper in the background of the rifle. This should capture how much the rifle moves from start to finish, but it seems like a crude approach.

Does anyone have  a better way to measure piston bounce without creating a see through compression chamber?

-Marty

[subscriber]

To track piston position over time, get a voltage source and oscilloscope that can measure voltage or electrical resistance at 10 kHz+ resolution:

Make a linear potentiometer out of a graphite pencil lead.    Use a lightweight wiper attached to the exposed underside of the piston at the cocking slot.   The graphite rod would be attached to the outside of the air cylinder (electrically isolated).   The electrical resistance would be proportional to the effective length of the current path through the graphite; thus would represent piston position captureable externally...


Here alternative methods to make potentiameters:
www.youtube.com/watch?v=aElHVTv-75M

https://electronics.stackexchange.com/questions/24420/how-can-i-make-my-own-potentiometer

www.instructables.com/id/Make-a-Pencils-Lead-Potentiometer-Experimentatio/

[MartyMcFly]

Subscriber, that is a very cool idea! I’m glad I asked,  its not something I would have thought of. Here I was wondering where to get plexiglass rods and whether they could be milled into a makeshift compression tube.

To track piston position over time, get a voltage source and oscilloscope that can measure voltage or electrical resistance at 10 kHz+ resolution:

Make a linear potentiometer out of a graphite pencil lead.    Use a lightweight wiper attached to the exposed underside of the piston at the cocking slot.   The graphite rod would be attached to the outside of the air cylinder (electrically isolated).   The electrical resistance would be proportional to the effective length of the current path through the graphite; thus would represent piston position captureable externally...


Here alternative methods to make potentiameters:
www.youtube.com/watch?v=aElHVTv-75M

https://electronics.stackexchange.com/questions/24420/how-can-i-make-my-own-potentiometer

www.instructables.com/id/Make-a-Pencils-Lead-Potentiometer-Experimentatio/

[Theoben HE]

Thanks lads for the information, Yes its all in the detail.

what I have tested so far:

with the vortek high picht spring it make less power : 150 ms 46.3 grain pellet
with the 85kg gasram make 170 ms
the 110kg gasram does 183 ms.

The thing is when I bought the gun new the QE .25 barrel is only 27cm 10/6 " long it piston slams hard with 20grain pellets.

So I replaced the QE barrel to h 125 .25 42cm barrel, The piston slam stops and the vo up 20 ms !
Thats why Im think the barrel lenght does matters, the .30 barrel is also 27cm maybe to short
for the power plant of the h135.

now with the long .25 barrel the gun is loud

the piston seal is little stiff inside bore.

the bigger the piston there more force needed to get the air to compress thats why a 25 mm piston is such good preformer on mid fpe guns. low piston weight  not to hard to cock and accurate. because its needs less mass and spring force.

I try a 31.4mm hatsan 135 with 120-125mm stroke, I have order a h135 cylinder to cutting to 31.4mm
I have a good arms shop here he can almost order everthing of hatsan.

[rsterne]

The difference in velocity and piston slam could be due to the bore diameter, rather than the length.... If the longer barrel is tighter, that would delay the pellet release, increasing the peak pressure, which would reduce or eliminate piston slam, and could also increase the velocity....

Bob

[Theoben HE]

the .25 barrel is not 100% tight bore its alright but I really see a better bore than now.
the .30 bore is better fitment of pellet also a choke.

now with the 3.5mm tp it lost 10 ms
so back to the 3.2 mm tp.

the pellet is also the relaese valve what I know, so a .30 46 grain pellet needs some air to get moving.
But the air throws to a bigger surface so less pressure is needed ? than a .25 ?

im working on difference in piston weights i can increase weight by 10 grams a time.
maybe the hole system is already over max fpe and im chasing a ghost.

only thing to try is a longer .30 barrel now 27cm 10.6 " so to a 35- 40 cm is to try.
Im can't get hold on such barrel with 15-16mm diameter.

If there a .30 barrel to try please contact me.

what choice in pellets .30 im looking for a tight fit .30 and .25 pellet
what size does the .25 .30 FX pellets have ?
and the h&n baracuda ,30 ,25

how do they fit ?

[subscriber]

Hi Bernard,

The only way to study the effect of barrel length as a single factor, is to start with a long barrel and cut off an inch at a time (or cm, if you prefer); without changing anything else (other than de-burring the muzzle).  Then to measure the pellet velocity at each discreet length.  Comparing two barrels and expecting the difference in length to be the driving factor for observed velocity differences, does not make it so.

The PDF link posted earlier in this thread showed the effect of barrel length as a function of pellet weight.   Image of velocity graph shown below.   Springers can work with surprisingly short barrels (if you can stand the cocking force).

For a springer powerplant with a given swept volume, the larger the caliber, the shorter the barrel required to reach peak pellet velocity.  This is simply because with a large bore, the expansion ratio is so much greater.  A .25 caliber has twice the bore area of a .177.  So, for the same powerplant, the .25 barrel length that would produce peak pellet velocity would tend to be half that of the .177. 

Yes, the exact pellet weight and "tightness" in the breech matter a lot.  However; the fundamental principle is the same as for efficiency due to "compression ratio"; just in reverse.  Once the air pressure drops below about 100 PSI, the pellet will actually slow down due to barrel friction.  This is a general value that works for all calibers with diabolo pellets (discussed in a few other threads on GTA).


.25 caliber 31 grain H&N Barracuda pellets I have measure:
Head: .2505" Very consistent.
Skirt: .254 to .259" around the same pellet skirt.   I checked a number from the same can, and none of them have round skirts.  May assume average effective diameter of .256 to .257".


The attached image compares measured VS calculated "springer" pellet velocity VS barrel length.
From:  www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns

[WyoMan]

Once the air pressure drops below about 100 PSI, the pellet will actually slow down due to barrel friction.]

I don't think so but I don't really know. I've shot a .22 pellet out of a barrel like a blowgun (lung pressure @ 2 psi). I've shot a .22 pellet out of a barrel with shop air (@ 100 psi) using a rubber tipped nozzle and it penetrated the drywall 

The dynamic friction will increase with pressure as the force normal increases (due to pellet obturation). But... as pressure declines, so does the force normal, and so does the dynamic friction. kind of a paradox. More pressure creates more friction. Difficult to measure.

My empirical results have shown that longer barrels on low powered springers (i.e. HW45s) will increase the fpe consistent with the increase in barrel volume. Does anyone really know? Interesting discussion, thanks-

Wyo

[subscriber]

I've shot a .22 pellet out of a barrel like a blowgun (lung pressure @ 2 psi).

WyoMan,

Let me guess; you pushed the pellet in via the muzzle, past the choke.  Thereby setting it up for loose, practically friction free fit; before blowing it out the chamber end...


A .22 caliber bore has an area of 0.038 square inches.  At two PSI that equates to 0.76 lb of force.  So, either your pellets were undersized, or your barrel oversized; or you pushed the pellets through multiple times before blowing them through.  The only way you are blowing a pellet through the choke with your mouth, is if it has been through before...

Now the driving force on a .22 pellet would be 3.8 lb at a steady 100 PSI.  What is the friction of a .22 pellet going down the barrel?  If you can overcome that, and maintain perhaps 2 lb over 2 feet of travel, then the pellet is going to accelerate "fast enough to penetrate drywall". 

A PCP and springer work by providing a limited pulse of high pressure air (not a nearly endless supply of low pressure air, like a shop compressor tank).  As such, with these airguns air the pressure keeps dropping as it expands down the barrel.  In the case of a springer, the pressure drops much faster due to piston bounce, and the fact that the air will be cooling rapidly after its initial adiabatic compression heating.

Now; the question that was also asked the context of how much is typical pellet friction; was how long should a barrel be to achieve full velocity, practically speaking?  The answer that came back from multiple practitioners of the art, was that making the barrel long enough in a PCP, to the point where the air pressure drops below 100 PSI, made for a ludicrously long barrel, that gained hardly any practical velocity.   

Now, the friction break even point may have been 50 PSI (rather than the 100 PSI my statement implied).  That said, the added FPE from a delta of 50 PSI (100 - 50) acting on a .22 pellet over another 6" of barrel would gain only 0.95 ft.lb. 

By the way; the HW45 is a pistol, rather than a rifle.  Just because you can gain a few FPS by increasing the barrel length of a springer pistol does not mean you will achieve the same gains with a rifle.  There are many threads on GTA of springer owners cutting several inches off their rifle barrels; then reporting "no loss in velocity"...

[MartyMcFly]

A quick update, lest anyone think the project was abandoned. The patient... ahem, I mean rifle, has arrived! It came in a beat-up box and was bleeding badly, oil was oozing out of her everywhere. I have cleaned her up (externally) and put her through the chrony. Its a Hatsan 125 .22 Vortex Sniper and after a 100 shot break-in she is shooting 885 fps with a standard deviation of 8 using 14.3g Crosman Domed Premiers. The rifle is making a honking noise when its cocked and a ping (like a tiny vibrating spring) when fired, so she needs to be opened up anyway.

Since she's a gas piston/ram gun I will be removing her heart and replacing it with a spring. For that I need to order a new longer backstop, a spring guide and of course the spring. I will reengage this thread once I have the parts and ready to operate.

-MartyMcFly

[subscriber]

That sounds like a good candidate, Marty.  More expensive than I thought you were going for...

[MartyMcFly]

I've got an old Benjamin Trail coming from family, but it won't arrive till past Thanksgiving, so when I saw the Hatsan new at a refurb price and free shipping I just had to have it to experiment with  Besides, I wanted to see the build quality of these rifles. So far I don't think I got ripped off.

That sounds like a good candidate, Marty.  More expensive than I thought you were going for...

[Jr007]

I always dream of owning Theoben Eliminator or Beemen Crow Magnum at the time I was 14yr old I couldn't afford $1300. Hopefully Rapid Air Worx (RAW) well bring it back to the market. I'm willing to pay 2-2.5K

New Improvement Design
*Reduce weight maybe like TI tube cylinder or other ways
*increase piston stroke from 100mm to 125mm Get over 30FPE
*Improve HE Gas System if possible or Coat the inside with DLC help reduce fiction.
*Hard Wood Ambidextrous Stock
*.22 cal .25 cal only - Walther or 4130 CM