Lift / Dwell Quiz

[Stubbers]

This is a trick question for some, and a simple one for others.

1).What is the biggest contributing factor that:

a) Changes the relationship between lift/dwell in a pcp at a GIVEN energy level (changing a variable while keeping energy output the same, so for example a 20 fpe .22 cal, what can you change while keeping power at 20fpe that alters the lift/dwell relationship the most)

2) Whats the second biggest contributing factor that does the above?

I'd make this multiple choice, but that is rather not fun.

-Matt

 

[Stubbers]

Oh c'mon, someone bite !

 

[nervoustrig]

To preserve a given energy level where scenario A is high lift + short dwell, and scenario B is low lift + long dwell, I would say manipulate the hammer weight. But that requires a complementary change in hammer spring tension, and I don’t know if making two changes is in the spirit of this exercise.

As for what elements produce a disproportionate effect on the relative amounts of lift and dwell, those would be the valve spring and the valve stem diameter so that may be closer to the answer you’re looking for.

BTW, I encourage you to leave it up for a bit longer before commenting. I would like to see other thoughts and comments.

 

[Stubbers]

To preserve a given energy level where scenario A is high lift + short dwell, and scenario B is low lift + long dwell, I would say manipulate the hammer weight. But that requires a complementary change in hammer spring tension, and I don’t know if making two changes is in the spirit of this exercise.

As for what elements produce a disproportionate effect on the relative amounts of lift and dwell, those would be the valve spring and the valve stem diameter so that may be closer to the answer you’re looking for.

BTW, I encourage you to leave it up for a bit longer before commenting. I would like to see other thoughts and comments.

First guess, incorrect, and here is why. To VASTLY effect the relationship of lift/dwell, this means a higher lift/shorter dwell being more efficient (or lower lift / longer dwell conversely being less efficient) on air and having released more (or less) air in a shorter (or longer) time period than in previous state. I have tuned with 50 gram hammers and 7 gram hammers, and efficiency is relatively the same between the two setups at the same power level, meaning the air delivery is the same weight of air (lift equal to air flow path) in the same time (dwell, or window the poppet is off the seat)...not to say there can't and won't be any change in lift/dwell when altering hammer weight, its that its rather insignificant compared to the top two answers I am looking for. The hammer weight would have to be disproportionately changed to get near the results of the other two and even then could not compete.

And I'll allow altering preload to get the energy level back to original state, I suppose that would be necessary in most of the scenarios although in one of the two it is not (hint)...

-Matt

 

[Vetmx]

Valve return spring tension. Reg pressure.

 

[Jasonmc]

(changing a variable while keeping energy output the same, so for example a 20 fpe .22 cal, what can you change while keeping power at 20fpe that alters the lift/dwell relationship the most)

Reg pressure can definitely affect lift/dwell.
Keeping a set reg pressure and adjusting the HS will do the same..

 

[ctshooter]

Curious what the OP is thinking, but reg pressure (along with HS adjustment to maintain same fpe) seems the obvious choice.

 

[Stubbers]

Valve return spring tension. Reg pressure.

Reg pressure can definitely affect lift/dwell.
Keeping a set reg pressure and adjusting the HS will do the same..

The question states that FPE / energy must remain the same, so adjusting HS alone is not an option.

Reg pressure is an acceptable answer, although its not 'reg pressure'...its pressure...think unregulated guns and their bell curve...also, no adjustment needed on HS to restore power to 20 fpe when adjusting...

An unregulated .22 can hit 20 fpe at 3000+ psi, and then have a bell curve that lands it BACK at 20 fpe with an entirely different lift/dwell profile at 2000 or lower psi...

If you were to tether that gun at either end of the bell curve, and check for efficiency, clearly the 3000 or higher pressure end of the curve would win, as its dwell is shorter, and lift lower, making for a sharper pulse of air that is far more efficient.

That leaves #2, still unanswered. Told ya it would be a trick question though

-Matt

 

[Stubbers]

Once you exceed your curtain area in a pcp (about 1/4 the valve seat diameter with stem area removed) the main factor that determines/meters the air delivered is dwell, or the window of time the valve seat is exposed above the curtain area. Valve springs and hammer weights are not known to greatly effect this, or spring-less pilot valves that are opened via the weight of air would clearly demonstrate a large power gap between them and heavily spring poppet valves opened with heavy hammers...

In most cases, you're exceeding the curtain area in a pcp, unless severely detuned, so its moot to say the relationship of lift is substantially changed, what matters is if dwell is substantially changed. Very few things accomplish this, very few, and its easy to confuse the effect of changing your hammer weight / spring rating versus the effect of altering your pressure, or ...... or ......

-Matt

 

[Stubbers]

For a 75% ported .22 cal, you only have to lift the valve off its seat around .04125" to achieve full flow, now how much do you think altering hammer weight or spring really effects you getting there to achieve the same, identical 20 fpe?

Huge hint in the above...

-Matt

 

[Scotchmo]

Lift and dwell are related but not quite the same. I’ll focus on dwell.

So - What variables affect dwell requirements the most for a given fpe? Biggest affect depends on some other factors but for sake of discussion:

#1 Projectiles mass.

For a given fpe, the heavy, slower accelerating projectile requires a longer dwell duration in order to achieve the same fpe as the the lighter projectile.

#2 Pressure

Higher pressure requires less dwell.

Most of the airguns that I currently play with have electronic hammers and assisted (balanced) valves. Lift is mostly predetermined, but dwell is variable.

 

[Stubbers]

Projectiles mass.

For a given fpe, the heavy, slower accelerating projectile requires a longer dwell duration in order to achieve the same fpe as the the lighter projectile.

Great response, was hoping someone would state this. Very much so, but I am not sure it takes the #2 spot, its a close call if you go to the extreme on the ends of super light projectile, and super heavy.

But I must say I think pellet weight effects dwell much more than hammer weight changes while maintaining the same 20 fpe level in a .22!

-Matt

 

[Stubbers]

Hammer weight change alone while restoring energy via increased spring preload in my experience and testing, including measuring lift, only changes the lift curve, and takes rather large changes to see little shifts in said lift curve, while your dwell remains relatively unchanged (again unless very disproportional weight changes are made outside our scope of use).

If the pressure wave or bell curve of your lift were vastly sharper with lighter hammers or even pilot valves opened with the mass of air, you would experience vastly less dwell time and either a huge sharp increase in power, or efficiency, much like going from 2000 to 3000 psi, or ......, or ......

Simply said, you can't vastly change the lift/dwell relationship in an air rifle without altering its peak power output...and its efficiency when operating well under it.

-Matt

 

[Scotchmo]

I was under the impression that you were looking for the relationship between dwell and fpe. Not what actually changes the dwell duration.

In my case with the electronic hammer and balanced valve, hammer (armature) mass is a minor issue when I want to increase the dwell duration. I increase dwell by increasing the pulse duration on the solenoid. Or by increasing the balance chamber bleed down volume.

Assuming a large enough plenum, projectile mass (i.e. pellet weight) has almost no affect on dwell duration. Dwell is fairly constant unless I change it directly. But the heavier pellet will require me to change the dwell to a longer duration in order to maintain fpe.

 

[Stubbers]

I was under the impression that you were looking for the relationship between dwell and fpe. Not what actually changes the dwell duration.

In my case with the electronic hammer and balanced valve, hammer (armature) mass is a minor issue when I want to increase the dwell duration. I increase dwell by increasing the pulse duration on the solenoid. Or by increasing the balance chamber bleed down volume.

Assuming a large enough plenum, projectile mass (i.e. pellet weight) has almost no affect in dwell duration. But the heavier pellet will require me to set the dwell to a longer duration in order to maintain fpe.

I am looking for dwell duration, not necessarily fpe, but the two do go hand in hand, you cannot achieve a large shift in your lift/dwell relationship without effecting the plateau power output of your pcp, no?

Very glad you chimed in with pellet weight, it's remarkable how much more this weight effects the relationship than hammer weight and spring when dialed in for the same power.

-Matt

 

[ctshooter]

IMO your original question involved staying at 20fpe. I'm not sure I agree with you using a heavier projectile as, yes this will create more back pressure and keep the valve open a tiny bit longer, but this will also generally result in a higher fpe for the heavier projectile. In fact if you tune a gun with a lighter projectile and then use a heavier one it will almost always go up in power.

This is a scenario that plays out in Airsoft a lot. Many fields limit power to 1 or 1.5 joules. Some people will tune and chrono before a game with, say, a 0.20gram or 0.25gram 6mm BB and then play with a 0.30gram + BB which will yield a joule number that would normally restrict their airsoft gun from that field.

 

[Scotchmo]

IMO your original question involved staying at 20fpe. I'm not sure I agree with you using a heavier projectile as, yes this will create more back pressure and keep the valve open a tiny bit longer, but this will also generally result in a higher fpe for the heavier projectile. In fact if you tune a gun with a lighter projectile and then use a heavier one it will almost always go up in power.

…

In the case of a valve with a constant dwell time duration, a heavier projectile will result in lower FPE.

FPE = force x distance.

The heavier projectiles will travel less distance down the barrel before the valve closes.

less distance = less FPE

This is true for system FPE. There are other factors that make this not so simple (for instance, the air mass that is included in the system FPE).

With my electronic balanced valves, unless I increase the dwell duration, heavier projectiles give less fpe.

 

[Stubbers]

IMO your original question involved staying at 20fpe. I'm not sure I agree with you using a heavier projectile as, yes this will create more back pressure and keep the valve open a tiny bit longer, but this will also generally result in a higher fpe for the heavier projectile. In fact if you tune a gun with a lighter projectile and then use a heavier one it will almost always go up in power.

This is a scenario that plays out in Airsoft a lot. Many fields limit power to 1 or 1.5 joules. Some people will tune and chrono before a game with, say, a 0.20gram or 0.25gram 6mm BB and then play with a 0.30gram + BB which will yield a joule number that would normally restrict their airsoft gun from that field.

Do you propose that if I try to shoot a 3 gr pellet at 20 fpe it will take the same lift/dwell relationship as a 30 gr slug shot at 20 fpe in 22 cal?

-Matt

 

[SpiralGroove]

Yeah Matt,
Right now I'm tuning my three RAW's (.20 - 13.73 JSB, .22 - 18.1 JSB & .25 - 33.94 JSB) for roughly 910 fps each, when 910 fps in each case represents .97 of max power desired (938 fps). I've lowered the regulator's set-point on each to be 105, 125 & 140 respectively, but find the .20 & .22 too hot even with drastically reduced hammer springs on my SSG's. Both the .20 & .22 can shoot in the low 800's with reduced hammer tension, but ultimately shoot at 1023 and 997 with max spring. Hence, 910 is .89 and .91 of max velocity - thus reducing ES. Maybe this is a marginal issue?

I was thinking of reducing the hammer weights on those two as their (OEM) 70 gram mass is just too heavy given the reg pressure.

However, now thinking of clipping a coil off the SSG to reduce hammer strike. If I reduce reg pressure any more it's going to increase dwell.

Right now each gun likely gets 150+ shots per fill, so not terribly worried about efficiency (cause it's good already), but the perfect mix of reg. pressure, spring tension and hammer weight would be Nirvana .

 

[Stubbers]

Yeah Matt,
Right now I'm tuning my three RAW's (.20 - 13.73 JSB, .22 - 18.1 JSB & .25 - 33.94 JSB) for roughly 910 fps each, when 910 fps in each case represents .97 of max power desired (938 fps). I've lowered the regulator's set-point on each to be 105, 125 & 140 respectively, but find the .20 & .22 too hot even with drastically reduced hammer springs on my SSG's. Both the .20 & .22 can shoot in the low 800's with reduced hammer tension, but ultimately shoot at 1023 and 997 with max spring. Hence, 910 is .89 and .91 of max velocity - thus reducing ES. Maybe this is a marginal issue?

I was thinking of reducing the hammer weights on those two as their 70 gram mass is just too heavy given the reg pressure.

However, now thinking of clipping a coil off the SSG to reduce hammer strike. If I reduce reg pressure any more it's going to increased dwell.

Right now each gun likely gets 150+ shots per fill, so not terribly worried about efficiency (cause it's good already), but the perfect mix of reg. pressure, spring tension and hammer weight would be Nirvana .

70 gram hammers, reminds me of my stock marauder. Sadly to greatly reduce hammer weight without making any other changes you have to greatly increase the spring rating to restore power. It's one reason people either run balanced valves, thinner valve stems that reduce the closing force imparted on the head of the poppet during the shot cycle, pilot valves, lower reg pressure, or simply lesser rated/no valve spring and thinner poppet sealing margins which all require reduced hammer weight/spring ratings to keep power the same upon changing.

For the .20 / .22 caliber..

You'd be best reducing the regulator pressure so that your target velocity is 97-98% just like the 25 cal, so 930 fps plateau for a 900-910 fps tune, and running lighter hammers or lighter hammer springs (hammer weight is better to chop off first). Either that or choke the flow with a smaller transfer port on those two smaller calibers, but this is the less efficient approach than regulator adjustment, just less work.

-Matt