Velocity vs Bore Drag for a Typical PCP

[rsterne]

First I need to tell you what I consider a "typical PCP".... I used a .25 cal running at 2500 psi with a 24" barrel, shooting at 1.10 FPE/CI efficiency at 900 fps.... That means the valve is closing when the pellet is about 30% of the way down the barrel.... I used a 37 gr. bullet, kind of mid-way between a 25 gr. JSB King and a 50 gr. bullet.... I did two graphs, one with breakaway values of 2 to 10 lbs. and a continuous bore drag after that of 1 lb..... and the other with a breakaway value of 10 lbs. and varied the continuous bore drag from 1 lb. to 5 lb.... Here are the results predicted by Lloyd's Internal Ballistics Spreadsheet....



You can see that the breakaway force doesn't have a lot of effect by itself on the velocity.... Obviously if it was a LOT higher (eg. 50 lbs.) then it would be important.... However, the sliding friction of the bullet going down the bore IS important, and robs velocity the higher it is.... In this example, each lb. of extra force required to slide the bullet down the bore costs about 12 fps (over 1%) in velocity.... It is therefore something to be considered, particularly with bullets, where the drag may be significantly more than with a diabolo (waisted) pellet....

Bob

[Frank in Fairfield]

So,
Bob are you saying the longer barrel will provide a slower shot than a shorter barrel where the pellet has a chance to leave the barrel BEFORE the valve closes?

[Frank in Fairfield]

Cause, if you are, then we are in agreement cause I mentioned this very same topic a few months ago where you argued a longer barrel will ALWAYS provide more speed. I knew friction would slow the pellet. 

[rsterne]

Nope, totally different argument.... Although increasing the bullet drag reduces the velocity in the above example, if you doubled the length of the barrel to 48", the lower graph, using 10 lb. breakaway and 1 to 5 lbs. of continuous drag would produce velocities of 1013 fps at 1 lb. (instead of 896) and 930 fps at 5 lb. (instead of 847).... In fact, even at 5 lbs. of continuous bore drag (the worst case I investigated), even with an 84" barrel the velocity (then 963 fps) is still increasing, albeit very slowly.... At that point, the spreadsheet runs out of room to calculate further....

Your statement is correct, that given a long enough barrel, and enough barrel friction the bullet will eventually slow down.... but not in any practical PCP, which was my point....

Bob

[Ribbonstone]

Might be more complicated.

 Only dealing with 2500psi, which is really a small amount of pressure compared to powder burners, but there is evidence of obturation with airgun pellets.



(BTW: in a soft media, expanding pellets are more likely to have measureable bases (unless they tumble).  Seems the expanding nose creates a “shadow effect” that that leaves undamaged bases.)

Not all airgun pellets.  Some are harder lead, some have thicker skirts, but at least some have thin, deep skirts and are soft lead.

Been thinking about this. BOTH were kicked in the butt by 2700psi.  Same pressure,  different valve dwell, resulting in a different degree of obturation.

Seems to me that if it were immediate/initial blast of air, would have expanded equally (same pressure launched them).

So Bob…what’s the deal?

Is it that the lower speed pellet really never got the full effect of the pressure in the valve?  Or is it that somehow longer exposure to the same pressure results in more obturation? Or that in the slower speed, the valve opened slower, and had a more gradual injection of air?

[rsterne]

No question, pellets obturate, and the softer ones with thinner skirts should obturate more.... Now it when it gets complicated.... Does that improve the seal and add to the acceleration, or increase the drag and reduce it?.... I'm guessing there is no one answer, both could happen, and either factor could dominate.... So, there is not necessarily a direct relationship between the static bore drag (measured by pushing the pellet with a dowel) and the actual dynamic drag under firing conditions.... With bullets, where obturation may not happen, maybe the results are more applicable?.... If the bullet is too tight a fit and the drag high, I would expect a direct loss in velocity....

Now to your question.... I assume those two pellets were the same pellet?.... just different dwell producing a different velocity?.... Anything I say is going to be a guess.... While the initial (and highest) pressure pulse should be the same (2700 psi), and we would assume that would cause the same obturation, we appear to have evidence to the contrary.... Is there any chance that the faster pellet decelerated quicker on impact, causing the skirt to flare out differently on impact?.... It doesn't look like that is the culprit, because the faster one seems to have rifling marks over a longer area.... My theory would be that since the pressure in the middle to last half of the pellet travel is greater with the greater dwell, then that additional obturation must be happening in the latter part of the acceleration.... Perhaps the lead is getting hot due to friction with the barrel and therefore subject to flowing (obturating) easier?....

Of your three suggested mechanisms, I don't think it's the first one; the second one, combined with any heating could easily be the culprit; and yes, your point three, the rise time with the shorter dwell would indeed be slightly slower, and I suppose the pellet inertia, being the same in both cases, may allow more obturation with the bigger blast of air, arriving very slightly quicker, before the pellet starts to move.... Bottom line, I suppose, is that you hit it harder (with more energy), and it distorted more....

Bob

[CarsonRatSniper]

Ok, this TOTALLY got me wondering so I had to actually measure drag vs guessing how much it 'felt like'.
I guess a lifetime of bodybuilding has skewed my sense of how much things weigh! LOL

Dry & Unlubed: (bore drag after it gets moving a few inches down the barrel)

JSB King = 4 ounces (as low as my Berkeley Fish Scale will measure)
'Cuda = 13 ounces
37gr S/HP = Continuous 1 lb 4 ounces / Break away = 6 lbs (tough to measure accurately - I split the difference as my scale read 5-7 lbs) My 37's show a bearing surface area of .150"

I'm sure lubing the bore and pellets/slugs like I do when I shoot will cut these numbers significantly.

If I got the twist rate right...well, I'll leave that up to the targets

I averaged a 15+ fps increase in my 1:14" twist just by lubing my pellets and slugs.
I also found with my 52gr BBT from my LBT mold that even .0005" over bore groove diameter affects power and efficiency to the tune of around 20-30fps with slugs and using only 50 psi per shot vs 70 psi.
The additional required air for oversized slugs is to swage them to bore size and get them moving - this affects your valve dwell (requires a longer duration).

Dave Corbin STRESSED to me how critical the fitment be to the bore groove diameter if you are seeking Supreme Accuracy.
It doesn't mean slightly oversized won't shoot well - but if you want World Class accuracy with no loss in efficiency (less start pressure = lighter hammer strike for same result) the fitment needs to be spot on.
He told me he has hundreds of letters from competitors around the World that are swaging slugs on his equipment (Yes! For air guns!) thanking him for the performance and multiple wins they've gotten.
Don't ask me who, where or what because I don't know...

This entire learning curve is a blast!   

[Ribbonstone]

Same pellet (CrowMag) shot from a Sumatra, so the difference is speed was by the different power wheel setting (and in a Sumatra, the power wheel changes the amount of valve stem is sticking up to be hit by the striker). 

[Pellgunfun]

Bob,

How are you getting your valve to slam open fully, and then slam shut before the pellet reaches the muzzle end of the barrel?  Seems logical that this would conserve air, and make the rifle a bit quieter.

[rsterne]

All PCPs, when tuned to be efficient, have a dwell under 50% of the barrel length, usually in the 25-33% range, and sometimes less, particularly at the beginning (high pressure end) of the shot string.... You are correct that the earlier the valve closes, the more efficient and quieter, but also it has less power.... I'm not doing anything different, just using Lloyd's spreadsheet to quantify and analyze things.... The valve travel is actually (very close to) a parabolic curve, although the flow rate is clipped at various points to "square" it off because of various factors....

Bob

[DOKF]

Is the algorithm for Lloyd's Ballistic Spreadsheet published?

Is near static drag (as measured with a spring scale) the same as drag measured at muzzle velocity (accounting any changes due to rifling or laminar flow)?

[rsterne]

Nope, Lloyd's spreadsheet is proprietary, only available from him, on a "need to know" basis.... *LOL*....

We have very little (no?) information about how bore drag changes during the firing cycle compared to a push-through measurement using a dowel.... Based on Physics, the coefficient of friction is based on load (how hard the lead presses on the bore) and the coefficient of friction (can be changed by using lube), and NOT on the velocity.... Starting friction is greater than sliding friction, something you will experience if you try and measure it.... The breakaway force is a combination of the (greater) starting friction AND the force required to size the bullet as it progresses through the chamber and into the rifling through the (usually) tapered leade....

I would expect that a bullet, without a hollow skirt, and virtually no chance of obturation at airgun pressures, would have a similar force required to slide it down the bore during the shot as what you find with a push-through.... A skirted pellet, designed to obturate at low pressures, should have more resistance during the firing cycle as the force pushing the skirt against the bore should be higher under pressure than static using a dowel.... How much more is anybody's guess....

Bob

[DOKF]

That's what I was thinking ...

You are just more eloquent.  ;-)

[Pellgunfun]

Thanks for the update Bob,

Is there any way one could test/determine that the valve is in fact slamming shut before the pellet would exit the barrel??? 

Concerning the Velocity vs Bore drag.
He is my "assumption".  Hehehe.  I feel the greatest drag is at the very beginning of the firing cycle.  One because the power needed to start the pellet moving is greater, and secondly the pressure is strongest which will also tend to press out the sides of the skirt into the rifling with greater force.  The question for me is, how many inches of barrel travel is needed before the pellet reaches it's 900 fps muzzle velocity?  That "acceleration" length from 0 to 900 fps of probably has the greatest drag of any part of the firing cycle.

I hope that made sense:-)

[rsterne]

Here is a typical output graph from Lloyd's spreadsheet.... This is for a Disco at high, medium, and low pressures....



The physics of how the acceleration occurs are pretty simple, really, just F=ma.... As you can see, acceleration is occurring all the way to the muzzle, initially from the full pressure in the valve, and then from that slug of air expanding as the pellet travels down the bore.... The shape of the curve is different, with more of the acceleration occurring in the first part of the travel when the pressure is higher.... Pumpers, if using a "pump and dump" style valve (no air remaining after the shot) wouldn't have that "step" in the pressure curve, it would be a smooth decline all the way to the muzzle, and that tends to be very inefficient by comparison to a PCP.... The greatest efficiency occurs with the short, sharp "sip" of high pressure air at the beginning of the shot string (the red lines above).... Note that the valve is closing at only about 2" of pellet travel at 2000 psi in a Disco.... This is calculated, not measured, BTW.... but physics leaves us little choice (and great understanding) of what is happening inside a PCP.... The agreement between calculated and real world results is quite astounding....

Bob

[Ribbonstone]

Have only seen obturation with pellets in a few examples, always when driven very fast.  But the pressure is the same, just the duration of that pressure, which (by my logic) shouldn’t matter.

Sometimes they actually get a little shorter as well.  Not much, but enough to see in this pair (along with the well expanded skirt):




Can see the difference diameter makes on bearing area pretty easily with thise 310/311/315” round ball.  The “fat” ones were considerably slower than the skinny ones.  The soft  .310 faster than the hard .311.



All does make logical sense.  The more bearing area, the more firction. the more power loss to friction.

[rsterne]

Lloyd sent me a photo of some JSB Exacts 15.9 gr. that showed a lot of obturation, fired from a Springer (IIRC) at ~ 850 fps.... pressure unknown, of course....

Bob

[Bullfrog]

Nope, totally different argument.... Although increasing the bullet drag reduces the velocity in the above example, if you doubled the length of the barrel to 48", the lower graph, using 10 lb. breakaway and 1 to 5 lbs. of continuous drag would produce velocities of 1013 fps at 1 lb. (instead of 896) and 930 fps at 5 lb. (instead of 847).... In fact, even at 5 lbs. of continuous bore drag (the worst case I investigated), even with an 84" barrel the velocity (then 963 fps) is still increasing, albeit very slowly.... At that point, the spreadsheet runs out of room to calculate further....

Your statement is correct, that given a long enough barrel, and enough barrel friction the bullet will eventually slow down.... but not in any practical PCP, which was my point....

Bob

So would the practical application of this be that in real world barrel lengths, a longer barrel is always going to give you higher velocities even from an airgun that is otherwise already efficient? If so, I'm having a hard time wrapping my mind around this. With powderburner .22LRs its generally accepted that in a typical super-sonic load the bullets start slowing down after 14 inches of barrel length due to friction with the barrel, which is why most .22LRs are made with 16 inch barrels today (the shortest federal law allows them to be made without registration). Is the physics behind a blast of air in a PCP that different than a blast from a .22LR shell?  Or is it the difference between a pellet and a bullet?

[rsterne]

I haven't studied the Physics of a .22LR, but I have a PCP.... and the bullet is still accelerating in a typical PCP in a much longer barrel than what is practical.... Nearly all the big bores (using bullets) are now running 28" barrels minimum, and 36" is not unusual, with gains still being worthwhile.... We are working with much lower pressures over a much longer time and distance than what occurs in a PB....

Bob

[Tomg]

So longer barrels will yield higher muzzle velocity (in most cases) with a PCP given the same setup as with a shorter barrel.
If thats the case, and i think it is (Bob will correct me if I am not) the upside of this is less hammer, less spring for the same velocities vs. a shorter barrel. Of Course the downside is, and it isn't negligible, the pellet stays in the barrel longer and require more of the shooter to be accurate.