Acceleration and velocity of a pellet inside a PCP - internal ballistics

[ericw]

Recording sonic cracks is problematic because of the inadequate dynamic range of the microphones , A/D converters (number of bits converted) used and possibly even the amplifiers between them.  If you can hear it, the sample rate would have to be very low (less than even cheap junk converters would have) to not sample it at least 2x per cycle of the highest freq you can hear(20KHz if your hearing is extrememly good).   If the microphone is far enough away, you could record the crack sound if the gain between microphone and A/D converter were not too high. 
Eric

[lloyd-ss]

This video is from several years ago.  It has the fastest shot that I have ever recorded.

[rsterne]

Wow, 42.6 gr. supersonic (141 FPE!).... Looks like the pressure after the shot was 2000-2100?.... How big was the reservoir?.... Did you ever figure the FPE/CI?....

Bob

[lloyd-ss]

Bob, 280 cc and I didn't want to know the efficiency 'cause it was  a dump shot.
The valve on that gun is very large, and coaxial with the barrel bore, and dumps directly to the back end of the pellet.  Picture pulling the stopper on a sink drain, where the drain has a radiused opening, and not an orifice-in-a-plate shape. In that configuration it was difficult to close the valve until after the pellet had left the bore and relieved some of the back pressure.
I am sure the stroke limit was set very high too. The test was just for maxing the thing out.
Lloyd

[rsterne]

About 0.13 FPE/CI.... *LOL*.... sounds like some of the recent maxed out shots with my .25 cal....

Bob

[Motorhead]

Gentlemen,
If I may cause a slight direction change for discussion ?

Just to make clear where I am coming from, My background is one of being a 2 stroke engine tuner where we work with a lot of Time/Area perimeters.  Ports opening and closing, sonic wave activity within engine and especially within the Expansion chamber / Tuned pipes.

So ... What is just nagging my brain is why I read so little about AMPLITUDE ( The strength and duration of the pressure pulse )
See: http://search.yahoo.com/r/_ylt=A0oGdN1Bh6ZQCzwAKEIPxQt.;_ylu=X3oDMTByZWgwN285BHNlYwNzcgRwb3MDMQRjb2xvA3NrMQR2dGlkAw--/SIG=11qvv7f7c/EXP=1353119681/**http%3a//en.wikipedia.org/wiki/Amplitude

Threads dealing with valving, transfer ports etc ... in at least threads I have read and understood ( Some serious metal masturbation at times in many of these threads ) The realm of making Amplitude of the pulse propelling our pellets a direct design attribute ... Or it does and have missed this because use of less descriptive nomenclature ?

Reason this is increasingly nagging my brain is tests I have been doing with hammer weights and increased hammer velocity getting valves open faster and shut sooner also.  Transfer passage area poppet to pellet quite large too.
Pulse AMPLITUDE has to my deductions were greatly increased resulting in greater pellet velocity on less air ... Go figure huh 

Anywho .. Just tossing it out and see what others may think, debunk, support to make me see my errors or insight on said subject.

Regards,
Scott

[rsterne]

Slightly hard to understand your thoughts Scott.... but if you are talking about the difference between a short sharp blast compared to a long slow push.... you are right on the money.... That is why higher pressures are more efficient for a given FPE level.... and also why larger ports tend to allow less pressure for a given FPE.... or conversely a shorter pulse (dwell) for a given pressure level....

Call it amplitude if you like, but what it amounts to is delivering more MASS of air right at the beginning of the pellets travel.... I don't think it much matters whether that comes from high pressure or high flow rate for a give pressure.... the trick is to get the duration (dwell) as small as possible to get the valve closed before the pellet has moved very far.... That gives the air in the barrel a longer time, and distance, to expand.... This results in a lower pressure at the muzzle, less muzzle blast, and therefore increased efficiency....

Bob

[Motorhead]

Slightly hard to understand your thoughts Scott.... but if you are talking about the difference between a short sharp blast compared to a long slow push.... you are right on the money.... That is why higher pressures are more efficient for a given FPE level.... and also why larger ports tend to allow less pressure for a given FPE.... or conversely a shorter pulse (dwell) for a given pressure level....

Call it amplitude if you like, but what it amounts to is delivering more MASS of air right at the beginning of the pellets travel.... I don't think it much matters whether that comes from high pressure or high flow rate for a give pressure.... the trick is to get the duration (dwell) as small as possible to get the valve closed before the pellet has moved very far.... That gives the air in the barrel a longer time, and distance, to expand.... This results in a lower pressure at the muzzle, less muzzle blast, and therefore increased efficiency....

Bob

I know ... convoluted thinking putting into words.

I relate it to taking a Party balloon and blowing it up to a given size. It's size is Volume and the pressure within created by the elasticity of the stretched rubber.
* So we have a vessel that contains confined energy.  The potential of how reactive this energy is comes down to how it is purged/released from containment.
Just let the air leave out the fill neck it fly's across the room because the Dwell / Time air takes to leave has little amplitude, but great duration.
Other extreme is simply POP the balloon and ALL the contained energy is released at once.  Amplitude is at MAXIMUM, dwell / pressure duration release is minimum. 

So guessing that in A REGULATED gun where the pressure and volume for the shot are FIXED, releasing it to drive the pellet is best done Fast as possible with short duration as practical ?

And what we see in NON regulated PCP's is a Very Large Sacrifice of efficiency so the pressure pulse driving pellet is more uniform over a greater pressure variance within valve.

I do realize K.I.S.S. go's to the unregulated design ... But in terms of efficiency, being able to predict what tunes changes will effect, the regulated design is vastly Superior is it not ? 

   
Was goofing around last night and did nothing more than some subtle changes in transfer tube I.D. changing nothing else.
.165" - .140" - .125" driving a 10.5gr .177 pellet.
Velocity were all within 35-40 fps ( 925 fps average)  ... but what was interesting was what shot report sounded like 
Larger the transfer tube size, quieter the report, Smaller got louder.
Figure this due too the Amplitude/Duration and distance pellet traveled down barrel when valve got closed.
* Bigger tube let the valve dump faster, smaller a tad slower. 
Do realize these transfer passage sizes are LARGE for a .177 ... but did somewhat enlighten me to the issues address in my query above. 

[lloyd-ss]

Scott,
Yes, I agree with what you are saying.  Working toward the "instantaneous open--dwell for a few millisecs--instantaneous close." A square wave.  We might have to explore that 2-stroke wave/pulse application.    It is probably pretty relevant to what we are doing.

The loud report with the small transfer port makes sense, just as you describe it. A slower release of air thru the skinny port means the vavle closes later, which means their is more residual pressure coming out of the muzzle, which means less efficient air usage.

Now, about the light hammer with a heavy spring.  How do you think that accomplishes getting the valve open and closed faster? I would think that a heavy slow hammer would take longer to make a U-turn and a light one could do that faster. What about the return spring?   Does it come into play? Please share what you think.
Lloyd-ss

[Motorhead]

Scott,
Yes, I agree with what you are saying.  Working toward the "instantaneous open--dwell for a few millisecs--instantaneous close." A square wave.  We might have to explore that 2-stroke wave/pulse application.    It is probably pretty relevant to what we are doing.

The loud report with the small transfer port makes sense, just as you describe it. A slower release of air thru the skinny port means the vavle closes later, which means their is more residual pressure coming out of the muzzle, which means less efficient air usage.

Now, about the light hammer with a heavy spring.  How do you think that accomplishes getting the valve open and closed faster? I would think that a heavy slow hammer would take longer to make a U-turn and a light one could do that faster. What about the return spring?   Does it come into play? Please share what you think.
Lloyd-ss

Lloyd,

Not implying a LIGHT hammer and Heavy spring,  But thinking if hammer fitted is already on the heavy side, getting it into motion faster with higher spring rate may work better than making an even heavier hammer.
My initial thought being with More spring energy stored in a higher rate spring, it would accelerate an otherwise Over Weight hammer faster.
 
* Such is the case I found with the M-rods hammer once going regulated. Spring rate was Too little for the mass of hammer and to get valve open far enough to get the sufficient volume, valve opened too slowly and dwell too long for efficiency. Thus going down the path to find sufficient lift using minimal hammer mass.  Speed of the hammer strike and just enough hammer mass to match spring rate for job at hand gave better results.
 

Not sure where you read

Now, about the light hammer with a heavy spring. 

Maybe had been a type error on my part being that statement is contrary to my findings .... Did I brain fart someplace

* Hey I'm new to this tuning thing and only thinking out loud here 


Regards,
Scott

[rsterne]

And what we see in NON regulated PCP's is a Very Large Sacrifice of efficiency so the pressure pulse driving pellet is more uniform over a greater pressure variance within valve.

Actually, the reverse is true.... Non-regulated PCPs tend to be MORE efficient, especially at the beginning of the shot string.... Not only that, but the pressure pulse is NOT more uniform over pressure, the opposite occurs.... The pulse is VERY short (and the gun consequently quieter and more efficienct) at the beginning of the shot string.... and gets progressively longer (and the gun louder and less efficient) as the pressure drops.... That is what self-regulation of the velocity is all about....

Bob

[lloyd-ss]

 [/i]

Not sure where you read

Now, about the light hammer with a heavy spring. 

Maybe had been a type error on my part being that statement is contrary to my findings .... Did I brain fart someplace

* Hey I'm new to this tuning thing and only thinking out loud here 

Regards,
Scott

Maybe I misread, or assumed too much, but in your thread about lightweight hammers, you went much lighter on the hammer weight, but you did not increase the spring rate?
Thanks,
Lloyd-ss

[Cal]

An interesting side note with Scott's two stroke relationships.

Keven Cameron, a noted competition motorbike tuner, wrote a series of technical articles for the periodical "Cycle World"  May I date myself  this was the 70's 80's and  perhaps the 90s.  (

Pertinent to  our interest was one column  devoted to the Acoustic signature of the exhaust note of a racing motorbike.

It was all in the SPEED of valve opening and closing.  Low and slow......fast and high.    Not the engine RPMs,  the valve event rate of opening!

Visualize a two cycle IC engine has a wide exit port that exposes a considerable gas transfer area with small displacements of the descending piston on opening.
Pulses on every stroke, coupled with high piston speeds,  we end up with the characteristic "two stroke scream".

Compared , at the time, with the relatively slow moving "Hardly- Able sons", with slow moving valve trains and less crisp exhaust opening that took place only 1/2 as frequently for the same engine RPM.  That was the true source that returned a thundering baritone so dear to America's heart.

The article included  the comparison with "high speed internal combustion engines" (and I don't mean Sir Harry Ricardo style) and how the four cycle valve event timing was approaching that of the two cycle engines so recently relegated off the tracks.   Couple the astronomical engine speeds with technologically advanced valve actuation, it is not hard to connect cause an effect  resulting in a similar acoustic signature.  A screaming howl. 

Not a bad sound either very close up, or quite far distant ;-)

So,  Valve event timing and evaluation by acoustic characteristics.  might be a tuners ear that turns the trick.   

Regards

Cal

[Cal]

Lloyd and Bob

Would you care to enter a discussion on valve event timing vs power and efficiency?  I would love to hear your views. 

Perhaps  the topic has been throughly covered on previous posts.

Perhaps also, such a discussion should have it's own thread.  The mention has come up here already ;-)

Regards

Cal

[Motorhead]

 [/i]

Not sure where you read

Now, about the light hammer with a heavy spring. 

Maybe had been a type error on my part being that statement is contrary to my findings .... Did I brain fart someplace

* Hey I'm new to this tuning thing and only thinking out loud here 

Regards,
Scott

Maybe I misread, or assumed too much, but in your thread about lightweight hammers, you went much lighter on the hammer weight, but you did not increase the spring rate?
Thanks,
Lloyd-ss

True ... never addressed MORE spring.
Test as it transpired was too try and detect a peak of kinetic energy of a known spring rate, known travel against a fixed resistance ( Valve poppet with 1850# behind it )
Velocity target tho mentioned really only was a fictitious number based upon knowing gun could shoot that fast with the factory valve/hammer components in there upper power range of adjustment.

Worst case scenario as hammer weight was altered is screwing up the power potential all together.  This however did not prove to be the case allowing some discovery to show up.
That being conflicting in my mind at least with what Bob has said within that thread

As to the issue of this being a "magical tune" because of the "ideal" hammer weight.... there is no question that you achieved it by that method.... However, you could also have done it with the standard hammer and less spring preload....

Which does puzzle me because less spring preload on the factory weight hammer causes the power to be greatly reduced ?
Where as using the SAME SPRING at minimum preload & certain lesser weights of hammer rifle shot substantially faster. 
Thus I am missing/lost something in translation ... or not.

So staying with Lloyds original topic ....

What I see has many parallels with our Powder burning brethren in the scope that Light weight small bore projectiles are most efficient when fueled by Very fast burning high amplitude propellent of short burn duration.  As calibers get larger, burn speed of propellent slows down so it creates a greater volume of pressure over a longer duration with the peak pressure happening shortly after or just at the point projectile go's into motion.
( If we stay for arguments sake in CF rifle and there operating pressures ... small bore to large bore the peak pressure of the pulse all are very close )

So ... our PCP's, pick a pressure and let just use that as a baseline to wrap our collective minds around propelling a pellet/bullet with it.
My take for what it's worth understanding basic PB ballistics is that we are not so different, small calibers would seem to want a propellent shove that hits it's peak pressure very quickly with low duration. projectile is dislodged and well on it's way with not much need for volume / duration of the pressure pulse.
Larger calibers still need the same peak of pressure within shot cycle, but likely just a tad later ( Slower valve opening perhaps ) In theory it would seem the pressure peak also should correspond to right as pellet starts moving.  Don't know ... only guessing.
* I know this is basic academic knowledge, but just wanted to be on the same page thought wise


Hammer SPEED getting valve up off it's seat for the Sharpest/Faster pressure wave rise behind pellet/bullet does appear to be most important.
Volume of air flowed, duration / dwell falls upon hammers mass holding valve open, pressure on stem from air pressure and poppet spring within valve, port area etc ... again basis academic PCP knowledge.


Again ... just thinking out loud and now talking in a circle 

Scott
 

[QVTom]

What I see has many parallels with our Powder burning brethren in the scope that Light weight small bore projectiles are most efficient when fueled by Very fast burning high amplitude propellent of short burn duration.  As calibers get larger, burn speed of propellent slows down so it creates a greater volume of pressure over a longer duration with the peak pressure happening shortly after or just at the point projectile go's into motion.
( If we stay for arguments sake in CF rifle and there operating pressures ... small bore to large bore the peak pressure of the pulse all are very close )

So ... our PCP's, pick a pressure and let just use that as a baseline to wrap our collective minds around propelling a pellet/bullet with it.
My take for what it's worth understanding basic PB ballistics is that we are not so different, small calibers would seem to want a propellent shove that hits it's peak pressure very quickly with low duration. projectile is dislodged and well on it's way with not much need for volume / duration of the pressure pulse.
Larger calibers still need the same peak of pressure within shot cycle, but likely just a tad later ( Slower valve opening perhaps ) In theory it would seem the pressure peak also should correspond to right as pellet starts moving.  Don't know ... only guessing.
* I know this is basic academic knowledge, but just wanted to be on the same page thought wise

I'm not so sure.  The primary reason we use slower powders for heavier calibers is to control the pressure with in safe limits of the gun ( it is more efficient only because we are pressure limited) .  A heaver projectile is going to require more energy to accelerate than a lighter one.  If the powder is to fast the pressure will build beyond safe limit before the bullet get moving.  The burning rate of a powder is selected  to match the bullet movement down the bore and prevent an overpressure.  If the action of a PB could take 2 or 3 times the pressure faster faster power would produce better performance or the same at a higher efficiency ( smaller charge ).

There may be some correlation between propellent and air but I don't see it as direct.

Tom