Jim's piston seal article in AGW...

[Loki_79]

If anyone can get a test rifle down to Exeter, then I have a contact with access to a Miro 120
55,400 fps @ 128 x 128 pixels
95,300 fps @ 128 x 64 pixels

https://www.phantomhighspeed.com/Pro...0-121-Cameras-

[RobF]

Just checked on a recent test, Rob, and in the last millisecond of the compression stroke the piston travels around 8.5mm. 1,000 fps (or near offer) is woefully inadequate, I'm afraid.

Seems that way...

We'll give it a go to satisfy curiosity.

[TonyL]

If anyone can get a test rifle down to Exeter, then I have a contact with access to a Miro 120
55,400 fps @ 128 x 128 pixels
95,300 fps @ 128 x 64 pixels

https://www.phantomhighspeed.com/Pro...0-121-Cameras-

Now, I'm sure that would be revealing.

[TonyL]

My friend Topdog's is the only credible attempt I'm aware of to directly measure piston position. Since then, I've made the first attempts to measure piston seal crush, which must be subtracted from the piston body position for any calculations regarding the volume of air in the cylinder. I have only been able to measure crush up to 150psi at present, so what follows is an educated guess (albeit supported by experiments last year with transfer ports that extended back into the cylinder, and consequent reductions in muzzle energy).

I think the seal face is probably in the region of 1mm - 1.5mm from the end wall at piston bounce for the rifle set ups I favour. Go short stroke or light piston, and it will have to be less for any given muzzle energy.

Thank you for clearing that one up for us, Jim.

[hmangphilly]

For .177"

Falcon Accuracy Plus ~120 psi.
Air Arms Express ~ 150 psi.
Air Arms Field ~ 200 psi.
The rest ~380 psi to >600 psi.

Piston bounce displacement is way less with low pressure start pellets, and all the available evidence points toward peak cylinder pressure being less, as well.

do low start pressure pellets give the same fps as higher ones or are we trading bounce for power ?

[TonyL]

The figures are in Jim's fine articles in AGW.

I don't have them in front of me right now, but one of the other pellets tested was the lightweight RWS Hobby. A pellet that many people said for years was the best for power efficiency in many springers, so a good pellet to set power with which would, hopefully, keep the riflr safely "under".

The Hobby has a higher start pressure (and, hence, more piston bounce), started later in the cycle and did one hell of a "catch-up" job, although muzzle energy, I seem to remember, was less.

I think the main factor bring that the pellets with lower release pressure were accelerating for longer whilst the piston was travelling forward.

[greenwayjames]

Measuring piston position was recorded in one of Gerald Cardews W&S Hawks that nobody was interested in seeing a picture of.
My previous post.
Some time ago I saw a photo of one of Gerald Cardews W&S Hawks that he used for development work. It was one he had been using to test anti-bounce piston devices. It still worked fine. The Hawk is owned by John Bowkett. As many know I am involved in his facebook fanpage. I will see if I can get permission to put a pic of it either here or over there if anyone is interested?
The pictures I saw showed a long thin rod that appeared through the rear of the antibounce device that was welded to the rear of the cylinder. I know that Bowkett has all of the masters masters plus a load of paperwork and photos concerning his experiments. I wonder if the results of his experimenting are amongst them? I am probably talking to myself again. San fairyanne as the frog eaters say

[bmp01]

Measuring piston position was recorded in one of Gerald Cardews W&S Hawks that nobody was interested in seeing a picture of

Ok. Please can you post a picture of the Gerald Cardrew's gun used for measuring piston position.

I'm sure the techniques used were the best of the day, might still be very useful. Any insight as to how the measured data is used would also be beneficial.

BMP01

[bmp01]

do low start pressure pellets give the same fps as higher ones or are we trading bounce for power ?

Not necessarily. It's all part of the big bag of variables that make up the springer mode of operation. Another closely related variable is the radius at barrel entry.

It's easy to see that a hard to start pellet can rob the rifle of power but it's equally true that a pellet that starts too easily won't be the best either.

That's because you need to make the most of the incredibly short burst of gas pressure that is available. Most of the pellet acceleration happens in the first several centimeters of the barrel with diminishing acceleration as the pellet progresses. So you can see it's worth building up a decent pressure behind the pellet before it sets off. If a pellet starts too early, the pressure can't build up to the same high pressure....lower pressure means lower force to accelerate pellet.

Contrast that to a PCP where the pellet is pushed into the barrel.....

Also closely related , pellet mass, lighter allows higher acceleration.....

BMP01

[hmangphilly]

For .177"

Falcon Accuracy Plus ~120 psi.
Air Arms Express ~ 150 psi.
Air Arms Field ~ 200 psi.
The rest ~380 psi to >600 psi.

Piston bounce displacement is way less with low pressure start pellets, and all the available evidence points toward peak cylinder pressure being less, as well.

yeah sorry let me put it another way ......

are the above figures are for pellets shooting the same fps ?

[BTDT]

yeah sorry let me put it another way ......

are the above figures are for pellets shooting the same fps ?

They are the measured pellet start pressures. The muzzle velocity depends on what happens after the pellet has started.

[hmangphilly]

They are the measured pellet start pressures. The muzzle velocity depends on what happens after the pellet has started.

thanks jim i'm coming at it from the wrong angle

[pieinthesky]

As someone who has been toying with the idea of building a spring air rifle (for a few days!) this thread is very interesting read.

I know almost nothing about airgun design so please be patient with me

I have sketched a few ideas with the cylinder mounted below the barrel and the piston firing backwards. This means the transfer port will be in the top of the cylinder.

To reduce piston bounce, could you then have a second opposing piston (shorter stroke/weaker spring) in the same cylinder that meets the main spring at (or near) the transfer port?

When the cylinder pressure reaches a certain point (before bounce occurs) the second piston is pushed back, increasing the volume of the chamber and preventing/reducing further pressure increase. At some stage it will then return to its rest position returning its stored energy into the system.

This would work in a similar way to the spring loaded piston seal mentioned earlier but would have several advantages:

- It could be adjusted/tuned from outside the rifle like a PCP hammer spring.
- It could possibly be used to soften the landing of the main spring
- Doesn't add weight to the main piston
- Easy and simple to make (maybe)

Does this sound feasible or is it over complicating matters?

[Blackrider]

I love Jim's articles, don't understand 'em but I love 'em !

[hmangphilly]

As someone who has been toying with the idea of building a spring air rifle (for a few days!) this thread is very interesting read.

I know almost nothing about airgun design so please be patient with me

I have sketched a few ideas with the cylinder mounted below the barrel and the piston firing backwards. This means the transfer port will be in the top of the cylinder.

To reduce piston bounce, could you then have a second opposing piston (shorter stroke/weaker spring) in the same cylinder that meets the main spring at (or near) the transfer port?

When the cylinder pressure reaches a certain point (before bounce occurs) the second piston is pushed back, increasing the volume of the chamber and preventing/reducing further pressure increase. At some stage it will then return to its rest position returning its stored energy into the system.

This would work in a similar way to the spring loaded piston seal mentioned earlier but would have several advantages:

- It could be adjusted/tuned from outside the rifle like a PCP hammer spring.
- It could possibly be used to soften the landing of the main spring
- Doesn't add weight to the main piston
- Easy and simple to make (maybe)

Does this sound feasible or is it over complicating matters?

your transfer port (cyl to barrel ) is going to be tricky if you are looking to build an efficient system , even more so if you intend to use your extra piston idea.

where would the exit for the port be ?

in the top of the cyl ? ..... the air from the opposite side has a long way to travel , and if i've read jim correctly , the pellet usually starts to go at 1.5 -1mm before end of stroke so quite a lot of your tp has been closed off by the piston .

if your tp is at the end of the cyl , then it will have to go through your secondary piston or the cyl end wall , and then make its way upwards and through 180degrees to the back of the pellet ......quite a long tp length I reckon

i'd be interested to compare the effects of an offset vs central tp though , this might be responsible for the preference of owners of hw's to tx 's.

maybe the offset port creates a teeny cushioning effect due to the air taking just that little bit longer to exhaust .

It's all about timing