Tx200 zero transfer port

[wonky donky]

so to conclude this, a transfer port is needed, we can not rely on the pellets to form the restriction to build the pressure consistently.

I am now thinking bore the end plug to form a 2mm long port, use a 300S conical breech seal and machine the barrel to suit, unless we can decide on a similar setup to what Dave is already running that would have zero air waste...i have one idea but will need to do a test machining on a scrap barrel to be sure it would work...it would be similar to how the HW100 barrels used to seal but with a little gripper groove for the O ring.

Boring the end plug maybe easier on the mill using an end mill, pilot drill first to size on the lathe then drop the end mill in to just flatten the bottom of the hole.

2mm is needed for strength, if we could get the barrel to sit exactly onto the face of the TP the O ring idea i have would work beautifully.

The 300s method is easy enough to replicate, tbh I'm thinking along the lines of a slightly longer port now due to the volume.

I have a HW100 somewhere, I'm struggling to visualise the sealing method, it can't be difficult to do, after all it's only machining.

[Shed tuner]

I think a couple of truths emerge from this. We do need a transfer port to provide the choke. Within sensible limits, the diameter of the port is more important than its length, although a minimum length is required to establish choking: the shorter the stroke, the smaller is the transfer port diameter required for maximum energy efficiency.

Mike

Hi Prof Mike,

So for the choke to function as such, presumably the port only needs to be smaller than the rifles calibre ? So 5mm could work on a .22, but no good on a .177 ?

Subject to it causing enough air acceleration to reach the required velocities / actually choking the airflow.

Thus if WD were to install a 1mm thick washer at the end of his comp tube, with say a 4mm port, that should provide adequate choking, whilst keeping the very low lost volume ?

You reference a minimum length to provide choking - is this just a case of being long enough to get some kind of "organised", or close to linear, airflow through the port ? Any feeling for what this length is - we know empirically that 4mm seems to work well (Tony's D52), but would say 1mm or 2mm be enough to get the required flow characteristics ?

Sorry for all the questions - but this (flow/choke) is an area we've not gotten into yet !

Rgds - JB

[bigtoe01]

Hi Tony, Assuming you have a milling cutter or flat bottomed drill of exactly the right o/d you could hold the milling cutter in a collet chuck fitted in the tail stock and save having to re-mount the plug on the mill.
Do it all in one operation then, especially if you fit a dro to the tail stock like the project in Model Engineers' Workshop May edition where they use a £10 digi vernier.
Alternatively there are some small diameter indexable boring bars, 5mm, available now that would make facing the bottom of the pocket easy.

Cheers

John

John I actually thought of that right after i posted my last post...my mind is else where however dealing with a huge issue over my car so i did not follow up.

[bigtoe01]

The 300s method is easy enough to replicate, tbh I'm thinking along the lines of a slightly longer port now due to the volume.

I have a HW100 somewhere, I'm struggling to visualise the sealing method, it can't be difficult to do, after all it's only machining.

I will draw my idea and post it for you

here you go... https://dl.dropboxusercontent.com/u/920660/wonky1.png

[Black Beard]

Some more stuff/ to think about:

I think choking the flow is a bad thing unless you are trying to limit performance/increase consistency on an air gun with too much capacity (eg 12ftlb HW80). By choking the flow you get less air going in to the barrel, so the pressure in the cylinder goes up and the piston starts moving back sooner and harder. This reduces the length of time the pulse of air acts on the pellet. Recoil surge will be sharper because of the increased pressure in the cylinder and also a big higher overall because the piston will be pushed back with more energy.

If you want to get a flow to choke then you need to get a certain pressure difference between the cylinder and barrel. This happens due to friction in the system and you can model it as a series of rough pipes. The critical dimensions are the entrance/exit to the transfer port as the friction in the small length of the port isn’t that big (unless you thread it or something equally dumb). Once the flow chokes then the only important dimension is the minimum diameter of the transfer port.

In a standard 80 the flow will choke for a short time with normal weight pellets. Once the flow has choked the limiting of air movement in the transfer port means the pressure in front of the moving piston goes up a lot higher than if it had not choked. The lower air flow in to the barrel means that the pressure in the barrel will be lower than for an unchoked flow (the pellet is also moving down the barrel to drop the pressure) and the pellet velocity will not increase as much as for a unchoked flow.
At some point the piston will run out of go and move backwards, the pressure in the cylinder and barrel will become more equal and the flow will unchoke. Once this happens the flow will acts like a series of rough pipes again.

By choking the flow Mr HW has limited the amount of air that can get in to the barrel. Really light pellets will accelerate down the barrel quickly but the amount of air in the barrel won’t go up like it would for an unchoked flow. So the pressure drops a lot as they get towards the front of the barrel. Energy of pellet = pressure * area* distance travelled so the pellet’s velocity is restricted by the dropped pressure. Your HW 80 stays under the limit with light pellets (but it still recoils like a git).

Choking might also increase the consistency of the air gun. It would take someone with an adjustable transfer port and a large amount of carefully weighed pellets (some with added weight) to determine if bigger transfer ports make less consistent air guns. Go on Jim- you know you want to.

BB
PS- that made sense when I wrote it.

[bigtoe01]

here is what I know, for this instance we will talk hw77

If you reduce stroke there comes a point where there is not enough air to push the pellet at the velocity you require. That said to a point you actually will increase the efficiency of the gun as you start to tune towards peak pressure and efficiency.
Stock 77's have to much air, they are closer to optimum but still not at optimum. Also the TP is not optimum dia, this depends on stroke and piston weight.

So, let us imagine we test stroke from 65mm to 80mm on a 25mm piston gun, we also adjust piston weight for each given stroke tested and adjust TP dia from 2.8 to 4mm in .1mm steps.. a lot of work!

I can tell you now from my limited testing at 80mm the TP will probably land somewhere between 3.8 and 4mm. However at 65mm stroke it will probably land around 2.8mm. One would deduce from this at 73mm stroke the TP would need to be around 3.3mm maybe 3.4.....which is close on what works

However piston weight now comes into play, with light weight and heavier pistons needing different size TP to tune the bounce point.

If you leave weight at around 220g for the piston, you will need a 3.9mm port at 81mm stroke, at 73mm stroke you will need a 3.4mm port and at 65mm you will need a 2.8mm port, even at 65mm stroke it is possible to see 11fpe in .177 however is will be bordering on tipping to not enough air and it will feel slammy as it will be heavily sprung and way to fast. Plus any slight imperfection in the seal or any leak will dramatically reduce efficiency, this is why a Diana 52 .177 sleeved down to 25mm can run 68mm stroke and make 13fpe+ easy as the comp tube is machined from a billet (and my sleeve is perfectly glued in ) and yet feels just to fast. The TP is 4mm which is way to big which as me thinking i may put a port restrictor back in and choke it down a little and see if things sweeten up maybe....or i just lengthen the stroke and set power with the 4mm dia port which will feel nicer anyway.

Regards de laval nozzles, the science points to this helping, plus you could add the Bernoulli effect and possibly add some increased efficiency into the mix, ideal candidate for these tests would be a hw35 or lgv as they have long TP which could be reshaped etc... probably best bet would be an old 35 with the breech plug removed so it could be machined easy and modified to take an O ring and be held by bolts so we know it was sealing 100%

All food for thought.

[Prof51mtw]

I think choking the flow is a bad thing unless you are trying to limit performance/increase consistency on an air gun with too much capacity (eg 12ftlb HW80). By choking the flow you get less air going in to the barrel, so the pressure in the cylinder goes up and the piston starts moving back sooner and harder. This reduces the length of time the pulse of air acts on the pellet. Recoil surge will be sharper because of the increased pressure in the cylinder and also a big higher overall because the piston will be pushed back with more energy.

Hi BB,

I think there is a misunderstanding here and I am not surprised because I think the physics is a bit counter-intuitive. The reality is that an airgun will not work with any semblance of efficiency unless choking occurs. In fact, any airgun with a respectable muzzle energy relies on air-flow choking as a key part of its functioning. The term 'choking' refers to the limiting of flow velocity to Mach 1, not to the limiting of mass flow. When choking takes place, the air is unable to increase in velocity, but is subject to further compression and, hence, an increase in density, coupled with a drop in temperature. The increase in density causes an uplift in the mass flow rate, such that more, not less, air flows into the barrel. So 'choked flow' implies a maximum amount of air transferred into the barrel bore. The temperature drop in the flowing air is associated with the conversion of stored thermal energy and in maintaining the airflow at Mach1. If the transfer port was opened up, so that choking did not occur, then the pellets muzzle energy would collapse, as the energy from the compression chamber was discharged from the barrel in the form of hot air.

Mike

[Prof51mtw]

Hi Prof Mike,

So for the choke to function as such, presumably the port only needs to be smaller than the rifles calibre ? So 5mm could work on a .22, but no good on a .177 ?

Subject to it causing enough air acceleration to reach the required velocities / actually choking the airflow.

Thus if WD were to install a 1mm thick washer at the end of his comp tube, with say a 4mm port, that should provide adequate choking, whilst keeping the very low lost volume ?

You reference a minimum length to provide choking - is this just a case of being long enough to get some kind of "organised", or close to linear, airflow through the port ? Any feeling for what this length is - we know empirically that 4mm seems to work well (Tony's D52), but would say 1mm or 2mm be enough to get the required flow characteristics ?

Sorry for all the questions - but this (flow/choke) is an area we've not gotten into yet !

Rgds - JB

Hi Jon,

I think the critical transfer port diameter is the inlet diameter, on the compression side of the port. This sets the critical pressure pulse length in the barrel. Jim has test results which support this idea and we are both of the view that the port/barrel interface is less critical and so the TP diameter should not need to be calibre specific. So what works for a given stroke length should be OK for both 0.177 and 0.22.......unless, of course, someone knows different!

I have asked an ex-colleague who has more expertise in air flow engineering than I do, about the minimum port length. He confirmed that a thin orifice will not support choked flow, but that a smooth tube with a length of at least three times the bore diameter (L/D >3/1) will. He has no more detailed information about shorter tubes, but expressed the opinion that a ratio of 1.5 might work for the flow velocities we are concerned with. As a favour, I am prevailing upon him to do some tests to find out. If this gets done, I'll let everyone know.

Mike

[Shed tuner]

Thanks Mike... so 1.5 x 4mm = 6mm, or 1.5 x 3.6mm = 5.4
We know that HWs with 6mm x 3.8mm ports work well, also my TX with 5.3 x 3.8

Both in the right kind of range...

Tony's D52 port is shorter though, I think 4x4mm ? Tony ?

Also from your response to BB, you're saying that choking the port causes the air density to increase, so the actual air mass flow increases - due the the density increase resulting from cooling more than compensating for the flow volume reduction caused by the choke ?

[Shed tuner]

h
I can tell you now from my limited testing at 80mm the TP will probably land somewhere between 3.8 and 4mm. However at 65mm stroke it will probably land around 2.8mm. One would deduce from this at 73mm stroke the TP would need to be around 3.3mm maybe 3.4.....which is close on what works

However piston weight now comes into play, with light weight and heavier pistons needing different size TP to tune the bounce point.

If you leave weight at around 220g for the piston, you will need a 3.9mm port at 81mm stroke, at 73mm stroke you will need a 3.4mm port and at 65mm you will need a 2.8mm port, even at 65mm stroke it is possible to see 11fpe in .177 however is will be bordering on tipping to not enough air and it will feel slammy as it will be heavily sprung and way to fast.

Yeah, all seems pretty well on the money to me Not tried anything less than 74mm though, so can't comment on 65... will be trying 70 shortly though

[Prof51mtw]

Thanks Mike... so 1.5 x 4mm = 6mm, or 1.5 x 3.6mm = 5.4
We know that HWs with 6mm x 3.8mm ports work well, also my TX with 5.3 x 3.8

Both in the right kind of range...

Tony's D52 port is shorter though, I think 4x4mm ? Tony ?

Also from your response to BB, you're saying that choking the port causes the air density to increase, so the actual air mass flow increases - due the the density increase resulting from cooling more than compensating for the flow volume reduction caused by the choke ?

Yes....that's about it. More info here http://www.grc.nasa.gov/WWW/k-12/airplane/mflchk.html

Mike

[bigtoe01]

Thanks Mike... so 1.5 x 4mm = 6mm, or 1.5 x 3.6mm = 5.4
We know that HWs with 6mm x 3.8mm ports work well, also my TX with 5.3 x 3.8

Both in the right kind of range...

Tony's D52 port is shorter though, I think 4x4mm ? Tony ?

Also from your response to BB, you're saying that choking the port causes the air density to increase, so the actual air mass flow increases - due the the density increase resulting from cooling more than compensating for the flow volume reduction caused by the choke ?

yeah John the D52 is around 4mm long and 4mm is dia, however the breech seal forms part of the port, the actual port (steel part is way thinner)

a D52 is a VERY efficient rifle, it will make power with spring rates other rifles will just not touch, however going below 75mm with a stock 4mm port either at 28mm bore or 25mm bore is not ideal....I think I may choke the .177 down to 3mm with a new seal and see how it does, stupid part is I think of this stuff all day and never considered the port on the old Diana was to large

[Shed tuner]

yeah John the D52 is around 4mm long and 4mm is dia, however the breech seal forms part of the port, the actual port (steel part is way thinner)

a D52 is a VERY efficient rifle, it will make power with spring rates other rifles will just not touch, however going below 75mm with a stock 4mm port either at 28mm bore or 25mm bore is not ideal....I think I may choke the .177 down to 3mm with a new seal and see how it does, stupid part is I think of this stuff all day and never considered the port on the old Diana was to large

Then again, I did this (5.3 long port sleaved from 3.8 to 3.2mm) with my TX (similar stroke, 76mm) and it didn't work out - I lost about 40fps across the board.. BBs model said it should help as it wasn't choking, and I could go down below 3.0.. apparently not though.

[bigtoe01]

Then again, I did this (5.3 long port sleaved from 3.8 to 3.2mm) with my TX (similar stroke, 76mm) and it didn't work out - I lost about 40fps across the board.. BBs model said it should help as it wasn't choking, and I could go down below 3.0.. apparently not though.

well I will just machine up some breech seals as they will be the part that does the port reduction size and see what works and hat does not...im thinking 3mm to start and push up from there if the velocity lowers to much...i do know 4mm is to large already

[Rich]

Are you able to make your TPs with a tapered bore, so the air enters at the small end (which should be radiused) and then expands as it leaves the TP?

Mike's point that the expansion brings about the cooling and the increased weight of air behind the pellet is for me the clue.

Also, the whole system needs some resistance to flow to work against, otherwise if it is all down to pellet tightness in the breech, the shot to shot consistency must suffer. A TP that absorbs some work is a constant that buffers those variations in fit.

In the world of PCPs, which may be a sin to mention (!), one of the most consistent rifles ever made was the Pro-Target, and if you look at the air path it is positively tortuous and not conducive to efficiency. It means however that the pellet fit in the breech is not all that relevant. At the other extreme we have the Stealth with its straight through concept, and it's no secret they can be devils to set up and keep stable.