Central transfer port vs offset transfer port, is central really the best?

[look no hands]

Of course, Pete.

And a few others.

I see, fallen out of love with the TX already then

Pete

[TonyL]

I see, fallen out of love with the TX already then

Pete

Nope.

Well, I need to spend more time with it to be fair, Pete. And I've had so little in the way of opportunities recently.

I do LIKE it. I do RESPECT it.

Not quite sure if I LOVE it as yet, but either way, it'll be a keeper.

Hoping to spend some quality time with it at The Bash in a couple of weeks and we'll see how the bond develops.

[look no hands]

Nope.

Well, I need to spend more time with it to be fair, Pete. And I've had so little in the way of opportunities recently.

I do LIKE it. I do RESPECT it.

Not quite sure if I LOVE it as yet, but either way, it'll be a keeper.

Hoping to spend some quality time with it at The Bash in a couple of weeks and we'll see how the bond develops.

Good idea, leave the HW's at home and let the TX show you what you've been missing

Pete

[RobF]

YEP

The ship would sink faster with the hole in the middle as the ingress of water would spread evenly and have less distance to travel to the extremities of the hull.

Its all about the time it takes for the distance to be covered

Depends if it's pressure that fires the pellet or flow of air.

If it's pressure there's no difference in having the TP offset, because pressure will be equal everywhere in the comp tube. If it's flow then it makes sense to have the TP central to allow as much air to get from inside the comp chamber down the TP.

I'm kind of thinking it's more to do with pressure than flow, just because the timescales are so short.

[TonyL]

Must agree with the above, Rob.
Nothing scientific or anything, but I've often thought about this and it must make sense? Lots of pressure in our little boingy toys towards the end of the stroke.

[Shed tuner]

Depends if it's pressure that fires the pellet or flow of air.

If it's pressure there's no difference in having the TP offset, because pressure will be equal everywhere in the comp tube. If it's flow then it makes sense to have the TP central to allow as much air to get from inside the comp chamber down the TP.

I'm kind of thinking it's more to do with pressure than flow, just because the timescales are so short.

initially it's pressure, but pressure with no flow won't get you far...

[Black Beard]

All the models of springer airguns suggest that normally choking does not occur. This is partly due to the very high gas temperatures which will increase the speed of sound and thus delay choking of the flow. Even going down to very small port sizes does not produce choking since it increases the maximum cylinder pressure which increases the gas temperature which of course increases the choking velocity. But it does reduce pellet energy.

Now the models may not be correct, but they would have to be way out for choking to occur which seems unlikely, as many are able to predict the effects of changes once they have been calibrated against a real rifle.

My model says the flow can choke with small transfer port sizes. It might only do it for less than a millisecond but it is possible. There is also lots of choked flow after the pellet leaves the barrel.

[Barryg]

A question for the experts

1 How much more FP would a extra 15 mm stroke give with the same piston and spring with the same size TP being central.
2 the same set up but with a offset TP.
The reason I ask is because the 52 and 460 have very similar dimensions apart from the stroke a the offset TP.
Diana give the same FPS for both but because of the extra 15 mm the 460 ususuly nuges ahead by around 1-2 FP, I am only guessing but I would expect at least a extra 5 FP from a 115 mm 52.
Is there any way to work it out?

[Rickenbacker]

Nope, not seeing the action laying lower Barry, just the barrel on the 52 sitting lower.

All the guns shown have the action equally inlet into the stock by half the cylinder diameter (as is the norm on any springer), so all actions sitting at the same height above the forend woodwork, as I've said before :-

http://www.airgunbbs.com/showthread....-97-if-not-Why



All the best Mick

Here's a thing Mick, if you're still looking in on this thread:

I can see how Barry has assumed the underlever actions 'sit lower in the stock' than the big 56TH in the picture in his opening post - because it seems that they do!

Looking at where the trigger block pins are in relation to the line of the stock, the 54 and 56 actions do sit a few mm higher than half-way.

Obviously it has bugger all to do with where the TP is... and likely has everything to do with fitting the recoil-less sledge and gubbins inside the stock of the 54 & 56.

Confirmed by the 52 action following the usual half-cylinder arrangement.

Here it is in pictures:

56

54

52

See what I mean?

Phil

[RobF]

initially it's pressure, but pressure with no flow won't get you far...

Definitely a head scratcher. It's a question of how much flow actually makes a difference. With PCP's we've done some interesting stuff with the chamber volume and some guns can get the power with quite low volumes compared with others.

I fully realise that pcp's and springers may not work the same, but I'm wondering if flow is more or less important depending on the type of tune.

[Black Beard]

Flow is always important. The initial volume of air begin the pellet is only enough to fill the skirt. Without flow the pellet will move a bit down the barrel, the pressure will drop and the pellet will stop accelerating.

Almost all the air in the cylinder will flow in to the barrel. Once the initial pulse of pressure is over (and the cylinder is going backwards), the pressures in the barrel and cylinder are roughly in equilibrium with not much gas flow. But in that initial part, when the pellet is moving for the first couple of inches, there is lots of flow across the transfer port. This is when the size, shape and position of the transfer port make a difference.

[T 20]

Here's a thing Mick, if you're still looking in on this thread:

I can see how Barry has assumed the underlever actions 'sit lower in the stock' than the big 56TH in the picture in his opening post - because it seems that they do!

Looking at where the trigger block pins are in relation to the line of the stock, the 54 and 56 actions do sit a few mm higher than half-way.

Obviously it has bugger all to do with where the TP is... and likely has everything to do with fitting the recoil-less sledge and gubbins inside the stock of the 54 & 56.

Confirmed by the 52 action following the usual half-cylinder arrangement.

Here it is in pictures:

56

54

52

See what I mean?

Phil

Hi Phil

Weirdly the thing that dictates the 54 and 56 sitting slightly higher in the stock than the 52's half a cylinder OD is the safety catch.

On the 54 and 56 there is an additional plastic plate between the underside of the safety and the woodwork to cover the void that the action recoils into.
The thickness of this plastic plate dictates the difference in action height between the 52 and 54 --- anyway, you'll be able to see for yourself next weekend.

Transfer ports as you say have bugger all to do with it, but the barrel centreline to scope centreline make a difference to the secondary zero on your aim points --- discuss






All the best Mick

[ballisticboy]

My model says the flow can choke with small transfer port sizes. It might only do it for less than a millisecond but it is possible. There is also lots of choked flow after the pellet leaves the barrel.

All most all of the several models I have seen or tried over the last 30 years have failed to produce choking at the port for sensible port sizes. This does not mean they are right just consistent. The highest speed I have seen is around 550m/s which represents about Mach 0.9 in the conditions in the port. Even when the variation in the values of Cv and Cp with temperature and pressure are taken into account there was still no indication of choking conditions. I believe a model produced by a student as a university project suggested choking many years ago but that was a relatively simple model based on Gerald Cardew's data.

What the air is doing in the port after the pellet has left the barrel is not going the affect the pellet flight or gun efficiency.

Flow is always important. The initial volume of air begin the pellet is only enough to fill the skirt. Without flow the pellet will move a bit down the barrel, the pressure will drop and the pellet will stop accelerating.

Almost all the air in the cylinder will flow in to the barrel. Once the initial pulse of pressure is over (and the cylinder is going backwards), the pressures in the barrel and cylinder are roughly in equilibrium with not much gas flow. But in that initial part, when the pellet is moving for the first couple of inches, there is lots of flow across the transfer port. This is when the size, shape and position of the transfer port make a difference.

It is true, flow has to take place into the barrel in order to produce/maintain the pressure needed to propel the pellet. However, when the piston reverses in the cylinder after reaching maximum travel (which does not equate to the maximum pressure point) the pressure drop in the cylinder is more rapid than that in the barrel leading to reverse flow in the port. As you say though the port velocities are low in this situation.

I do not personally believe the position of the port will make much discernible difference to the efficiency of the gun system. Port shape and size will be the main deciders. Also the most efficient gun system will not necessarily give the smallest dispersion at the target or be the nicest to fire.

[RobF]

Also the most efficient gun system will not necessarily give the smallest dispersion at the target or be the nicest to fire.

That's also true of pellets, ones which are the most efficient in terms of realising the power of the rifle, aren't always the ones which are the best at grouping.

It depends on what the point of the exercise is, but if you have a gun that can only do X power with Y pellet which is no good beyond Z range, or cannot be consistent over a decent amount of shots or common temperature ranges, then it's really just forum fodder.

[Turnup]

Jumping in on this interesting thread.......has anyone tried using a piston with a tapered nose, and corresponding tapered comp tube. I am speculating that as the piston reaches the end of its travel the cone of air might offer a bit more buffering. It might even be advantageous to put a "pip" on the nose which enters the TP to increase TP choking in the last few mm of piston travel. Has it been done?