Infinitengines "Dreamair" pneumatic speargun

[tromic]

That would be also interesting to have some diagram showing the force that accelerate the spear from cocking position to release position with respect to CVT.

 

[popgun pete]

Yes, but for that the "equation" for the spiral form track would need to be known for both the inner and outer winding drum sets. I have a photo of them provided by the author of the "Dreamair". The force calculation is easy as it will be the product of the pressure and the piston cross-sectional area. Effect of friction and drag can only be estimated, but will affect the result as may gun temperature with such a large piston being used. Not a gun to allow to "cook under the Sun" on a hot day! Same applies to any pneumatic speargun, keep them in the shade or under a cover when laying in an exposed location.

NB. for component naming I have used my own judgement, hence they may not be entirely correct

 

[tromic]

Nice new image! Yes, calculation of the input force is easy, but the output force is different case.

 

[foxfish]

Good quality components!

 

[popgun pete]

Some information supplied by the "Infinitengines" Company concerning the current gun's physical dimensions.

The size of the "Dreamair" speargun is 110 cm (130 cm overall length). The dimensions of the oval barrel tube are 55 mm x 38 mm. (Note that a "standard" pneumatic gun, e.g. Mares "Sten", has a 40 mm OD tank tube with a 38 mm ID.) The weight of the gun without the spear is 1800 g and the gun can shoot spears from 6.5 mm to 8 mm in diameter with 150 cm length; spear shafts of a diameter larger than 8 mm can be used if a flotation element is added to the gun. The "Dreamair" speargun can be manufactured in various barrel lengths.

 

[tromic]

After some drawing (based on number of pixels from the image above) and calculation in excel I got some results:

 

[popgun pete]

If you multiply your Po by 2.5 then you arrive at the 10 bar that the "Dreamair" gun is intended to start at. Which in turn makes for some big numbers in the force level and thus the effort required to cock the gun!

 

[tromic]

Yes, that is why I took for the pressure to be only 4 bars... or I made a mistake in calculation...? Possible...

 

[popgun pete]

The numbers when multiplied by 2.5 appear to be not that far away from the chart shown for the gun.

or as shown on the web-site, note that start pressure is 11 atm here.

 

[tromic]

I checked the calculation and found error in "Y" column.
This could be more accurate, but something is still strange regarding operating pressure...?

Fs on the drawing up-right, points in oposite direction, to the right side, before cocking the gun.

 

[tromic]

Here is a diagram of the force (kgf) necessary to load the gun on 4 bar inicial pressure - blue.
1200 on x-axis means pulling the wishbone for 1200 mm. Red line is the force on the piston (kgf).

 

[popgun pete]

The inner drum appears to be of a simple conical form, but the outer drums may be more like a "beehive", getting rapidly larger in the mid-section of the drum width and then tapering off to the maximum diameter at the "base" of the drum which is the section of the drum nearest to the gun body. This "beehive" shape increases the leverage in the latter half of the wishbone pull. We will soon know once the testing is completed as such data will be an indicator of how useful the system is and will no doubt be used to convince potential buyer as to the merits of the design.

 

[tromic]

According to my data it might work well even on less than 4 bar. It could be easily pressurised by bicycle pump, like a tire.

 

[foxfish]

Such a clever design, I cant wait to find out if really works well.....

 

[tromic]

I did some aditional calculation regarding the shooting performance for two cases.
I took the average force on the spear that is from above table 32 kgf. If we lower this force for 10% (estimation) because of friction we have the force of 29 kgf. I took for spears to be 140 cm long.

Here are results:
1. Spear OD/weight = 7.5 mm/0.483 kg, Inicial speed = 37.6 m/s, Energy = 348 J, Momentum (m*v) = 18.18 kg m/s
2. Spear OD/weight = 8.0 mm/0.550 kg, Inicial speed = 35 m/s, Energy = 341 J, Momentum (m*v) = 19.37 kg m/s

As could be seen 8.0 mm spear has lower speed but higher momentum so would have higher range and/or impact.

 

[popgun pete]

I have been told by inventor Andreas Zournatzis that the shock absorber unit for the piston is currently being sorted out and that this may be the reason for any delay. The sliding oval shaped piston is large, hence it has significant momentum when it flies back to the rear end of the gun, although at the same time it is hauling on the inner cables which in turn spin the axles and outer drums which wind in the wishbone cables that are pulling the spear shaft forwards. I had thought the pointy nose which we can see projecting forwards from the rear bulkhead was part of a damper, but in these photos, which Andreas has supplied, we see a moving column unit with an oil damping system.

I would think that the more "work" the gun has to do, such as driving a 9 mm shaft which can potentially deliver a knock-out blow (to the fish!), then the piston would be more restrained than if the gun was shooting much lighter 6.5 mm shafts. The old "Nemrod" pneumatic guns were equipped with 9 mm shafts, as a standard fitment, to knock over the denizens of the reef who came out to confront the intruders in their domain and were greeted instead with a barrage of heavy iron flung in their direction.

 

[tromic]

If the piston were 35 g, made in Delrin (38 mm OD x 20 mm length, density 1.5) at half of the speed of the spear it might have the similar impact as the piston of 10 g has at 30 m/s - about 4 J? I do not know what would be the real weight of the piston...

 

[tromic]

I would try something like this on the image as shock absorber. I believe it would work.

More accurate, 5 J is acctually after 7 mm of breaking (not 5 mm)

 

[popgun pete]

I think the reasoning that the shock absorber is done in the way shown in the photos is that it offers absolutely no resistance until the piston hits the black “anvil”, whereas a gas compression absorber as you have shown would oppose the piston from the very beginning of the shot. I had thought of a hydraulic shock absorber, such as is used in wet barrel pneumatic muzzles to slow down pistons, but the problem is gravity would keep the oil at the bottom of the oval barrel tank until only the last part of the piston movement where the oil would then be forced up into the central axis area, but by then it would be too late to offer any progressive deceleration.

The sliding metal tube or column shock absorber that he has chosen has its own captive oil supply, hence the attitude of the gun in the water will not affect where that oil goes as it would if the oil was sloshing around freely inside the rear tank space.

I suspect that this is a recent addition to the gun as there is no sign of it in the earlier concept drawings such as this one.

 

[tromic]

Yes, the force on the piston would be somewhat less with pneumatic shock absorber. But the influence of that would be practically negligible. After the firs turn of spiral track cone (6.6 cm wishbone move) the force would be about 0.5 kgf less. After next turns the influence becomes really negligible. Ive tried to show it on diagram but the difference can not be seen. Advantage would be less complex design but maybe would not be reliably over time, if there would be some tiny air leakage from the pneumatic shock absorber air volume.

Additional space for piston movement would be another benefit.