DIY Air Cannon for tree (and roof) work

[e9999]

so, my newer activity/venture/hobby is tree climbing. Been learning on my own for a while now about climbing and working in trees. Quite a bit more complicated to do safely that one may think at first. Very rewarding. Especially the rope and knot work. Fascinating. How often does your life literally hang (!) on one single knot every weekend if you're not a mountain climber? And saves tons of money (wanna make tons of money - become an independent arborist! But know this is one of the most dangerous jobs around to do.)

Anyway, one of the things you need to do is to throw a lead weight and a line in trees to allow you to climb more easily and more safely. Unless you are way more gifted than I am, throwing by hand a line high up while doing so accurately, is really a (tedious) crapshoot. (Basically, you want to throw a small line over a suitable limb just at the right spot so that you can then pull up your main climbing line using the first little one.) So pro arborists either practice a lot throwing them by hand - quite difficult for large trees IME- or use commercial devices like huge slingshots or compressed air devices for more control. Call me intrigued...

[Also, I need to do some roof work and unlike when I was young and foolish, now I'd rather be safely secured while doing that. So I'm contemplating throwing a rope over the house to attach myself to while on the other side of the roof. (Been thinking about using a drone for that, but no progress yet on that front.) ]

So, I need to throw a lead shot bag quite accurately at significant heights (think 100+ feet in some cases).

And what self-respecting DIY guy hasn't dreamed about making his own air cannon, right?

So, enter the AiR15 prototype:

I'll save you some time if you want to make your own: The most important point to know here is that a 16oz lead shot bag from Weaver fits quite tightly into the bore of a 1 1/2" sch40 PVC pipe, and that tight fit is critical. I can then go over most of the large trees in my yard with only 60 psi or so of pressure in the chamber. And I got plenty more than that.

Some other considerations: I started out without the air chamber, with just the valve connected to the tank. Used a biggish and short hose but still felt that was not optimal and I was losing too much pressure in the hose under flow. IOW, shorter range. So added the chamber and made it relatively large wrt to the barrel. Still using the air line to supplement it while firing but I think I get much more range now for the same tank pressure with that air chamber in. The funky heavy lever on the valve handle is to ensure fast valve opening. Ugly but works. Suggestion: make the chamber strong enough cuz it may be next to your face when shouldering the weap... errrr.... device.

Quite rewarding to see that weight bag fly way up there... Now to practice aiming and judging distances a bit. But at least now I can sort of really aim instead of praying and cursing...

Intriguing possibilities for family vacations/reunions too...

 

[Bob Morazes]

Love it!
Can you comment on the picture to define what you did?
as a thrower of lead bags, I bow with admiration to your ingenuity.
Bobmo

 

[e9999]

thank you, but I can't claim that I invented the concept, they do sell conceptually similar devices at arborists stores, although they are over $200 I think. And what kid hasn't dreamed of making a potato cannon, same thing. Alternatively, the large slingshots are around $120 or so.
There really is not much to it, just pick the correct barrel size for your bag or vv. That is the one critical thing. I tried to use wads also but that was not too successful in prelim trials. I made this from bits from the scrap pile. With the proper fitting you can also use a bicycle pump but that may take a while to charge. The tank is a freebie old RO tank, but any propane tank would work too I think, they are strong enough if you stay under the 100 psi range or so, I believe (try at your own risk - I did some tests with mine while out of harm's way). Quick opening of the valve is also needed/desirable, that is a bit tricky. Some valves have a one-sided asymmetric handle, which is easier to operate than mine, but the problem with that is that you then jerk the device off aim if you go fast and it's not buttery smooth. It also helps to have a butt to rest on your shoulder or on the ground. When I had just the valve without the chamber earlier, that thing was wildly swinging around when I was firing it, not great for proper aim.
I should also add that I have no idea if that barrel length is optimal. I suspect it may be better if it were shorter TBH.

 

[ntsqd]

I built an air cannon for attempting to get potatoes up to the speed of sound. It works well, but they're nowhere near that fast. Which is just as well, it is a scary implement. I too used 1.5" PVC for the barrel. My reservoir is a coaxial piece of 3" PVC about 30" long, with a diaphragm valve at the breach and a lawn sprinkler valve for the dump valve. Originally the barrel was 72" long, but it fell over one very, very cold morning and is now about 36" long. Performance does not seem to have suffered, the spuds still completely disintegrate at high speed contact with a wall. I wrapped the reservoir tube with several wraps of kelvar cloth and used several spiral layers of clear packing tape to hold it in place. I suspect that the valve arrangement that I copied off the interwebs from somewhere would be a vast improvement for lock speed and shot placement.

Propane goes liquid at around 300 psi depending on local conditions so taking one of those tanks up to normal air compressor pressure shouldn't be a problem. Unless the tank is out of hydro, old and/or rusty. Then it's unsafe for anything.

 

[e9999]

One big advantage of the potato projectile is that they will conform precisely to the bore, much easier to achieve a decent seal. Seen some vids of some potato cannons based on fuel and ignition devices, those are scary.
Yes, have been thinking about some sort of electrical valve to help with the fast opening. Will look some more at the electronics scrap pile. Vaguely remember some 24V valves someplace and may have some old sprinklers too. That would be fun. Although not necessary for my needs I think, it goes plenty high as is, but maybe would help with less aiming disruption. Gotta carry a battery then too. So not that high on the priority list, but interesting.

I thought you'd have to go well above 100F to get 300 psi of propane vapor pressure, but could be wrong on that. Anyway, the propane tanks surely (hopefully?) have a safety factor too, so yes, I think that they are normally safe at typical compressor pressures if in good repair. But it's easy to check them hydraulically first, just be sure to dry them well. I will say that I have seen little 1lb propane containers (like for camping stoves) that were noticeably bulged after refills, so those seem sketchier (and are too small for this type of device anyway).

 

[ntsqd]

According to: How much pressure is in a propane tank? - https://www.ferrellgas.com/tank-talk/blog-articles/how-much-pressure-is-in-a-propane-tank/ 200 psi, which is not at all what I remember from working R&D on the World's Most Expensive Patio Heater.

Hang tight, I'll build a SW model to get a jpg of the valve arrangement. It is a very simple thing to build though it does require a little bit of tuning when first assembling it.

 

[ntsqd]

I'm blanking on who it was that came up with this basic design theory. May have been deCartes (sp?), but common pressure regulators work on the same principle. The basic pneumatic spud gun design I found on the net, it is not my creation. The fine tuning points are mine.

Spud-Be-Gone® view A: (mcmaster's sprinkler valve model lacks a LOT of internal details!)

Not shown in that model, but in the real assembly are four short sections of FC332 hose near the breach end that center the breach end of the barrel in the reservoir.

Detail view:

There are some key features shown here. The first is the black part. It is a disc of rubber and part of the tuning is getting it so that the pipe thread reducer screwed in the rear of the reservoir causes it to bow or cup over the breach. In the static, uncharged condition it needs to make contact in both locations firmly enough to seal, but no more than that. Another is the aluminum disc bolted to the rubber. It needs to be enough bigger than the bore of the barrel that it doesn't allow the rubber disc to end up in the barrel. You can guess how I figured out this feature. Yet another feature is that the fill valve is between the dump (sprinkler) valve and the rubber disc.

In charging it up air forces the rubber to cup enough more that it can get to the other side of the disc while still sealing the breach of the barrel resulting in equal pressure on both sides of the rubber disc.

When the dump valve is activated all of the pressure on the backside of the rubber disc goes away. That allows the pressure in the reservoir to blow the rubber disc back and escape down the barrel. The "Lock Time" of this system is impressively fast. I tried reducing the air volume on the backside of the rubber disc with various stuffers to see if I could make it even faster and while I'm sure that I had a positive effect that with sophisticated enough sensors could be measured, but I don't recall a perceptible improvement.

Note that the sprinkler valves work in an extremely similar way. I added a push-button pressure relief valve in a strategic location on the sprinkler valve body and that is my "trigger". I've since removed the 24 VAC valve coil and plugged the hole.

 

[e9999]

Wow, thanks for taking the time. You are pushing the envelope there. Just about went out and kicked my prehistoric launcher for being so ugly... I am not trying to break Mach 1 myself, though, so that may have to wait until I get a lathe and a lot more free time. The concentric tube idea is very clever to save space, that's kinda like a bullpup air cannon. Nice!

I will have to console myself by thinking that my gizmo is like WW2 russian engineering: It's crude but works (sort of).

 

[ntsqd]

That launcher above could send a spud just about out of sight on 100 psi. One year my then ~12 y.o. nephew wanted to see it work so my mother, grandfather, and I took him to a section of the beach where the road is right next to the sand and we could launch out into the ocean. Based on the splashes I'd estimate max range of about 250 yards if a rainbow trajectory is used. Nephew thought it was cool. So did grand dad, but he couldn't understand why I'd build such a thing.

I built a pneumatic launcher for t-shirts that was more like what you built. Plan was to use it at 4x4 event raffles to launch t-shirts out into the crowd. I put a lawn sprinkler valve between the reservoir and the launch tube. In the lessons-learned file for that one I think that it would benefit from having as big of a reservoir volume as reasonably possible. You want enough volume that it maintains the bore pressure as high as possible until the projectile exits the barrel. Use the largest pipe size sprinkler valve that you can find. Couple both the reservoir and the barrel close to the valve, reduce the extra volume as much as possible. It's just volume that you have to pressurize that isn't doing anything for you.

Mine didn't survive the trip home from the first event. I glued a pipe stub into the launch tube that the valve threaded onto. The stub broke leaving me with a stuck piece of threaded pipe in the valve and a useless launch tube. Were I to build another one only the launch tube would be PVC. The valve would have a steel pipe nipple that it screwed onto.

 

[e9999]

so you have some sort of battery or inverter/transformer you carry along with the thing then to activate the valve?

the barrel on mine is just inserted in the female receiver. Turns out the dry friction fit is very good, it's actually quite hard to remove it. I don't think it leaks at all.

The last trial I did I put 60 psi in the tank and therefore a bit less in the air chamber, so by the time the lead shot bag is exiting the pressure must be something like 30+ psi in the barrel and it's accelerated by maybe 50 lbf or so, guessing, that's quite a bit and indeed it shoots up something like 75' maybe. One issue is that I can only insert the bag in the barrel with the line trailing, and with the big end first, so it must be doing a 180 in flight, which is probably not great aerodynamically.

 

[ntsqd]

All that the electric coil does is open a vent that creates ambient pressure on one side of the diaphragm, which allows the pressure on the other side to move it far enough to unseal and open the valve. I substitute a push-button pressure release valve for the electric pilot valve. No electricity needed.

Some (maybe all?) of the sprinkler valves have a manual operation mode. Been a while ago, but what I recall is that you could fiddle with the coil somehow and that would vent the valve. It was an awkward and slow fiddle though, so in operation I replaced it with the vent valve. Don't recall where I got those vent valves and I've been stymied so far in finding one to link to.
Charging my search term got me this:

and finally a link!

1/8 NPT Push Button Bleeder Valve

1/8 NPT Bleeder Valve. Manually bleed pressure out of a system. Thumb button activated 200 psi max. For low pressure applications only. Warning....This is not a safety valve.

palmerspursuit.com

There are other valves made that operate the same way, but the sprinkler valves are usually the price leaders.

Would it be possible to put some of the line in the barrel first? Then load the shot bag.

 

[e9999]

Thanks for the link. Very clever, you built a pneumatic amplifier, small signal, big result. Very nice. Kinda like the trigger on an antique HD crossbow, always been impressed with those.

It does not look like it would be very easy/good to put the line in first, the bag fit is already too much, really, I have to massage it in and then push it to the breech with a rod. The line made that difficult/impossible when I tried cuz it's a bit thick. Maybe with a much thinner Dyneema line.

 

[ntsqd]

If I recall the name correctly that is how all Pilot Valves function, though not all of them use a diaphragm.

If all it is, is a haul line, then it doesn't need to be huge and it's strength likely only needs to be ~3X that of what the bag weighs. That should exceed the strength needed to haul the safety line up and over the limb by a significant margin. Maybe some generic 550 cord is small enough?

 

[e9999]

550 would be too thick, I think, but I have significantly thinner Dyneema lines that may work. Not a huge issue, though, the bag is flying plenty far enough.

 

[e9999]

Wow! I'm impressed. Crude as the gizmo is, it blew me away today. I had to put the bag up about 50' in a 1'x1' or so area through several crotches and some light foliage, almost straight up, in order to get the line where I needed it. Sheer luck maybe, but I got it the very first try. I could just aim carefully and fire smoothly. There is absolutely no way I could have done that with a throw. Just not possible. Goes to show ya, that's Tech that'll make even hapless newbies looks good!

[I will say, though, that there is a fundamental problem with the Physics here. I can get the bag to fly fairly straight for a bit with enough pressure, so reasonably easy to aim, but then, not so surprisingly perhaps, it keeps on going for quite a bit farther, with possibly unexpected consequences... Gotta figure a way to make it stop it while in flight, just after it crosses over the desirable limb. Oh well....]

 

[ntsqd]

Seems to me that if your haul line's length were temporarily limited to a little past the desired limb that it would be a bit like Wile E. Coyote hitting the end of his leash.

My first thot was that by playing with charge pressure that you could control the distance, but my immediate analysis of that says that it's a very fussy and inconsistent way to try to do that.

Perhaps a combination of the two ideas?

 

[e9999]

yes, thought about the fixed length but the difficulty there is to estimate the distance accurately enough. And that may take longer than doing a couple of tries. And yes, controlling the pressure for distance is a lot of trial and error. If I just go with high pressure to flatten the trajectory, its easier to aim. But then you gotta hope you don't end up with the bag stuck high up in a farther part of the tree, as almost happened yesterday. When the bag is good and stuck, it's a pain, now you got to climb up somehow to free it, kinda defeats the purpose altogether. I have actually made a lead ball that has a release mechanism of sorts to free it from the line under high tension, but can't adapt that to the bag easily. Maybe a thin fragile sacrificial bit of line but then again a lot of fussing around. Might as well adopt the brute force method and actually enjoy the powerful firing...

 

[KLF]

Back in '07 I was working (mostly for fun) at a Haunted House attraction, building props and effects, then doing maintenance to keep it running. Boss asked me to make an air cannon, just to make a loud noise as they were exiting, and to "poof" up at them from the floor. I ran it down at about 40 psi, with a remote tank right behind it. Sprinkler valve to a section of 3" PVC. Used a pressure mat on the floor to trigger it. Very effective.

Allegedly we discovered if we put the air up to about 100 psi, it would really launch a 16 oz plastic soda bottle 1/2 full of water a lot farther than we expected.

Allegedly.

 

[ntsqd]

Those alleged discoveries are sometimes the most fun to find.

Maybe?
Amazon product ASIN B08PVZ3834

 

[e9999]

I should try and see if mine makes any noise with nothing in the barrel. Probably will be a PFFFT, though. However, it does make a very nice Pop / Bloop when the bag leaves the barrel at speed. I think the rather tight fit of the bag does help mitigate a bit the less than very fast opening valve, probably takes a while to start moving.

Yes, I should get a laser rangefinder, always wanted one of those.