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Tested some of my ideas today:
- Longer tube from mixer to initiator chamber to promote mixing, that doesn't do any good.
- Spiral to promote mixing, no good either. Maybe the wire gauge is too much for the tube ID. Tried the spiral on both tube lengths.
This might be interesting. Seems pretty much related.
That IS very interesting. It could be fun to build that model. From what I can understand, it is a 2.1 liter chamber (could be 14cm ID pipe, 14cm long = 2154cm³), filled with stoichiometric mixture of gas/air, with a ID 9cm pipe length of 60cm and a ID 3.5cm hole from chamber to pipe. There seems to be a special shape around the hole to the pipe, and they don't mention where in the chamber the ignition takes place.
I have no idea whether it would be possible to downscale that model and by how much. The chamber is 2098cm³ compared to my chamber of 50cm³.
Since last post, I have purchased a slip roll for sheet metal rolling, so now I can actually make the Gasdynamic-Acoustic Model of a Bird Scare Gun if I want to.
Today I went testing. Very interesting day. I borrowed a 12V-230V inverter so I could use an air madras pump to blow into the mixer using my cars battery as power supply as I don't have access to utility power at the test site. I had 3D printed an adapter to sit between the pump hose and the mixer. The pump actually supplied so much air that I had to partially cover around 75% of the pumps air intake to limit the airflow into the mixer, just to get the sound cannon to fire. As I suspected, when forcing air into the mixer, I can open up the propane valve completely, and still get a good loud boom. It was not easy to cover the correct amount of the pumps air intake with only my fingers but I managed to get some good repetitive booms from the cannon, and I upped the ignition frequency to 6.3 shots per second and still got some good loud shots. Unfortunately my receiver battery ran out of power while testing limiting the extend of the test.
During testing, the cannon sound suddenly changed, the gun barrel piece had melted and broke. That has never happened before.
The air pump was actually a big help during testing, as I could let it run when not shooting to cool down the cannon parts, the air that then came out of the barrel end was surprisingly hot.
After driving home, and unloading the stuff from the trunk, I noticed a bad smell and blue smoke coming from the trunk my ignition battery had short circuited, some metal part had touched the ignition controller PCB and shorted the battery. No damage other than to the battery, I need to be more careful with those LiPo batteries
Bloody hell, close call there. I've seen what LiPo batteries can do when they let go, it's not pretty.
Been sick with a bad man-flu lately. What I forgot to mention about my latest testing, was that I also tested using a piece of DN12 corrugated tubing between mixer and initiator chamber. I tried 3 different lengths, 5cm being the smallest and some 15cm the longest, that is the piece seen in the latest pictures. There was no real difference between using 5cm or 15cm (without the air pump). All 3 lengths seemed to work equally well to the 9cm straight tubing from the weed burner.
This makes me think that I can just use 5cm corrugated tubing an not a straight piece of carbon fiber tubing, as initially planned, between mixer and chamber. I will be doing some more testing of this in combination with different mixer geometries. I need to find something that works, and is easy for me to manufacture from long-lasting materials, here the unknown is the mixer, I hope I end up with something I can mill from aluminium.
Hopefully I get more time to work on this in between all my other small projects
Went testing a new mixer idea today. I wanted to make an adjustable mixer with less restriction of airflow to the mix, so I thought maybe it could help me turn up the propane flow. After testing, I don't think this will work. I still get the feeling that an too open mixer will let too much pressure from the combustion escape, and this decreases the chances of a loud boom.
The mixer version I tested today, does not have that portion with a smaller diameter like the versions I screw onto the weed burner tube piece has. I will do more testing to see if this is a big factor.
I kind of like using the DN12 currugated tubing between mixer and chamber since it's flexible, and this will make it easier to install the final system in the RC plane. What length of this tubing I will use is still undetermined, I'll have to do more testing in this area too, even though it seems like 5cm will work just as well as 15cm, so maybe this is something I can choose based on what fits best in the RC plane.
The new spark plug idea seems to be good, I can now install the spark plug from outside the chamber because the spark gap area can pass through the 4mm threaded hole that fits the brass bolt. I need to add this to my future testings so it can be tested. So much testing
Re: Noise cannon for RC planes
New spark plug prototype finished. Very compact and it works great.
New orange one-piece initiator chambers has been printed, this time the finish is very smooth as it should be.
Regarding the spark plugs, anyone know if brass and copper are the best choice regarding resistance to fouling from the spark? What I mean is the materials get this dark burned layer on them where the spark jumps. Maybe some kind of coating can prevent this. Tinning does not work.
Platinum is about the only metal that might resists fowling by the spark. You might be able to hack a platinum car spark plug.
jimmy101, thanks. I actually hacked an old spark plug apart, took some effort with the angle grinder. My problem here would be that I don't have a reliable way to attach the electrode material to my spark plug, and any attachment would require some extra space, so the spark plugs won't be as compact.
The effect of fouling is noticeable when taking a newly made plug and hooking it up to the HV module for a continuous spark. The sparks jump at a faster rate (I can hear that), and the rate drops as the black coating starts to form after a few seconds.
The spark can still jump through the black coating, but it does have a harder time doing so, I can see that when the spark jumps at another gap in the system, but it is a larger gap than that of the spark plug electrodes, but it is a clean, non-black-coated gap.
I think I'll have to settle with what I have, and then maybe clean the spark plug, or replace the thin copper wire, doing whatever maintenance is needed. At least now I have a system where the spark plug is easy to mount or remove from the initiator chamber. I'll just have to be more careful with unintentional spark gaps, a coat of epoxy resin on exposed conductors will be very helpful
OK, how about a wild-ass idea? Reverse the polarity every one in a while. That might blow the corrosion off one or both electrodes.
Do you tend to get deposits on one or both electrodes? Given your design, if the central wire was Platinum then only the much larger screw would corrode and that has such large area that things might be OK for a bit longer.
Great idea, but unfortunately the HV modules does not support reversing the polarity of the output voltage.
Yes, I get deposits on both electrodes.
The more I think about it, the more I can see that this is fine as it is right now. The spark plug is something like $0.5 each in materials and it will probably last a few thousand shots before it even needs cleaning. I'll just keep in mind not to glue the center electrode to the brass bolt with epoxy as I planned to, it needs to be easy to remove for service or replacement.
Next task for me is making the carbon fiber mold from 3D printed PVA. I already did some test prints with PVA last night, I think it's going to work out very well
Got the PVA mold printed now. Hopefully I can do some fiber work tomorrow. I keep melting the PLA parts due to excessive shooting when testing, but it just sounds so damn good, I can't help myself
1 layer of carbon fiber, followed by 1 layer of glass fiber and then 6.7g epoxy resin. The thread in the middle of the chamber is keeping a piece of steel plate in place for the spark plug. It's cool to see the glass fiber become all transparent when it gets wet by the epoxy resin.
After hardening and when I got time again, I put on another 1 layer of glass fiber and finished off with 1 layer og carbon fiber, this time adding a bit more epoxy resin, 10g. as I felt a bit short on the stuff for the first 2 layers. I wrapped the whole thing in some white 20µm PE foil, this should help push the fiber layers on to the mold, or in this case, the previous layers, and it also squeezes out excess resin. It will probably need some sanding when done, to remove all the white foil, but I'll have to wait till tomorrow evening before I can unwrap it, the epoxy will need 24 hours to harden before I start messing with it.
When it's done hardening, I'll also have to trim the excess fiber material from the ends. I prepared for this by added some painters tape to the steel tubes before adding the first layer of fiber, so when carefully cutting the fiber with my Proxxon rotary grinder/cutter I should be able to stop cutting when seeing the painters tape and avoid cutting into the tubing. If not, I'll need some more layers of tape for the next casting
So I have some update. The carbon fiber part is almost done, I have had it submerged in water for almost a week, and also used hot water, but the PVA mold does not seem to dissolve that easy. Maybe the PVA part had tiny holes that the epoxy resin could leak into. I don't know unless I cut it open and by that ruin all my work. So I'll post cure the epoxy, mount the spark plug and test the part with shooting before I destroy it to investigate.
I think it is very hard to get good results with the carbon fiber, and it takes a really long time to make just 1 of these parts. During all my carbon fiber testing, I actually got hold of some silver solder, and some 0.5mm thin steel sheet, both normal steel and stainless steel. I was planning on using it for a custom made exhaust for my plane, but I see it could also prove to be helpful with the noise cannon. So I wanted to try making the initiator chamber from steel plate. I got the idea to use my hydraulic press to form the steel sheet to a correct shaped punch, so I made a setup where the steel blank is held in place by 10mm thick steel plates and a hard wood punch shaped on my lathe. I got a great deal of help and suggestions from a thread I made on a metal working forum (allmetalshaping.com).
The idea is to make 2 half spheres the same diameter as the current 3D printed initiator chamber and silver solder them together and silver solder them to the corrugated steel tubes as well.
Pictures of the manufacturing process of the steel half spheres are attached
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