Noise cannon for RC planes

[Futterama]

During my testing of gas burners, I wanted to try out a weed burner. So I bought one on sale and cut away the long and bent part of the tube so it would fit my initiator chamber. I was surprised how well it worked! And it's dead simple, more simple than the Express burner. So I hooked up one of my HV modules to my new homemade spark plug and mounted the spark plug in the middle of the initiator tube. The HV module is powered by a small LiPo cell with a switch for ignition. The long main tube is 120cm.

The firing rate is quite good and I think I can improve it further, by using a fresh gas can (when new they are around 85psi), the one in the video is used and has around 60psi. I'm also going to try to increase the flow with a bigger propane nozzle and bigger air intake holes, but I need to start building my own stuff as it gets quite expensive to keep buying gas burners

Enjoy the video. It turned out alright even though it was windy and raining a bit.

[youtube][/youtube]

[jimmy101]

Regarding circuit ground, the HV wires from the module probably carry AC, so is ground just one of the HV wires or are you referring to another ground?

If both circuits are running form the same set of batteries then "ground" is which ever of the battery's leads you want to call "ground".
If you want to reduce the Rf getting to the RC system you can just try wiring the caps across the transmitter's power leads (and as close to the RC module as possible).

[Futterama]

From the video in my previous post, I extracted the soundtrack and used an online sound editor to analyze the sound so I could get an idea of how fast I am able to shoot the sound cannon. It turns out I can shoot it every 400ms, thats 2.5 times a second. So I made a microcontroller board with SMD components, and with such small traces, an optocoupler, seperate supply for the HV modules, my homemade spark plug and short wires, I have no problems with the microcontroller and RF noise from the spark or HV module.

So I went to the garden to shoot my microcontroller timed sound cannon, and it rips! I had it firing maybe 10 times, and then the grey plastic tubing got soft from the heat and broke.

So I found some corrugated stainless steel hose online, and found a place where they used it for solar panels for heating water, and the price was good compared to other places but still set me back what equals to $46 for 4 meter hose.

I bought 2 meter DN20 (ID 21mm) and 2 meter DN16 (ID 17mm). I'm pretty sure the DN20 will work just fine, but I'm not sure about the DN16, but the smaller size of DN16 would suit the plane better, so I got it anyway so I can test it.

I'm gonna test the DN20 tomorrow

[Futterama]

I have tested the new stainless steel hose with a small initiator chamber of 130ccm. Both the DN20 and the DN16 works with 1 meter hose. This is great news! 8)

More testing is needed as I just confirmed that the new hoses will generate a loud boom, but I haven't tested how fast I can shoot them with the existing gas mixer (weed burner) yet. Neighbors start to wonder where the sounds come from, so I'll wait till its dark with more testing, maybe drive outside of town for testing

[Futterama]

Small update on big progress!

I went into the woods to shoot using the DN16 metal hose, I was down to 365ms. Small progress.

I then made a version of the sound cannon that I could mount on one of my 1100mm foam airplanes. The total weight of the plane without the cannon was 1.4kg, with the cannon almost 2kg. The first attempt to take off ended with a broken wing and fuselage 2 days ago. I glued the plane together again and tweaked some small things including the center of gravity of the plane and tried again yesterday. BIG progress, the thing works in the air too! And it sounds really awesome even though the rate of fire is lower than what I can achieve on the ground.

The electric motor of the plane burned out mid flight, probably due to the fact that I had to run it at full power, but I got it down safely anyway.

Watch the video!

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[mobile chernobyl]

Just noticed this thread pop up to the main page again - all I can say is WOW! Great progress!

Love seeing more work done in the field of detonations!

Good luck in your future ventures - Do you have a picture of how the noise maker is installed in/on the RC plane? And what is your theory/hypothesis regarding the rate of fire being lower in flight vs. on the ground?

[Futterama]

mobile chernobyl, thanks, it's just so much fun to build this and I hope you all enjoy me sharing my experiences.

I just took some photos of the installation. The plane was a model I had crashed previously and was not the most suitable for the job, but I couldn't wait to test it in the air any longer so I fixed the plane and installed the sound cannon.
The 150ml propane tank should be mounted as upright as possible to avoid liquid propane entering the blue hose. But the plane was not big enough for a full upright, and I wanted to be able to fit the canopy afterwards so it does sit at an angle. I modified the valve from the weed burner to accept the blue hose, which is actually a gasoline hose from gasoline RC cars, but it seems to work fine. Also I attached an arm for servo actuation so I could open and close the gas flow by remote. The 1 meter long detonation tube was cut in two and the square pink box in the pictures was 3D printed and used for connecting the tubes so they could better fit on the small plane. The propane nozzle, nozzle housing and mixer tube was real heavy, so I 3D printed those parts too, except the actual propane nozzle. Since I want the detonation tube opening to point forward on the plane, I made an adapter for the air intake, so that the exhaust hole and intake hole gets the same amount of air pressure from the air passing by due to the plane moving forward. This way, the propane nozzle can actually enable flow from intake to exhaust using the venturi effect, but the air pressure inside the initiator chamber and detonation tube will be slightly larger than atmospheric pressure.
The rate of fire is controlled by my microntroller board, the one wrapped in yellow painters tape. The controller board has a microcontroller reading the signal from the RC receiver and the position of the small pot. The pot is used to adjust the rate of fire, or actually the rate of spark ignition, and the signal from the RC receiver is the same as the signal for the gas valve actuation servo and is used to enable/disable ignition. So when I flip the switch on my RC transmitter, the servo opens up the gas flow, and the controller board starts igniting the gas at the set interval. The ignition module is powered by a small separate battery and the microcontroller uses an optocoupler to control the ignition module, so that the module cannot send noise back to the RC receiver through the servo wire.
Before flight, I had set the fire rate to a safe level where I was quite sure enough gas/air mix would have entered the initiator chamber and detonation tube. I only had that one flight due to the motor failure so I could not test it again with a faster rate of fire by adjusting the pot.

Now, I have ordered another new RC plane, a cheap trainer plane with a larger wingspan and more room inside the fuselage. I hope to be able to put more stuff into the fuselage so I can reduce the drag from stuff hanging off the bottom of the plane and I hope the shape of the wing and the bigger wingspan will enable the plane to carry the extra load with more ease than the warbird in the video. Because I need more test flights, a lot of them. I need to test rate of fire, intake vs. exhaust hole sizes, propane nozzle sizes (I hope a bigger propane nozzle hole and air intake hole can increase the gas/air mix flow so I can increase the rate of fire), I need to take the steel detonation tube into the air too ect. I want a completely proven design working before I start cutting up the wing and making changes to the big wooden plane seen in my first post of this thread.

[Futterama]

A little update. I have built a larger propane nozzle, with a 0.5mm hole instead of the 0.3mm hole from the weed burner nozzle - in an attempt to increase rate of fire. Bigger propane nozzle hole equals bigger air intake holes, but when the holes get bigger, more of the pressure from the burning gas mix escapes back through the air intake, and this prevents the loud boom from being generated. So I was thinking, how do I prevent the pressure from escaping backwards? I tried building a small two-piece flap valve from 0.3mm tin can plate, it is lightweight enough to get pushed open from the gas mix pressure from the new propane nozzle, and I was hoping the rapid pressure rise from the ignition inside the initiator chamber would close the valve, but this does not seem to be the case from my testing. It seems like the pressure from the combustion is opening the valve and locking it up in an open position.

So this made me thinking, maybe the valve action should not rely on the pressure from the combustion, but rather be accurately controlled by the ignition microcontroller. I'm thinking something like an electric motor spinning and through some mechanics, this opens and closes the valve so that the valve is closed just before ignition, and opened again right after the loud boom. I can have the microcontroller controlling the motor speed and the motor/valve position could be detected using hall sensors or optical sensors so the ignition and valve action is accurately controlled.

I need to go to thinking mode again...

[Futterama]

So I came up with the idea of using a throttle valve (like the one used on a carburetor) as an inlet valve, I was thinking I could easily control such a valve with a motor or fast RC servo. So I did a test (using the proven weed burner) where I fired the noise cannon with the valve open, and one where I close the valve with the gas still flowing and igniting very quickly after closing the valve. The result is very strange, I get a lot bigger boom with the valve open, and I can see a much bigger exhaust flame when the valve is open. I would have thought is was the other way around!

What can be the reason for this? Shouldn't the closed inlet valve help produce a much higher pressure inside the initiator chamber when igniting (because then none of the burning gas can escape through the air intake) and this would then help accelerate the flame through the tube?

I'm thinking that the closed valve somehow affects the propane/air mix, because there were times during my testing where the cannon wouldn't fire at all with the valve closed which must be due to an un-ignitable mixture. Maybe I'm using a lean or rich mix, and the closed valve somehow creates a too lean or too rich mix that won't ignite.

Pictures of the throttle valve.

[Futterama]

Ok, so I kinda went away from the idea with the valve, it gets too complicated really quick.

I'm currently still investigating how to get the fastest shooting possible, and I'm at 4 shots per second now which is only 240 shots per minute, but with 2 cannons, one in each wing, I get double of that. I'm also considering 4 cannons, each shooting in turn, that would get me to almost 1000 shots per minute
But the rate of fire is highly dependent on gas pressure. The small propane cans that I plan to use, drops very quickly in pressure when used. Also the temperature is of great importance. I got myself a MAPP gas can which was at 6.6bar/96psi when new, now I have used 10% of the gas by weight, and the pressure has dropped to 4.9bar/71psi. This is the highest pressure can I have had my hands on. Maybe the MAPP gas is a factor for the pressure, I will try to fill one of the small cans with MAPP gas, and only fill it till it weighs the same as when new, and compare the pressure.
By increasing the can temperature just 10°C, the pressure will almost double. So I have been thinking, maybe I can rig up a system to heat the can using the heat from the exhaust from the gasoline engine on the RC plane. But I need to be able to turn heating on and off so I can either keep a steady temperature at a safe level, or find an electronic pressure gauge so I can have a microcontroller keeping a steady gas pressure, this would be the most optimal solution. Then I would have the highest rate of fire possible with the lowest expense on gas canisters.

During my testing, the corrugated stainless steel tube proves to be very reliable. It gets burning hot, just look at my thumb from when I grabbed the tube too soon after shooting it.
The heat is also too intense for both the 3D printed parts and the grey drain pipe after a series of maybe 10 shots. So I was in thinking mode on how to make a heat resistant initiator chamber that is also lightweight. Thin welded steel plate, no, too heavy because I can't weld such thin plate with the welder I have available. Silver solder the thin steel plate, could work, but it seems hard to find silver soldering tools around here. TIG welding aluminium plates, yes, but I don't have access to a AC/DC TIG welder.
Then it suddenly came to me. Carbon fiber! Lots of people make fiberglass stuff for RC planes, carbon fiber is not any different in manufacturing methods, only a different material which gives a more stiff product. I even have a guy at the club making fiberglass fuel tanks for RC jets, he gave me some hints, and I found some braided carbon fiber sleeve which would be easy to work with, and I found some affordable high temp epoxy resin. Resin has arrived, but not the carbon fiber, so I won't be making any real stuff from it before next weekend.

I also ordered some small 0.3-0.4mm high quality tungsten carbide drills so I can make my own propane nozzles and get an accurate hole. Those chinese HSS drills are no good, it needs to be good quality when we are down to that size. The pictures shows 0.5mm/0.02" diamter drills, the black is poor quality, the blank is tungsten carbide from a German manufacturer. Look at that awesome drill, shaped cutting edge and a relief along the spiral, how can they do that on such a small drill?

Another progress is the size of the initiator chamber, I'm down to around 70cm³/4.3ci volume still keeping the 50mm/2" diameter. The sound does get a bit more high pitch to it when going down in chamber size, but that does not matter to me, it can sound like a cannon or machine gun, I'm happy either way, as long as it is LOUD

Last thing, I had an accident with my smaller ignition module, the 3D printet part came off the initiator chamber and the sound cannon turned into a blowtorch and the wires from the igniter got caught in the flame, whoops. So I'm using the bigger type igniters, they are more powerful, more than I really need, and they are twice the weight of the smaller one. Unfortunately it seems like most ebay sellers only carry the bigger one now, or the small square one, which is not really powerful enough for my liking. I want to save weight where I can, but I'm having a hard time finding those smaller modules

[Futterama]

I'ts been a month since my last update and I noted the thread views, more than 200 views in the last month, someone must be following my thread but very silently

I have found that I can easily heat the smaller propane cans with resistive wire and a small LiPo battery, so I can increase the pressure and I also got a digital pressure sensor for controlling the pressure increase. But when heating the propane can, it won't just release gas, is will also release the liquid propane, even when the can is in the upright position as it should be. This is no good as the liquid propane will then be sprayed into the initiator chamber and giving a way too rich mix.
I don't know why the propane can will release the liquid propane, the only idea I have is that the liquid propane inside the bottle, is boiling so heavily that it splashes around and some of the splashes, enters the outlet. I'm not sure how to solve this yet, but I have a few ideas to try out, including a special propane can with built-in foam that just doesn't release any liquid propane at room temperature, even when upside-down:
http://www.guilbert-express.net/product ... 44-76.html

I haven't made any carbon fiber parts yet, I'm still thinking of how to shape it, the mold to use, how to mount the spark plug and how to connect them to the corrugated stainless steel tube and the propane nozzle. I have a few ideas to try out in the near future, the weather is real fine this week so I can work with the epoxy resin outdoor and avoid inhaling too many fumes

The new tungsten carbide drills are great, I now use my own homemade propane nozzles, and they work just great.

With regards to the initiator chamber, my most recent edition have a volume of 50cm². I also tried to shorten the corrugated stainless steel tube, the sound did change, and was a bit boring at 70cm tube, compared to the 100cm tube. But at the same time, I was testing some gun barrels, which is the visible part of the sound cannon on the airplane wing, and those barrels was 13mm inner diameter, which is 3mm smaller than the corrugated stainless steel tube, so they will affect the sound, but they actually made the sound better when using 70cm tube, so that's great since 70cm tube fits my plane a lot better than 100cm tube 8)

Also, I ordered 4 more ignition modules, they have arrived, and I just ordered more of the corrugated stainless steel tube, 5 meters of the proven DN16 and 2 meters of DN12 which is 12mm diameter, I want to see if I can get a loud sound using those smaller 12mm tubes, I need to test the limits of this

More updates will come as I progress.

[Futterama]

So I was just outside for some more testing. I attached the noise cannon to a test airplane, just to get some wind from the propeller into the air intake. I had bent the steel tube 120 degrees with a nice soft radius. I was having a hard time getting a good sound from it. So I bent the steel tube back to almost straight and now it was easier to get a good sound from it. So bends on the steel tube is also a factor to consider. Holy cow, this is getting very complicated, too many factors to consider! I think it is time for me to get rid of some of the variables and make them constants. Like, the length and shape of the tube, this is limited by the airplane wing so that is easy to make constant. Also the initiator chamber, I need to start making those carbon fiber chambers.

[CpTn_lAw]

Really nice work!

I was thinking... couldn't you come up with a geometry that allows preferably air from one side in the chamber?
I'll try to make a small drawing of what i'm thinking about right now. (whem i get back from work)

[Futterama]

Really nice work!

I was thinking... couldn't you come up with a geometry that allows preferably air from one side in the chamber?
I'll try to make a small drawing of what i'm thinking about right now. (whem i get back from work)

Thanks, I'm working hard on this project and it's getting expensive, but I'm getting there! Also, the motivation is still there, I haven't been able to find anything remotely similar for RC planes, the closest thing is a loudspeaker and sound module, but that's just drowned by the noise from the gasoline engine, so this will surely become an attraction at the airfields, hopefully world's first gas powered RC sound cannon

So, do you mean a 90 degree angle on the initiator chamber?

[Futterama]

I have been trying out a lot of various sources of fuel for the noise cannon. From early on, I have been looking more for propane than butane, because of the higher pressure that propane will deliver, higher pressure means higher firing rate. But all those weed burner cans are a mix of butane and propane, and the pressure will drop when the content is consumed, simply because the propane will evaporate inside the can before the butane, so the propane will be consumed before the butane, so when the propane content goes down, so does the pressure. Butane will give a pressure of around 2.1 bar (30 psi) at room temperature, where propane will give a pressure of around 9 bar (130 psi).
So, I found a clean source of propane, with a minimal content of butane, so now I have small canisters with 8-9 bar pressure at room temperature.
I did a quick test of these new high pressure, pure propane canisters, and I didn't get good results, the gas started to burn continuously inside the initiator chamber so I guess I'm not near the stoichiometric ratio anymore. I think I really need to redesign the air intake for the initiator chamber at this point, so that I can regulate the amount of air that is drawn into the chamber by the propane stream because I have no idea whether the mix is lean or rich.

Oh, and I did my first carbon fiber thingy today, probably need 2 or 3 layers of sleeve before it's good. Pictures are before and after resin has been applied.