Noise cannon for RC planes

[Futterama]

I have just discovered that the telemetry sensor data protocol for my radio system is published documentation, so I can make use of my radios built-in telemetry functions to monitor stuff like propane pressure and temperature, heating battery voltage and so on. Most sensors are even associated with an alarm so I can have my transmitter beep or vibrate when a sensor limit is reached, like empty heater battery or empty propane canister.

This is so cool

[Futterama]

I did a destructive test today.

When searching for information on propane/butane canisters and refill of those, many people state that the cans can't hold up to the pressure of pure propane. So I took a look at the datasheet on my foam filled canisters from Express:

http://www.guilbert-express.net/product ... 44-76.html
http://www.guilbert-express.net/mediath ... 4_indF.pdf

It says: 18 bar steel case...

So I bought myself a 25 bar gauge, hooked it up to a 444 canister with heater wires mounted, put it in a foam box, wrapped the canister with old fabric around it, including a piece of some pants and tried to secure it best as possible.
I hooked a thermometer to the can and used a long wire for the heater.

I took the box to the garden and secured it with a wheelbarrow ect. and aimed some cameras at the pressure gauge. I had a wireless video feed from an old smartphone pointed at the thermometer.

And so I started heating the can with the same battery I plan on using in the RC plane.

I feared a massive explosion when the canister would give in (no fire of course, just escaping pressure), but hoped for the best
The result is that the canister can take upwards of 20 bar before failing. The first failure was the concave bottom that went convex, it is seen as a drop in pressure and heard as a puff. Seconds later, the canister shoots out the bottom. No total destruction of garden or surrounding houses
It did however rip open the pants and filled the box with foam pieces, but it didn't even break a box made from stiff foam for putting up posters.

The temperature reading was 65°C when I last checked, but I was handling the zoom camera when the fun part happened and didn't have enough cameras to record the temperature reading, and the battery was disconnected from the thermometer when the canister failed. But with the speed of temperature increase in mind, I would suspect the canister to be somewhere around 75-80°C when it failed.

I made a video :albino:

With this result in mind, I have no trouble putting one of these canisters filled with propane in my RC plane.

Someone asked what would happen if my heating controller would just keep heating the canister. It would blow within 8 minutes. So I will monitor the temperature and make sure I get an alarm soon enough to be able to land the plane and remove the heater battery. Or I could make a safety function that activates the valve servo without activating the HV modules, so as to be able to empty the canister in-air in case of emergency.

Sorry, video deleted during cleanup

[Futterama]

I started building the controller for the sound cannon. This one can control up to 6 cannons.

PCB ready for soldering 8)

[Futterama]

The controller board takes a lot of time to build. Probably because I want all the bells and whistles I can get

So I was wondering:
Question 1: How hot does the sound cannon parts actually get when shooting?
Question 2: How hot can I allow the sound cannon to get inside the wing of the RC plane?
Question 3: How many shots can I do or how long can I shoot before needing a cooling break?
Question 4: How long does it need to cool before next round of shots?

Answer 1: I borrowed a thermal imaging camera from my workplace and went out shooting. It can get REALLY hot if I just keep shooting, but with the weather we have here now, it cools very rapid too. But inside the wing, it won't get nearly as much cooling.
I also wanted to know which part of the sound cannon got hottest. The thermal imaging helped figure out the hot spots but they are not 100% accurate due to the mild wind during the testing. I didn't have a wind-free area to test, but I think I got the big picture.
I have added some temperature sensors to my controller, they can measure up to 150°C. These should be located at the hottest parts of the sound cannon, so the controller can prohibit shooting when temperature is too high.

Answer 2: I would have to look at the materials used to build a plane wing. The most sensitive material would probably be the balsa wood, as it ignites at 200°C according to what I found online. The covering film can withstand 250°C, but the glue actives at 92°C, but the film will have all the cooling from the airflow over the wing. I don't know about the wood glue, but I won't be gluing the sound cannon in place, I will use some metal standoffs bolted in place.
Then there is the temperature sensors, so I definitely won't go above 150°C on any part of the sound cannon.

Answer 3: I let the sound cannon cool way down, and fired it for 10 seconds at 250 shots/min. The hottest place was just below 100°C.
10 seconds would be a long time to shoot during flight as the plane would pass in just 5 seconds at full speed (I checked one of my videos). And you don't attack a target flying slow, do you? So I don't think it would be any problem to fire the sound cannon for the duration I would desire during flight as long as the sound cannon gets to cool down.

Answer 4: In open air, with a bit wind, less than 1 minute. Inside a plane wing, probably with still air around it, I have no idea. Only time will tell.
If cooling takes forever, I could add some forced cooling. I could have a servo operated hatch behind the air intake that would close when shooting, letting pressure rise around the gas mixer, which helps to flow the gas the right way into the sound cannon and not out the intake. The hatch would then open after shooting, letting the airflow from the speed of the plane both push air into the sound cannon barrels, but also suck air out of the intake and out the hatch. On the P-51 Mustang, the air intake and adjustable air scoop outlet is scale, but on my current plane, it would not be scale, but I don't mind that right now.

How cool are those pictures, huh?

[Futterama]

More update!

I made a neoprene jacket for the gas canister. The idea is to keep the excess heat from the heating wire close to the gas canister. Another feature is the canister will now be mounted with vibration dampening material.

The blue lead is for the heating wire that's wrapped around the canister. The other lead is for a temperature sensor glued to the bottom of the canister for monitoring it's temperature.

[hectmarr]

Excellent project and very good work. I liked it a lot. :bounce: :bounce:

[Futterama]

Excellent project and very good work. I liked it a lot. :bounce: :bounce:

Hi hectmarr, I'm glad you liked it.

The project is still going, but I have had some setbacks. The system is mounted in my RC plane in one wing only at this point.
I had to make a new exhaust for the engine to let fresh air from the propeller enter the cannon air intake.
When testing the system on the ground, I found that a long air intake path is bad as some of the burning gasses during a shot is pushed back out through the gas mixer, hence using up some oxygen in the air in the intake path. So I needed a lot of throttle on the engine to get a good firing of the cannon (the engine propeller pushes air into the air intake and more engine throttle = faster air intake and replacement of consumed oxygen).

Also, while testing, I found that the servos for the control surfaces on the plane was twitching, and they are very old, so they needed replacement. This plane is not just money any more, it is also a big time investment after mounting the cannon in the wing. The servos set me back $350 and some time for install and new improved wiring. This time I routed the servo wires away from any spark wires from the high voltage spark module. I don't want the RC receiver to malfunction from shooting while in an attack dive, I need to be able to pull up

I haven't worked on the project for the past few months - I got myself a brand new TIG welder which I got because I got tired of silver solder a whole exhaust for the plane, I've always wanted one and I had the funds. So I needed to make a corner for welding in my workshop, and TIG welding is fun, so I also started welding stuff. At the moment I'm building my own remote welding current foot pedal control for the machine, a good welding project to learn from. It's also needed to get some good welds on aluminium, and I really would like to build an aluminium exhaust for the plane at some point.

Anyway, when welding gets boring, I'll finish up the details on the RC plane and sound cannon so I can go test it. It has been a pretty bad summer for RC planes in my area, so I'm hoping for better weather so I can go testing when ready.

[Futterama]

Here are some pictures of the mounting process in the plane.

I had to first remove the plastic covering from the wing, then remove some of the wood covering to get into the structure.

I made a custom drill for making the hole in the wing for the cannon barrel. The front edge of the wing has been reinforced with extra wood.

I made a 90 degree angle on the cannon tubing, to better be able to mount the tubing through the ribs. Anywhere I weakened the structure to make room for the tubing, the surrounding wood was reinforced with plywood.

When I had the installation in the wing worked out, I put all the covering back and made a nice finish around the barrel.

[Futterama]

Here is a few pictures of the center part of the wing where the air intake is placed where I also had to remove material to fit the cannon.

I got some nice hose clamps off ebay, the wirewound type was replaced by the flatter type as they gave me more room to work with inside the tight areas of the wing.

[Futterama]

I don't want to ruin the looks of the RC plane, so I wanted the air intake for the cannon to be the scale engine air intake on the model. Problem was, it is located right behind the exhaust stingers from the gasoline engine. I can't really feed the cannon with exhaust gasses, it needs fresh air.

So I decided to make a whole new exhaust for the engine, and I made it so the actual exhaust pipe was located as on the real plane. It took ages to silver solder a whole exhaust, so I'm glad I now have a TIG welder, because at some point, I might want to make a lighter exhaust from aluminium, it could also be a lot smaller, but I went for the bigger volume to get another sound from the engine.

[youtube][/youtube]

[Futterama]

I also need to mount the propane canister in the fuselage along with an extra battery and the gas valve.

New servos needs mounting and as I mentioned, new wiring with capacitors built into the connectors inside the wings and better connectors where I need to connect and disconnect when going to the airfield.

I also needed new strut legs for the gear, homemade of course

[hectmarr]

All this work is going very well ! :bounce:
One question: The electronic device to regulate the temperature of the propane tank, did you buy it? Or did you make it? How does it work?

[jackssmirkingrevenge]

Not normally a fan of non-projectile weapons but that is some next level stuff, fitting it into a functional RC aircraft is quite an achievement, bravo!

[Futterama]

All this work is going very well ! :bounce:
One question: The electronic device to regulate the temperature of the propane tank, did you buy it? Or did you make it? How does it work?

Thanks, yes it's going pretty well but takes a long time because I keep hitting bumps like making a new exhaust, replacing servos and so on. But it's still fun!

I don't really regulate the temperature of the propane tank, I only add heat faster when the pressure drops due to temperature drop when consuming gas. It would be no problem to leave out the heating element and just let the propane tank heat up by itself, but it could take several minutes after firing the cannon for 5-10 seconds. So it's only there to allow shooting again much faster.

The control is just a MOSFET transistor that turns on the battery voltage from the extra 4-cell LiPo battery. This is controlled by the main controller chip. At the moment, the idea is to turn on the heater as soon as a shooting is commanded from the RC remote because I know it will need heat. And the heat will continue after the shooting is done for a specified amount of time. This time needs to be determined by experimentation and should probably be determined from the amount of time the gas valve was opened during shooting. Like if I shoot for 1 second the heat needs to be turned on for 5 seconds.
Since the heating element is on the outside of the can, the heat takes a little time to travel from the heating wire and into the tank and into the gas. This also needs to be taken into account.
I have also noticed that if I let the heater be turned on for more than 25-30 seconds, the temperature of the tank walls will become so high that the liquid propane inside the tank will start to boil violently and liquid propane could escape the tank which I need to avoid (that's why I use a special tank with foam inside), so the controller will not allow more than e.g. 25 seconds of heating from each shooting to avoid this.

The temperature sensor is not for archiving a certain temperature, it is only there to warn about too high or too low a temperature.

All this controller stuff is something I make myself. I use Microchip brand microcontrollers which I program myself. I also make the printed circuit board myself. Electronics and programming microcontrollers is another hobby of mine.

The picture is of my latest version of the controller for controlling 2 cannons. The one on the table is a bigger version for 6 cannons which is something I'm going to use on a bigger RC plane. I'm currently in the progress of revising the versions as I had made some mistakes in the design. This also takes a very long time to make because I want everything perfect

[Futterama]

Not normally a fan of non-projectile weapons but that is some next level stuff, fitting it into a functional RC aircraft is quite an achievement, bravo!

Thanks a lot, your feedback is highly appreciated 8)