Noise cannon for RC planes

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
Futterama
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Fri Jul 06, 2018 2:53 pm

hectmarr, thanks.

The propane pressure is 8bar and this would blow more than a pump. I would need the pump to blow into the nozzle in reverse direction with the propane feed hose detached so the dirt gets out of the system. But during all my extensive testing, this is the first time I have had a nozzle clog.

I have made a cleaning tool with some 0.3mm piano wire so I can clean another time if needed.
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hectmarr
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Fri Jul 06, 2018 4:18 pm

Yes, indeed, a mechanical system can also work properly. It happens that when the airplane model is on the ground, the turbulence generated by the propeller,Lift up dust, can dirty and cover such a tiny hole. :(
Ideas don't make holes, guns do!
Futterama
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Mon Aug 13, 2018 1:54 pm

This project is still moving forward.

The test plane can fire in the air but with a low rate of fire and sometimes not very loud, it depends a lot on the propane/air mix.

If I add a radial fan at the air intake of the cannon, I can turn all the way up for the propane and also the rate of fire, and I get really loud bangs. So this is what I'm working on right now. See this video.

[youtube][/youtube]

I have made a really lightweight radial fan from 3D printed PLA and a small 17mm brushless motor and 10A motor controller. Total weight only 57g. And it is really powerful :D

Now that I have forced air at the intake, I'm going to try to add a reed valve and see what that does to the combustion process - I hope that it will help direct most or all combustion pressure out through the main tube and barrel outlet, and not let some of the pressure out through the intake as it does now. Hopefully this will also add to the noise level :lol:

I'm using the thinnest of my feeler gauges (0.05mm) as a reed for the initial test. I can get thin stainless steel foil from ebay in even thinner sizes if 0.05mm is too stiff.
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hectmarr
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Mon Aug 13, 2018 6:35 pm

It sounds spectacular !! I like how this project progresses. :)
Ideas don't make holes, guns do!
Futterama
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Tue Aug 14, 2018 1:22 am

hectmarr wrote:It sounds spectacular !! I like how this project progresses. :)
Thanks! Yes, the sound is getting really close to my end goal with the radial fan at the intake. This also opens up a lot of possibilities like the intake mixer can be redesigned to be way simpler as it no longer has the job of sucking in air anymore, the fan does that.
If the reed valve has a good effect, it might also allow to shorten the main tube some more, making it more flexible with regards to mounting in the plane. Not that I need it shorter for my current plane, but others might need it for theirs.
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jrrdw
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Tue Aug 14, 2018 7:00 am

Will yours be as loud as these I wonder?

[youtube][/youtube]
Look if you must.
Futterama
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Tue Aug 14, 2018 8:03 am

jrrdw wrote:Will yours be as loud as these I wonder?
Check out the video description:
Brrrttt sound effects added in post-production just for a bit of fun
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jackssmirkingrevenge
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Tue Aug 14, 2018 12:41 pm

This just keeps getting better, kudos as usual :D
Brrrttt sound effects added in post-production just for a bit of fun
... as should have been obvious from the fact that we were hearing the noise of the shell impacts as well ;)
hectmarr wrote:You have to make many weapons, because this field is long and short life
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jrrdw
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Tue Aug 14, 2018 2:19 pm

All the added in is at the end of each sequence. Listen again.
Look if you must.
Futterama
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Wed Aug 15, 2018 2:12 am

I tested my quick and dirty reed valve yesterday and it did not give any noticeable difference in sound performance. I guess the radial fan is powerful enough alone to overcome most or all of the back pressure from firing the cannon.
I also confirmed this by comparing fan temperature with and without the reed valve, the difference was minimal so the intake flames does not reach the fan much.

The last thing I would like to test before starting the work of adding the fan to my big plane and re-designing my controller board, is the intake mixer design. On the smaller test-plane it takes up a lot of space and weight and with the fan added to supply air, the intake mixer could be a lot simpler and smaller in design, I want to test this.
Along with this test, I am also keen on testing a bigger propane nozzle (0.4mm vs. 0.3mm) now that I have the possibility to supply the air needed.

Stay tuned :D
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farcticox1
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Wed Aug 15, 2018 4:31 am

going off on a tangent a bit it would be cool if you could turn the whole plane into a speaker with one of those amplifier magidgets and just play mp3 audio files of anything you wanted, don't know if you would get the volume though :?

[youtube][/youtube]
Futterama
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Wed Aug 15, 2018 5:04 am

farcticox1 wrote:going off on a tangent a bit it would be cool if you could turn the whole plane into a speaker with one of those amplifier magidgets and just play mp3 audio files of anything you wanted, don't know if you would get the volume though :?
That is already possible using this: https://www.mrrcsound.com/tt-25-info

But it is mainly for electric powered RC vehicles. From what I can find, people suggest that a gas engine will drown out any sound from such devices, so I think my system is louder. It's hard to test as my sound meter only records 4 times a second, but I was actually considering to give it a try, I just forgot yesterday.
Last edited by Futterama on Wed Feb 06, 2019 7:04 am, edited 1 time in total.
Futterama
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Wed Aug 15, 2018 1:23 pm

So I tested 2 things today: Reed valve function and minimal mixer assembly.

First I 3D printed a white tube where I fitted the propane nozzle. Plain and simple construction. See pictures.

Then I fired the sound cannon indoors (with hearing protection) and turned off the lights. Now I could clearly see how far back the flame got, and I could clearly feel the blast of hot air coming out the radial fan. I repeated the test with the reed valve in place. After this, I'm reconsidering to dump the reed valve. I'm sure the radial fan will last longer with a reed valve in place. I just need to work a bit on the design.

And the simple propane feed is working perfectly fine with the radial fan feeding the air to the sound cannon.

[youtube][/youtube]
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