Noise cannon for RC planes

[Futterama]

Hi spudders!

I came to this forum because I got that crazy idea of putting a noise cannon on my big RC warbird plane.
The plane is 209cm/82" in wingspan and is powered by a 26cc gasoline engine. Top speed is around 160kph/100mph and it weighs around 5kg/11lbs.

The plane makes a nice sound when flying by at high speed and low altitude, like a few meters above the ground. I thought it would sound even nicer, if it could make a cannon or machine gun sound, since it is a warbird.

So I'm trying to figure out how to build a combustion noise cannon for the plane. I have a few requirements/specifications:

- It needs to be small so it can be mounted in the wing (at least one on each wing). Cannon chamber estimation is around 200ml/12cu in.
- It needs to be light-weight.
- It needs to be louder than the noise from the gasoline engine.
- Fuel should be butane/propane mix, whatever I can find locally in a suitable small container (I have seen 150ml containers with 70% butane/30% propane).
- It would be nice if I could fire the cannon more than once per flight, so auto-reloading is the goal.

I'm very good with electronics and microcontrollers, and I have a lathe and mill.

I have a few ideas already, and I'll try to describe them the best I can.

The cannon chamber is fed with fuel from the fuel tank through a fast-acting solenoid valve, which is open for a certain amount of time, controlled by a microcontroller. The open time needed to obtain a good blend of fuel/air will depend on a few things, I know, like fuel tank pressure (which is temperature dependent).
I expect to make the solenoid valve myself.
The open end of the cannon chamber, or the "barrel" is pointing forward, and I think I need a burst disk to get any decent sound level from this. Ok, so the fuel is ignited by those 400KV Boost Step-up Power Module High-voltage Generator from ebay and the burst disk is ruptured, hopefully giving a good pop.
Now, for reloading, I need to get fresh air into the cannon chamber. I'm thinking of opening the chamber in the opposite end of the barrel, using a rotary plate with holes, aligning to holes in the chamber, and then the airspeed of the plane should push fresh air into the chamber. This would again be a microcontroller timed event, and I can use a small RC servo for rotating the hole-plate. When the rotary hole-plate is rotated back to close the holes in the chamber, I need to re-apply a burst disk. I'm thinking of a system that uses a roll of thick paper or similar, with a mechanism that presses the paper over the barrel, and after a shot, the mechanism would release the press agains the barrel, and roll up the paper that now got a hole in it, and the roll-up will move a fresh part of the paper in front of the barrel, and the mechanism should then re-apply pressure to the paper and barrel to hold the paper in place as a burst disk.

At the moment, I'm trying to make an early prototype so I can figure out how to get a good sound volume on the cannon.

A few questions comes to mind:

- Is it realistic to use a timed fuel delivery system using a solenoid valve to obtain a reasonable consistent fuel/air mix?
- Would I need some sort of pressure regulator in front of the solenoid valve to get good results?
- Regarding fuel tank pressure, I have read that a butane tank will have a constant pressure of around 30 psi at room temperature due to the vapor pressure of butane. Propane is more like 4-5 times that, according to what I could find online, so would a canister with a higher propane content, have a higher internal pressure?
- What do you think is the best material for the cannon chamber, PVC pipe or 1mm aluminium sheet formed and TIG welded? The weight is probably similar, but the aluminium would be easier to form for a better fit in the wing.

I have attached a couple pictures of the plane for you to enjoy

[mobile chernobyl]

How many noise shots are you looking to produce per flying session? You may be over complicating it by trying to do a repeating design - which albeit cool - may be overkill.

If you're trying to simulate high repetition main gun fire - I can understand the desire for repeatability though!

I'm thinking more along the lines (and this is way against my normal inclination haha) of having multiple pre-assembled cartridges, strapped to the wings like air-to-ground weaponry. An internal relay mechanism (rotary if you're fancy and want to fabricate it yourself to save weight) could dictate which cartridge receives the ignition spark from the HV source.

Again, I'm normally all about ditching the cartridge and trying to make something repeatable - but it tends to make things overly complex, heavy and less reliable. But if it's your desire to challenge yourself with the task of accomplishing this, disregard my idea above lol.

[Futterama]

Hi mobile chernobyl, thanks for replying.

I would like to have many shots per flying session. These types of planes were equipped with fast shooting machine guns, so it's not going to be 100% scale like sound, but I was thinking a few chambers in each wing, which I could fire rapidly one by one during a flyby, and then let the cannons "reload" while I'm flying around for the next flyby.

The idea with "multiple pre-assembled cartridges, strapped to the wings like air-to-ground weaponry" is actually really good, I haven't thought about that.

I'm currently waiting for the ignition modules, and while doing so, I'm thinking about the gas valve design. A solenoid might get too heavy, so I'm thninking about using a small high-speed RC servo, they come in various speed, power, sizes and weight, and they are pretty cheap too.

Another aspect I'm tinkering with, is the chamber volume. I'm trying to figure out how small I can make the chamber and still have enough power from the combustion to rupture the burst disk and make a reasoable noise. The chamber needs to be very solid, it won't work if the chamber can give in or temporarily deform due to the pressure from the combustion, I already learned that. Also, a smaller chamber requires for more accurate fuel delivery, so I'll have to make a prototype valve soon.

I'll keep posting as I move forward with tests and prototypes.

[mobile chernobyl]

Another property of mixed gases you may want to explore is detonation - something that will surely make noise and not require a rupture disc to do so.

Typically detonations are not something we spudders want - but it is a commonly used effect of rapid burning gasses that is employed everywhere from farming to avalanche mitigation devices.

I'm not 100% sure how the guy is doing it in this video, but this looks like a great system if you can down-scale it and still achieve a detonation.


Essentially you'd want a long tube, with a rough interior wall finish (really rough). In that tube, a proper air-fuel mix and a source of ignition at one extreme end, with the other end open, or somewhat ported. This is not the be-all/end-all recipe, but one that is commonly employed in detonation studies.

There are a few members on this forum that are familiar with some of the science behind detonation, and maybe they'll chime in! I think using small detonation chambers, you may actually be able to make a (very) low frequency pulse jet engine style 'noise maker' for this project - and in doing so - create something reallllllly cool in the process that I don't believe currently exists in the hobbyist world!

[Futterama]

Hi mobile chernobyl, thanks for the suggestion.

I have been looking for information about detonation, but did not really come up with an answer to the first question that popped into my mind: Can it be done with an air/propane mix or does it need something more like oxygen/acetylene?

I wrote the guy from youtube who posted that video you linked to, and asked him about his "exhaust cannon", he has made a lot of videos with a device that seems to be powered from two hoses, probably oxygen and some fuel. But he also posted 2 videos where it seems like the cannon is powered by an air/fuel mixture. So I'll see if he will answer me.

[Fnord]

Hi Futterama!
Getting a machine-gun sound from a small combustion cannon is super simple, the only hard part is if you actually want it to shoot something!
The most important question is... what's your payload capacity? It would save a lot of headaches if you could carry a small hpa tank.

Where you want to start experimenting is a small metal chamber (100-200ml) with two needle valves to regulate fuel and air flow into the chamber. Use a compressor/hpa tank for your test model. The igniter can run continuously.
Have a spring-loaded flap valve seal the barrel end of the chamber- no need for a burst disk and it doesn't need to be a perfect fit.

I did something like this years ago when I was experimenting with bbmgs but never found a practical application for it. There's no need for micro controllers or any timing circuits on a first gen prototype.
It does produce a very satisfying "DAKKADAKKADAKKA" and the refire rate can be controlled both by gas flow and tension on the flap valve. Your spark gap could be wired in series with your engine's plug to save weight.

Getting it working purely on forced air from the craft could be more difficult, since your input flow/pressure is going to vary quite a bit during maneuvers. If this is your only option, look to that gatling gun on the SF homepage. He's doing something very similar there, and the design is passive in operation, with only a valve, motor and a fan running it.

[Futterama]

Hi Fnord! Thanks for replying.

I don't want to shoot anything, I'm only in for the sound 8)

I'm not sure what my payload capacity on the plane is, but since the lift in the wings is dependent on the wing profile and airspeed, a heavier plane would need a higher airspeed (or another wing, but then I might just be better off building another plane) and high airspeed is not good when landing, so I have to keep the weight down.
I have been reading a bit about HPA tanks for paintball guns, and they are rated for thousands of psi, but it would be a problem for me to pressurize it to more than 115 psi, I simply don't have access to the correct equipment. Also, a cheap paintball air tank is way too heavy, and a lighter tank is way too expensive. Maybe there are smaller tanks out there, but then I would still rely on having someone fill the tank for me.

The idea with a spring loaded flap valve is great, I'll have to check that out.

Do you have any video of a flap valve system running?

Regarding air flow during maneuvers, I will be able to keep the airspeed pretty constant when firing the guns, since my plan was to only fire the guns when passing by the airfield at low altitude and high speed (full throttle and a dive). But I can see the challenge in refilling the cannon chamber with air from wind pressure alone if I use a flap valve.

I did see the thread about a gatling gun, I'll have to read it again to pick up some more ideas.

[Shyasuna]

Hi mobile chernobyl, thanks for the suggestion.

I have been looking for information about detonation, but did not really come up with an answer to the first question that popped into my mind: Can it be done with an air/propane mix or does it need something more like oxygen/acetylene?

I wrote the guy from youtube who posted that video you linked to, and asked him about his "exhaust cannon", he has made a lot of videos with a device that seems to be powered from two hoses, probably oxygen and some fuel. But he also posted 2 videos where it seems like the cannon is powered by an air/fuel mixture. So I'll see if he will answer me.

Hi, Futterama.
I'm a maker of "Exhaust Cannon" and "Detonation Cannon" that you mentioned the other day.
Thanks for your watching my creations, and I'm sorry I couldn't answer your questions. After I noticed your post here, I tried to found your message at my YouTube message box and gmail mail box, but there was no message from you. So if you don't mind, please tell me how did you send me the message?

Anyway, your creation and project is really cool! Because I'm in Japan and it is illegal to shoot something, I have been creating "Exhaust" or "Detonation" cannons for the purpose of generating intense sounds like real cannons. So I'm very interested in your project.

Now, I answer your questions.
The "Exhaust Cannon" which I create has similar mechanism what common pneumatic cannon has. So, its power source is only compressed air that is up to 1 MPa. By applying unique designs to exhaust cannons, they can generate their intense sounds.

[youtube][/youtube]
Is this the movie you watched then? The two hoses which connected to the cannon are common air-hose, and it works by compressed air. This full-automatic action is extremely like real machine-gun, but the mechanism is also extremely complicated, and also its weight is so heavy. So this type of pneumatic cannons with full-automatic mechanism will be not available for your project.

The devise you mentioned which uses air/fuel mixture is called "Detonation Cannon" in Japan, and It can easily generate extremely intense sounds. You will be able to understand the principle of detonation cannons by searching "Detonation Tube" on the internet. Detonation Tube has studied as a heat engine with high heat efficiency, and now, the study for realizing of new rocket engine which uses the mechanism of detonation tube is conducted all over the world. The new engine is called "PDE:Pulse Detonation Engine", and you will be get the technique to realize iterative detonation by searching "PDE".

By the way, to ignite detonation, it is important to use mixed gas of fuel and air at stoichiometric mixture ratio. Using tanks of fuel and air is OK, but you can easily create stoichiometric mixed butane/propane gas by using gas torch. The gas blown from nozzle of torch is already mixed with air perfectly. In fact I used the gas torch for my detonation cannon to create fuel and ignite. (For more stable mixing on the flying plane, you should isolate the mixing system from outside atmosphere, so you will maybe have to prepare air and fuel tank.)

[Futterama]

Hi Shyasuna!

It's very nice to meet you here and thanks for the very nice reply to my thread here

I was trying to contact you through youtube. I went to the "About" page on your user, here:
https://www.youtube.com/user/Shyasuna/about
And I just clicked the "Send message" button in the right corner.

Regarding your video, I have watched all of your videos I think

The videos of most interest was the ones with mixed fuel/air:



I tried to search for "Detonation Tube" but I didn't really find anything really useful, or maybe those papers are just too hard for my understanding

A few questions you might be able to answer:
1. Does the tube have to be a specific length/diameter ratio?
2. Can it be done with butane and/or propane?

I just tried with my butane gas torch by connecting it to a metal tube, but it didn't really make any sound. But the tube was 17mm inside diameter and 1300mm in length, so maybe it should be shorter for this diameter? In your 2 videos above, it seems like your tube is shorter and bigger in diameter than mine.

[mobile chernobyl]

Hi Shyasuna!

"it seems like your tube is shorter and bigger in diameter than mine."

That is interesting

[Shyasuna]

Sorry for my late reply.　I'm now investigating how to design the optimum configuration for detonation cannon. So, I tell you my current view briefly.

Although I don't know about the existence of length/diameter ratio, there is minimum value of both length and diameter for realizing detonation. The more important element is the existence of turbulence promoter inside of the tube. Turbulence promoter is used for the purpose of accelerating propagation of deflagration and it results in shortening the distance of the DDT(Deflagration to Detonation Transition).

I used this spiral as a turbulence promoter. This spiral is fixed near the gas torch. (This spiral which used as a turbulence promoter for DDT is called "shchelkin spiral" ) You will be able to get intense sound by inserting this spiral inside of tube as a turbulence promoter.

With respect to its length, it is said that once the DDT achieved, the upstream spirals works as a obstacle that decay the detonation propagation and we have to cull optimum length. However in our situation that use propane or butane and air mixture, it may be originally impossible to generate fully transited detonation by using hand-held tube. This means that the longer the spiral's length, the better for the intense sounds.

I had thought that the my detonation cannon's sounds was attributable to detonation, but it might have been caused by fast deflagration jet. (I'm now planning to conduct the experiment with additional spirals.)

Summarizing above, for creating intense sounds by fast deflagration, the larger pipe which contains spiral inside is better. If you want to realize the real detonation, it will be needed to use pure oxygen as a oxidant instead of air, use high detonatable fuel such as hydrogen or acethylene, or using very long tube.

In additon, basis of detonation are well summarized in this paper. If you interested, please check this.
http://commons.erau.edu/edt/124/

I forgot to mention my cannon's dimension.
The inner diameter is 30 mm, and the length is 900 mm, and it uses propane and butane mixed fuel.

[Futterama]

Shyasuna, thanks a lot, this is very useful information, I'll have to read that 60 page paper when I get the time

[Shyasuna]

Futterama

Don’t mention it. Thanks to you, I got the motive to investigate about detonation.

I'm now reading this paper.
http://www.sciencedirect.com/science/ar ... 8089900655
This paper seems to have information about the general dimensional condition for detonation. I tell the contents when I finished to understand this.

[Futterama]

Shyasuna has posted a few more videos on youtube regarding detonation cannons, and I just happened to find some plastic tube that wasn't smooth, so I did a few tests. I can get a good bang but the high pitched sound is not very repeatable with my setup, sometimes it sounds more like a blob than a bang. Perhaps it has something to do with the wind, it's pretty windy tonight.

The spiral in the picture was no good, so I widened the spiral pitch to about 20-30mm and shortened it up to half the length of the tube and this is what could give me a detonation-like sound.

I will have to find some more of this tube, it seems like a good foundation for more testing. The size/diameter is also be very suitable for my RC plane.

[Futterama]

I now have a 2 meter piece of the tube, and tested it in its full length, both with and without an inner spiral. The results are still un-repeatable. It seems like the amount of time I let the gas flow from the burner before igniting it, has a large influence on how the sound is going to be. It's still windy, so this could still be a limiting factor. At one point, without the inner spiral, I got the loudest bang so far, it gave a satisfying light ear-ringing afterwards

I also tried with another burner (and gas), but that couldn't even ignite the gas in the tube for some reason, probably because of poor quality or poor mixing of gas/air.

I'm suspecting both the windy conditions outside and the length of the tube to be important factors in my tests. I don't have any wind-free places to go, so I might try to put a box around the burner so it can have some more "stable" air to mix into the gas.