Noise cannon for RC planes

[hectmarr]

Interesting experience you made with the tank that I hold 20 bar, and then failed by the base of the can, and electronics very well. Your answer is clear. Thank you

[Futterama]

So the project has not had much progress the past few months, I have so many other projects going on, some of which I'm helping others, and when I promise to help someone, I'll do it.

Also, I got into electrolysis of water to get hydrogen and oxygen gas, something I like a lot because it goes boom with a nice deep sound to it. I won't be using any of this on the RC plane but I did get the idea of remote detonation of a few liters of hydrogen-oxygen gas in a plastic bag using the HV spark modules. This would be fun to place on the RC airfield and when a RC bomber plane passes by, set off the bags with gas to simulate explosions, maybe even hide the bags in paper buildings, blowing them up for spectators to enjoy

Anyway, this Sunday, there is a meeting for large scale RC plane pilots where you can talk about the large RC models, building techniques and show off any projects you're working on. So I'm planning to bring my sound cannon for the RC world to see. I hope it will turn heads and bring out a smile on most peoples faces 8)
I'm very exited to see how people will react to my project

This project showoff has got me working a bit more on the project. I just finished a new adapter for the large "barbecue" propane tank for filling the Express canisters with pure propane. The old adapter was way bigger and had too many pockets where atmospheric air could be trapped, and I think some of it has gotten into the small Express canister. It's not causing any safety hazard regarding combustion of the propane gas inside the small canister, but it could introduce moisture to the inside of the canister walls, making them corrode from the inside and weakening the canisters. It has definitely had an impact on the static pressure of the canister, it should be around 8 bar at room temperature, but was lower at around 7 bar. So I emptied the canister and used the new adapter for filling the canister with pure propane. The pressure is now the expected 8 bar.

When the canister was brand new, I took a note of it's weight and wrote it on the bottom of the canister. It is important not to overfill the canister, so I will fill it to the original weight. The strange thing is, that on the canister, it says it has a gas content of 60g. But when I fill 60g of propane into the canister, it's weight is more than when it was new, approximately 20g too much. So I had to take out the excess gas to avoid an overfill condition (if it is overfilled, the liquid gas will take up too much volume and when the canister temperature is rising, like on a hot summer day, the pressure could increase to unsafe levels, so I'm taking safety serious). 40g of gas is also plenty for shooting. And I have spare canisters. And I will completely replace the canisters regularly so I'm sure the canisters I use are not corroded inside.

So I went through some of my previous posts, and I can see I mention that I will mount the corrugated tubing using metal standoffs. This idea was discarded when I decided on a max temperature of 100°C, this is not high enough to cause severe damage to plywood, so the tubing is mounted using plywood which is also what can be seen on the pictures of previous posts.

Currently I'm working on a good way to regulate the gas flow from the canister to the gas mixer. I have been using the valve from the weed burner to regulate the flow directly from the canisters used when testing. The problem with this is, I never really know exactly how much to open the valve to get a good shooting sound. Too low or too high a flow will prohibit the loud sound from the combustion. And it is really unhandy to adjust the valve every time I open it as I will have to have the sound cannon shooting to find the correct setting.
Also I made a big mistake with the valves. They work by seating a taper in a hole, limiting the flow this way. At one point, I was convinced it was a good idea to make the angle on the taper steeper, so it would take less turns on the valve knob to fully open the valve. Of course, I made the change to all my valves. But now that I know I can't have the valve fully opened, I need some kind of flow regulation, and I have been thinking of making a simple flow regulator, based on the design of the weed burner valve, but it kind of doesn't make sense to do all that extra work if I can just modify the valve taper. I can't really modify the existing taper, it does not have enough material to change the taper angle again, so I will be making a whole new taper screw, because with the steep angle the tapers have now, they are just too sensitive to adjustments on the knob.

Hopefully I can get something done tomorrow so I have a good pre-adjusted valve ready for Sunday when I'm going to show people my project.

Pictures of the valve with the new taper screw will follow. Stay tuned! 8)

[jackssmirkingrevenge]

I hope it will turn heads and bring out a smile on most peoples faces

I'm confident it will, make sure to document it

[Futterama]

Today was awesome!

Everyone I talked to, was impressed with the sound cannon and the loud shots it made. Not a single negative comment about it. People wanted to know more on how it worked and one even asked me where I bought it, and I could proudly tell him I made it myself

I had 2 demonstrations during the day. The first demo started OK with a handful of spectators, but then I got some electronics problems with noise from the spark module which then only allowed me to fire 1 shot, then wait 3-4 seconds, then fire another and so on. And it started raining a bit. So I took it inside again and started fiddling a little with the wiring to make it work better. I succeeded and at the second demo, I placed the sound cannon closer to the door so people inside could better hear that something was going on outside. I had around 10 spectators at first and I had to tell some of them to back off a bit, otherwise it will hurt your ears, they didn't really believe it until I started firing

Soon after, I had around 30 spectators, and every time I fired the cannon, I could tell they were impressed. Unfortunately I did not have a buddy to record it, so you'll have to trust my words.

Several people expressed that they would like a sound cannon of their own and one guy even gave me his card as he wanted to get in contact with me. Very cool. I'm not going to sell it as a readily available product, but I will be happy to help people build their own sound cannon.

I got some good chats with people that had ideas on how to improve some aspects of the sound cannon, especially around the electrically noisy HV spark module, so I even got some new ideas to try out.

Another cool thing that happened today was a guy bringing his P-51 Mustang RC model, the exact same model as I have on the shelf, waiting to be built. He was at the table next to me and we got a good talk about the plane.

[Futterama]

So yesterday was a big step for me, showing off my project to everyone.

During the demonstration, I also demonstrated that I can control the rate of fire from the RC remote using what we call subtrim. It was set around 80% to begin with. So I took it to 90%, which makes the firing rate higher. I also wanted to demonstrate that there is a limit to the firing rate at which the noise generated would become sub-optimal. At 100% subtrim, the sound changed to a little less impressive, so the highest setting I tried yesterday where the sound wouldn't suffer, was 90%. Today I measured the frequency of the 90% setting and it is 6.9Hz. That's 414 shots per minute. I think this is a new record!

[jackssmirkingrevenge]

Always good to share the fruits of your labor with like-minded individuals, bravo!

That's 414 shots per minute.

Impressive! Aircraft machineguns are typically twice-three times higher in rate of fire but the vast majority of spectators would not know the difference.

The kinetic energy fetishist in me still wonders what would happen if you put a barrel on it... my feeling is that it could potentially accelerate an airsoft BB to a reasonable velocity, which could make for a very interesting display.

[Futterama]

Always good to share the fruits of your labor with like-minded individuals, bravo!

Thanks!

That's 414 shots per minute.

Impressive! Aircraft machineguns are typically twice-three times higher in rate of fire but the vast majority of spectators would not know the difference.

Thanks again! My final goal is to mount 6 of these sound cannons in my P-51D Mustang RC model. According to Wikipedia, the P-51D was armed with:

six .50 caliber (12.7 mm) M2/AN Browning machine guns.

Furthermore:

The AN/M2 had a cyclic rate of 750–850 rounds per minute.

So even with just 2 sound cannons, firing in between each other, I would be close to the real firing rate, sound wise 8)

The kinetic energy fetishist in me still wonders what would happen if you put a barrel on it... my feeling is that it could potentially accelerate an airsoft BB to a reasonable velocity, which could make for a very interesting display.

A guy actually asked me this yesterday, and I'm sorry, but I don't have the same kinetic energy fetish, so I just replied him that if you stuff a projectile into the small barrel, you wouldn't really be able to fill the initiator chamber and tubing with fresh gas/air mix. But it does give a good kick and it does have some recoil even with just air as projectile.

Sorry, video deleted during cleanup

[jackssmirkingrevenge]

I'm sorry, but I don't have the same kinetic energy fetish

No need to apologize, we welcome innovators with all kinds of beliefs here, however odd!

so I just replied him that if you stuff a projectile into the small barrel, you wouldn't really be able to fill the initiator chamber and tubing with fresh gas/air mix.

If you really wanted to make it work, I envision a separate air intake with a one-way valve in the manner of the V-1 flying bomb's Argus As 014 pulse jet that shuts at the moment of combustion:



I appreciate however that you are just content with the acoustics

But it does give a good kick and it does have some recoil even with just air as projectile.

That's quite the overpressure, I'll wager if you actually had to try and fire something it would pack quite a punch.

Does the aircraft visibly slow down with guns blazing or is there no need to compensate on the throttle?

[Futterama]

I did have an idea to use a valve, but it needs to be able to open with the pressure from the intake alone, so it can't contain any return spring, and it needs to be able to open/close very quickly, and I really couldn't come up with a simple design that could do that. And it works without a valve, so why bother? Except if the valve would increase the volume of the loud noises...

That could be a research task for version 2. Right now, I am concentrating on the spark system, it radiates way too much electrical noise, so much it will interfere with the radio control system

I haven't finished the plane yet, but I don't think the recoil will be noticeable on airspeed.

[mrfoo]

Reed valves can work at extremely high frequencies. The main problem with a valved pulse jet approach would be getting it to fire *slowly* enough. The dominant frequency of a pulse jet is largely dictated by the natural frequency of its "pipe", it's a (mostly) singled ended resonant volume, you can get a rough guess at frequency from here : http://hyperphysics.phy-astr.gsu.edu/hb ... locol.html

If we take the concrete example of a dynajet, it's about 20 inches long, we can assume a temperature of around 450°C, and runs (in real life) at around 250 - 260 cycles / sec. That's about 30 times faster than you want already.

Here's one running :



Something you can hide in the wing of a model is going to run waaaaay too fast (if you can get it to run at all without special fuelling). A 1 foot long valved pulse jet would theoretically run at ~500 cycles / sec.

Now, a valved (or, indeed, valveless, if you're careful) combustor will happily self-fuel via the kadency effect, but your problem is going to be the opposite of a pulse jet designer's job - stopping it from going into pulsed combustion mode as you want to drive it from the spark. Thats a hard thing to do.

[mrfoo]

On the other hand, of course, making a very small pulsejet that sustains is also exceptionally hard to do, it might be possible to do everything you're not "supposed" to do in order to make a thing that only fires once but still self-fuels. The easiest valver to make is the "craft jet", at standard size (and smaller, even with a straight pipe) they're easy to get running (and fun, too, but don't forget your ear defenders)

http://www.john-tom.com/JetTurbinePlans ... rcraft.pdf

The main downside in your usage is the need for some ram air to get the first "charge" in, especially as your supply of ram air is running "the wrong way".

[Futterama]

Here is a video from one of the spectators where I demo the sound cannon.

[youtube][/youtube]

[mrfoo]

Oh yeah!

:thumbs:

[Futterama]

At the demo, I had troubles with my spark controller. It would often reset after the first spark.

I have later found that it is not a good idea to use the RESET pin of the microcontroller as input, as it will easily pick up noise from the spark and even though it is not enabled as reset, it will still "reset" the microcontroller (as the pin voltage spikes above VDD I guess). At least, it became much more immune to noise when I changed this.

Also these noise issues got me testing whether the spark could affect the RC system, and it certainly can, it can make servos twitch, and it can even reset the RC receiver, rendering the RC plane uncontrollable for a few seconds, possibly resulting in a crash.

So at the RC meeting, a guy came up and gave me some hints. Along with what I found myself, I have these considerations to make:

- Don't let any of the high voltage lines be electrically connected to the metal parts of the cannon. At the moment, one of the wires are in contact with the initiator chamber through the spark plug assembly. This will make the initiator chamber act as an antenna for the noise. I'll have to change this and I have a few ideas on how to do that.

- Use ferrite beads or toroids on both the power supply input and HV output of the HV module. I have ordered some samples to test.

- Use a resistor in the HV path to limit the current in the spark. This requires a high voltage resistor, and those are not easy to find and pretty expensive at the electronics stores. I knew regular spark plugs often comes with an internal resistor, exactly to reduce electric noise, and by researching this a bit, I found that you can get spark plug caps with built-in resistors. My plan was then to break open a spark plug cap and hopefully be able to salvage the resistor. So I bought 2 different types of resistor caps at the engine store, and what do you know, one of them could be disassembled using only a screwdriver and out comes the perfect little high voltage 5kΩ resistor, even with solderable tabs at each end! It did set me back around $8 US, and I'll need 1 resistor for each cannon. I'll have to see if ebay can help with cheaper resistor spark plug caps. The spark plug cap is a NGK XD05F, stock no. 8072.

[Futterama]

New spark plug where the high voltage lines are isolated from the steel ball