Noise cannon for RC planes

jackssmirkingrevenge · Wed Jan 10, 2018 7:37 am

Futterama wrote:Now the question is, do I get too much air into the air intake or not enough? The easiest to do would be to restrict the air flow and start there, see if less air will make a difference. If not, I will have to make a bigger air intake or start fiddling with forced air from a blower but I hope the natural air intake will do.

If it works when static but not in the air, the logical conclusion would be that it's getting too much air when in flight. With you level of expertise it shouldn't be too hard to cook up a carburetor type valve that can be adjusted in flight with a servo to limit the amount of air fed into the system, or alternatively make a blower that can simulate in-flight airflow on the ground so you can pre-set the restriction for a typical airspeed before takeoff.

Futterama · Wed Jan 10, 2018 7:54 am

jackssmirkingrevenge wrote:With you level of expertise...

Thanks!

Great idea with the servo controlled carburetor type valve!
I'll start by testing on the ground and use a blower to blow air into the intake and barrel and see what that gets me.

Remember, when the plane is in the air, the barrel on the wing is pointing forward and also functions as a kind of air intake.

My theory is that if the airspeed is equal around both the barrel and the air intake, I will need the same exposed area on the intake as on the barrel to even out the pressure created from the air hitting the barrel. Barrel area is around 133mm². Intake area is at the moment much larger. And the propeller probably also adds to the airspeed at the intake, along with some turbulence, so my guess is I'm getting too much air. A simple test can verify, which I'm going to perform as soon as weather is good for it. It will only include blowing air with a blower as I need to fix my engine exhaust before I can run the engine again and that requires a bit of work.

jackssmirkingrevenge · Wed Jan 10, 2018 8:03 am

Futterama wrote:My theory is that if the airspeed is equal around both the barrel and the air intake, I will need the same exposed area on the intake as on the barrel to even out the pressure created from the air hitting the barrel.

That would be my feeling too.

Good news is I did not crash, the frozen ground did undo all my foam tires from the wheel hubs, but no damage to the plane.

Retraction would be difficult but you could consider tracks

hectmarr · Wed Jan 10, 2018 9:26 am

What we use for the pulsorreactores, not to impoverish the mixture of combustible air, when it is in flight, especially in chopped, is a "static air bell". It is a simple element that deflects the air inside the diffuser of the carburetor, and this carburetor works with static air, In the photo is together with the engine stop mechanism. Independently of the speed of flight they are normally about 250 300 km / h. In certain models it is not necessary but in our case, the engine stopped abruptly in this type of maneuvers.

Futterama · Wed Jan 10, 2018 9:38 am

hectmarr, can you post a bigger picture and maybe a link to some reading of this "static air bell" because Google does not seem to find anything relevant.

hectmarr · Wed Jan 10, 2018 10:09 am

Ok I'm looking for some info and I'll add it here. But the "idea" is very basic. You just have to avoid the direct entry of air with a stopper that should let air in only on the sides. You can test with the air of a compressor. You must determine approximately the speed of your model, and simulate that speed with compressed air, and see if you stop having that problem. I imagine it will be solved.

hectmarr · Wed Jan 10, 2018 10:26 am

In figure 1 is the fully open admission of the pulsorreactor.
In figure 2, it is with the obstruction system, which I added to you.
In figure 3, an old, real drone apparently with the system incorporated.

Futterama · Wed Jan 10, 2018 10:31 am

Is this the way the air moves, some of it into the holes for the intake and some moves past the holes?

I have ordered some barometric air pressure sensors on ebay, with those I can log the pressure in different locations, and I plan on using them with a WiFi module, then I can see the pressure in real time from my mobile phone. But it takes 4 weeks for them to arrive.

hectmarr · Wed Jan 10, 2018 11:59 am

It is right. The dynamic pressure of the air does not directly affect the air intake. This allows any system to work with "calm" air. You can, if necessary, make a hole or some, in the cover, to achieve calibrate the system with precision. I have the Castle Creature pitot tube and it works properly. Measures the differential pressure and the maximum, is recorded in the on-board indicator. To have patience until the sensors arrive.

Futterama · Wed Jan 10, 2018 2:37 pm

Thanks, I'll keep this in mind.

I went testing tonight, and my sound cannon is very sensitive to air blown into either the output or input. I have a few ideas to check out to fix this, but now I need to concentrate on fixing my exhaust and hope for good weather for more in-flight testing.

Futterama · Thu May 31, 2018 6:50 am

It has been quite some time since I last updated this thread, sorry about that. But there is not much to tell.

I'm still trying to make the sound cannon work on the plane in the air. I'm sure it's an aerodynamic issue regarding air intake and the forward pointing barrel.

The plane keeps setting me back. It seems like for every time I fly the plane for testing, something breaks. First it was my homemade exhaust. Since it broke and also decreased the engine power a bit, I made a new and smaller exhaust, which works OK but it decreases the power even more, but it works.
Then the landing gear got stuck in the up position on my latest flight, so I had to belly land the plane, no damage done but I need to fix the landing gear before I can go fly the plane again.

I'm currently building a second full metal cannon which I want to mount on another plane, a smaller and more simple plane, one I actually got just for this purpose. It's with an electric motor, foam wings and plastic fuselage. It's very lightweight but I have recently tested it's ability to carry a dummy load of 600g. with no issues so it should be able to carry the sound cannon but at a slower airspeed.
The plan is to mount the cannon on the belly of the plane and put the propane canister inside the fuselage. Then I have more freedom to change the aerodynamics around the air intake and barrel because it is easy accessible and I can 3D print some shapes.

I still have my successful in-air experiment from earlier to look back upon, which is basically what I'm trying to reproduce now with the all metal cannon but on a more weight-carrying-capable plane.

The picture attached is the plane I'm going to do the next tests with.

Futterama · Thu May 31, 2018 7:07 am

Oh, for those interested in the actual workings of DDT (Deflagration to Detonation Transition), here is a video I found which shows the flame burning in a tube with obstacles:

[youtube][/youtube]

Futterama · Tue Jul 03, 2018 3:21 am

Sound cannon has now been mounted on the test plane. During only 1 evening of testing, I found the sweet spot that generates loud bangs. More testing is planned to optimize rate of fire and testing shapes that better resembles the bigger plane installation.

Meanwhile, I'll need to order some aluminium tubing for a new landing gear for the big plane.

This project is still in progress, now I just got a lot further as I have a good idea what was wrong with the setup on the big plane

Futterama · Wed Jul 04, 2018 5:07 am

I went testing again last night where a lot of my club buddies came to see and hear the cannon in the air.

At first, I couldn't get any ignition from the cannon, it turns out the propane nozzle was blocked off by a tiny piece of dirt. I managed to remove it, but would like to avoid something similar happen when we are talking the cannon mounted in the big RC plane.

The nozzle has a cylindrical cavity of 2.5mm in diameter and 7mm in length which the propane passes just before it is entering the small 0.3mm orifice on the nozzle. I was hoping to find something I could put into this space to filter out any dirt that might get into the system.

Any ideas on material choice for this?

Maybe just stuff the hole with cotton?

Since this happened on my test plane, I'm wondering where the dirt came from. If it was from me attaching and detaching the propane tubes and laying them aside on the dry ground, the same wouldn't happen in the big plane as the tubes would mostly stay in place and probably never be taken apart in the field, only at home in the "clean" environment of my workshop.
If it was from the propane canister or from me refilling the canister or even from the propane itself (unlikely), it would be easier to make an inline filter for putting onto the propane feeding tube between the canister and open/close valve.

hectmarr · Fri Jul 06, 2018 9:53 am

It occurs to me to clean the nozzle with compressed air from a small manual air pump when necessary. You would have to leave a connection for this that should be blocked when the noise gun is operating. Maybe, a "T" connection. On the one hand the propane, on the other hand the auxiliary entrance lara to clean and the other way, to the mouthpiece. It's just an idea, I do not know if it's possible to apply this in your design, but I think so. Your work is very nice

Noise cannon for RC planes

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