Hi there 
It's been kind of a while since we published the last article. Actually the topic of this one is pretty cool and can be used for many purposes such as mole hunting 2.0 :D.
So everything started with an idea. A simple idea. "Hey! we should drop fake bombs from our R/C plane!". Then, "Hey, smoke would look great on this R/C plane!", and finally, "Hey! We should launch fake rockets from our R/C plane!" (and yes, we are grown up people with employement... )
We started doing it by hand, firing the smoke and the firecrackers with matches just before to take-off. It was nice, but we had to hurry and we could not decide on when the smoke/firecracker should be launched.
The principle
After looking on the Internet for automatic ignition systems, we came up with our personnal DIY system, which, while not perfect, gives pretty neat results!
There isn't that many options to automatically light a firecracker's wick:
- We can use the principle of a lighter, that is, having an embedded tank of flammable gaz, a system to generate a sparkle, and an automatic valve.
- Or we can use a thin steel wire and put a large current run it so it would become incandescent.
The first solution would involve a too complicated system for the sunday DIYers that we are. Also, we were not sure how it would behave with all the relative wind there is on a plane. We decided to adopt the second method.
To have a reliable and effective ignition system, the key is to reach a temperature that would effectively light the wick, but not TOO hot in order not to destroy the wire or even the plane's battery.
First version
The coil
So we started by using a wire that came from a stranded wire (itself coming from an old computer power supply unit). We took 3 or 3 strands, twisted them, cut around 10cm (4inches) of it and shaped it as a coil around the wick. then, we put our protective glasses, gloves, and put the power. the coil instantly melted, ligthing up the wick! So it worked
The switch
We only had to find a way to put the power when the plane was in the air. That was a tricky question because in an R/C plane even if you have a very powerfull battery that can provide 300 or 400W easily, you still need a switch that will withstand this kind of power. But in a plane you also have the PWM output of servos comming from the R/C receiver. Several solutions here : the most aesthetic one is to buy/design an electrical relay that is triggered by a channel of your R/C controller. but as I said, we where in a hurry to make this plane in the air, so we opted for the most epic switch ever: a servomotor with hot-glued wires on it that make contact when we trigger a channel on the transmittor. Effective, very ugly, that was perfect!
So it worked like this for a few flights, with the obligation of crafting a new coil for each successful ignited firework. kind of boring after a few ones... So we decided to improve a little the system.
The improved version
Basic physics..
I had some steel wire remaining from an old project (a hot-wire cutting table to cut airfoils out of foam blocks). I knew that it could become very hot without breaking by experiment it back then. So we first got some wire, and measured the electrical resistance per meter which was around 10ohm per meter. Let call it Rw.
The resistance of a wire is proportionnal to its length. So we have:
By experimentation, we also knew that a power of around 100W would do the trick : it's enough to provide very big sparks, but not enough to damage the battery and to reduce the autonomy of the plane. So to calculate the length of the wire we need, it's really easy physics :
it's a simple resistive circuitry, so we have the famous
Also the power consumed is given by
Injecting the ohm equation gives us
We inject the very first equation giving us
Only very basic math and physics here. Applying it to our problem gives us (the average voltage on a plane that is flying with mid throttle on 3S is 11V).
On the Bixler...
That is a neat result because it allows for a small enough wire to be shaped as we want to make it easier to lock the wick. We decided to cut it in half and put it each side of the Bixler's wing. We cut a can and glued it to protect the wing from the rocket's projections. We also doubled the circuitry with 2 switches to have another ignition system to light up smoke
Here is an amazing video demonstrating the rockets and the smoke bombs, it also features some night flights with the Bixler 2!
