Comment by Harry on July 15, 2012 at 3:26pm

Glad it wasnt worse for you!  I cant help but notice the stickers; "Electrifry" and "RimFire".  I'd say you need to look for the hot spot and try to get to the cause.  Maybe then with some help you can pinpoint it and see what is salvageable of the elctronics.  First thought was electronic spray cleaner, but safety first.

Comment by Harry on July 15, 2012 at 3:43pm

I read it again.  Is it possible you had motor wires touching like maybe connectors were exposed a little and they touched from being jostled?  I had an ESC smoke once from a motor short.  I hit throttle and got a smoke and light show. 

Comment by Vernon Barry on July 15, 2012 at 3:45pm

FIrst off, since you don't have a BEC and further, were powering the APM via a second battery, the only explanation for the main battery and ESC to fail is the ESC and or wiring was bad. In other words, it's not like the APM was able to send a "kill" signal to the ESC and cause this fire just because you turned the transmitter off.

It's an RC airplane first, and then the APM only adds functionality. Had you not had the APM, it still likely would have done exactly the same thing. There is no signal that the APM could have put out, and further, that your ESC interpreted to cause it to fail.

Several wires leading to the ESC (two motor wires and battery positive wire) were melted right through.

Exactly as I am saying, you had a wiring issue totally unrelated to the APM or any other controls.

Does anyone know of a condition whereby the autopilot would supply max current to the ESC if the connection between TX and RX is severed?

As I stated, there is no signal other than max PWM AKA full throttle. The same signal that your RC receiver would put out. There is no Max current, only the same PWM as the radio, and in fact, when you calibrate the radio in mission planner, you can compare the input and output singals looking at the raw data screen. You can also see what outputs do in failsafe when you turn off the transmitter to prove to yourself there is nothing funky about the APM.

 

What I did notice from the photos, is that your main batteries are in series. Maybe I missed it but you didn't post the main battery specs for each one.

From this thread here http://www.rcuniverse.com/forum/m_7341988/printable.htm I see the ESC and Motor you mentioned are used by others but one thing stands out, it's rated for 5 cells MAX. If you plugged in 2 each 3 cell batteries, that's 6 cells or one more cell than the ESC is rated for. This would be the exact thing to blow the ESC sky high.

Comment by Jiro Hattori on July 15, 2012 at 3:47pm

It looks like wires from ESC? get burned seriously. The connector to the LiPo looks still has shape.

Comment by Jesse on July 15, 2012 at 3:49pm

This is exactly why I will never venture from nitro/gas engines... no crazy, overpriced, spontaneously-combusting electronics to deal. Just fuel and go! :-D

Comment by Ritchie on July 15, 2012 at 4:03pm

Lets quantify the situation.

APM is 5v and less than 1A - safe

ESC is whatever your LiPOs are -  create a spark

LiPOs - create spark or fire

Its doubtful that the LiPOs did this as when they go up a fire extinguisher would not have helped.

If a spark was all that occured then what did it burn? the electronics are not flammable so look for something else was the fire fuel. You need to isolate all forms material within your craft to check for flammability. Some plastics can produce fire if heated sufficiently so you may have the ESC too close to some plastic.

ESCs have been placed inside airframes with "sufficient" space and deformed the EP around it very easily (my first easystar). Also dont forget fibreglass has resin to harden it and that is highly flammable.

These are just off the top of my head but i hope it helps you narrow your focus

Comment by Matthew Schroyer on July 15, 2012 at 4:23pm

First, thank you all for your responses.

@Harry - I had tested this RTF kit before without the APM 2.0, and no such issue came about. If it had a wiring issue, I think it would have exhibited odd behavior at that time.

@Vernon - I would chalk the melted wires to the fact that the ESC blew, and I'm not ruling out the ESC as being a part (or even the major part) of the issue, but I am confused about something you said. The APM was still powered even as the transmitter was shut off, as it had a power source independent of the main battery and the ESC -- are you sure there's no way for the APM to send a throttle signal? Maybe I'm not understanding you 100 percent, but doesn't the calibration screen only register inputs from the receiver (not what it's outputting)? I've gone back to the mission planner radio calibration to check the behavior of the receiver when the transmitter is cut, and the throttle drops out every time. It doesn't appear the receiver is sending max throttle when the TX is cut, at any rate.

Comment by Harry on July 15, 2012 at 4:40pm

That's the thing about motor shorts, they can be intermittent with changes in torque or how the bearings are seating.  If it is an internal short in the motor, first place to look is the front of the motor where a winding could touch the bell.  A telltale sign of that is the wire is shiny where the insulation came off from rubbing and is flat spotted. The windiings on a motor can be fragile before they go into those beefy wires on the outside.  

I was also wondering if bullet connectors had been bumped and enough exposed to short if they touched accidentally.

Comment by Matthew Schroyer on July 15, 2012 at 5:11pm

@Harry - A short in the motor wasn't something I hadn't considered, but I need to inspect the motor anyway. The bullet connectors actually appeared fine - this RTF came with the connectors already connected, and the connection was shrink-wrapped. It's actually a pretty nice package. But again, the motor was still at the time the TX was turned off, and I don't think the motor would turn at all unless it had a current coming from the ESC.

Oh, and to clarify my last comment, the APM actually appears to be working after all of this -- that's how I was able to connect it to the PC and the RX and check the calibration again. I suppose what I really need to do from here is connect it to a new ESC and a different motor and see if I can get it to replicate this behavior. I might just be able to find some time tonight to do that. Outside, of course.

Comment by Björn Geir Leifsson on July 15, 2012 at 5:14pm

There's a sizeable electrolytic capacitor in  ESC's. It looks like a small drum, usually sitting between the feeding wires. From the pictures it looks like this  capacitor burnt up and torched the hole in the fuselage side.

Here's a video that shows how these things can fire if overloaded:

http://www.youtube.com/watch?v=M8nT97CrgcU

As suggested, you should check if you fed the ESC with too high tension. I agree that it is extremely unlikely that the APM can have caused this.

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