I have a large octo With powerful motors. It's been in the air many times.

I did some work on it today, changing the camera gimbal. I also installed a New 5,8ghz video tx and a trigger Circuit for operating the shutter on the camera. The shutter and video tx were both Connected to the camera. Shutter was Connected to ch6 on the receiver.

I decided to Power it up in the office to adjust the camera shutter channel and test that everything worked. The telemetry system(Frsky Taranis) warned about low RSSI. Since the only new Connection I had made to receiver/apm was the shutter cable, I removed the servocable from the receiver ch6. The telemetry system stopped warning about low RSSI. So, just to check again, I reconnected the servo cable to ch6. I know I should not do this kind of stuff on a live machine With propellers, and luckily I was taking precautions by keeping out of Reach of propellers. Then all hell broke loose...

8 Heavy lift motors With large APC propellers, capable of lifting a total of 20kg went into full throttle. I just let it go and took shelter, while the machine was doing its thing to the ceiling and Office Furnitures, before shutting Down. Most of the propellers and one motor destroyed. Glad to be alive With all my body parts intact. I've been working on multirotors since "the beginning", and had my share of propeller cuts, but thats a long time ago..  It's obvious I needed a reminder again... :)

So... what happened? I'm not concluding With anything, this may not be related to the APM at all. I suspect that the camera shutter in some way made the receiver bring channels high/low. Logfile Attached. Anyone care to do some analyzing?

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Ausdroid, are you saying you've had instances of props spinning without arming with the A2?

It might not always seem practical to remove props, but at least disconnect one (best to do two) of the 3-phase wires on each motor. Then the motor just stutters instead of going into flying death mower mode.

Another option would be to make a connector with a incandescent light bulb wired in series to act as a current limiting device. Then if the motors try to spin up they won't, you'll just get a total brown out

That sounds like a trouser transforming moment. I once held a smaller F450 sized quad in a death grip as it dismantled itself under full power when it hit a failsafe indoors (first thing I built and I did not know to use thread lock, among many other things).

I then purchased a 0-30V 0-10A bench power supply. A 0-3A one is quite cheap and it would work as well. I set it to current limit before any serious fun can start.

Why has someone not produced a tailor-made device that can be put in line with a 2S-10S battery and current limit to 2A so that systems can be safely checked with props on?

I have also always wanted to put a mini DC jack on my power module with appropriate diodes so that I can connect either a DC adaptor indoors or a small battery outdoors to act as a ground-based UPS so I can power the electronics 20 minutes before connecting a flight battery. I could also then plug it in and swap flight batteries without powering down.

That's a good idea Jolyon, and it also puts you in a position to occasionally unplug and re-plug all of those bullet connectors, which is a good idea for maintaining a solid connection both mechanically and electrically in the long run.

Ausdroid, that's a really nifty idea, no possibility of setting a current-limiting power supply incorrectly etc., bone-simple and solves the whole hassle of removing props. I suggest sacrificing a fuse or two to test that the amperage is indeed appropriate. I'm gonna build me one today!

They have, its called a 2A fuse with a connector at either end. Or use some cheap non rechargeable AA cells, that works just fine if you buy the really cheap ones that cant supply any current :-)

Haha! No comments about the trouser. I was just crouching under mye Office desk, praying the machine would not find the way there. Felt a little like John Connor for a brief moment there.. :)

Indeed. The bigger the props, the bigger the reason to remove them on the bench.

@Mike Well I think I will make me one of those fancy fuse devices! Always thinking of ways to complicate things.

@Kristian - In my case I may choose a quick death at the hands of a 17inch CF prop instead of the reaction of my wife when she would see the ceiling and crown molding 'alterations'.

I had a similar event in my apartment a few months back.  The UHF receiver would arm and then lose connection, forcing the Naza into failsafe, which directs the drone to climb 20 ft before RTL.  After some trouble shooting it seemed to work fine.  A few weeks later I was outside and it did the same thing, but a quick change of the battery, and the problem didn't repeat itself.  Ultimately a few months later it did it again, this time with a large crowd watching from nearby.  The results were almost disastrous, as instead of slowly returning to the ground, the unit started to toilet bowl in a large circle as it slowly came down.  I grabbed it out of the sky at waist height...The motors went full throttle...10 stitches later I replaced the receiver and it has been working smoothly ever since. 

Absolutely. On more than one occasion.  The last time, I had it just outside the workshop, powered up, trying to get the damn telemetry to work. I was no where near the transmitters and it just started to spin up. It was on one skid before the fuse blew.

I don't understand some of these incidents. If the Pixhawk isn't armed then how can the motors spin up?

Also, for those of you who find taking your props on/off tedious have a look in to quick release prop mounts. I use them on all my copters now and they are just great. It's very fast to de prop even an octo and you never have to worry about them being secure as they tighten when the motor spins. 

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