How I lost my DJI F550 Hexacopter on its first day!

(This is a repost of a blog I wrote back in September 2014 on another site…)

DJI f550 Flamewheel

DJI f550 Flamewheel

Mine is a long and sad tale, and I recount it here that others may learn from my bitter experience and thus avoid the same tragic events from befalling themselves. It is a story of a series of unfortunate events and disastrous coincidence for which my inexperience left me woefully unprepared. These words of warning come at a heavy price – read ‘em and weep.

Prelude

So Drone Number 1 looked just like this (only with the stock landing feet) and after two days of lovingly soldering and screwing it all together I took it out for its maiden flight in the Churchyard round the corner. Everything went fine, and for a first ever flight I was quite pleased. However, there was not a lot of space here, so decided to take it down to Baiter Park for the next flight which is just a 20 minute walk down the road.

The battery was fully charged and having just seen how stable and responsive the Naza flight controller and GPS stabilization were, I was full of confidence. I marched down to the park, fully equipped with

  • DJI Flamewheel F550 drone (+Zenmuse gimbal and GoPro)
  • Futaba T8J 2.4GHz RC Transmitter
  • Black Pearl 7″ video receiver/monitor (5.8GHz)
  • Great expectations

The Tragedy

I set up on a large patch of grass about 50m from the narrow beach that separates dry land from Poole Harbour. I didn’t plan on flying over the water, and there was plenty of space and thankfully no dogs. I fixed the monitor to the top of my transmitter with a Gorilla Pod (as I had yet to order a clamp made for the purpose), fired everything up, and lifted off.

All started well, and after a few seconds hovering around 10m high I began a wide circle out towards the beach which ended up with the drone just skimming the beach line as it came around in front of me. The sun was just setting and the views from the GoPro on the Black Pearl monitor were spectacular.

Then all of a sudden, with no warning, the drone stopped moving and hung in the air for a moment. I jiggled the sticks around but there was no response. Then without warning it shot up into the air another ten or twenty feet and I began to panic a little, and at that point it stopped again. Except it didn’t quite stop. It began a slow and gentle descent while at the same time drifting out over the water at a leisurely pace. In horror I slammed the throttle up, tried to spin it around, bring it back, and when all that failed I flipped the ‘Fail Safe’ switch to put it into ‘Return to Home’ mode. Nothing had the slightest effect. Gently, gently it drifted away getting always lower and lower until about 30m offshore it silently slipped serenely below the surface of the sea.

The Rescue

As I watched it gently kiss the waves I was already running across the beach and splashing into the water after it. Swimming out to where it went down I searched as best I could in silty salty water with no goggles, but to no avail. I then ran home and back to get my mask and snorkel, but by the time I returned the sun had set and visibility under water was almost nil. Totally devastated I returned home to the most morose evening I have ever had.

Early the next morning my wife and I both donned masks & snorkels and went back out to the little beach, trying to identify where it went down. After half an hour of fruitless searching I was on the verge of giving up when whoops of excitement from Megan announced her totally awesome discovery.

However, having carefuly washed and soaked it first in tap water then de-ionised water to flush the last of the salt from the electronics the sad news became clear. Practically nothing was salvageable.

An Expensive Lesson in Electrolysis

Electrolysis is the action whereby electricity flows from one electrical terminal (the anode) through an ‘electrolyte’ (in this case, salty water) to the other terminal (the cathode). In doing so it removes metal from the anode and deposits it on the cathode. This is kind of the reverse of how a battery works, and is used in silver and gold plating jewellery etc. Unfortunately for the drone, the battery was connected the whole twelve hours it spent on the sea floor, and the results were (electronically speaking) horrific.

Result of electrolysis on the video transmitter.

Result of electrolysis on the video transmitter.

It actually cleaned up quite well, but on closer inspection you can see the result. The ceramic blocks in the picture below should have a metal end cap on both ends (like the ones you see above). However, the metal has been eaten away from the positive end of these two caps and deposited as the crystalline mess you see above. All the parts on the circuit which were connected to the positive end of the battery have suffered more or less the same fate.

All metal connected to battery +ve is eaten away.

All metal connected to battery +ve is eaten away.

Sadly, everything electronic was written off. Several of the speed controllers has actually burned out (try spinning a prop at 9000 rpm under water!) and all the rest would have suffered similar to that above. Even if some still worked, would it ever be reliable enough to trust it to flight? I don’t think so!

The Post-Mortem

So, what went wrong? I slowly began piecing together fragments of the puzzle, but did not fully understand what happened until some time later when I had built Drone 2 and was flying it down at Baiter (a little further from the water this time!)

Radio Interference

I had the same configuration with the Black Pearl attached to my transmitter so I could view the first person video while flying. I got out to about 20 or 30 meters and suddenly lost all control. The drone stopped and hovered for a few seconds, and then shot up into the air. Sound familiar? But this time it completed its ‘Return to Home’ manoeuvre by climbing to 20 meters, flying back to its take off point, and sweetly landing itself.

Perplexed at this action (but pleased that it actually did return home – and landed within a meter of its take off point!) I took off again. Again at 20-30m out it did the same. Conclusion, the radio range was terrible! It was losing the transmitter signal at just 30m range, and then quite correctly executing its failsafe return to home sequence.

In an epiphany of understanding I ripped the Black Pearl monitor away from the transmitter and flew again. This time I took it out hundreds of meters with no mishap. The monitor, attached to my transmitter, was interfering and killing the signal – this is just how I had it set up that fateful evening a week before.

Battery Voltage Settings

But that, of course is only half the story, because why didn’t is just up and return home instead of drifting out to sea?

The Naza flight controller has a battery voltage monitor with two levels. At the first warning level it flashes the red led to tell you its time to bring it home. At the second (lower) level, if you are still flying and it gets that low, then rather then just fall out of the sky it executes a controlled descent and landing. This happens wherever it is, whatever it is doing. Rather than try to complete a return to home, if the battery hits this critical level it will just give up and descend. Of course you can still override this behaviour by cranking the throttle right to the top, so you do have some control just in case it happens to descend over a busy road (or into the sea).

Now, to set the battery warning levels you have to fly the machine and measure the difference between the no-load voltages and the in-flight voltage so you can work out the losses that occur in the battery while it is under load and account for them in the settings. The instructions say ‘set a reasonable level first, and then fly…’ so I figured the default levels should be reasonable to start with, at least for the purposes of doing the measurements. In fact they were a little high, but I didn’t know at the time and actually it wouldn’t have mattered if not for the radio interference.

So what actually happened?

The monitor killed the transmitter signal and the drone lost contact.

The drone correctly went into failsafe return to home mode and started to climb to 20m.

At this time the critical battery level triggered (the extra juice required to climb just tipped it over the limit).

The drone correctly went into emergency controlled descent mode.

Normally I should have been able to give it some stick and regain control and bring it home. However, I had no radio control, so the drone didn’t see my stick input. Instead it just kept descending.

The gentle breeze was the last thing that was needed to complete the disaster. In the fifteen seconds or so it took to come down, the breeze carried it further out over the bay.

Here endeth the first lesson.

And a bloody expensive lesson at that. Moral of the story? Do not obstruct your transmitter in any way. Until you are confident that your battery levels are set correctly, expect an emergency descent at any time. Keep it low and close until you are familiar and know how it will behave. Stay clear of the water until your are sure its going to behave. Oh, and if you’re going to get married, pick a wife like mine who never once berated my stupidity or the cost of my mistakes, and who picked me up at my lowest ebb and took me back out to find it. Megan, you’re a brick!

xXx

Author: sp_admin

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