Transmitter failure, but APM saves the day

3689402527?profile=originalI finished my SkyWalker build last night, and went out to CMAC to take it for its first flight. All the ground tests looked good (including the new 'system check' in mavproxy) so we took it for the first flight. About 60 meters into the flight I heard "MODE STABILIZE" from mavproxy, and I lost control. Jack and I watched as the SkyWalker glided gracefully to a perfect landing in the grass while we wondered what had happened.

After looking at the mavlink logs, we saw that the APM had started to receive failsafe values from the receiver soon after takeoff. Jack thought the antenna on the receiver might be a bit dodgy, so we replaced it with a 2nd receiver I happened to have in my box of spares, and we tried again. It was almost an exact repeat of the first flight, losing transmitter control soon after takeoff, APM going to STABILIZE mode, and the SkyWalker landing perfectly in the grass.

We then started to do more extensive ground tests and found that the Turnigy TGY 9X transmitter had failed. It has previously had good range, but for some as yet unexplained reason it now doesn't do more than 60 meters or so.

We had a good look inside the transmitter module, and we can't see anything wrong. The antenna cable is a little bit abraded on the transmitter, although it isn't clear if that is the cause of the issue. Strangely, the range test button doesn't find the problem. I get the same range with the range test button pressed as when its off. When I got home I checked the range test switch in case it was faulty and it seemed to be working fine (at least as far as my multimeter can tell). It looks like I might need to buy a more expensive transmitter. Pity, as otherwise the TGY 9X has performed well!

The real hero of the flight was the APM. The failsafe values in my receiver had the mode switch set to STABILIZE, and it glided the plane down to a perfect landing both times. I don't have it set to RTL for failsafe as it would be a breach of the club rules to circle above the takeoff point (you'd be going over the clubhouse).

Tomorrow we're going to try a different transmitter and see if the SkyWalker can live up to its reputation as a great UAV. We've got a "mow the lawn" search pattern setup to simulate the type of search we will be doing for the outback challenge. If that goes well we will put the pandaboard and ptgrey camera in the plane and get back to testing our algorithms for finding Joe.

Once things look OK with the SkyWalker we'll see about putting all the equipment in our CyberHawk UAV, which is what we plan to use for the OBC next year.


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  • i'm trying to build a skywalker platform too, but stumped with what motor to get, what are you using to power your skywalker?

  • The antenna feed cable is often carelessly manipulated as it was a normal wire and it is not, it's a delicate thin coax cable.

  • I took the module off and there was a fair amount of slack in the wire. Also, There was a 90 degree kink in it and it looks like the metal part of the wire could be shorting out. It is likely the transmitter as stated above.
  • I had a 6ch from HK fail in the exact same way. I never bend my antenna on my my HK Txs anymore.
  • Developer

    yes, I did try the range test button. The strange thing was that I got exactly the same range with the range test button pressed as without it. That made me suspect that it was the range test functionality that was broken, by being permanently engaged. The only thing I could think to test in that regard was to use a multimeter to test the range test switch, and make sure that it did make/break on press. I did that, and the switch seems to be working fine. It is still possible that something else in the range test functionality of the transmitter is broken.

    As Jack said, there is quite a lot abrasion on the antenna cable (due to me not carrying the transmitter in a proper case). It is hard to tell if that is the cause of the problem, although it is an obvious suspect. The main thing making me a little reluctant to just say that is definately the problem is that if that was the only problem I'd expect the range test function to still work, and to cut the range by even more. (anyone know how the range test button works? does it de-tune the antenna? or maybe lower the transmit power?)

    Anyway, as Jack says, I'm now thinking of getting a new transmitter module and hopefully when I plug it in it will have good range. I'm thinking of either this FrSky one or this Corona one. If that doesn't work, then I think I'll just get a new Spektrum DX8 (assuming my wife approves!).

  • On review at the scene, the problem was almost certainly as I.S. suggests - a fault in the the coax antenna feedline where it passes the hinge in the base of the antenna. Indeed, a bare patch in the feedline covering can be seen at this point.

    The top part of the antenna is known to become dislocated too easily, and has endured periods of time during transport where the only thing stopping it being lost is the feedline.

    Tridge is considering replacing the transmit module rather than repairing the old because for our insurance to apply, our equipment must have the "C-tick" label showing compliance Australia. The modules come with this compliance label.


  • @ I.S

    I tried a different receiver (flysky from nitroplanes) and the range was much better. I never walked far enough away for it to fail w/ the new rx because I got to the end of the park - 100yds

  • @Jimmy

    Did you try using a different Tx module? I mean is it a controller or module issue?

    My #1 suspect is the hard wired RF wire which is prone to snap & twist (and finally break inside?) which could result in a diminished range, though all the circuitry doing fine.

  • Hi, sorry, but I have to comment.

    I had the exact same problem with my Turnigy 9x  transmitter/receiver the weekend before last which resulted in a seemingly minor crash that ripped the connecting rod for the wing out of the fuselage (major damage). I still don't know what caused the problem, but I was able to replicate the lack of range by holding the plane together and walking 60 feet from the transmitter w/ a battery pressing down the right stick.

  • Moderator
    The STABILIZE mode is the better option for failsafe. RTL would put you circling around with no transmitter to land with... FBW keeps you flying to where you have pointed it to go, again without a transmitter. As explained it safely glided is way down to a safe landing.

    Good job.
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