We lost and luckily found our Shrike this week! So what happened? Long story, but we believe the following occurred to create the situation:


1.) We were flying from inside a car (Tx only, vRx was mounted on top of the car) which was no doubt reducing the range of our Tx.

2.) APM was set for a 45% throttle cruise speed for Auto & RTL, which for our airplane, was not enough to safely return it home in the 20mph head wind we were facing during RTL.

3.) When the airplane was at it's furthest point, it lost signal from our Tx, causing it to kick into Failsafe, which is to Loiter once, and then RTL.

4.) The airplane went into loiter, gained a little connection from our Tx, returned to back to Auto (the mode we were in), then lost connection, when to loiter, gained signal return to auto, etc. all the while a 15-20 mph wind is carrying it further and further away.

5.) When we finally turned the Tx off to allow the APM to only RTL, it was too late, the battery was too weak and the wind too strong for it to RTL.

It eventually lost enough power to safely land with stabilization carefully returning it to level and controlled decent.

What did we learn & what are we doing to fix it? A ton. We literally have over 50 tasks to make prevent this from happening and to aid it recovery. A few of the main ideas:

1.) Get the Tx out of the car to improve range

2.) Set failsafe to skip the loiter and go straight to RTL

3.) Increase RTL throttle rate to allow for headwind recovery

4.) Place a buzzer on the plane to assist in recovery

5.) OSD/Telemetry to assist in navigation

6.) Elevate vRx to increase signal quality

and many, many more.

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  • Here is a video of a UDB3 jumping between Waypoint and RTL modes. I was using 35Mhz radio and I think the various radio frequencies in use at the lighthouse on the cliffs behind the plane and the telephone masts on the hills in the distance were the cause. I switched to 2.4GHz and never had a problem again...


  • Developer

    I thought we had fixed that failsafe trigger issue. Sorry about that. 

    I hear we found an issue with the chip erase function from Atmel on the Flash. Tridge has implemented a fix for that to check for out of spec chips.


  • Wow, that is incredibly lucky.  Not just that you found it, but that it landed safely.  How come you guys have been operating so long without the Telemetry?  Definitely need that.

    And a pretty good video, I learned some things too.  The "what happens when it goes in and out of failsafe" definitely needs consideration!

    The buzzer is a great idea.  Maybe I'll take that on.  Simply turn on a buzzer if groundspeed is 0 for more than 2 minutes or something like that.  What do you guys think?

  • My understanding is that the MIN_GNDSPD_CM parameter is supposed to force the APM to adjust the throttle in order to maintain a given minimum ground speed while flying into a head wind. Unfortunately I can't get it to work myself, but you may have better luck? Its definitely an extremely important feature, especially in cases like this.

    There is a forum topic regarding it here: http://www.diydrones.com/forum/topics/airspeed-vs-ground-speed-with...

  • A car roof is actually a really good place for an antenna. Mounted in the center it will use the roof as a ground plane and considerably extend the range. An antenna inside the car will have the opposite effect.

    Also use the APM minimum ground speed feature. In wind this will increase the throttle to ensure you don't go backwards.
  • Definitely go to CP antennas on both video transmitter and receiver and consider using a helical antenna on the receiver (much more directional) to assist in direction finding and extending the receive range. Not sure the frequency you are using (900mHz, if I recall correctly). Antenna size might become an issue with a helical unless you are considering going to a shorter wavelength (5.8gHz, perhaps).

  • I know that feeling, including the against-all-odds recovery. Include your contact details in the aircraft right away and look into a defense-in-depth strategy for failsafe and recovery. Independent methods of locating and perhaps direction finding, and so on.

  • You were incredibly lucky to locate the plane - wow, just another 50 feet into that tall grass and it would have been really tough to visually locate ... or a gated road or private property with dogs. Glad you were able to locate it and it all worked out okay.

    And respectfully I would like to suggest that flying right next to a freeway or for that matter, now that you're showing us your maps planning your flights over the freeway is not a good idea. I know it's highly unlikely but just imagine if your plane came down on the freeway and caused an accident. You always think it might never happen but just like you had amazingly good luck finding your plane - luck can also take a different turn. The best way to help prevent an accident is to try and avoid it in the first place. Looks like you have lots of rural areas to fly so please consider planing your flights over farmland.

    I always enjoy your videos, your passion and determination - keep improving your system, learning and having fun.

  • Maybe a future feature for APM a buzzer that comes on after a settable length of time, useful if you forget to unplug the battery or loose your aircraft ! - Or to beep at any other event low battery, time, etc
  • Moderator

    Yep weather is a big factor,  I try not to fly in wind speeds that are greater than 50% of my cruise speed. 

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