APM 2.6 - AP3.2.3 - Spiral of death in FBWA

Hi Everyone,

Very weird crash today. I have 12-15 flights all flawless before this incident. When it occurred I immediately thought I lost a servo - but upon inspection both servos were fine. Watching the video (below), I did not input the left roll that began the spin. I tried to switch to 'manual' but that made no difference; it's like I had no attitude control what-so-ever.

-APM 2.6 Plane 3.2.3, GPS with external compass
-3DR power module
-APM airspeed sensor

Here's a video of the crash - winds were about 10-15mph hitting the aircraft from the right side. I left the sound in, so maybe lower your speakers before clicking the link:

Attached are both the telemetry log from ground station & dataflash log.  

Any help would be greatly appreciated! Trying to rule out the autopilot as the cause before venturing back out. It almost looks like a stall, but I'm doing at least 20mph when the spiral begins.. I've flown this plane at 10-15mph without any issue.

logs.zip

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Replies

  • Typical stall, one wing drops, then spiral.
    The ONLY way to get out of a spin, is DOWN elevator. Yes, it sounds crazy, but it will work. Also, use RUDDER, not alerions. Once spin is stopped, reduce power, then up elevator, alerions, as normal.
  • How did you generate that graph? Thanks!

    Raph said:

    Hi Scott,

    Thank you for your input. Are you aware of a good way to check if the airspeed sensor is badly tuned? I ran through the calibration as directed; on the ground it shows a value of 3-4, and if I blow on the tube it does increase..

    I attached a graph from the crash showing altitude, GPS speed & airspeed. To an untrained eye (me) it looks ok? I also increased my FBW min speed as well..

    download?id=705844%3AUploadedFile%3A2002229

  • I always do a calibration because as the board warms up there tends to be some drift. This is especially true for the baro.
  • What type of plane is that?
    I'm on my phone so can't veiw the logs, but looking at the video it looks like a classic tip stall to spin. In FBW the autopilot will react opposite to what is required to stop a spin. The left wing was very close to stall and started to lose lift so it dipped. FBW dutifully attempted to lower the left aileron which increases the Angle of Attack (AOA) to create more lift on that wing, and the opposite on the other wing. Since the AoA was already critical, it caused the left wing into a deep stall. It simutaneously lowered the AoA on the right wing, preventing it from also stalling. So now you have one wing deep stalled and the other one flying. To make things worse, the throttle went to max creating torque and gyroscopic effect which increases the rate of rotation and flatten out the spin making it hard to recover. The proper recovery is to quickly switch to manual, cut the throttle, push the nose DOWN, and if you have the skill (few do) apply rudder opposite to the spin direction. If you have a flying wing (especially the X8)you may need to put the stick full nose down and roll into the spin to get full down Elevon deflection.
    Why did you stall at a higher speed than expected? It could be:
    1. You hit some turbulence
    2. You were flying in a slip because your yaw control is not tuned, or you have a wing with no rudder.
    3. Load factor (pulling more than 1G)

    Did you have the new Stall_Prevention code activated? It's a new parameter created by Tridge to prevent this scenario.
    Is your ARSP_FBW_Min set too low?
    • just a correction - double checked my params and STALL_PREVENTION was enabled (set to 1) when this crash occurred. 

      • I've had this happen more than once in the past (before stall prevention was added).  If you switch to manual fast enough you can recover.

        Since you had it happen with stall_prevention enabled, you either didn't have your airspeed sensor well calibrated or your min speed is set too low.  The airspeed sensor is not super accurate, especially at low speed so you need to give it a few MPH of margin.

        • Hi Scott,

          Thank you for your input. Are you aware of a good way to check if the airspeed sensor is badly tuned? I ran through the calibration as directed; on the ground it shows a value of 3-4, and if I blow on the tube it does increase..

          I attached a graph from the crash showing altitude, GPS speed & airspeed. To an untrained eye (me) it looks ok? I also increased my FBW min speed as well..

          download?id=705844%3AUploadedFile%3A2002229

          baro-airspeed-gpspeed.JPG

          https://storage.ning.com/topology/rest/1.0/file/get/3702786354?profile=original
          • It appears that just before your altitude peaked your airspeed dropped to 9 m/s. Since your airspeed is reading 2-3 m/s when zero, it seems to be reading faster than it really is. It could be that you were actually flying 6-7 m/s when the stall occurred. There can be a bit of airspeed noise when the airspeed is close to zero, so I can't definitively say what the speed was.
            Did you do a preflight calibration before the flight? I do this calibration before every flight to be sure the airspeed is zeroed out after the APM warms up. Do you have the digital or analog airspeed sensor?
            • hi iskess,

              I did not perform a preflight calibration - on the field I'm using a tablet for telemetry; im not aware of a way to do this calibration with the android app (haven't found it).. the wiki only gives instructions for mission planner.

              preflight-257x300.jpg

              Is there any way to enable this calibration on boot every time? Seems to be a requirement for safe flight.. i'm surprised it doesn't do that by default..

              So the bottom line is, on the ground the airspeed should read 0? or is there a range that is 'ok' like 1-2?

              I'm using the analog airspeed sensor. I didn't know I could use the digital one with the APM - is it worth it in your opinion?

              • I also use the analogue one but I never fly with a groundstation, so instead of doing that calibration I made a cover for the pitot tube out of heat shrink. I just leave the cover on while plugging in the battery, once everything's settled and ready for flight I take the cover off.

                This has worked well for me.

This reply was deleted.

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