Unexpected left roll leading to crash

Hello all,

I had a maiden flight today with a Cube Orange and Herelink onboard a 3m wingspan aircraft. Takeoff in FBW-A without incident. Performed numbers circles while performing autotune. As I prepared for landing, the aircraft banked hard left without command and recovery was unsuccessful. It resulted in a total loss of airframe.

I tried to look at the log and from what I could tell, it received RCIN signals for left roll while I know for sure I was leveling out for base leg for landing. I’m not sure if I’m looking at the right thing.

Bin file

please post the bin log from the microSD card

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Sorry for posting the wrong file.
BIN file

Hi Dougie,

Looking at the log around the time of the crash event suggests that the GPS speed drops off from 16 to 10m/s, and altitude drops - the RCIN and RCOUT shows discrepancies - which can be the case with FBWA - the RC inputs are still being processed, its not a direct passthrough mode. I also have to assume that you use the standard channel mapping of:

  • Roll stick should control channel 1
  • Pitch stick should control channel 2
  • Throttle stick should control channel 3
  • Yaw stick should control channel 4

And that you have correctly calibrated your radio system and airspeed sensor.

Here are some graphs of the data leading up to and including the crash event.

First thing I note is that the GPS speed drops but the airspeed sensor speed increases. I tend to believe GPS speed more than airspeed, but what is shown here is that the GPS represents ground speed and the airspeed sensor obviously represents airspeed. The two are usually not the same. Related to this, altitude is not maintained for a time period before the roll event and then through the event. Airspeed would increase and groundspeed potentially decrease with a high nose down pitch.

I included the RCIN and RCOUT plots to see what you asked for and what the autopilot sent out. RCIN asks for throttle up and this is represented in the RCOUT, so you are asking for more throttle whilst the aircraft is descending and this is passed on by the autopilot - but you don’t go above around 75% throttle. You are asking for a lot of pitch (channel 2) and are not getting it - you max the pitch channel on RCIN but RCOUT shows not much in the way of pitch command at all. The autopilot is also asking for increasing amounts of yaw which, with a V-tail will be using up some control throw (yaw pitch mix?). Note the increasing offset on roll for RCIN - for neutral stick roll should be around 1500, which it is and then starts reducing towards 1000 (which is full roll one way) and then there are some opposite roll inputs. This is most odd to me. The autopilot tends to deal with the roll offset ok but does respond to the last two roll inputs - maxxing out roll in the opposite direction.

The internal estimations seem to be performing alright from Attitude/RP Comparison chart.

The last two charts display a comparison of desired roll and pitch against measured roll and pitch. The second graph is a zoom in around the crash event to get a better idea of the angles attained.

Both pitch and roll become increasingly divergent which suggests the aircraft was busy doing its own thing and not responding to the autopilot commands. Also if the units of this chart are degrees then some relatively large attitudes were being asked for and not being achieved.

If you weren’t asking for roll or pitch inputs at this time, I think there was a problem with RCIN - where that problem is, I don’t know.

It would be good to have a ‘direct’ control mode to change to here. This is a direct passthrough of commands from transmitter to servo/esc. The aircraft you were flying should be conventionally stable and be able to be flown in direct mode easily - in fact I would do that first if you repair the aircraft so that you can understand the aircraft behaviours (for example learning how and when it stalls without the autopilot interjecting).

Others may be able to comment more on your settings/params or may have a different opinion. Interested to read alternative explanations.

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Thank you so much for your in-depth analysis of the log.

Regarding airspeed vs ground speed, 3-5 mph (1.3-5.5 m/s) wind was coming from the 10 o’clock direction at the time of the unexpected behavior. I’m not sure if it would agree with the data you observed.

Question: at the time of the crash, the last two seconds of the log, which direction of roll was recorded in RCIN? If I read that chart correctly, RCIN received input to full left roll and full pitch down, which I did not do. The two observers assisting at the time, experienced builders and pilots, also described the aircraft behaving similarly to a loss of RC.

Lessons learned from your report/observation: Use manual mode. Manual mode was definitely on the checklist but was deferred because the mode button was not mapped as planned.
If or when the plane is repaired, I will swap out Herelink and use a traditional RC transmitter. I had difficulty judging throttle input as I only had half-throttle travel.

Thanks again for taking the time to look at the log.

No problem, like I say, I would be interested in the opinion of others as I’m still learning.

Re. your question, Ithink full right roll would equate to a pwm time of 2000 and full right roll would then be 1000. The only way to really know, for your set up would be to check the herelink calibration in missionplanner - it will tell you in the radio calibration window. You would need the receiver (herelink) plugged in to the cube and the cube linked to mission planner either by a telemetry link or by a USB cable.

I have to say, I am a little worried regarding your statement that you only had half throttle on the herelink. That would lead me to suggest the calibration of the herelink controls wasn’t complete or otherwise in error.

For the cost of these airframes I hope you do repair and have another go. Perhaps check in here before flying again so we can be sure about basic setup and control checks etc.

Good luck.

I meant that i enabled “center throttle 0” option on the Herelink so that center = 1000 and top = 2000. It has been calibrated after the option was enabled.

Most of my aircrafts do not require 50% throttle to cruise, and the self-centering throttle stick wasn’t quite useful in Manual or FBWA. The downside of that option is that muscle memory takes over and the stick goes to center and shut off the motor.

Ah ok, self centring stick for throttle on a conventional aircraft isn’t that great as you know.

I know… Why do you have to remind me how silly I am? LOL

I bought the Herelink 20 months ago and put together 4 aircrafts in that time frame without installing it on any of them. I have no idea what i was thinking using it on this one.

Here’s the first link of the accident chain…

Hi @Dougie,

My observations are as follows. I would like to hear others’ views on whether this is correct.

The rudder movement is reversed.
In FBWA, the rudder normally moves to yaw in the same direction as the aircraft is rolling. However, your plane is yawing in the opposite direction of the roll direction. This can be clearly seen in AutoTune mode, where you are repeatedly rolling the aircraft (see the following video). I believe this is because you have set SERVO4_REVERSED=1.

Just before the crash, the aircraft was moving the aileron in the right roll direction in an attempt to bring the attitude from left roll to level. At the same time, however, the aircraft was yawing significantly to the left. Yawing to the left increased the speed of the right wing, and the lift of the right wing was extremely large. This caused the aircraft to roll to the left. In this case, the left yawing had a greater effect on the left roll than the aileron’s effect on the right roll.

I recommend that you check the servo reversal settings according to this document.


Looks like you nailed it, hatnac. (my opinion)

We did not notice any skidding behavior while it was in the air (a white plane at a safe altitude in overcast), but it makes perfect sense!

Thinking back, I checked manual pitch, roll, and yaw; then, I checked FBW pitch and roll. FBW yaw was not checked thoroughly because I couldn’t induce the V tail to make enough deflection.

I don’t recall setting up any reversed servo for the aileron, elevator, throttle, and rudder. I remember being happy about not having to mess with any channels during bench testing as I have messed up channel outputs before. But I may have reversed the wrong channel when trying to reverse ground steering.

The airframe is almost repaired, and it looks like a cracked egg… I shall name it Humpty Dumpty from now on.