Help with log analysis after maiden crash in Autotune


I am new to AP and I am hoping someone could help me analyse my [very short] log to see what went wrong.

Plane = Bixler 3
FC = Matek F405-wing
AP version = 3.10.0-dev

I have flown this plane a lot since I got it in February. I installed iNav [bc pixhawks are $$s] and it had been flying great until a launching mishap about 2 months ago broke the engine mount off. I decided to fix up a number of things on the plane that have been bugging me for a while but the main change was to install the new Matek F405-wing with arduplane.

I have spent a lot of time bench testing the setup including extended running times. I did have some issues with randomly getting the “No RC Receiver” failsafe. The only way I could find to fix that was to switch the SBUS wire to R2 (serial 2) where it would work fine, or to re-flash arduplane. I did the latter and, after a week of bench testing, the only issue I was still having was that occasionally SPort would drop the GPS sensor. According to mission planner the GPS is still working however, so I don’t think the GPS is at fault.

The flight.
Going against the plane docs, I decided to take off in manual mode. Take off was normal although the plane did seem a little sluggish. At about 20m altitude, I selected autotune. The plane pitched up vertically, stalled and nose dived straight into the ground. I was too slow to re-select manual mode and the plane didn’t seem to respond to any control inputs. In hindsight I should have climbed much higher than 20m but I was conscious that the docs recommend taking off in autotune and I wanted to get the tuning dialled in ASAP. Rookie mistake!

The carnage was the fuselage broken into two main pieces, the tail snapped off, engine mount broken off [again!] and two batteries crushed. The FC had some bent pins, which have straighten ok, but otherwise seems to be working fine. The interesting thing is that the camera (Runcam split 2) stopped recording at the exact moment that I selected autotune. I don’t know how these two things could be related, I don’t have any control setup for the camera, it is just on when the FC is powered.

I have looked at the log but there is nothing that jumps out at me as to the cause. Does any one have any ideas?

Thanks in advance.

2018-10-26 15-20-14.bin (432 KB)

I personally recommend getting your plane flying well in Manual mode before attempting any other flight modes.

I see that you wrote:


Would you mind linking for me where the in the documentation you find this?

I don’t yet have time to look at your log, but I once crashed in FBWA because I forgot to verify that the control surfaces went the correct directions. (When you put it in FBWA mode, and tilt the nose UP, you want to see the elevator go UP. If instead the direction is reversed, and the elevator goes down, you have a major problem.)

Thanks, the controls were definitely going the right way, both in manual mode and in fbwa, prior to launch.

It’s important to be clear here - when you say the surfaces moved
appropriately, we believe you when you say that when you (e.g.) pulled
back on the elevator stick the elevator moved up.

However - the implied question was, “when you tilted the nose of the plane
up with the controls nuetral, did the control surface move up or did it
move down?” It’s a very easy mistake to make.

Hi, yes I understood the question. Perhaps I should have been more clear. In any of the stabilised modes that I tried during bench testing, the control surfaces moved correctly. I am sure of this, I know it is an easy mistake to make having used FCs before. I must admit that I didn’t check it again immediately prior to launch but I don’t know why it would have suddenly changed in the interim.

The elevator was definitely inverted. You can tell by looking at the correlation between elevator output and Y gyro when you are flying in MANUAL mode:

can you see that when the elevator goes up, the Y gyro goes down?
Now look at the aileron:

there we see the positive correlation that we expect for control surfaces.
Another way to look at it is your stick movement on launch in MANUAL:

that shows the elevator stick moving towards lower PWM values on takeoff. As you had RC2_REVERSED=0, that means it was asking for down elevator. It would be very unusual to use down elevator on takeoff in MANUAL.
As elevator was on channel 6, you needed to set:

    and you also needed to set

so that RC input was correct.
I really wish we could find some way to detect this in pre-flight and stop this from happening. It is just too easy to get this wrong.

Wow! Thanks tridge. I was pretty sure it was an incorrect setting somewhere but that really surprises me bc I was positive it was working correctly, but that top graph is pretty damning!

Did you create that graph by selecting AETR.Elev and the IMU gyro from the selection on the right? (I’m at work at the mo so can’t experiment but I will later when I get home)

Fuse’ is back together so hopefully I’ll get the chance to re-maiden it later this week.

yes, elevator on left, Y gyro on right

good luck with it!

Great. I’ll experiment later. I really want to learn how to make use of the logs but there’s so much info there!

Many thanks for taking a look at it for me.