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Crash analysis support

(alcala) #1

Hi everybody
2 days ago I had a crash with an easyglider 4 (pixracer + xrs frsky + pitot tube + gps)

suddenly I fbwa mode the plane started to spin and I was not able to recover

I suspect a bad COG (center of gravity)

Any idea ?

Adolfo

(alcala) #2

Update
Elevator is channel 3
During the spin RCIN & RCOUT are inverted
Strange
Did I tune something wrong ?

Thanks
Adolfo

(George Zogopoulos Papaliakos) #3

Okay, so here is my guess. It goes without saying that I could be wrong.

So, you took off at 261s in Stabilize, full throttle and climbed to 110m.
At that point you had around 11m/s in airspeed.
image
image

Then, you closed the throttle all the way and witched to FBWA at 277s.
image
I assume you wanted to glide and started doing so at around 7.5m/s airspeed.
I’m not sure how slow this airframe can fly at.

Then, at 396.5s I think I see a tip stall. Signs:

  • even since 380s your pitch rate oscillates rapidly, probably due to a badly tuned pitch PID.
  • at 396.5s your airspeed falls to 6.4m/s
  • at the same time you start losing altitude
  • at the same time, your autopilot starts pulling full up elevator. At that point, your roll is 11degrees
  • at 397.5s your roll departs. The autopilot tries to compensate, but roll channel is saturated.
  • at 398.7s airspeed has now dipped to 5.5m/s. The airplane seems to be in freefall, judging from altitude loss.
  • Naturally, the autopilot was not in position to raise the throttle and gain airspeed, because it had no authority over it (FBWA mode).
    image
    image
    image

Summing up:

  1. It is a bad idea to glide with FBWA. The airplane will try to keep level throughout a stall, even if its natural tendency is to drop a wing into a dive and gain airspeed.
  2. Know your airspeed limits.
  3. Monitor your aircraft performance. Pitch rate is oscillating fast. This is an indicator of bad tuning.
(alcala) #4

George
first many thanks for your deep analysis.

I share your conclusion and recommendations

I will adjust the tuning and way of flying (no glide in FBWA)

nice regards

Adolfo

(alcala) #5

George

what is the software you are using for log analysis ?

Adolfo

(George Zogopoulos Papaliakos) #6

It’s a MATLAB library we made for Ardupilot .bin log conversion to native MATLAB structures.
After that, it was all about plotting the right quantities at the right time frame.

You can find it here: https://github.com/Georacer/ardupilog