Unexplained crash with 2.74b , need help

Hey Guys

Just came back from flying field after a bad unexplained crash. it was a new flying wing airframe and started this morning by following the manual on roll/pitch and yaw tuning.  First of we calibrated the Baro meter to report 0 meters on the ground and then we calibrated the airspeed sensor. What was strange was that we had to lower the airspeed ratio to 0.1 to get the airspeed sensor to report 0 ms a sec. ( my main opinion for the crash)

We proceeded with the first test fly by wire A to see if the roll and pitch was working correctly. we saw that the reaction time was very slow and I proceeded to try change the rate for this. Before i could change anything loiter seemed to pick its own direction and headed full speed( 144kmp) in that direction. when we switched to RTL the plane made two loops in the air and crashed. can anyone look at the Tlogs and confirm reason for crash? the red block below shows the uncontrolled flight path before crash.

totalflight.jpg?width=700

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Replies

  • Hi,

    I think nobody else wrote this (here): A not quite zero airspeed reading while the plane is stationary is okay. It just has to agree with groundspeed at flight speeds and at no wind.

    The reason for the nonzero readings is that the airspeed is proportional to the square root of the ram pressure. The square root of a very small number is a not really so small number, so even the slightest sensor noise near zero will calculate to a few km/h.

    EDIT: Okay a 10 m/s reading on the ground is not normal.

    Regards

    Soren

  • Developer

    Hi Johann,

    That isn't version 2.74b, it looks like version 2.73.

    What was strange was that we had to lower the airspeed ratio to 0.1 to get the airspeed sensor to report 0 ms a sec. ( my main opinion for the crash)

    yep, that was indeed the main issue. The smallest reasonable ARSPD_RATIO is 1.0. It means the airspeed was reading completely wrong and you're plane didn't really stand a chance.

    To fix the zero you need to redo the airspeed preflight calibration (there is a menu item in mission planner for preflight calibration). Reducing the ratio isn't the right fix.

    Cheers, Tridge

  • Johann, You were right in your theory about the air speed  cal being wrong. From your own words you set the airspeed ratio to get zero on the ground. You need to re read the instructions

    I didn't plot any graphs, just played back you tlog. You air speed was working on the ground whist your plane was sitting pointed into the wind, ~ 4 m/s from NNE?  I fast forward to where you started flying and saw an AS of ~-4 m/s, due to your calibration  error. This surely did mess with any navigation modes(loiter, auto), but not FBWA or STAB.

    There is some confusion on the modes you where in. After the loiter where it was heading south with a strong wind, it entered stab, then manual, did some unplanned aerobatics and finally leveled out heading southwest and probably at a range where you could not see it very well from behind. It briefly went to STAB mode and then to a short failsafe. You lost your TX at ~ 385 meters and kinda started turning back, then it must have got signal again and you were in FBWA and surely couldn't see it at 667 meters. Did you crash into a swimming pool?

    I always have audible  alerts from the MP using a usb loud speaker on my GS computer. I can here it from 50 ft. I consider it a necessity for safe operations. Or you need a reliable assistant on the GS while the pilot keeps the plane in view. Don't have a spotter what the plane and you operating the GS with the TX around your neck.

    Any how be safe and preplan a little better. There is a lot that can go wrong very quickly. You need to be prepared, I have never crashed due to auto pilot errors, not because I haven't had any, but because I was always ready to fly in manual mode. And always have a plight plan set up around your field and use AUTO instead of LOITER. It can get crazy easier if things are not tuned.

    Good luck and be safe.

  • Never mind I think I have it now

    2013-09-15 16-59 174.log

  • Pieter please explain how and i will do it
  • Johan, Pieter

    Here are the two logs on the flash that i can see in the same time our fly way / crash happend.

    If it is not the correct one please tell me and i will look for the correct ones

    2013-09-15 08-21-02.rlog

    2013-09-15 08-21-02.tlog

    https://storage.ning.com/topology/rest/1.0/file/get/3692830275?profile=original
  • The thing is.. in the params I extracted from your log the airspeed ratio is 1.9936..
    Do you have a dataflash log? The telemetry log graph in missionplanner is a PITA to work with.

This reply was deleted.

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