Flying wing rolling left on giving roll right input

We made a 4.5 kg MTOW wing with 1.9 m span, 0.75 sqm wing area with PW51 airfoil, 22 degrees sweep, 45 cm root chord and 30 cm tip chord

Few seconds after takeoff it started rolling left and crashed. The pilot inputed roll right as soon as it is rolling left but of no use. It hapened twice and I am still stuck what is the issue.

We verified control stick inputs and stabilisation before takeoff. There are no RC input reversal
The servos are providing enough torque.
The elevator input is good and plane is responding fine.

I am attaching the log. Since this is my first post, please ignore any mistakes and request you to help me with this.

Here are the logs.
Log 1

Log 2

Is this the maiden flight for this aircraft? Have you already flown it successfully without the flight controller and directly with R/C system? Unless you are sure of the design and it’s controllability manually, it doesn’t make sense to go into the logs.

Hi @Albcem, I designed the plane in XFLR. Unfortunately we never flew planes above 1.5 kg MTOW and we are quite concerned about flying it without stabilisation.So, we only tried in FBWA mode. It is made of EPP and reinforced with a layer of fiber glass. I am attaching a picture for reference. DO u think flying it in complete manual mode will help us figure out exactly what the problem is?

this is probably the wrong forum to discuss potential design related issues, but here are a few potential things to check before messing with Ardupilot or any gyro stabilizers:

  • validate the design’s dynamic stability within XFLR5 in pitch, roll and yaw! If the design is unstable, you will have to go back to design board as soon or later this will cost you a total loss when your electronic stabilizers fail,
  • validate the CG position and stability margin,
  • check the flight speed at your current MTOW and see if you are even abe to launch the aircraft at that speed, 4.5kg for 0.75m2 is a heavy wing loading but not unflyable.
  • check whether you have adequate motor power to sustain the cruise speed after launch
  • get rid of batteries and propulsion unit and test controllable flight as a glider / if everything checked OK above, but things still fail, you may have roll authority problem. This may require increasing your control throws or enlarging the control surfaces used for roll. For instance you may use all 4 control surfaces as elevons as a start.

A prerequisite for success with Ardupilot is to have a healthy design that flies well manually. So yes, try flying in Manual mode for sure and more ideally take the flight controller physically offline.

I think you will find the weakest link through this process.

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Thanks a ton for the suggestions@Albcem. I increased the gains and got it flying in FBWA mode.
Now we are gonna autotune the aircraft. I didnt realise throws would be reduced when throttle is high, so could not figure that out in ground checks.

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