As a rule my maiden flight is ALWAYS in manual mode.
Always check your CG. I built this arplane, it’s pretty slick but the CG is a PITA.
I had to add a 5oz lead ballast by filling the nose of the aircraft behind my runcam nano.
I insulated the lead ballast with electrical tape in stuck in the gap in the middle of the nose and secured it in place. When the 3300ma 4S battery is installed I can balance it with my fingers on the cg points perfectly. That was the first test. The second test is to be SURE ARDUPILOT has RCMAP_ROLL and RCMAP_PITCH set correctly. ALSO check SERVO1_FUNCTION, SERVO2_FUNCTION, and SERVO3_FUNCTION. Be sure left elevon, right elevon, and throttle are set correctly. It’s entirely possible that you have the SERVO Functions rand the rmap settings reversed. If it’s reversed you can have proper control surface movements by r/c but incorrect from the autopilot. I know first hand because when I was a newbie I did it. But my preflight checklist included checking r/c action with the control surfaces, r/c range test, roll plane to left, right, up, and down while checking the control surface movement.
Here’s what you need to test:
Input - Action
Roll right - Left elevon goes up and right elevon goes down
Roll left - Right elevon goes up and left elevon goes down
Pitch down - Both elevons go up
Pitch up - Both elevons go down
If your movements are incorrect then you need to adjust which servo output is left/right and the reversals of each elevon.
From the video it appears that either your CG is tail heavy, Ardupilot servo config is wrong, combination of both …
So the easiest thing to do is double check your CG
Next: Double check your ardupilot config, specifically SERVO functions and RC Map.
Hopefully this helps and you will be in the air in no time.