I’ve built Flightory’s Super Stingray (Super Stingray - Flightory) and although it managed to perform a takeoff and a transition on it’s maiden flight, it crashed directly after in a sudden uncommanded nose dive and roll to the left. It was supposed to start a clockwise rising loiter.
I have been going over the logs with multiple people, trying to interpret them in an attempt to determine what has gone wrong, but we are simply not able to determine conclusively what seemed to have been the cause of the problem.
In some serious aircrash investigation efforts we have managed to salvage the wings and found that the elevons were still operational and we tested them and found them to have been wired correctly and configured correctly with the still functional flight controller.
Motors were properly fixed and tightened, no signs of loose propellors, the COG was set to 60mm behind the wing to fuselage connection to the body, as per the models manual. The battery was well mounted in the body using velcro and fully charged.
Flight controller channel details:
S1 - Motor 3(D)
S2 - Motor 2(C)
S3 - Motor1 (A)
S4 - Motor4(B)
S7 - Elevon right
S8 - Elevon left (Inverted)
S12 - Throttle
Is yours X frame? are you sure about motor numbering/letters?
What can be seen is that first of all you have a disbalance between motors 1 and 3 and motors 2 and 4 from the start (much more power is applied to these than to the other two).
If A and D are correct i.e. these are the two front motors, that means that you probably have CG way off, i.e. the distance from CG to motors A and D (1 and 3) would be about half of the distance to motors 2 and 3.
It is not the CG with respect to the wing which matters for hover, but rather CG to motor distance, and this one should be the same between the front and rear motors.
you had a motor disbalance, with much more power applied to what I assume are two front motors.
The elevons seem not to be working, i.e. they never took control of the attitude, there seems to be no response in the output to elevons when pitch and roll began to go out of bounds.
At higher speed, the motors reduced power, but because of the disbalance, the rear motors were reduced much more, and one motor was at zero, making the whole thing very unstable.
I do not understand why elevons were not operating correctly during the transitions.
The flight controller was requesting a climb, and as soon as the transition was done it started to request the steep turn to the left to start the loiter circle. While the control performance in roll is poor it seems to be following, but pitch is messed up.
I’ll suggest two theories:
1 - There was a control setup issue. I see you said the controls were working, but I’m going to assume that is with the RC manual test. Did they also function correctly when tested with FBWA? The controller is demanding full deflection for both elevons as the plane dives. So either the controls are not set up correct or theory 2.
2- The plane stalled. I’m not totally convinced myself, but I’ll throw it out there. High bank angle, low end of the speed range when the turn started (as stated by the manufacturer). Perhaps if Q_ASSIST was on that might have helped but unclear. You mentioned the C of G was correct, but was the weight higher? The specs listed seem to focus on the traditional version so do they account for the extra weight and drag of the VTOL?
A maiden flight in auto mode is pretty risky. There’s pretty good guidance on better ways to conduct a first flight that might have prepared you for better outcomes.
So you think that motor disbalance had nothing to do with it? Because what I saw in the motor output was initial drop of the power to motors, followed by two surge waves during the transition, i.e the motors were the only ones trying to keep attitude, but probably due to imbalnce were not able to (at high speed there was a lot of lift, so the motors had to reduce the power, but at lower power they did not have enough authority to stabilize, and the problem is happenning before the transition was complete. And the elevons were not operating to keep attitude…
About stall: a while ago I was testing stall behaviour of Ardupiot on a foamy plane with traditional control surfsaces, and while it is very possible that in this case there was a stall, the normal response of ardupilot would be to still put in full up elevator, which did not happen here. Also, one of the problems of ardupilot was that once in a stall, and a small yaw deviation occurred, the AP tried to correct that with rudder and ailerons, , basically leading to a perfect spin from which AP does not try to exit (kept elevator up, ailerons deflected, rudder to one side)…
Also, here the speed was ever increasing, so that is not how a stall usually goes it was a spiral dive, not a stall.
