Fixed wing, vertical dive, crash (culprit: frozen servo?)

#1

https://logs.px4.io/plot_app?log=9f41169f-7418-4281-a19c-81b3e981662d

I would appreciate any observations or hints that might help us understand this outcome. Upon switching to mission mode at 4:09, aircraft correctly pitched over to descend 40 meters

Pitch setpoint seemed correct, as did actuator outputs. After the landing (ok, crash, after a 90 meter vertical dive!) the elevator control linkage was intact.

Any other ideas about what might have happened?

#2

The vehicle is not responding well to the pitch command. It seems stuck in the dive, commanding pitch up and up elevator, but very little response from the vehicle. What airframe? what servos? any chance the elevator servo is stalling out or electrically intermittent? Pictures may be helpful.

#3

The pitch servo is frozen in place. I removed it from the airplane and used a servo tester, and nothing - no movement at all. I can’t even move the servo manually by pushing on or turning the control arm.

While it’s possible that this happened at impact, we have concluded that the pitch servo failed by just freezing in place - at the most inopportune time. We think the pitch servo simply froze with the elevator in a neutral position as it was at around 4:09.5… never to respond to signal again. Perhaps we’ve learned our lesson on using cheap Tower Pro SG90 9g servos.

Picture here These days we’re building with foamcore and hotglue, it’s cheap, light, strong and quick to build. 2-3 hours build time for this aircraft.

#4

you’re not kidding about cheap servos. As low as $0.25 each from Ali Express! Great looking easy foamy build. I wonder if you could get away with slightly smaller control surfaces (or a slightly lower travel linkage). Less torque on the tiny servos and it might be a better match for the default PX4 gains. The downside would be less authority at very slow speeds.