I’m analyzing a VTOL log running ArduPlane v4.6.3. The aircraft was flying in AUTO mission mode, then transitioned to QRTL. Shortly after, a Critical Volt Battery failsafe at 39.59 V, which forced a switch to QLAND. However, during the final VTOL descent in QLAND, the aircraft descended much faster than configured.
At the end of the flight, the batteries were checked and showed approximately 40% remaining capacity.
Since throttle was at 100% (QTUN.ThO = 1.0) while the aircraft was still descending, could this indicate thrust saturation due to voltage sag under load?
Is it expected for QLAND to exceed the configured descent behavior when available thrust becomes insufficient?
Is the divergence between Desired and Actual Climb Rate in the final phase a consequence of insufficient available thrust?
Per cell voltage dropped below 3.3v. That’s way too low for a LiPo and they will fade at that level. If it’s LiIon then I’d look at the C rating of the battery. Landing and take off are 170-180 amp with spikes over 200a. That might be too much for the battery to support.
Motor outputs are nearly maxed for take off and landing, so not a lot of reserve power there. I don’t see any motor saturation, but there isn’t a lot of room. If its windy or turblent when you’re trying to land, or approaching to transition down wind, there might not be much power left for control or stabilization. Means that even on a good day this is going to be on the edge.
In this case I think you’re correct that the problem is just battery sag. The plane needs all it’s got and leaving the battery to it’s max like that didn’t leave enough to land.
