Help analyse Stall (logs etc included)

ArduPlane Plane 3.3
1

APM 2.6 running 3.30 on a Skywalker airframe which has had 30+ flights and seems to loiter and fly basic missions pretty well. It was ‘tuned’ on autotune=5, I haven’t worked thru any TECS tuning process. Stall Prevention is disabled as I have struggled with hand launches in FBWA since updating to firmware supporting this feature, tho I’m not sure whether this affects that or not.

Wind on the day was NW, light winds (haven’t tried to work it out from the logs).

In the minute leading up to the stall…

Flying in FBWB mode (generally into wind), it is slowly losing altitude ( approx 30ft/min), during this time it’s applying throttle on and off and gradually increasing the amount of elevator applied all with no effect on altitude hold. Over the same period of time, airspeed falls from around 14.5m/s to 13 m/s (TRIM_ARSPD_CM :1100 )

I turn downwind and it loses a little airspeed (down to just above 12m/s) at which point the APM applies a boot load of elevator and throttle which starts a stall. Airspeed increases to over 20m/s as I engage manual mode to eventually recover the model after losing about 120 meters alt and a little more hair.

For comparison, a flight earlier in the day which involved 4 minutes of ‘loiter’ whilst re-calibrating the airspeed sensor (for no particular reason), it maintained it’s altitude ok, but the airspeed was dropping despite the APM applying increasing amounts of elevator/throttle.

A few settings :

ALT_CTRL_ALG 0
ALT_HOLD_FBWCM 5000
ALT_HOLD_RTL 15000
ALT_MIX 1
ARSPD_ENABLE 1
ARSPD_FBW_MAX 16
ARSPD_FBW_MIN 7
FBWB_CLIMB_RATE 2
FBWB_ELEV_REV 1
MIN_GNDSPD_CM 0
TRIM_ARSPD_CM 1100 (24.6mph/40kmh)
TRIM_AUTO 0
TRIM_PITCH_CD 0
TRIM_THROTTLE 45

Video : youtu.be/IoXS5p2h9HI

[attachment=0]Alt+RC2Out+RC3Out+AirSpd.png[/attachment]

If anybody could take some time to have a look and offer any advice on this or anything else you can see in the logs in general it will be greatly appreciated. I understand the time required to trawl thru logs so if you get to take a good look, name your charity and I’ll make a donation.

TIA

Chris

2

I always make sure to keep my FBW_MIN at least 3 M/S greater than the actual stall speed to avoid stalls. The airspeed sensor is not very accurate at low speeds.

Unless you have “THROTTLE_PASS_STAB” set in FBWA and (I believe always in FBWB), the throttle value selects a proportional speed between FBW_MIN and FBW_MAX.

I’m at work, so I haven’t looked at your log files. 7 M/S is only about 15MPH which seems very low for FBW_MIN to me, especially since you can easily have 5-6 MPH of noise on the airspeed sensor at low speeds.

I would push your min up to around 11 M/S, cruise up to 14 or so and max up to something like 20.