How to set up FBW mode to prevent plane stall?

[jcajhen]

My FPV testing airplane, modified TwinStar, »passed away« after another stall flying in FBW mode. On the latest addition, I installed on it also Airspeed sensor – I have mixed experienced with it; true airspeed indication was not accurate enough. – indicate too low speed and was not responsive enough (flying up/down wind comparing to GPS speed). Maybe the angle of pitot tube was not right. See the video: http://www.youtube.com/watch?v=1NQCB...feature=relmfu

On one of my previous flight, plane stalled in FBW mode after my too slow flight downwind with throttle on Idle, but I was managed to recover the plane just above the ground (I managed to switch off FBW mode 7m above the ground..), see the video (at 1:14): http://www.youtube.com/watch?v=0zAYx...feature=relmfu

I tried to avoid this using Alex instruction on how to set up parameters to prevent FBW to stall the plane in such conditions – discussion on the following thread:
http://rangevideo.com/forum/showthre...-takeoff/page2

I only had set cruise and glide throttle to zero. RVOSD always try to pitch plane up, but since I adjust autopilot authority for the airplane flying powered, when not powered the autopilot is not authoritative enough on the elevator to be able to stall the plane. Other options are to set max pitch angle low enough so it never stalls or like Frank said cruise altitude and altitude control limit to zero

On my last flight I tested new setup at low speed flight in FBW mode. Plane stalled and I was not able to recover from the spin (I missed to switch off FBW mode and was unable to recover the plane from the spin in FBW mode). See the video of this last flight: http://www.youtube.com/watch?v=45UwI...=youtube_gdata (low resolution video, I used my back up Video device, run out of battery of my Canon recorder..).
When flying my Twinstar normal, without FBW, it was no problem to recover plane from the spin – throtlle, put elevator a bit down, maybe a bit of rudder/aileron to the opposite from the spin direction.

I try to pull out some conclusions and learning experiences:
- Flying FBW (same for Level flight, Position hold mode, RTH..) at low speed/low altitude is tricky and dangerous if the FBW is not set correctly (…the autopilot is not authoritative enough on the elevator to be able to stall the plane)
- Once airplane enter to the spin under FBW (level flight, Position hold) mode, it’s impossible to recover the plane out of the stall; the only solution is to switch off FBW mode to regain full manual control on elevator and aileron/rudder axis. Once enter to the spin FBW autopilot keep pulling the full elevator trying to level the plane
- Thus it’s not recommended to use FBW in landing maneuvers or in low altitude/low speed flight unless correct FBW setup has been tested.
- Autopilot set up – to be not authoritative enough on the elevator to be able to stall the plane is very important and should be mandatory part of the set up/testing procedure. But, perform these tests high enough (300+ m) to have enough time to react properly…

[Martin Y]

Correct me if I'm wrong but FBW doesn't have control over the throttle.

[ommadawn]

I have had stall/spin happen twice with FBW mode. Both times it occurred because I wasn't allowing FBW to fly the plane for me. In FBW mode all you need to use is the aileron stick. FBW will balance the turn for you. Both times I spun, I was turning and pulling back on the elevator stick, out of habit. Both times, releasing the controls allowed FBW to recover the plane.
I think a lot of it has to do with how you set up the plane to fly hands off, unstabilised. Once the plane is trimmed to fly at a safe margin above stall speed, with a conservative CofG and the neutrals are set (and saved,) then FBW should IMHO prevent stall/spin incidents, if it is allowed to fly the plane.
As far as landings go,I open flaps and get a pitch up from the flap deployment, but FBW compensates instantly, and maintains safe flight attitude. The only changes I have made from the default settings are to increase max bank to 60 deg, and max pitch to 45 deg to allow a bit more control authority in FBW mode.
Cheers
Frank

[Alex Villa]

You have a more serious problem here, the vibrations on the plane are upsetting the AHI. Need to work that before everything else.
About autopilot control there is another issue that could happen, If roll control(stabilization roll gain), or elevator control(stabilization pitch gain) is too low, the autopilot could lose control at low speeds because will not be able to move the control surfaces enough to keep the desired attitudes, with the low airflow. I think this could be happening on your video, not an actual stall, but lack of aileron control.

All this depends a lot on the plane surface control area and flight speed, on FBW mode you can avoid stall by keeping the throttle set at cruise. On my very heavy flying wing I never set throttle to less than 8-10A unless I know I will keep the attitude leveled or diving, so the speed is high enough to keep flying. The flying wing is a Maxi swift which has very over-sized elevons, because is meant to be a light and slow flyer, this really helps. One time I was flying at more than 4km altitude and I did noticed a stall, I guess it was caused by thinner air at those altitudes.

The most important thing of all this is to keep in mind that RVOSD will not prevent the plane from stalling, you still have to manage the throttle control wisely, just like you do in manual mode.