Nose dive after retract of flaps

I am a beginner pilot, so I am not sure if this is pilot error or Plane error.

I have had two crashes the same week caused by the same problem, last time was yesterday. I am flying a Bix3 with a Pixhawk (without airspeed sensor). Yesterday I flew for 30min on one battery without problems, did the Autotune again since I wanted more iterations than last time (this time I did 80 roll and 80 pitch iterations).

Takeoff on first battery was very nice, I used full flaps and full throttle. I haven’t thougth about it before, but reviewing the logs it seems that the plane is climbing nicely without me adding any elevator. After climbing a bit a I retract the flaps completely, the plane pitches down a bit (from the sudden loss of lift I am guessing) and I pull back on the elevator to bring it back up. Nothing dramatic.

Takeoff on second battery was not good. Same as before, but this time I reduced the flaps to half some time before retracting them all the way. When I retracted the flaps from 50% to 0% the plane went into a nosedive, I input max elevator, but the plane keeps on diving for 80m until it crashes in the most spectacular way, sending styrofoam, battery, camera, wings everywhere.

I am mostly thinking that this is an aerodynamic phenomena from me retracting flaps at “high speed”, but I am not sure.

I have seen that another user here has posted the same problem here, so I thought maybe there was something else to it.

Anyway I am happy for help, even if it’s just pointing out the correct way of taking off with flaps :slight_smile:

Logs are here:

The first one is the successful flight, last one is crash.

P.S.: Flaps are on channel 5, elevator is channel 2, aileron is channel 1, throttle is channel 3.

I am pretty sure that it is not a stall, I don’t have an airspeed sensor, but there was no wind and the ground speed was sufficient.

Maybe I should throttle back before retracting flaps?

What are your takeoff procedures?

For what it’s worth, I fly a Bix3. I have never used (nor felt like I needed) the flaps.

When doing a ground-takeoff (with wheels on a runway) and throttling up slowly, it takes a sharp yaw-turn just before liftoff. I suspect this is a ground-loop perhaps aggravated by the thrust being behind the front wheels? I achieve best performance when I slam the throttle forward, and have the plane liftoff before the hard-yaw (ground loop?) causes problems.

When I hand-launch at full-throttle, I don’t have to throw too hard and while I see a small dip, I have never contacted the ground. No flaps in this case either.

I have done plenty of takeoffs without flaps, and I am fine doing so, but that doesn’t help me understand what’s happening in this case. Sometime I use flaps for slow flying, and I am now concerned that this dive might happen in such a case.

I really want to understand what’s happening, why retracting flaps cause an uncontrollable nosedive. My best bet is that it is related to the large airspeed, I have never experienced it when disengaging flaps after slow flying.

P.S.: Flaps are on channel 5, elevator is channel 2, aileron is channel 1, throttle is channel 3.

I’m pretty sure this is the pitch integrator not adjusting to the new
flight dynamics fast enough.

How good was the COG on this vehicle?

Interesting, haven’t thought of that.

Does that support the hypothesis that one should indeed not retract flaps with relative large air-speed (as in my case)?

The CG was spot on, I always check before takeoff.

Had a quick look at the crash log, motor was running full throttle for the entire flight except the last little bit. At the point that the throttle goes to zero, the pitch recovers from -44° to -11° but too late before the impact.

The Bixler type of aircraft have a horrible tendency for the nose to be pushed down by the motor thrust which is why half throttle launches on these types are much preferred and are more successful to full throttle launches.

I suspect that the retracting of the flaps reduced the pitch-up tendency that flaps cause and the foam elevator did not have enough authority to keep the nose up until the throttle stopped.

Thanks @Graham_Dyer, very insightful comment, and I did not catch the relation between pitch and throttle when I reviewed the logs myself.

I do hand launches and I always launch at full throttle, which is in reality 75% throttle (set by THR_MAX), because I am scared of stalling. Until now I have had this type of crash a couple of time, not knowing what I am doing wrong, but it is quite obvious now. I usually takeoff with flaps and full throttle and climb, then I throttle back and retract flaps, which goes fine. I can also cruise at full throttle without flaps without diving, so the problem must be connected to both throttle push and sudden loss of lift from retracting flaps.

Solution: Decrease throttle before retracting flaps.