Takeoff not completing under TKOFF_OPTIONS[0]=0

[Georacer]

User @menschel reported that

• in a Plane
• without an airspeed sensor -> running under TKOFF_OPTIONS[0]=0
• in an AUTO mission with a takeoff item to 50m
  the aircraft would climb to 47m and just hang there, not completing the mission item.
  
  The final reach of 50m was accidental.

He was flying a small ZOHD flying wing.
He kindly provided a flight log for us here.

He attempted to file a fix here: #28352, but what he proposes isn't the solution.

This affects our 4.6.0 beta.

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While it was unable to climb the last 3m, the airframe was at full throttle and at ~6deg pitch up (3deg nav pitch + 3deg PTCH_TRIM_DEG)
Still, that wasn't enough to make the last few meters.

[tridge]

we discussed on the EU dev call, and think we should gradually bring in TECS control during the takeoff, with a widening pitch window available to TECS, so in initial part of takeoff the TECS min/max pitch would be completely constrained, then it widens during the takeoff so that at the end of the takeoff TECS has full control

[menschel]

Just for clarification.
It's a plane with a tail 1, 2 not a flying wing, customized sub 250g build (246g). It has almost no mass, is therefore vulnerable to wind but can literately turn on a dime.

The reason why it does not gain any altitude after level-off may be related to over-speeding the wing profile.
50km/h is what ZOHD names as the recommended max speed. The increase to 50m was caused by an accidental switch miss, flaps instead of FBWA.

[Georacer]

Just for clarification.
It's a plane with a tail 1, 2 not a flying wing, customized sub 250g build (246g)

Thanks!