Plane: TKOFF_THR_MIN is applied to SLT transitions

[Georacer]

This PR applies TKOFF_THR_MIN throttle to forward transitions of SLT-type quadplanes.

Changes

• Apply TKOFF_THR_MIN throttle to transitions of SLT-type quadplanes.
• Split quadplane.in_transition() into forward and backward cases. SLT and tiltrotor never go into a back-transition. Tailsitter can return in_back_transition()==true.

Tests

In SITL the following has been tested:

• In SLT, the TKOFF_THR_MIN is applied during the forward transition.
• In SLT, in_frwd_transition is never true during a back-transition.

Notes

• By default TKOFF_THR_MIN is 60%. We had put it thus for classic FW takeoffs. Is it a problem that all SLT-quadplanes will now transition at 60% min throttle?

[Georacer]

Case where QAssist will employ TKOFF_THR_MIN:

[tridge]

@Hwurzburg can you explain how the flare code works, what type of aircraft is it used on?

[tridge]

this is specifically aiming for TVBS landing, so not fwd transition

[tridge]

we will need @Hwurzburg to have a look at how this is defined, and for what classes of aircraft

[Georacer]

tilt rotor VTOLs landing conventionally as fixed wing....with switch active in pilot throttle controlled modes it will force tilts up at low throttle to clear props...at highest switch position it also sets pitch to flare pitch .....use it on all my tvbs for emergency fw landing when batts are low

@Hwurzburg the reason we ask is that we want to decide if the quadplane.in_transition() test needs to be replaced with a check for forward transition only.
Based on your description, I understand now that the !quadplane_in_transition check is meant to protect against the case where the pilot is currently commanding a forward transition while on low throttle. In this case we don't want to force the landing pitch.

@tridge, on second inspection, I see that there is an additional check here for !control_mode->is_vtol_mode(). When quadplane.is_back_transition()==true then tautologically is_vtol_mode()==true as well.
So adding a definition of and using is_back_transition() is superfluous.

I think we can simply replace in_transition() with in_frwd_transition() and spare us the definition of in_back_transition().

[Georacer]

I just tested the feature. I loaded Convergence in RealFligth, and set long DO_AUX_FUNCTION 89 1. This is the flare switch.
Took off in QSTAB, then I transitioned to FBWA.
Every time I closed the throttle to zero, the front tilt servos would raise the motors vertically.

[Hwurzburg]

great!

[Hwurzburg]

@Hwurzburg can you explain how the flare code works, what type of aircraft is it used on?

tilt rotor VTOLs landing conventionally as fixed wing....with switch active in pilot throttle controlled modes it will force tilts up at low throttle to clear props...at highest switch position it also sets pitch to flare pitch .....use it on all my tvbs for emergency fw landing when batts are low

fixed

[tridge]

@Georacer good catch on the default for TKOFF_THR_MIN, I think change to zero, and where the variable is used, if it is zero then use TRIM_THROTTLE, I think it is reasonable that a takeoff always uses at least trim (cruise) throttle
on some aircraft 60% throttle may lead to overspeed on takeoff

[Georacer]

@Georacer good catch on the default for TKOFF_THR_MIN, I think change to zero, and where the variable is used, if it is zero then use TRIM_THROTTLE, I think it is reasonable that a takeoff always uses at least trim (cruise) throttle on some aircraft 60% throttle may lead to overspeed on takeoff

I've set TKOFF_THR_MIN to 0. When it is set to 0, the transition will now use TRIM_THROTTLE.

On the other hand, for a normal plane takeoff (not transition), when TKOFF_THR_MIN==0, TRIM_THROTTLE will be used instead. as we have been discussing in #27758