Plane: added airspeed based pitch limit check

prevent using too much pitch when at low airspeed, which can lead to
severe instability in SLT quadplanes

ArduPlane/quadplane.cpp

@@ -3900,25 +3900,75 @@ bool SLT_Transition::active() const
     return quadplane.assisted_flight && ((transition_state == TRANSITION_AIRSPEED_WAIT) || (transition_state == TRANSITION_TIMER));
 }
 
+/*
+  limit VTOL roll/pitch in POSITION1, POSITION2 and waypoint controller. This serves three roles:
+   1) an expanding envelope limit on pitch to prevent sudden pitch at the start of a back transition
+
+   2) limiting roll and pitch down to the Q_ANGLE_MAX, as the accel limits may push us beyond that for pitch up.
+      This is needed as the position controller doesn't have separate limits for pitch and roll
+
+   3) preventing us pitching up a lot when our airspeed may be low
+      enough that the real airspeed may be negative, which would result
+      in reversed control surfaces
+ */
 bool SLT_Transition::set_VTOL_roll_pitch_limit(int32_t& roll_cd, int32_t& pitch_cd)
 {
+    bool ret = false;
+    const int16_t angle_max = quadplane.aparm.angle_max;
+
+    /*
+      we always limit roll to Q_ANGLE_MAX
+     */
+    int32_t new_roll_cd = constrain_int32(roll_cd, -angle_max, angle_max);
+    if (new_roll_cd != roll_cd) {
+        roll_cd = new_roll_cd;
+        ret = true;
+    }
+
+    /*
+      always limit pitch down to Q_ANGLE_MAX. We need to do this as
+      the position controller accel limits may exceed this limit
+     */
+    if (pitch_cd < -angle_max) {
+        pitch_cd = -angle_max;
+        ret = true;
+    }
+
+    /*
+      prevent trying to fly backwards (negative airspeed) at high
+      pitch angles, which can result in a high degree of instability
+      in SLT aircraft. This can happen with a tailwind in a back
+      transition, where the position controller (which is unaware of
+      airspeed) demands high pitch to hit the desired landing point
+     */
+    float airspeed;
+    if (pitch_cd > angle_max &&
+        plane.ahrs.airspeed_estimate(airspeed) && airspeed < 0.5 * plane.aparm.airspeed_min) {
+        const float max_limit_cd = linear_interpolate(angle_max, 4500,
+                                                      airspeed,
+                                                      0, 0.5 * plane.aparm.airspeed_min);
+        if (pitch_cd > max_limit_cd) {
+            pitch_cd = max_limit_cd;
+            ret = true;
+        }
+    }
+
     if (quadplane.back_trans_pitch_limit_ms <= 0) {
-        // disabled
-        return false;
+        // time based pitch envelope disabled
+        return ret;
     }
+
     const uint32_t limit_time_ms = quadplane.back_trans_pitch_limit_ms;
 
     const uint32_t dt = AP_HAL::millis() - last_fw_mode_ms;
     if (last_fw_mode_ms == 0 || dt > limit_time_ms) {
+        // we are beyond the time limit, don't apply envelope
         last_fw_mode_ms = 0;
-        // past transition period, only constrain roll
-        int16_t limit_cd = MIN(quadplane.aparm.angle_max,plane.aparm.pitch_limit_max_cd);
-        roll_cd = constrain_int32(roll_cd, -limit_cd, limit_cd);
-        return false;
+        return ret;
     }
 
     // we limit pitch during initial transition
-    const float max_limit_cd = linear_interpolate(MAX(last_fw_nav_pitch_cd,0), MIN(quadplane.aparm.angle_max,plane.aparm.pitch_limit_max_cd),
+    const float max_limit_cd = linear_interpolate(MAX(last_fw_nav_pitch_cd,0), MIN(angle_max,plane.aparm.pitch_limit_max_cd),
                                             dt,
                                             0, limit_time_ms);
 
@@ -3938,7 +3988,7 @@ bool SLT_Transition::set_VTOL_roll_pitch_limit(int32_t& roll_cd, int32_t& pitch_
         to prevent inability to progress to position if moving from a loiter
         to landing
     */
-    const float min_limit_cd = linear_interpolate(MIN(last_fw_nav_pitch_cd,0), MAX(-quadplane.aparm.angle_max,plane.aparm.pitch_limit_min_cd),
+    const float min_limit_cd = linear_interpolate(MIN(last_fw_nav_pitch_cd,0), MAX(-angle_max,plane.aparm.pitch_limit_min_cd),
                                                   dt,
                                                   0, limit_time_ms);
 
@@ -3947,7 +3997,7 @@ bool SLT_Transition::set_VTOL_roll_pitch_limit(int32_t& roll_cd, int32_t& pitch_
         return true;
     }
 
-    return false;
+    return ret;
 }
 
 /*