Plane: reduce height gain in auto landing for tiltrotors

avoid running the motors while tilted past the tilt max if we are in
AIRBRAKE state. This stops a large amount of forward thrust from the
tilted motors while trying to slow down, while still maintaining
attitude control

ArduPlane/quadplane.cpp

@@ -2199,6 +2199,10 @@ void QuadPlane::vtol_position_controller(void)
         poscontrol.last_run_ms = now_ms;
     }
 
+    // avoid running the z controller in approach and airbrake if we're not already running it
+    // and tilt is more than tilt max
+    bool suppress_z_controller = false;
+
     // horizontal position control
     switch (poscontrol.get_state()) {
 
@@ -2231,6 +2235,20 @@ void QuadPlane::vtol_position_controller(void)
             aspeed = groundspeed;
         }
 
+        if (tiltrotor.enabled() && poscontrol.get_state() == QPOS_AIRBRAKE) {
+            if ((now_ms - last_pidz_active_ms > 2000 && tiltrotor.tilt_over_max_angle()) ||
+                tiltrotor.current_tilt >= tiltrotor.get_fully_forward_tilt()) {
+                // use low throttle stabilization when airbraking on a
+                // tiltrotor. We don't want quite zero throttle as we
+                // want some drag, but don't want to run the Z
+                // controller which can result in high throttle on
+                // motors that are tilted forward, thus increasing
+                // speed
+                suppress_z_controller = true;
+                hold_stabilize(0.01);
+            }
+        }
+        
         // speed for crossover to POSITION1 controller
         const float aspeed_threshold = MAX(plane.aparm.airspeed_min-2, assist_speed);
 
@@ -2253,9 +2271,10 @@ void QuadPlane::vtol_position_controller(void)
 
         const float stop_distance = stopping_distance();
 
-        if (poscontrol.get_state() == QPOS_AIRBRAKE && !(tiltrotor.enabled() && !tiltrotor.has_vtol_motor() && (tiltrotor.current_tilt >= tiltrotor.get_fully_forward_tilt()))) {
-            // don't ouput VTOL throttle on tiltrotors if there are no fixed VTOL motors and the tilt is still forward
+        if (!suppress_z_controller && poscontrol.get_state() == QPOS_AIRBRAKE) {
             hold_hover(0);
+            // don't run Z controller again in this loop
+            suppress_z_controller = true;
         }
 
         /*
@@ -2428,8 +2447,9 @@ void QuadPlane::vtol_position_controller(void)
             // if continuous tiltrotor only advance to position 2 once tilts have finished moving
             poscontrol.set_state(QPOS_POSITION2);
             poscontrol.pilot_correction_done = false;
-            gcs().send_text(MAV_SEVERITY_INFO,"VTOL position2 started v=%.1f d=%.1f",
-                                    (double)ahrs.groundspeed(), (double)plane.auto_state.wp_distance);
+            gcs().send_text(MAV_SEVERITY_INFO,"VTOL position2 started v=%.1f d=%.1f h=%.1f",
+                            (double)ahrs.groundspeed(), (double)plane.auto_state.wp_distance,
+                            plane.relative_ground_altitude(plane.g.rangefinder_landing));
         }
         break;
     }
@@ -2601,8 +2621,14 @@ void QuadPlane::vtol_position_controller(void)
     case QPOS_LAND_COMPLETE:
         break;
     }
-    
-    run_z_controller();
+
+    /*
+      run the z controller unless something has already run it or set a target throttle
+     */
+    if (!suppress_z_controller) {
+        // otherwise run z controller
+        run_z_controller();
+    }
 
     if (now_ms - poscontrol.last_log_ms >= 40) {
         // log poscontrol at 25Hz

ArduPlane/tiltrotor.cpp

@@ -530,8 +530,7 @@ void Tiltrotor::vectoring(void)
         return;
     }
 
-    float tilt_threshold = (max_angle_deg/90.0f);
-    bool no_yaw = (current_tilt > tilt_threshold);
+    const bool no_yaw = tilt_over_max_angle();
     if (no_yaw) {
         // fixed wing  We need to apply inverse scaling with throttle, and remove the surface speed scaling as
         // we don't want tilt impacted by airspeed
@@ -674,4 +673,11 @@ bool Tiltrotor_Transition::show_vtol_view() const
     return show_vtol;
 }
 
+// return true if we are tilted over the max angle threshold
+bool Tiltrotor::tilt_over_max_angle(void) const
+{
+    const float tilt_threshold = (max_angle_deg/90.0f);
+    return (current_tilt > MIN(tilt_threshold, get_forward_flight_tilt()));
+}
+
 #endif  // HAL_QUADPLANE_ENABLED

ArduPlane/tiltrotor.h

@@ -42,6 +42,7 @@ public:
     void bicopter_output();
     void tilt_compensate_angle(float *thrust, uint8_t num_motors, float non_tilted_mul, float tilted_mul);
     void tilt_compensate(float *thrust, uint8_t num_motors);
+    bool tilt_over_max_angle(void) const;
 
     bool is_motor_tilting(uint8_t motor) const {
         return (((uint8_t)tilt_mask.get()) & (1U<<motor));