ArduPlane: add thr position trigger to ThrLandCtrl Q_OPTION

ArduPlane/config.h

@@ -286,3 +286,9 @@
 #ifndef FS_EKF_THRESHOLD_DEFAULT
  # define FS_EKF_THRESHOLD_DEFAULT      0.8f    // EKF failsafe's default compass and velocity variance threshold above which the EKF failsafe will be triggered
 #endif
+
+/////////////////////////////////////////////////////////////////////////////
+//  Landing Throttle Control Trigger Threshold
+#ifndef THR_CTRL_LAND_THRESH
+ #define THR_CTRL_LAND_THRESH 0.7
+#endif

ArduPlane/quadplane.cpp

@@ -1223,7 +1223,7 @@ bool QuadPlane::is_flying_vtol(void) const
   smooth out descent rate for landing to prevent a jerk as we get to
   land_final_alt. 
  */
-float QuadPlane::landing_descent_rate_cms(float height_above_ground) const
+float QuadPlane::landing_descent_rate_cms(float height_above_ground)
 {
     if (poscontrol.get_state() == QPOS_LAND_FINAL) {
         // when in final use descent rate for final even if alt has climbed again
@@ -1237,19 +1237,24 @@ float QuadPlane::landing_descent_rate_cms(float height_above_ground) const
     if ((options & OPTION_THR_LANDING_CONTROL) != 0) {
         // allow throttle control for landing speed
         const float thr_in = get_pilot_land_throttle();
-        const float dz = 0.1;
+        if (thr_in > THR_CTRL_LAND_THRESH) {
-        const float thresh1 = 0.5+dz;
+            thr_ctrl_land = true;
-        const float thresh2 = 0.5-dz;
-        const float scaling = 1.0 / (0.5 - dz);
-        if (thr_in > thresh1) {
-            // start climbing
-            ret = -(thr_in - thresh1)*scaling*max_climb_speed;
-        } else if (thr_in > thresh2) {
-            // hold height
-            ret = 0;
-        } else {
-            ret *= (thresh2 - thr_in)*scaling;
         }
+        if (thr_ctrl_land) {
+            const float dz = 0.1;
+            const float thresh1 = 0.5+dz;
+            const float thresh2 = 0.5-dz;
+            const float scaling = 1.0 / (0.5 - dz);
+            if (thr_in > thresh1) {
+                // start climbing
+                ret = -(thr_in - thresh1)*scaling*max_climb_speed;
+            } else if (thr_in > thresh2) {
+                // hold height
+                ret = 0;
+            } else {
+                ret *= (thresh2 - thr_in)*scaling;
+            }
+        }    
     }
 
     if (poscontrol.pilot_correction_active) {
@@ -2302,6 +2307,9 @@ void QuadPlane::PosControlState::set_state(enum position_control_state s)
         } else if (s == QPOS_AIRBRAKE) {
             // start with zero integrator on vertical throttle
             qp.pos_control->get_accel_z_pid().set_integrator(0);
+        } else if (s == QPOS_LAND_DESCEND) {
+            // reset throttle descent control
+            qp.thr_ctrl_land = false;
         }
     }
     state = s;

ArduPlane/quadplane.h

@@ -122,6 +122,9 @@ public:
 
     // return true if the user has set ENABLE
     bool enabled(void) const { return enable != 0; }
+    
+    // is throttle controlled landing descent active?
+    bool thr_ctrl_land;
 
     uint16_t get_pilot_velocity_z_max_dn() const;
     
@@ -239,7 +242,7 @@ private:
     bool should_relax(void);
     void motors_output(bool run_rate_controller = true);
     void Log_Write_QControl_Tuning();
-    float landing_descent_rate_cms(float height_above_ground) const;
+    float landing_descent_rate_cms(float height_above_ground);
     
     // setup correct aux channels for frame class
     void setup_default_channels(uint8_t num_motors);
@@ -396,7 +399,7 @@ private:
 
     // are we in a guided takeoff?
     bool guided_takeoff:1;
-    
+
     struct {
         // time when motors reached lower limit
         uint32_t lower_limit_start_ms;