Plane: improve target airspeed in landing approach

if the user hasn't set TECS_LAND_ARSPD then we can use an airspeed
between ARSPD_FBW_MIN and TRIM_ARSPD_CM when on approach

this also fixes landing_desired_closing_velocity() to never go above
the landing target speed, so we don't try to speed up if we are
starting the landing sequence too early

ArduPlane/quadplane.cpp

@@ -2381,7 +2381,11 @@ void QuadPlane::vtol_position_controller(void)
         // use nav controller roll
         plane.calc_nav_roll();
 
-        const float stop_distance = stopping_distance();
+        // work out the point to enter airbrake mode. We want enough
+        // distance to stop, plus some margin for the time it takes to
+        // change the accel (jerk limit) plus the min time in airbrake
+        // mode. For simplicity we assume 2 seconds margin
+        const float stop_distance = stopping_distance() + 2*closing_speed;
 
         if (!suppress_z_controller && poscontrol.get_state() == QPOS_AIRBRAKE) {
             hold_hover(0);
@@ -3973,6 +3977,16 @@ Vector2f QuadPlane::landing_desired_closing_velocity()
 
     // base target speed based on sqrt of distance
     float target_speed = safe_sqrt(2*transition_decel*dist);
+
+    // don't let the target speed go above landing approach speed
+    const float eas2tas = plane.ahrs.get_EAS2TAS();
+    float land_speed = plane.aparm.airspeed_cruise_cm * 0.01;
+    float tecs_land_airspeed = plane.TECS_controller.get_land_airspeed();
+    if (is_positive(tecs_land_airspeed)) {
+        land_speed = tecs_land_airspeed;
+    }
+    target_speed = MIN(target_speed, eas2tas * land_speed);
+
     Vector2f target_speed_xy = diff_wp.normalized() * target_speed;
 
     return target_speed_xy;
@@ -3983,26 +3997,38 @@ Vector2f QuadPlane::landing_desired_closing_velocity()
 */
 float QuadPlane::get_land_airspeed(void)
 {
-    if (poscontrol.get_state() == QPOS_APPROACH ||
+    const auto qstate = poscontrol.get_state();
+    if (qstate == QPOS_APPROACH ||
         plane.control_mode == &plane.mode_rtl) {
-        float land_airspeed = plane.TECS_controller.get_land_airspeed();
-        if (!is_positive(land_airspeed)) {
-            land_airspeed = plane.aparm.airspeed_cruise_cm * 0.01;
+        const float cruise_speed = plane.aparm.airspeed_cruise_cm * 0.01;
+        float approach_speed = cruise_speed;
+        float tecs_land_airspeed = plane.TECS_controller.get_land_airspeed();
+        if (is_positive(tecs_land_airspeed)) {
+            approach_speed = tecs_land_airspeed;
+        } else {
+            if (qstate == QPOS_APPROACH) {
+                // default to half way between min airspeed and cruise
+                // airspeed when on the approach
+                approach_speed = 0.5*(cruise_speed+plane.aparm.airspeed_min);
+            } else {
+                // otherwise cruise
+                approach_speed = cruise_speed;
             }
-        float cruise_airspeed = plane.aparm.airspeed_cruise_cm * 0.01;
-        float time_to_landing = plane.auto_state.wp_distance / MAX(land_airspeed, 5);
+        }
+        const float time_to_pos1 = (plane.auto_state.wp_distance - stopping_distance(sq(approach_speed))) / MAX(approach_speed, 5);
         /*
           slow down to landing approach speed as we get closer to landing
         */
-        land_airspeed = linear_interpolate(land_airspeed, cruise_airspeed,
-                                           time_to_landing,
+        approach_speed = linear_interpolate(approach_speed, cruise_speed,
+                                            time_to_pos1,
                                             20, 60);
-        return land_airspeed;
+        return approach_speed;
     }
-    Vector2f vel = landing_desired_closing_velocity();
 
-    const float eas2tas = plane.ahrs.get_EAS2TAS();
+    // calculate speed based on landing desired velocity
+    Vector2f vel = landing_desired_closing_velocity();
     const Vector3f wind = plane.ahrs.wind_estimate();
+    const float eas2tas = plane.ahrs.get_EAS2TAS();
     vel.x -= wind.x;
     vel.y -= wind.y;
     vel /= eas2tas;