Plane: remove tailsitter VTOL transition race condition

ArduPlane/Attitude.cpp

@@ -377,7 +377,8 @@ void Plane::stabilize()
     }
     float speed_scaler = get_speed_scaler();
 
-    if (quadplane.in_tailsitter_vtol_transition()) {
+    uint32_t now = AP_HAL::millis();
+    if (quadplane.in_tailsitter_vtol_transition(now)) {
         /*
           during transition to vtol in a tailsitter try to raise the
           nose rapidly while keeping the wings level
@@ -386,7 +387,6 @@ void Plane::stabilize()
         nav_roll_cd = 0;
     }
 
-    uint32_t now = AP_HAL::millis();
     if (now - last_stabilize_ms > 2000) {
         // if we haven't run the rate controllers for 2 seconds then
         // reset the integrators
@@ -411,7 +411,7 @@ void Plane::stabilize()
                 control_mode == &mode_qrtl ||
                 control_mode == &mode_qacro ||
                 control_mode == &mode_qautotune) &&
-               !quadplane.in_tailsitter_vtol_transition()) {
+               !quadplane.in_tailsitter_vtol_transition(now)) {
         quadplane.control_run();
     } else {
         if (g.stick_mixing == STICK_MIXING_FBW && control_mode != &mode_stabilize) {

ArduPlane/quadplane.h

@@ -83,8 +83,8 @@ public:
     /*
       return true if we are a tailsitter transitioning to VTOL flight
     */
-    bool in_tailsitter_vtol_transition(void) const;
-    
+    bool in_tailsitter_vtol_transition(uint32_t now = 0) const;
+
     bool handle_do_vtol_transition(enum MAV_VTOL_STATE state);
 
     bool do_vtol_takeoff(const AP_Mission::Mission_Command& cmd);

ArduPlane/tailsitter.cpp

@@ -262,9 +262,19 @@ void QuadPlane::tailsitter_check_input(void)
 /*
   return true if we are a tailsitter transitioning to VTOL flight
  */
-bool QuadPlane::in_tailsitter_vtol_transition(void) const
+bool QuadPlane::in_tailsitter_vtol_transition(uint32_t now) const
 {
-    return is_tailsitter() && in_vtol_mode() && transition_state == TRANSITION_ANGLE_WAIT_VTOL;
+    if (!is_tailsitter() || !in_vtol_mode()) {
+        return false;
+    }
+    if (transition_state == TRANSITION_ANGLE_WAIT_VTOL) {
+        return true;
+    }
+    if ((now != 0) && ((now - last_vtol_mode_ms) > 1000)) {
+        // only just come out of forward flight
+        return true;
+    }
+    return false;
 }
 
 /*