Plane: check for high ratio of rmsD to rmsP in tuning

ArduPlane/tuning.cpp

@@ -277,21 +277,45 @@ float AP_Tuning_Plane::controller_error(uint8_t parm)
     if (!plane.quadplane.available()) {
         return 0;
     }
+
+    // in general a good tune will have rmsP significantly greater
+    // than rmsD. Otherwise it is too easy to push D too high while
+    // tuning a quadplane and end up with D dominating
+    const float max_P_D_ratio = 3.0f;
     
     switch(parm) {
     // special handling of dual-parameters
     case TUNING_RATE_ROLL_PI:
     case TUNING_RATE_ROLL_P:
     case TUNING_RATE_ROLL_I:
-    case TUNING_RATE_ROLL_D:
         return plane.quadplane.attitude_control->control_monitor_rms_output_roll();
+
+    case TUNING_RATE_ROLL_D: {
+        // special case for D term to keep it well below P
+        float rms_P = plane.quadplane.attitude_control->control_monitor_rms_output_roll_P();
+        float rms_D = plane.quadplane.attitude_control->control_monitor_rms_output_roll_D();
+        if (rms_P < rms_D * max_P_D_ratio) {
+            return max_P_D_ratio;
+        }
+        return rms_P+rms_D;
+    }
+        
         
     case TUNING_RATE_PITCH_PI:
     case TUNING_RATE_PITCH_P:
     case TUNING_RATE_PITCH_I:
-    case TUNING_RATE_PITCH_D:
         return plane.quadplane.attitude_control->control_monitor_rms_output_pitch();
 
+    case TUNING_RATE_PITCH_D: {
+        // special case for D term to keep it well below P
+        float rms_P = plane.quadplane.attitude_control->control_monitor_rms_output_pitch_P();
+        float rms_D = plane.quadplane.attitude_control->control_monitor_rms_output_pitch_D();
+        if (rms_P < rms_D * max_P_D_ratio) {
+            return max_P_D_ratio;
+        }
+        return rms_P+rms_D;
+    }
+        
     case TUNING_RATE_YAW_PI:
     case TUNING_RATE_YAW_P:
     case TUNING_RATE_YAW_I: