Plane: add comment on speed scalar calculation

 it needs to be a constant so that when the user changes TRIM_THROTTLE (eg. via DO_CHANGE_SPEED or via the parameter) that it doesn't change the tuning of their PID loops by changing the speed scaler. The aircraft could either start oscillating badly or could become very sluggish in control

Author: peterbarker

ArduPlane/Attitude.cpp

@@ -42,7 +42,11 @@ float Plane::calc_speed_scaler(void)
     } else if (arming.is_armed_and_safety_off()) {
         // scale assumed surface movement using throttle output
         float throttle_out = MAX(SRV_Channels::get_output_scaled(SRV_Channel::k_throttle), 1);
-        speed_scaler = sqrtf(THROTTLE_CRUISE / throttle_out);
+        // we use a fixed value here as changing the trim throttle
+        // value is often done at runtime - and changing that
+        // shouldn't change the speed scaler here (it will change the
+        // effective PID values)
+        speed_scaler = sqrtf(AP_PLANE_TRIM_THROTTLE_DEFAULT / throttle_out);
         // This case is constrained tighter as we don't have real speed info
         speed_scaler = constrain_float(speed_scaler, 0.6f, 1.67f);
     } else {

ArduPlane/Parameters.cpp

@@ -426,7 +426,7 @@ const AP_Param::Info Plane::var_info[] = {
     // @Range: 0 100
     // @Increment: 1
     // @User: Standard
-    ASCALAR(throttle_cruise,        "TRIM_THROTTLE",  THROTTLE_CRUISE),
+    ASCALAR(throttle_cruise,        "TRIM_THROTTLE",  AP_PLANE_TRIM_THROTTLE_DEFAULT),
 
     // @Param: THROTTLE_NUDGE
     // @DisplayName: Throttle nudge enable

ArduPlane/config.h

@@ -138,8 +138,11 @@
 #ifndef THROTTLE_MIN
  # define THROTTLE_MIN                   0 // percent
 #endif
-#ifndef THROTTLE_CRUISE
- # define THROTTLE_CRUISE                45
+#ifdef THROTTLE_CRUISE
+#error THROTTLE_CRUISE was renamed to AP_PLANE_TRIM_THROTTLE_DEFAULT
+#endif
+#ifndef AP_PLANE_TRIM_THROTTLE_DEFAULT
+ # define AP_PLANE_TRIM_THROTTLE_DEFAULT                45
 #endif
 #ifndef THROTTLE_MAX
  # define THROTTLE_MAX                   100