APM: added parameter FBWB_ELEV_REV

this fixes issue 343

ArduPlane/ArduPlane.pde

@@ -1137,15 +1137,19 @@ static void update_current_flight_mode(void)
                 // Thanks to Yury MonZon for the altitude limit code!
 
 				nav_roll_cd = g.channel_roll.norm_input() * g.roll_limit_cd;
-				altitude_error_cm = g.channel_pitch.norm_input() * g.pitch_limit_min_cd;
+                
+                float elevator_input;
+                elevator_input = g.channel_pitch.norm_input();
 
+                if (g.flybywire_elev_reverse) {
+                    elevator_input = -elevator_input;
+                }
 				if ((current_loc.alt >= home.alt+g.FBWB_min_altitude_cm) || (g.FBWB_min_altitude_cm == 0)) {
-	 				altitude_error_cm = g.channel_pitch.norm_input() * g.pitch_limit_min_cd;
+	 				altitude_error_cm = elevator_input * g.pitch_limit_min_cd;
 				} else {
-					if (g.channel_pitch.norm_input()<0) {
-						altitude_error_cm =( (home.alt + g.FBWB_min_altitude_cm) - current_loc.alt) + g.channel_pitch.norm_input() * g.pitch_limit_min_cd;
-					} else {
-                        altitude_error_cm =( (home.alt + g.FBWB_min_altitude_cm) - current_loc.alt);
+                    altitude_error_cm = (home.alt + g.FBWB_min_altitude_cm) - current_loc.alt;
+					if (elevator_input < 0) {
+						altitude_error_cm += elevator_input * g.pitch_limit_min_cd;
                     }
 				}
 				calc_throttle();

ArduPlane/Parameters.h

@@ -87,6 +87,7 @@ public:
         k_param_flybywire_airspeed_min = 120,
         k_param_flybywire_airspeed_max,
         k_param_FBWB_min_altitude_cm,  // 0=disabled, minimum value for altitude in cm (for first time try 30 meters = 3000 cm)
+        k_param_flybywire_elev_reverse,
 
         //
         // 130: Sensor parameters
@@ -253,6 +254,7 @@ public:
     //
     AP_Int16    flybywire_airspeed_min;
     AP_Int16    flybywire_airspeed_max;
+    AP_Int8	    flybywire_elev_reverse;
 
     // Throttle
     //

ArduPlane/Parameters.pde

@@ -199,6 +199,13 @@ const AP_Param::Info var_info[] PROGMEM = {
 	// @User: Standard
 	GSCALAR(flybywire_airspeed_max, "ARSPD_FBW_MAX",  AIRSPEED_FBW_MAX),
 
+    // @Param: FBWB_ELEV_REV
+	// @DisplayName: Fly By Wire elevator reverse
+	// @Description: Reverse sense of elevator in FBWB. When set to 0 up elevator (pulling back on the stick) means to lower altitude. When set to 1, up elevator means to raise altitude.
+	// @Values: 0:Disabled,1:Enabled
+	// @User: Standard
+	GSCALAR(flybywire_elev_reverse, "FBWB_ELEV_REV",  0),
+
     // @Param: THR_MIN
 	// @DisplayName: Minimum Throttle
 	// @Description: The minimum throttle setting to which the autopilot will apply.