APM_Control: reduce elevator when past LIM_ROLL_CD

this reduces elevator control when rolled over hard in fixed
wing. Using the elevator when on the side just caused earth frame yaw
and is counter productive. It can also prevent some aircraft from
recovering from inverted flight.

libraries/APM_Control/AP_PitchController.cpp

@@ -201,7 +201,27 @@ int32_t AP_PitchController::_get_rate_out(float desired_rate, float scaler, bool
     }
 
 	_last_out += _pid_info.I;
-	
+
+    /*
+      when we are past the users defined roll limit for the
+      aircraft our priority should be to bring the aircraft back
+      within the roll limit. Using elevator for pitch control at
+      large roll angles is ineffective, and can be counter
+      productive as it induces earth-frame yaw which can reduce
+      the ability to roll. We linearly reduce elevator input when
+      beyond the configured roll limit, reducing to zero at 90
+      degrees
+    */
+    float roll_wrapped = fabsf(_ahrs.roll_sensor);
+    if (roll_wrapped > 9000) {
+        roll_wrapped = 18000 - roll_wrapped;
+    }
+    if (roll_wrapped > aparm.roll_limit_cd + 500 && aparm.roll_limit_cd < 8500 &&
+        labs(_ahrs.pitch_sensor) < 7000) {
+        float roll_prop = (roll_wrapped - (aparm.roll_limit_cd+500)) / (float)(9000 - aparm.roll_limit_cd);
+        _last_out *= (1 - roll_prop);
+    }
+    
 	// Convert to centi-degrees and constrain
 	return constrain_float(_last_out * 100, -4500, 4500);
 }