Plane: allow for higher roll limits on takeoff

only restrict to LEVEL_ROLL_LIMIT below 5m. Above that start to allow
larger roll limits, reaching the full roll limit at 15m

ArduPlane/Attitude.cpp

@@ -593,6 +593,7 @@ bool Plane::suppress_throttle(void)
         if (auto_takeoff_check()) {
             // we're in auto takeoff 
             throttle_suppressed = false;
+            auto_state.baro_takeoff_alt = barometer.get_altitude();
             return false;
         }
         // keep throttle suppressed

ArduPlane/Plane.h

@@ -486,6 +486,9 @@ private:
 
         // time stamp of when we start flying while in auto mode in milliseconds
         uint32_t started_flying_in_auto_ms;
+
+        // barometric altitude at start of takeoff
+        float baro_takeoff_alt;
     } auto_state;
 
     struct {

ArduPlane/commands_logic.cpp

@@ -334,6 +334,7 @@ void Plane::do_takeoff(const AP_Mission::Mission_Command& cmd)
     // zero locked course
     steer_state.locked_course_err = 0;
     steer_state.hold_course_cd = -1;
+    auto_state.baro_takeoff_alt = barometer.get_altitude();
 }
 
 void Plane::do_nav_wp(const AP_Mission::Mission_Command& cmd)

ArduPlane/takeoff.cpp

@@ -98,9 +98,22 @@ void Plane::takeoff_calc_roll(void)
 
     calc_nav_roll();
 
-    // during takeoff use the level flight roll limit to
-    // prevent large course corrections
-    nav_roll_cd = constrain_int32(nav_roll_cd, -g.level_roll_limit*100UL, g.level_roll_limit*100UL);
+    // during takeoff use the level flight roll limit to prevent large
+    // wing strike. Slowly allow for more roll as we get higher above
+    // the takeoff altitude
+    float roll_limit = roll_limit_cd*0.01f;
+    float baro_alt = barometer.get_altitude();
+    // below 5m use the LEVEL_ROLL_LIMIT
+    const float lim1 = 5;    
+    // at 15m allow for full roll
+    const float lim2 = 15;
+    if (baro_alt < auto_state.baro_takeoff_alt+lim1) {
+        roll_limit = g.level_roll_limit;
+    } else if (baro_alt < auto_state.baro_takeoff_alt+lim2) {
+        float proportion = (baro_alt - (auto_state.baro_takeoff_alt+lim1)) / (lim2 - lim1);
+        roll_limit = (1-proportion) * g.level_roll_limit + proportion * roll_limit;
+    }
+    nav_roll_cd = constrain_int32(nav_roll_cd, -roll_limit*100UL, roll_limit*100UL);
 }