Plane: prevent stalled takeoff with bad TKOFF_TDRAG_SPD1

go to level pitch if pitch rises by 10 degrees

ArduPlane/ArduPlane.pde

@@ -530,12 +530,16 @@ static struct {
     // the highest airspeed we have reached since entering AUTO. Used
     // to control ground takeoff
     float highest_airspeed;
+
+    // initial pitch. Used to detect if nose is rising in a tail dragger
+    int16_t initial_pitch_cd;
 } auto_state = {
     takeoff_complete : true,
     land_complete : false,
     takeoff_altitude_cm : 0,
     takeoff_pitch_cd : 0,
-    highest_airspeed : 0
+    highest_airspeed : 0,
+    initial_pitch_cd : 0
 };
 
 // true if we are in an auto-throttle mode, which means

ArduPlane/system.pde

@@ -337,6 +337,7 @@ static void set_mode(enum FlightMode mode)
         auto_throttle_mode = true;
         next_WP_loc = prev_WP_loc = current_loc;
         auto_state.highest_airspeed = 0;
+        auto_state.initial_pitch_cd = ahrs.pitch_sensor;
         // start or resume the mission, based on MIS_AUTORESET
         mission.start_or_resume();
         break;

ArduPlane/takeoff.pde

@@ -142,6 +142,14 @@ static int8_t takeoff_tail_hold(void)
         // level pitch. Return 0 meaning no fixed elevator setting
         return 0;
     }
+    if (ahrs.pitch_sensor > auto_state.initial_pitch_cd + 1000) {
+        // the pitch has gone up by more then 10 degrees over the
+        // initial pitch. This may mean the nose is coming up for an
+        // early liftoff, perhaps due to a bad setting of
+        // g.takeoff_tdrag_speed1. Go to level flight to prevent a
+        // stall
+        return 0;
+    }
     // we are holding the tail down
     return g.takeoff_tdrag_elevator;
 }