Plane: added TKOFF_THR_MINACC option

this is used for triggering auto takeoff with a hand launched plane

ArduPlane/Attitude.pde

@@ -379,6 +379,32 @@ static void throttle_slew_limit(int16_t last_throttle)
     }
 }
 
+/*
+  check for automatic takeoff conditions being met
+ */
+static bool auto_takeoff_check(void)
+{
+    if (g_gps == NULL || g_gps->status() != GPS::GPS_OK) {
+        // no auto takeoff without GPS lock
+        return false;
+    }
+    if (g_gps->ground_speed < g.takeoff_throttle_min_speed*100.0f) {
+        // we haven't reached the minimum ground speed
+        return false;
+    }
+    if (g.takeoff_throttle_min_accel > 0.0f &&
+        (ins.get_accel().x < g.takeoff_throttle_min_accel) &&
+        ahrs.pitch_sensor > -3000 && ahrs.pitch_sensor < 4500 &&
+        abs(ahrs.roll_sensor) < 3000) {
+        // we haven't reached the minimum acceleration or we are not
+        // anywhere near flat. Thanks to Chris Miser for this
+        // suggestion
+        return false;
+    }
+    // we're good for takeoff
+    return true;
+}
+
 
 /* We want to supress the throttle if we think we are on the ground and in an autopilot controlled throttle mode.
 
@@ -387,7 +413,7 @@ static void throttle_slew_limit(int16_t last_throttle)
    *       AND
    *       2 - Our reported altitude is within 10 meters of the home altitude.
    *       3 - Our reported speed is under 5 meters per second.
-   *       4 - We are not performing a takeoff in Auto mode or takeoff speed not yet reached
+   *       4 - We are not performing a takeoff in Auto mode or takeoff speed/accel not yet reached
    *       OR
    *       5 - Home location is not set
 */
@@ -403,11 +429,7 @@ static bool suppress_throttle(void)
         return false;
     }
 
-    if (control_mode==AUTO &&
-        takeoff_complete == false &&
-        g_gps != NULL &&
-        g_gps->status() == GPS::GPS_OK &&
-        g_gps->ground_speed >= g.takeoff_throttle_min_speed*100) {
+    if (control_mode==AUTO && takeoff_complete == false && auto_takeoff_check()) {
         // we're in auto takeoff 
         throttle_suppressed = false;
         return false;

ArduPlane/Parameters.h

@@ -79,6 +79,7 @@ public:
         k_param_alt_offset,
         k_param_ins,                // libraries/AP_InertialSensor variables
         k_param_takeoff_throttle_min_speed,
+        k_param_takeoff_throttle_min_accel,
 
         // 110: Telemetry control
         //
@@ -356,6 +357,7 @@ public:
     AP_Int8 stick_mixing;
     AP_Int8 rudder_steer;
     AP_Float takeoff_throttle_min_speed;
+    AP_Float takeoff_throttle_min_accel;
 
     // RC channels
     RC_Channel channel_roll;

ArduPlane/Parameters.pde

@@ -97,6 +97,15 @@ const AP_Param::Info var_info[] PROGMEM = {
     // @User: User
     GSCALAR(takeoff_throttle_min_speed,     "TKOFF_THR_MINSPD",  0),
 
+    // @Param: TKOFF_THR_MINACC
+    // @DisplayName: Takeoff throttle min acceleration
+    // @Description: Minimum forward acceleration in m/s/s before un-suppressing throttle in auto-takeoff. This is meant to be used for hand launches with a tractor style (front engine) plane. If this is set then the auto takeoff will only trigger if the pitch of the plane is between -30 and +45 degrees, and the roll is less than 30 degrees. This makes it less likely it will trigger due to carrying the plane with the nose down.
+    // @Units: m/s/s
+    // @Range: 0 30
+    // @Increment: 0.1
+    // @User: User
+    GSCALAR(takeoff_throttle_min_accel,     "TKOFF_THR_MINACC",  0),
+
     // @Param: RUDDER_STEER
     // @DisplayName: Rudder steering on takeoff and landing
     // @Description: When enabled, only rudder will be used for steering during takeoff and landing, with the ailerons used to hold the plane level