Copter: keep manual throttle mix at zero throttle when airmode is on

allow landing detection in airmode
increase landing detection timeout in airmode.

ArduCopter/config.h

@@ -417,6 +417,9 @@
 #ifndef LAND_DETECTOR_TRIGGER_SEC
  # define LAND_DETECTOR_TRIGGER_SEC         1.0f    // number of seconds to detect a landing
 #endif
+#ifndef LAND_AIRMODE_DETECTOR_TRIGGER_SEC
+ # define LAND_AIRMODE_DETECTOR_TRIGGER_SEC 3.0f    // number of seconds to detect a landing in air mode
+#endif
 #ifndef LAND_DETECTOR_MAYBE_TRIGGER_SEC
  # define LAND_DETECTOR_MAYBE_TRIGGER_SEC   0.2f    // number of seconds that means we might be landed (used to reset horizontal position targets to prevent tipping over)
 #endif

ArduCopter/land_detector.cpp

@@ -60,6 +60,7 @@ void Copter::update_land_detector()
         land_detector_count = 0;
     } else {
 
+        float land_trigger_sec = LAND_DETECTOR_TRIGGER_SEC;
 #if FRAME_CONFIG == HELI_FRAME
         // check for both manual collective modes and modes that use altitude hold. For manual collective (called throttle
         // because multi's use throttle), check that collective pitch is below land min collective position or throttle stick is zero.
@@ -70,9 +71,17 @@ void Copter::update_land_detector()
         const bool landing = flightmode->is_landing();
         bool motor_at_lower_limit = (flightmode->has_manual_throttle() && (motors->get_below_land_min_coll() || heli_flags.coll_stk_low) && fabsf(ahrs.get_roll()) < M_PI/2.0f)
                                     || ((!force_flying || landing) && motors->limit.throttle_lower && pos_control->get_vel_desired_cms().z < 0.0f);
+        bool throttle_mix_at_min = true;
 #else
         // check that the average throttle output is near minimum (less than 12.5% hover throttle)
-        bool motor_at_lower_limit = motors->limit.throttle_lower && attitude_control->is_throttle_mix_min();
+        bool motor_at_lower_limit = motors->limit.throttle_lower;
+        bool throttle_mix_at_min = attitude_control->is_throttle_mix_min();
+        // set throttle_mix_at_min to true because throttle is never at mix min in airmode
+        // increase land_trigger_sec when using airmode
+        if (flightmode->has_manual_throttle() && air_mode == AirMode::AIRMODE_ENABLED) {
+            land_trigger_sec = LAND_AIRMODE_DETECTOR_TRIGGER_SEC;
+            throttle_mix_at_min = true;
+        }
 #endif
 
         uint8_t land_detector_scalar = 1;
@@ -99,9 +108,9 @@ void Copter::update_land_detector()
         const bool WoW_check = true;
 #endif
 
-        if (motor_at_lower_limit && accel_stationary && descent_rate_low && rangefinder_check && WoW_check) {
+        if (motor_at_lower_limit && throttle_mix_at_min && accel_stationary && descent_rate_low && rangefinder_check && WoW_check) {
             // landed criteria met - increment the counter and check if we've triggered
-            if( land_detector_count < ((float)LAND_DETECTOR_TRIGGER_SEC)*scheduler.get_loop_rate_hz()) {
+            if( land_detector_count < land_trigger_sec*scheduler.get_loop_rate_hz()) {
                 land_detector_count++;
             } else {
                 set_land_complete(true);
@@ -174,7 +183,7 @@ void Copter::update_throttle_mix()
 
     if (flightmode->has_manual_throttle()) {
         // manual throttle
-        if (channel_throttle->get_control_in() <= 0 || air_mode == AirMode::AIRMODE_DISABLED) {
+        if (channel_throttle->get_control_in() <= 0 && air_mode != AirMode::AIRMODE_ENABLED) {
             attitude_control->set_throttle_mix_min();
         } else {
             attitude_control->set_throttle_mix_man();