AC_AttitudeControl:TradHeli - fix for alt_hold angle limiter active at hover collective in Traditional Helicopters

add definition for max collective/throttle for helicopters that compensates for negative collective pitch range
and sets proper collective overhead before alt_hold angle limiter becomes effective

libraries/AC_AttitudeControl/AC_AttitudeControl_Heli.cpp

@@ -268,7 +268,7 @@ void AC_AttitudeControl_Heli::rate_controller_run()
 // Update Alt_Hold angle maximum
 void AC_AttitudeControl_Heli::update_althold_lean_angle_max(float throttle_in)
 {
-    float althold_lean_angle_max = acos(constrain_float(_throttle_in/AC_ATTITUDE_CONTROL_ANGLE_LIMIT_THROTTLE_MAX, 0.0f, 1.0f));
+    float althold_lean_angle_max = acos(constrain_float(_throttle_in/AC_ATTITUDE_HELI_ANGLE_LIMIT_THROTTLE_MAX, 0.0f, 1.0f));
     _althold_lean_angle_max = _althold_lean_angle_max + (_dt/(_dt+_angle_limit_tc))*(althold_lean_angle_max-_althold_lean_angle_max);
 }
 

libraries/AC_AttitudeControl/AC_AttitudeControl_Heli.h

@@ -22,6 +22,7 @@
 #define AC_ATC_HELI_RATE_YAW_FF                     0.024f
 #define AC_ATC_HELI_RATE_YAW_FILT_HZ                20.0f
 
+#define AC_ATTITUDE_HELI_ANGLE_LIMIT_THROTTLE_MAX   0.95f    // Heli's use 95% of max collective before limiting frame angle
 #define AC_ATTITUDE_HELI_RATE_INTEGRATOR_LEAK_RATE  0.02f
 #define AC_ATTITUDE_HELI_RATE_RP_FF_FILTER          10.0f
 #define AC_ATTITUDE_HELI_RATE_Y_VFF_FILTER          10.0f