AP_TECS: Add a seperate stall speed parameter used for minimum speed …

…scaling

When stall prevention is enabled we were scaling from the aircraft's
minimum flight speed. However this is normally already picked as being
above the stall speed, and for a variety of reasons we may want to pin
the aircraft at a higher minimum speed. But if the aircraft was
commanded to fly to close to that minimum speed as soon as it banked for
a pattern it would command a increase in speed to keep it away from
stalling. However if your minimum speed is far from stalling this
increase was incorrect. To make it worse what this actually results in
happening is an aircraft diving for more speed (over 10 m/s on some
aircraft) as well as descending to gain that speed resulting in over 200
foot deviations in altitude control.

libraries/AP_TECS/AP_TECS.cpp

@@ -283,6 +283,15 @@ const AP_Param::GroupInfo AP_TECS::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("HDEM_TCONST", 33, AP_TECS, _hgt_dem_tconst, 3.0f),
 
+    // @Param: STALL_SPEED
+    // @DisplayName: Stall speed used for scaling minimum speed
+    // @Description: If stall prevention is enabled this speed is used to calculate the minimum airspeed while banking. If this is set to 0 then the speed is scaled from the minimum speed. Typically set slightly higher then true stall speed.
+    // @Range: 5.0 75.0
+    // @Units: m/s
+    // @Increment: 1
+    // @User: Advanced
+    AP_GROUPINFO("STALL_SPEED", 34, AP_TECS, _stall_speed, 0),
+
     AP_GROUPEND
 };
 
@@ -415,7 +424,11 @@ void AP_TECS::_update_speed(float DT)
     if (aparm.stall_prevention) {
         // when stall prevention is active we raise the minimum
         // airspeed based on aerodynamic load factor
-        _TASmin *= _load_factor;
+        if (is_positive(_stall_speed)) {
+            _TASmin = MAX(_TASmin, _stall_speed*EAS2TAS*_load_factor);
+        } else {
+            _TASmin *= _load_factor;
+        }
     }
 
     if (_TASmax < _TASmin) {

libraries/AP_TECS/AP_TECS.h

@@ -196,6 +196,7 @@ class AP_TECS {
     AP_Int32 _options;
     AP_Float _flare_holdoff_hgt;
     AP_Float _hgt_dem_tconst;
+    AP_Float _stall_speed;
 
     enum {
         OPTION_GLIDER_ONLY=(1<<0),