tecs: convert tas error and soft bounds to percentage of trim airspeed

src/lib/tecs/TECS.cpp

@@ -216,8 +216,17 @@ void TECS::_detect_underspeed()
 		return;
 	}
 
-	const float tas_fully_undersped = math::max(_TAS_min - TAS_ERROR_BOUND - TAS_UNDERSPEED_SOFT_BOUND, 0.0f);
-	const float tas_starting_to_underspeed = math::max(_TAS_min - TAS_ERROR_BOUND, tas_fully_undersped);
+	// this is the expected (something like standard) deviation from the airspeed setpoint that we allow the airspeed
+	// to vary in before ramping in underspeed mitigation
+	const float tas_error_bound = kTASErrorPercentage * _equivalent_airspeed_trim;
+
+	// this is the soft boundary where underspeed mitigation is ramped in
+	// NOTE: it's currently the same as the error bound, but separated here to indicate these values do not in general
+	// need to be the same
+	const float tas_underspeed_soft_bound = kTASErrorPercentage * _equivalent_airspeed_trim;
+
+	const float tas_fully_undersped = math::max(_TAS_min - tas_error_bound - tas_underspeed_soft_bound, 0.0f);
+	const float tas_starting_to_underspeed = math::max(_TAS_min - tas_error_bound, tas_fully_undersped);
 
 	_percent_undersped = 1.0f - math::constrain((_tas_state - tas_fully_undersped) /
 			     math::max(tas_starting_to_underspeed - tas_fully_undersped, FLT_EPSILON), 0.0f, 1.0f);

src/lib/tecs/TECS.hpp

@@ -219,12 +219,8 @@ public:
 
 private:
 
-	// [m/s] bound on expected (safe) true airspeed tracking errors, including TAS = TAS_min - TAS_ERROR_BOUND
-	static constexpr float TAS_ERROR_BOUND = 2.0f;
-
-	// [m/s] true airspeed soft boundary region below the accepted TAS error region (below TAS_min - TAS_ERROR_BOUND)
-	// underspeed mitigation measures are ramped in from zero to full within this region
-	static constexpr float TAS_UNDERSPEED_SOFT_BOUND = 1.5f;
+	// [0,1] percentage of true airspeed trim corresponding to expected (safe) true airspeed tracking errors
+	static constexpr float kTASErrorPercentage = 0.15;
 
 	static constexpr float _jerk_max =
 		1000.0f;