mc pos control: add MPC_THR_CURVE param

The hover thrust param also changed the thrust scaling in Stabilized mode.
However if the hover thrust is very low (e.g. below 20%), the throttle
stick input becomes very distorted.

src/lib/FlightTasks/tasks/ManualStabilized/FlightTaskManualStabilized.cpp

@@ -145,16 +145,22 @@ void FlightTaskManualStabilized::_updateSetpoints()
 
 float FlightTaskManualStabilized::_throttleCurve()
 {
-	/* Scale stick z from [-1,1] to [min thrust, max thrust]
-	 * with hover throttle at 0.5 stick */
+	// Scale stick z from [-1,1] to [min thrust, max thrust]
 	float throttle = -((_sticks(2) - 1.0f) * 0.5f);
 
-	if (throttle < 0.5f) {
-		return (_throttle_hover.get() - _throttle_min_stabilized.get()) / 0.5f * throttle + _throttle_min_stabilized.get();
+	switch (_throttle_curve.get()) {
+	case 1: // no rescaling
+		return throttle;
 
-	} else {
-		return (_throttle_max.get() - _throttle_hover.get()) / 0.5f * (throttle - 1.0f) + _throttle_max.get();
+	default: // 0 or other: rescale to hover throttle at 0.5 stick
+		if (throttle < 0.5f) {
+			return (_throttle_hover.get() - _throttle_min_stabilized.get()) / 0.5f * throttle + _throttle_min_stabilized.get();
+
+		} else {
+			return (_throttle_max.get() - _throttle_hover.get()) / 0.5f * (throttle - 1.0f) + _throttle_max.get();
+		}
 	}
+
 }
 
 bool FlightTaskManualStabilized::update()

src/lib/FlightTasks/tasks/ManualStabilized/FlightTaskManualStabilized.hpp

@@ -77,6 +77,7 @@ private:
 					(ParamFloat<px4::params::MPC_MAN_TILT_MAX>) _tilt_max_man, /**< maximum tilt allowed for manual flight */
 					(ParamFloat<px4::params::MPC_MANTHR_MIN>) _throttle_min_stabilized, /**< minimum throttle for stabilized */
 					(ParamFloat<px4::params::MPC_THR_MAX>) _throttle_max, /**< maximum throttle that always has to be satisfied in flight*/
-					(ParamFloat<px4::params::MPC_THR_HOVER>) _throttle_hover /**< throttle value at which vehicle is at hover equilibrium */
+					(ParamFloat<px4::params::MPC_THR_HOVER>) _throttle_hover, /**< throttle at which vehicle is at hover equilibrium */
+					(ParamInt<px4::params::MPC_THR_CURVE>) _throttle_curve /**< throttle curve behavior */
 				       )
 };