Quadratic thrust for SIH hexarotor (#67)

* sih: add thrust model hexacopter model

* fix: add missing line break

* sihsim_hey airframe: add THR_MDL_FAC

to accomodate for simulated quadratic motor thrust.

---------

Co-authored-by: Matthias Grob <maetugr@gmail.com>

ROMFS/px4fmu_common/init.d-posix/airframes/10044_sihsim_hex

@@ -45,3 +45,5 @@ param set-default PWM_MAIN_FUNC5 105
 param set-default PWM_MAIN_FUNC6 106
 
 param set-default EKF2_GPS_DELAY 0
+
+param set-default THR_MDL_FAC 1 # simulated quadratic motor thrust

src/modules/simulation/simulator_sih/sih.cpp

@@ -337,6 +337,10 @@ void Sih::generate_force_and_torques(const float dt)
 		       m3    m2
 		          ├1/2┤
 		          ├  1  ┤    */
+		for (int i = 0; i < 6; ++i) {
+			_u[i] *= _u[i]; // quadratic thrust model
+		}
+
 		_T_B = Vector3f(0.0f, 0.0f, -_T_MAX * (+_u[0] + _u[1] + _u[2] + _u[3] + _u[4] + _u[5]));
 		_Mt_B = Vector3f(_L_ROLL * _T_MAX * (-.5f * _u[0] - _u[1] - .5f * _u[2] + .5f * _u[3] + _u[4] + .5f * _u[5]),
 				 _L_PITCH * _T_MAX * (M_SQRT3_F / 2.f) * (+_u[0] - _u[2] - _u[3] + _u[5]),