feat(vtol_att_control): increase front transition throttle to compensate for weight ratio

src/modules/vtol_att_control/standard.cpp

@@ -204,15 +204,17 @@ void Standard::update_transition_state()
 			roll_body = Eulerf(Quatf(_fw_virtual_att_sp->q_d)).phi();
 		}
 
+		const float transition_throttle = getFrontTransitionThrottle();
+
 		if (_param_vt_psher_slew.get() <= FLT_EPSILON) {
 			// just set the final target throttle value
-			_pusher_throttle = _param_vt_f_trans_thr.get();
+			_pusher_throttle = transition_throttle;
 
-		} else if (_pusher_throttle <= _param_vt_f_trans_thr.get()) {
+		} else if (_pusher_throttle <= transition_throttle) {
 			// ramp up throttle to the target throttle value
 			const float dt = math::min((now - _last_time_pusher_transition_update) / 1e6f, 0.05f);
 			_pusher_throttle = math::min(_pusher_throttle +
-						     _param_vt_psher_slew.get() * dt, _param_vt_f_trans_thr.get());
+						     _param_vt_psher_slew.get() * dt, transition_throttle);
 
 			_last_time_pusher_transition_update = now;
 		}

src/modules/vtol_att_control/vtol_att_control_params.yaml

@@ -46,6 +46,7 @@ parameters:
     VT_F_TRANS_THR:
       description:
         short: Target throttle value for the transition to fixed-wing flight
+        long: Scaled by the square root of the weight ratio (WEIGHT_GROSS / WEIGHT_BASE).
       type: float
       default: 1.0
       min: 0.0

src/modules/vtol_att_control/vtol_type.cpp

@@ -539,21 +539,26 @@ float VtolType::pusher_assist()
 
 float VtolType::getFrontTransitionTimeFactor() const
 {
-	// assumptions: transition_time = transition_true_airspeed / average_acceleration (thrust)
-	// transition_true_airspeed ~ sqrt(rho0 / rh0)
+	// assumptions:
+	// transition_time = transition_true_airspeed / average_acceleration (thrust)
+	// transition_true_airspeed ~ sqrt(rho0 / rh0) * sqrt(weight_ratio)
 	// average_acceleration ~ rho / rho0
-	// transition_time ~ sqrt(rho0/rh0) * rho0 / rho
+	//   independent of weight: standard VTOL throttle is weight scaled,
+	//   tiltrotor and tailsitter weight scale via hover thrust
+	// transition_time ~ sqrt(rho0/rh0) * rho0 / rho * sqrt(weight_ratio)
 
 	// low value: hot day at 4000m AMSL with some margin
 	// high value: cold day at 0m AMSL with some margin
 	const float rho = math::constrain(_attc->getAirDensity(), 0.7f, 1.5f);
 
+	float density_factor = 1.0f;
+
 	if (PX4_ISFINITE(rho)) {
 		float rho0_over_rho = atmosphere::kAirDensitySeaLevelStandardAtmos / rho;
-		return sqrtf(rho0_over_rho) * rho0_over_rho;
+		density_factor = sqrtf(rho0_over_rho) * rho0_over_rho;
 	}
 
-	return 1.0f;
+	return density_factor * sqrtf(getWeightRatio());
 }
 
 float VtolType::getMinimumFrontTransitionTime() const
@@ -574,15 +579,26 @@ float VtolType::getTransitionAirspeed() const
 {
 	// Since the stall airspeed increases with vehicle weight, we increase the transition airspeed
 	// by the same factor.
+	return sqrtf(getWeightRatio()) * _param_vt_arsp_trans.get();
+}
+
+float VtolType::getFrontTransitionThrottle() const
+{
+	// Thrust needs to scale with weight_ratio to keep the acceleration weight-independent. Assuming
+	// a quadratic throttle -> thrust map, this requires scaling the throttle by sqrt(weight_ratio).
+	return math::min(sqrtf(getWeightRatio()) * _param_vt_f_trans_thr.get(), 1.0f);
+}
 
+float VtolType::getWeightRatio() const
+{
 	float weight_ratio = 1.0f;
 
 	if (_param_weight_base.get() > FLT_EPSILON && _param_weight_gross.get() > FLT_EPSILON) {
 		weight_ratio = math::constrain(_param_weight_gross.get() /
 					       _param_weight_base.get(), kMinWeightRatio, kMaxWeightRatio);
 	}
 
-	return sqrtf(weight_ratio) * _param_vt_arsp_trans.get();
+	return weight_ratio;
 }
 
 float VtolType::getBlendAirspeed() const

src/modules/vtol_att_control/vtol_type.h

@@ -223,17 +223,17 @@ class VtolType : public ModuleParams
 	mode get_mode() {return _common_vtol_mode;}
 
 	/**
-	 * @return Minimum front transition time scaled for air density (if available) [s]
+	 * @return Minimum front transition time scaled for air density (if available) and vehicle weight [s]
 	*/
 	float getMinimumFrontTransitionTime() const;
 
 	/**
-	 * @return Front transition timeout scaled for air density (if available) [s]
+	 * @return Front transition timeout scaled for air density (if available) and vehicle weight [s]
 	*/
 	float getFrontTransitionTimeout() const;
 
 	/**
-	* @return Minimum open-loop front transition time scaled for air density (if available) [s]
+	* @return Minimum open-loop front transition time scaled for air density (if available) and vehicle weight [s]
 	*/
 	float getOpenLoopFrontTransitionTime() const;
 
@@ -249,6 +249,12 @@ class VtolType : public ModuleParams
 	 */
 	float getTransitionAirspeed() const;
 
+	/**
+	 *
+	 * @return The target front transition throttle scaled for vehicle weight
+	 */
+	float getFrontTransitionThrottle() const;
+
 	virtual void parameters_update() = 0;
 
 
@@ -366,10 +372,15 @@ class VtolType : public ModuleParams
 	void stopBlendingThrottleAfterFrontTransition() { _throttle_blend_start_ts = 0; }
 
 	/**
-	 * @return Transition time scale factor for density.
+	 * @return Transition time scale factor for density and vehicle weight.
 	*/
 	float getFrontTransitionTimeFactor() const;
 
+	/**
+	 * @return Gross weight over base weight ratio, constrained to sane limits.
+	*/
+	float getWeightRatio() const;
+
 };
 
 #endif