Airspeed Selector: changes in airspeed library to account for new EAS…

… state (equivalent airspeed)

Signed-off-by: Silvan Fuhrer <silvan@auterion.com>

src/lib/airspeed/airspeed.cpp

@@ -55,8 +55,8 @@
  * @param differential_pressure total_ pressure - static pressure
  * @return indicated airspeed in m/s
  */
-float calc_indicated_airspeed_corrected(enum AIRSPEED_COMPENSATION_MODEL pmodel, enum AIRSPEED_SENSOR_MODEL smodel,
-					float tube_len, float tube_dia_mm, float differential_pressure, float pressure_ambient, float temperature_celsius)
+float calc_IAS_corrected(enum AIRSPEED_COMPENSATION_MODEL pmodel, enum AIRSPEED_SENSOR_MODEL smodel,
+			 float tube_len, float tube_dia_mm, float differential_pressure, float pressure_ambient, float temperature_celsius)
 {
 
 	// air density in kg/m3
@@ -184,16 +184,15 @@ float calc_indicated_airspeed_corrected(enum AIRSPEED_COMPENSATION_MODEL pmodel,
 
 
 /**
- * Calculate indicated airspeed.
+ * Calculate indicated airspeed (IAS).
  *
  * Note that the indicated airspeed is not the true airspeed because it
- * lacks the air density compensation. Use the calc_true_airspeed functions to get
- * the true airspeed.
+ * lacks the air density and instrument error compensation.
  *
  * @param differential_pressure total_ pressure - static pressure
- * @return indicated airspeed in m/s
+ * @return IAS in m/s
  */
-float calc_indicated_airspeed(float differential_pressure)
+float calc_IAS(float differential_pressure)
 {
 
 
@@ -207,32 +206,47 @@ float calc_indicated_airspeed(float differential_pressure)
 }
 
 /**
- * Calculate true airspeed from indicated airspeed.
+ * Calculate true airspeed (TAS) from equivalent airspeed (EAS).
  *
  * Note that the true airspeed is NOT the groundspeed, because of the effects of wind
  *
- * @param speed_indicated current indicated airspeed
+ * @param speed_equivalent current equivalent airspeed
  * @param pressure_ambient pressure at the side of the tube/airplane
  * @param temperature_celsius air temperature in degrees celcius
- * @return true airspeed in m/s
+ * @return TAS in m/s
  */
-float calc_true_airspeed_from_indicated(float speed_indicated, float pressure_ambient, float temperature_celsius)
+float calc_TAS_from_EAS(float speed_equivalent, float pressure_ambient, float temperature_celsius)
 {
-	return speed_indicated * sqrtf(CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C / get_air_density(pressure_ambient,
-				       temperature_celsius));
+	return speed_equivalent * sqrtf(CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C / get_air_density(pressure_ambient,
+					temperature_celsius));
 }
 
 /**
- * Directly calculate true airspeed
+ * Calculate equivalent airspeed (EAS) from indicated airspeed (IAS).
+ * Note that we neglect the conversion from CAS (calibrated airspeed) to EAS.
  *
- * Note that the true airspeed is NOT the groundspeed, because of the effects of wind
+ * @param speed_indicated current indicated airspeed
+ * @param scale scale from IAS to CAS (accounting for instrument and pitot position erros)
+ * @return EAS in m/s
+ */
+float calc_EAS_from_IAS(float speed_indicated, float scale)
+{
+	return speed_indicated * scale;
+}
+
+/**
+ * Directly calculate true airspeed (TAS)
+ *
+ * Here we assume to have no instrument or pitot position error (IAS = CAS),
+ * and neglect the CAS to EAS conversion (CAS = EAS).
+ * Note that the true airspeed is NOT the groundspeed, because of the effects of wind.
  *
  * @param total_pressure pressure inside the pitot/prandtl tube
  * @param static_pressure pressure at the side of the tube/airplane
  * @param temperature_celsius air temperature in degrees celcius
  * @return true airspeed in m/s
  */
-float calc_true_airspeed(float total_pressure, float static_pressure, float temperature_celsius)
+float calc_TAS(float total_pressure, float static_pressure, float temperature_celsius)
 {
 	float density = get_air_density(static_pressure, temperature_celsius);
 
@@ -254,3 +268,19 @@ float get_air_density(float static_pressure, float temperature_celsius)
 {
 	return static_pressure / (CONSTANTS_AIR_GAS_CONST * (temperature_celsius - CONSTANTS_ABSOLUTE_NULL_CELSIUS));
 }
+
+/**
+ * Calculate equivalent airspeed (EAS) from true airspeed (TAS).
+ * It is the inverse function to calc_TAS_from_EAS()
+ *
+ *
+ * @param speed_true current true airspeed
+ * @param pressure_ambient pressure at the side of the tube/airplane
+ * @param temperature_celsius air temperature in degrees celcius
+ * @return EAS in m/s
+ */
+float calc_EAS_from_TAS(float speed_true, float pressure_ambient, float temperature_celsius)
+{
+	return speed_true * sqrtf(get_air_density(pressure_ambient,
+				  temperature_celsius) / CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C);
+}

src/lib/airspeed/airspeed.h

@@ -58,7 +58,7 @@ enum AIRSPEED_COMPENSATION_MODEL {
 };
 
 /**
- * Calculate indicated airspeed.
+ * Calculate indicated airspeed (IAS).
  *
  * Note that the indicated airspeed is not the true airspeed because it
  * lacks the air density compensation. Use the calc_true_airspeed functions to get
@@ -68,12 +68,12 @@ enum AIRSPEED_COMPENSATION_MODEL {
  * @param static_pressure pressure at the side of the tube/airplane
  * @return indicated airspeed in m/s
  */
-__EXPORT float calc_indicated_airspeed_corrected(enum AIRSPEED_COMPENSATION_MODEL pmodel,
-		enum AIRSPEED_SENSOR_MODEL smodel,
-		float tube_len, float tube_dia_mm, float differential_pressure, float pressure_ambient, float temperature_celsius);
+__EXPORT float calc_IAS_corrected(enum AIRSPEED_COMPENSATION_MODEL pmodel,
+				  enum AIRSPEED_SENSOR_MODEL smodel,
+				  float tube_len, float tube_dia_mm, float differential_pressure, float pressure_ambient, float temperature_celsius);
 
 /**
- * Calculate indicated airspeed.
+ * Calculate indicated airspeed (IAS).
  *
  * Note that the indicated airspeed is not the true airspeed because it
  * lacks the air density compensation. Use the calc_true_airspeed functions to get
@@ -83,32 +83,45 @@ __EXPORT float calc_indicated_airspeed_corrected(enum AIRSPEED_COMPENSATION_MODE
  * @param static_pressure pressure at the side of the tube/airplane
  * @return indicated airspeed in m/s
  */
-__EXPORT float calc_indicated_airspeed(float differential_pressure);
+__EXPORT float calc_IAS(float differential_pressure);
 
 /**
- * Calculate true airspeed from indicated airspeed.
+ * Calculate true airspeed (TAS) from equivalent airspeed (EAS).
  *
  * Note that the true airspeed is NOT the groundspeed, because of the effects of wind
  *
- * @param speed_indicated current indicated airspeed
+ * @param speed_equivalent current equivalent airspeed
  * @param pressure_ambient pressure at the side of the tube/airplane
  * @param temperature_celsius air temperature in degrees celcius
- * @return true airspeed in m/s
+ * @return TAS in m/s
  */
-__EXPORT float calc_true_airspeed_from_indicated(float speed_indicated, float pressure_ambient,
-		float temperature_celsius);
+__EXPORT float calc_TAS_from_EAS(float speed_indicated, float pressure_ambient,
+				 float temperature_celsius);
 
 /**
- * Directly calculate true airspeed
+ * Calculate equivalent airspeed (EAS) from indicated airspeed (IAS).
+ * Note that we neglect the conversion from CAS (calibrated airspeed) to EAS.
  *
- * Note that the true airspeed is NOT the groundspeed, because of the effects of wind
+ * @param speed_indicated current indicated airspeed
+ * @param scale scale from IAS to CAS (accounting for instrument and pitot position erros)
+ * @return EAS in m/s
+ */
+__EXPORT float calc_EAS_from_IAS(float speed_indicated, float scale);
+
+
+/**
+ * Directly calculate true airspeed (TAS)
+ *
+ * Here we assume to have no instrument or pitot position error (IAS = CAS),
+ * and neglect the CAS to EAS conversion (CAS = EAS).
+ * Note that the true airspeed is NOT the groundspeed, because of the effects of wind.
  *
  * @param total_pressure pressure inside the pitot/prandtl tube
  * @param static_pressure pressure at the side of the tube/airplane
  * @param temperature_celsius air temperature in degrees celcius
  * @return true airspeed in m/s
  */
-__EXPORT float calc_true_airspeed(float total_pressure, float static_pressure, float temperature_celsius);
+__EXPORT float calc_TAS(float total_pressure, float static_pressure, float temperature_celsius);
 
 /**
 * Calculates air density.
@@ -118,6 +131,19 @@ __EXPORT float calc_true_airspeed(float total_pressure, float static_pressure, f
 */
 __EXPORT float get_air_density(float static_pressure, float temperature_celsius);
 
+/**
+ * Calculate equivalent airspeed (EAS) from true airspeed (TAS).
+ * It is the inverse function to calc_TAS_from_EAS()
+ *
+ *
+ * @param speed_true current true airspeed
+ * @param pressure_ambient pressure at the side of the tube/airplane
+ * @param temperature_celsius air temperature in degrees celcius
+ * @return EAS in m/s
+ */
+__EXPORT float calc_EAS_from_TAS(float speed_true, float pressure_ambient,
+				 float temperature_celsius);
+
 __END_DECLS
 
 #endif

src/modules/mavlink/mavlink_receiver.cpp

@@ -1887,8 +1887,10 @@ MavlinkReceiver::handle_message_hil_sensor(mavlink_message_t *msg)
 	{
 		airspeed_s airspeed{};
 
-		float ias = calc_indicated_airspeed(imu.diff_pressure * 1e2f);
-		float tas = calc_true_airspeed_from_indicated(ias, imu.abs_pressure * 100, imu.temperature);
+		float ias = calc_IAS(imu.diff_pressure * 1e2f);
+		float scale = 1.0f; //assume no instrument or pitot placement errors
+		float eas = calc_EAS_from_IAS(ias, scale);
+		float tas = calc_TAS_from_EAS(eas, imu.abs_pressure * 100, imu.temperature);
 
 		airspeed.timestamp = timestamp;
 		airspeed.indicated_airspeed_m_s = ias;