Rotor effects: Add more realistic aerodynamic effects for helicopters

This commit performs a first pass at implementing more realistic helicopter
physics particularly at low speeds due to rotor effects:

- Improved heading changes when pitch/banking, ported from @metiscus fork
- In-Ground-Effect (IGE): thrust boost when close to ground
- Transverse Flow Effect: roll,bank effects between 5-25 knots
- ETL: thrust penalty below 24knots, roll-bank effects
- Torque: heading changes at lower speeds + higher collective

The effects make landings more realistic and challenging.

data/aircrafts/as350.3d

@@ -66,6 +66,9 @@ belly_height 1.7
 # Length in meters
 length 10.92
 
+# Rotor diameter in meters
+rotor_diameter 10.8966
+
 # Gear (down position) to belly height in meters
 gear_height 0.35
 

data/aircrafts/b47.3d

@@ -60,6 +60,9 @@ belly_height 0.5
 # Length in meters
 length 9.63
 
+# Rotor diameter in meters
+rotor_diameter 9.4742
+
 # Gear (down position) to belly height in meters
 gear_height 0.0
 

data/aircrafts/ec145.3d

@@ -55,6 +55,9 @@ belly_height 1.55
 # Length, in meters
 length 13.03
 
+# Rotor diameter in meters
+rotor_diameter 11
+
 # Gear (down position) to belly height, in meters
 gear_height 0.44
 

data/aircrafts/hh60.3d

@@ -71,6 +71,9 @@ belly_height 2.2
 # Length in meters
 length 19.81
 
+# Rotor diameter in meters
+rotor_diameter 16.4592
+
 # Gear (down position) to belly height in meters
 gear_height 0.5
 

data/aircrafts/hh65.3d

@@ -69,6 +69,9 @@ belly_height 2.38
 # Length in meters
 length 13.54
 
+# Rotor diameter in meters
+rotor_diameter 11.938
+
 # Gear (down position) to belly height in meters
 gear_height 0.35
 

data/aircrafts/ka27.3d

@@ -67,6 +67,9 @@ belly_height 2.35
 # Length in meters
 length 10.49
 
+# Rotor diameter in meters
+rotor_diameter 16.637
+
 # Gear (down position) to belly height in meters
 gear_height 0.3
 

data/aircrafts/s64.3d

@@ -61,6 +61,9 @@ belly_height 3.55
 # Length in meters
 length 21.41
 
+# Rotor diameter in meters
+rotor_diameter 21.9456
+
 # Gear (down position) to belly height in meters
 gear_height 0.45
 

data/aircrafts/uh1d.3d

@@ -60,6 +60,9 @@ belly_height 1.7
 # Length in meters
 length 12.75
 
+# Rotor diameter in meters
+rotor_diameter 14.7828
+
 # Gear (down position) to belly height in meters
 gear_height 0.3
 

data/aircrafts/v22.3d

@@ -70,6 +70,9 @@ belly_height 2.8
 # Length in meters
 length 17.47
 
+# Rotor diameter in meters
+rotor_diameter 11.5824
+
 # Wingspan in meters
 wingspan 15.24
 

src/cmdset.c

@@ -579,6 +579,17 @@ void SARCmdSet(SAR_CMD_PROTOTYPE)
 		got_match = True;
 	    }
 	}
+	/* Rotor diameter */
+	else if(!strcasecmp(parm, "rotor_diameter"))
+	{
+	    sar_object_aircraft_struct *obj_aircraft_ptr =
+		SAR_OBJ_GET_AIRCRAFT(obj_ptr);
+	    if(obj_aircraft_ptr != NULL)
+	    {
+		obj_aircraft_ptr->rotor_diameter = (float)ATOF(val);
+		got_match = True;
+	    }
+	}
 	/* Gear height */
 	else if(!strcasecmp(parm, "gear_height"))
 	{

src/obj.h

@@ -861,6 +861,9 @@ typedef struct {
 	/* Wingspan of aircraft in meters */
 	float		wingspan;
 
+	/* Rotor diameter in meters */
+	float		rotor_diameter;
+
 	/* Height of landing gear in meters */
 	float		gear_height;
 
@@ -961,6 +964,9 @@ typedef struct {
 	 */
 	float		center_to_ground_height;
 
+	/* Rate of heading change due to torque (radians/cycle) */
+	float torque_velocity;
+
 
 	/* Moveable parts */
 	sar_obj_part_struct	**part;

src/objio.c

@@ -3243,6 +3243,19 @@ static void SARObjLoadLine(
 		if(aircraft != NULL)
 		    aircraft->wingspan = wingspan;
 	    }
+	    else if(!strcasecmp(parm, "rotor_diameter"))
+	    {
+		/* Arguments:
+		 *
+		 * <wingspan>
+		 */
+		float	rotor_diameter = 0.0f;
+
+		arg = GET_ARG_F(arg, &rotor_diameter);
+
+		if(aircraft != NULL)
+		    aircraft->rotor_diameter = rotor_diameter;
+	    }
 	    /* Landing Gear Height */
 	    else if(!strcasecmp(parm, "gear_height"))
 	    {

src/sfm.h

@@ -42,6 +42,18 @@
  */
 #define SFMDefaultGravity	9.80665
 
+
+/*
+ *	Effective transactional lift speed value in m/s.
+ */
+#define SFMETLSpeed 12.35  // 24 knots
+
+/*
+ *	Transverse Flow Effect (TF) values in m/s.
+ */
+#define SFMTFStart 2.57 // 5 knots
+#define SFMTFEnd 12.86  // 25 knots
+
 /*
  *	Core structure:
  */

src/sfmmodel.c

@@ -297,6 +297,10 @@ SFMBoolean SFMModelChangeValues(
 	{
 	    model->wingspan = value->wingspan;
 	}
+	if(flags & SFMFlagRotorDiameter)
+	{
+	    model->rotor_diameter = value->rotor_diameter;
+	}
 	if(flags & SFMFlagGearState)
 	{
 	    model->gear_state = value->gear_state;

src/sfmmodel.h

@@ -78,6 +78,7 @@
 #define SFMFlagStopped			((SFMFlags)1 << 41)
 #define SFMFlagLength			((SFMFlags)1 << 42)
 #define SFMFlagWingspan			((SFMFlags)1 << 43)
+#define SFMFlagRotorDiameter		((SFMFlags)1 << 44)
 
 /*
  *	Flight model types:
@@ -128,7 +129,7 @@ typedef struct {
 	 */
 	SFMFlags	flags;
 
-	int			type;		/* One of SFMFlightModel*. */
+	int			type;	/* One of SFMFlightModel*. */
 	SFMPositionStruct	position;	/* Meters. */
 	SFMDirectionStruct	direction;	/* Radians. */
 	SFMPositionStruct	velocity_vector;	/* Meters/cycle. */
@@ -146,8 +147,9 @@ typedef struct {
 	SFMPositionStruct	accel_responsiveness;
 	double			ground_elevation_msl;	/* Meters. */
 	double			service_ceiling;	/* Meters. */
-	double			length;		/* Meters */
+	double			length;	/* Meters */
 	double			wingspan;	/* Meters */
+	double			rotor_diameter;	/* Meters */
 	double			belly_height;	/* Undercarrage to center, meters. */
 	SFMBoolean		gear_state;	/* True when down. */
 	int			gear_type;	/* One of SFMGearType*. */
@@ -165,11 +167,12 @@ typedef struct {
 								 * ground height,
 								 * in meters.
 								 */
+	double			torque_velocity;	/* Radians/cycle internal */
 	double			heading_control_coeff;	/* -1.0 to 1.0. */
 	double			pitch_control_coeff;	/* -1.0 to 1.0. */
 	double			bank_control_coeff;	/* -1.0 to 1.0. */
 	double			elevator_trim_coeff;	/* -1.0 to 1.0. */
-	double			throttle_coeff;		/* 0.0 to 1.0. */
+	double			throttle_coeff;	/* 0.0 to 1.0. */
 	SFMBoolean		after_burner_state;
 	double			after_burner_power_coeff;	/* Times engine power. */
 	double			engine_power;	/* In kg * m / cycle^2. */