Fixed and verified wind effect on trajectory.

py_ballisticcalc/conditions.py

@@ -128,7 +128,7 @@ def get_density_factor_and_mach_for_altitude(self, altitude: float):
 @dataclass
 class Wind(TypedUnits):
     """
-    Stores wind data at the desired distance.
+    Wind direction and velocity by down-range distance.
     direction_from = 0 is blowing from behind shooter. 
     direction_from = 90 degrees is blowing from shooter's left towards right.
     """

py_ballisticcalc/trajectory_calc.py

@@ -92,6 +92,7 @@ def __neg__(self):
 
 
 class TrajectoryCalc:
+    """All calculations are done in units of Feet and fps"""
 
     def __init__(self, ammo: Ammo):
         self.ammo = ammo
@@ -220,14 +221,14 @@ def _trajectory(self, ammo: Ammo, atmo: Atmo,
         else:
             if len_winds > 1:
                 next_wind_range = winds[0].until_distance() >> Distance.Foot
-            wind_vector = wind_to_vector(shot_info, winds[0])
+            wind_vector = wind_to_vector(winds[0])
 
         if Settings.USE_POWDER_SENSITIVITY:
             velocity = ammo.get_velocity_for_temp(atmo.temperature) >> Velocity.FPS
         else:
             velocity = ammo.mv >> Velocity.FPS
 
-        # x - distance towards target, y - drop and z - windage
+        # x: downrange distance (towards target), y: drop, z: windage
         velocity_vector = Vector(math.cos(barrel_elevation) * math.cos(barrel_azimuth),
                                  math.sin(barrel_elevation),
                                  math.cos(barrel_elevation) * math.sin(barrel_azimuth)) * velocity
@@ -254,7 +255,7 @@ def _trajectory(self, ammo: Ammo, atmo: Atmo,
 
             if range_vector.x >= next_wind_range:
                 current_wind += 1
-                wind_vector = wind_to_vector(shot_info, winds[current_wind])
+                wind_vector = wind_to_vector(winds[current_wind])
 
                 if current_wind == len_winds - 1:
                     next_wind_range = 1e7
@@ -362,36 +363,20 @@ def calculate_stability_coefficient(twist_rate: Distance, ammo: Ammo, atmo: Atmo
     return sd * fv * ftp
 
 
-def wind_to_vector(shot: Shot, wind: Wind):
-    """
+def wind_to_vector(wind: Wind) -> Vector:
+    """Calculate wind vector to add to projectile velocity vector each iteration:
+        Aerodynamic drag is function of velocity relative to the air stream.
+
     Wind angle of zero is blowing from behind shooter
-    90-degree is blowing towards shooter's right
-
-    Looks like we only start with data on wind direction in the x-z plane, not any vertical components.
-    Therefore, given wind velocity v with angle d:
-        The downrange "range" component is v*cos(d)
-        The orthogonal "cross" component is v*sin(d)
-    When the bullet has a vertical velocity component, meaning velocity angle to horizon a > 0
-        the sin(a) range component acts to lift it in the y axis, and only cos(a) acts in the x axis
-        ... of course this is reduced by any amount of the velocity that points in the z axis
-        which we determine using  the cant_angle
-    TODO: Right now this takes the initial (launch) elevation and cant_angle components.
-        This is an OK "flat-fire" approximation but we actually have the angle of travel at every instant
-        so we should use that instead.
-    TODO: Correct terms now that Shot class factors cant out of barrel elevation
+    Wind angle of 90-degree is blowing towards shooter's right
+
+    NB: Presently we can only define Wind in the x-z plane, not any vertical component.
     """
-    sight_cosine = math.cos(shot.barrel_elevation >> Angular.Radian)
-    sight_sine = math.sin(shot.barrel_elevation >> Angular.Radian)
-    cant_cosine = math.cos(shot.cant_angle >> Angular.Radian)
-    cant_sine = math.sin(shot.cant_angle >> Angular.Radian)
-    range_velocity = (wind.velocity >> Velocity.FPS) * math.cos(
-        wind.direction_from >> Angular.Radian)
-    cross_component = (wind.velocity >> Velocity.FPS) * math.sin(
-        wind.direction_from >> Angular.Radian)
-    range_factor = -range_velocity * sight_sine
-    return Vector(range_velocity * sight_cosine,
-                  range_factor * cant_cosine + cross_component * cant_sine,
-                  cross_component * cant_cosine - range_factor * cant_sine)
+    # Downrange (x-axis) wind velocity component:
+    range_component = (wind.velocity >> Velocity.FPS) * math.cos(wind.direction_from >> Angular.Radian)
+    # Cross (z-axis) wind velocity component:
+    cross_component = (wind.velocity >> Velocity.FPS) * math.sin(wind.direction_from >> Angular.Radian)
+    return Vector(range_component, 0, cross_component)
 
 
 def create_trajectory_row(time: float, range_vector: Vector, velocity_vector: Vector,

py_ballisticcalc_exts/py_ballisticcalc_exts/trajectory_calc.pyx

@@ -124,7 +124,7 @@ cdef class TrajectoryCalc:
     def zero_angle(self, shot_info: Shot, distance: Distance):
         return self._zero_angle(shot_info, distance)
 
-    def trajectory(self, shot_info: Shot, max_range: Distance, dist_step: [float, Distance],
+    def trajectory(self, shot_info: Shot, max_range: Distance, dist_step: Distance,
                    extra_data: bool = False):
         cdef:
             object step = Settings.Units.distance(dist_step)
@@ -242,7 +242,7 @@ cdef class TrajectoryCalc:
         else:
             if len_winds > 1:
                 next_wind_range = winds[0].until_distance() >> Distance.Foot
-            wind_vector = wind_to_vector(shot_info, winds[0])
+            wind_vector = wind_to_vector(winds[0])
 
         if Settings.USE_POWDER_SENSITIVITY:
             velocity = ammo.get_velocity_for_temp(atmo.temperature) >> Velocity.FPS
@@ -274,7 +274,7 @@ cdef class TrajectoryCalc:
 
             if range_vector.x >= next_wind_range:
                 current_wind += 1
-                wind_vector = wind_to_vector(shot_info, winds[current_wind])
+                wind_vector = wind_to_vector(winds[current_wind])
 
                 if current_wind == len_winds - 1:
                     next_wind_range = 1e7
@@ -382,18 +382,11 @@ cdef double calculate_stability_coefficient(object twist_rate, object ammo, obje
         double ftp = ((ft + 460) / (59 + 460)) * (29.92 / pt)
     return sd * fv * ftp
 
-cdef Vector wind_to_vector(object shot, object wind):
+cdef Vector wind_to_vector(object wind):
     cdef:
-        double sight_cosine = cos(shot.barrel_elevation >> Angular.Radian)
-        double sight_sine = sin(shot.barrel_elevation >> Angular.Radian)
-        double cant_cosine = cos(shot.cant_angle >> Angular.Radian)
-        double cant_sine = sin(shot.cant_angle >> Angular.Radian)
-        double range_velocity = (wind.velocity >> Velocity.FPS) * cos(wind.direction_from >> Angular.Radian)
+        double range_component = (wind.velocity >> Velocity.FPS) * cos(wind.direction_from >> Angular.Radian)
         double cross_component = (wind.velocity >> Velocity.FPS) * sin(wind.direction_from >> Angular.Radian)
-        double range_factor = -range_velocity * sight_sine
-    return Vector(range_velocity * sight_cosine,
-                  range_factor * cant_cosine + cross_component * cant_sine,
-                  cross_component * cant_cosine - range_factor * cant_sine)
+    return Vector(range_component, 0., cross_component)
 
 cdef create_trajectory_row(double time, Vector range_vector, Vector velocity_vector,
                            double velocity, double mach, double windage, double weight, object flag):

tests/test_computer.py

@@ -46,7 +46,7 @@ def test_cant_positive_elevation(self):
         self.assertGreater(t.trajectory[5].windage.raw_value, t.trajectory[3].windage.raw_value)
 
     def test_cant_zero_sight_height(self):
-        """Cant_angle = 90 degrees with sight_height=0 should match baseline with:
+        """Cant_angle = 90 degrees with sight_height=0 and barrel_elevation=0 should match baseline with:
             drop+=baseline.sight_height, windage no change
         """
         canted = Shot(weapon=Weapon(sight_height=0, twist=self.weapon.twist),