BUG: Add reference area factor correction to aero surfaces (solves #557)

rocketpy/prints/rocket_prints.py

@@ -114,9 +114,13 @@ def rocket_aerodynamics_quantities(self):
         print("\nAerodynamics Lift Coefficient Derivatives\n")
         for surface, position in self.rocket.aerodynamic_surfaces:
             name = surface.name
+            # ref_factor corrects lift for different reference areas
+            ref_factor = (surface.rocket_radius / self.rocket.radius) ** 2
             print(
                 name
-                + " Lift Coefficient Derivative: {:.3f}".format(surface.clalpha(0))
+                + " Lift Coefficient Derivative: {:.3f}".format(
+                    ref_factor * surface.clalpha(0)
+                )
                 + "/rad"
             )
 
@@ -135,6 +139,9 @@ def rocket_aerodynamics_quantities(self):
         print(
             f"Center of Mass position (time=0): {self.rocket.center_of_mass(0):.3f} m"
         )
+        print(
+            f"Center of Pressure position (time=0): {self.rocket.cp_position(0):.3f} m"
+        )
         print(
             "Initial Static Margin (mach=0, time=0): "
             + "{:.3f}".format(self.rocket.static_margin(0))

rocketpy/rocket/aero_surface.py

@@ -349,12 +349,20 @@
         # If base radius is not given, the ratio between base radius and
         # rocket radius is assumed as 1, meaning that the nose cone has the
         # same radius as the rocket
-        if self.base_radius is None or self.rocket_radius is None:
+        if self.base_radius is None and self.rocket_radius is not None:
             self.radius_ratio = 1
+            self.base_radius = self.rocket_radius
+        elif self.base_radius is not None and self.rocket_radius is None:
+            self.radius_ratio = 1
+            self.rocket_radius = self.base_radius
         # If base radius is given, the ratio between base radius and rocket
         # radius is calculated
-        else:
+        elif self.base_radius is not None or self.rocket_radius is not None:
             self.radius_ratio = self.base_radius / self.rocket_radius
+        else:
+            raise ValueError(
+                "Either base radius or rocket radius must be given to calculate the nose cone radius ratio."
+            )
 
         self.fineness_ratio = self.length / (2 * self.base_radius)
         return None

rocketpy/rocket/rocket.py

@@ -517,9 +517,13 @@ def evaluate_center_of_pressure(self):
         # Calculate total lift coefficient derivative and center of pressure
         if len(self.aerodynamic_surfaces) > 0:
             for aero_surface, position in self.aerodynamic_surfaces:
-                self.total_lift_coeff_der += aero_surface.clalpha
-                self.cp_position += aero_surface.clalpha * (
-                    position - self._csys * aero_surface.cpz
+                # ref_factor corrects lift for different reference areas
+                ref_factor = (aero_surface.rocket_radius / self.radius) ** 2
+                self.total_lift_coeff_der += ref_factor * aero_surface.clalpha
+                self.cp_position += (
+                    ref_factor
+                    * aero_surface.clalpha
+                    * (position - self._csys * aero_surface.cpz)
                 )
             self.cp_position /= self.total_lift_coeff_der
 
@@ -871,7 +875,9 @@ def add_tail(
         self.add_surfaces(tail, position)
         return tail
 
-    def add_nose(self, length, kind, position, bluffness=0, name="Nose Cone"):
+    def add_nose(
+        self, length, kind, position, bluffness=0, base_radius=None, name="Nose Cone"
+    ):
         """Creates a nose cone, storing its parameters as part of the
         aerodynamic_surfaces list. Its parameters are the axial position
         along the rocket and its derivative of the coefficient of lift
@@ -892,6 +898,8 @@ def add_nose(self, length, kind, position, bluffness=0, name="Nose Cone"):
         bluffness : float, optional
             Ratio between the radius of the circle on the tip of the ogive and
             the radius of the base of the ogive.
+        base_radius : int, float, optional
+            Nose cone base radius in meters. If not given, use rocket radius.
         name : string
             Nose cone name. Default is "Nose Cone".
 
@@ -907,8 +915,8 @@ def add_nose(self, length, kind, position, bluffness=0, name="Nose Cone"):
         nose = NoseCone(
             length=length,
             kind=kind,
-            base_radius=self.radius,
-            rocket_radius=self.radius,
+            base_radius=base_radius or self.radius,
+            rocket_radius=base_radius or self.radius,
             bluffness=bluffness,
             name=name,
         )
@@ -983,8 +991,9 @@ def add_trapezoidal_fins(
             with its base perpendicular to the rocket's axis. Cannot be used in
             conjunction with sweep_length.
         radius : int, float, optional
-            Reference radius to calculate lift coefficient. If None, which is
-            default, use rocket radius.
+            Reference fuselage radius where the fins are located. This is used
+            to calculate lift coefficient and to draw the rocket. If None,
+            which is default, use rocket radius.
         airfoil : tuple, optional
             Default is null, in which case fins will be treated as flat plates.
             Otherwise, if tuple, fins will be considered as airfoils. The
@@ -1064,8 +1073,9 @@ def add_elliptical_fins(
             Fins cant angle with respect to the rocket centerline. Must be given
             in degrees.
         radius : int, float, optional
-            Reference radius to calculate lift coefficient. If None, which
-            is default, use rocket radius.
+            Reference fuselage radius where the fins are located. This is used
+            to calculate lift coefficient and to draw the rocket. If None,
+            which is default, use rocket radius.
         airfoil : tuple, optional
             Default is null, in which case fins will be treated as flat plates.
             Otherwise, if tuple, fins will be considered as airfoils. The