diff --git a/aero_astro_calc/Inertia.py b/aero_astro_calc/Inertia.py
index 535e556..dd0c297 100644
--- a/aero_astro_calc/Inertia.py
+++ b/aero_astro_calc/Inertia.py
@@ -1,4 +1,5 @@
 from typing import Tuple
+from numpy import tan, pi
 
 
 class Inertia:
@@ -6,9 +7,68 @@ class Inertia:
         self.area = area
         self.centroid = centroid
 
-    def __add__(self, other):
-        AX = self.area * self.centroid[0] + other.area * other.centroid[0]
-        AY = self.area * self.centroid[1] + other.area * other.centroid[1]
-        self.area = self.area + other.area
-        self.centroid = (AX / self.area, AY / self.area)
+        self.sections = [self]
+
+    def __add__(self, other):
+        sections = self.sections[:]
+        sections.extend(other.sections[:])
+
+        A = 0
+        AX = 0
+        AY = 0
+
+        for section in sections:
+            A += section.area
+            AX += section.area * section.centroid[0]
+            AY += section.area * section.centroid[1]
+
+        centroid = (AX / A, AY / A)
+        inertia = Inertia(A, centroid)
+        inertia.sections = sections[:]
+
+        return inertia
+
+    def __str__(self):
+        return f"Area: {self.area}, Centroid: {self.centroid}"
+
+    def Ixx(self):
+        return sum(
+            [
+                section.area * (section.centroid[1] - self.centroid[1]) ** 2
+                for section in self.sections
+            ]
+        )
+
+    def Iyy(self):
+        return sum(
+            [
+                section.area * (section.centroid[0] - self.centroid[0]) ** 2
+                for section in self.sections
+            ]
+        )
+
+    def Ixy(self):
+        return sum(
+            [
+                section.area
+                * (section.centroid[0] - self.centroid[0])
+                * (section.centroid[1] - self.centroid[1])
+                for section in self.sections
+            ]
+        )
+
+    def Iyx(self):
+        return self.Ixy()
+
+    def smiley(self):
+        return self.Ixx() * self.Iyy() - self.Ixy() ** 2
+
+    def x_factor(self, Mx, My):
+        return (My * self.Ixx() - Mx * self.Ixy()) / self.smiley()
+
+    def y_factor(self, Mx, My):
+        return (Mx * self.Iyy() - My * self.Ixy()) / self.smiley()
+
+    # def alpha(self, Mx, My):
+    #     return 180 / (tan(self.x_factor(Mx, My) / self.y_factor(Mx, My)) * pi)