From 224e0ea3fdce9de4a74de4fd7ee4bf0c8689ae92 Mon Sep 17 00:00:00 2001
From: JoePfeiffer <joseph@pfeifferfamily.net>
Date: Mon, 6 Jul 2020 19:16:55 -0600
Subject: [PATCH] In getLongitudinalUnitInertia(), modify so parallel axis
 theorem is only applied to Izz (yaw), not Iyy (pitch).

Modify code for getLongitudinalUnitInertia() and getRotationalUnitInertia() so similar functions have more similar code.

Add comments to clarify getLongitudinalUnitInertia() and getRotationalUnitInertia()
---
 .../sf/openrocket/rocketcomponent/FinSet.java | 54 +++++++++++--------
 1 file changed, 31 insertions(+), 23 deletions(-)

diff --git a/core/src/net/sf/openrocket/rocketcomponent/FinSet.java b/core/src/net/sf/openrocket/rocketcomponent/FinSet.java
index 71b339a08..8b462dc6f 100644
--- a/core/src/net/sf/openrocket/rocketcomponent/FinSet.java
+++ b/core/src/net/sf/openrocket/rocketcomponent/FinSet.java
@@ -643,12 +643,12 @@ public abstract class FinSet extends ExternalComponent implements RingInstanceab
 	 * 
 	 * 1. Approximate the fin with a rectangular fin
 	 * 
-	 * 2. The inertia of one fin is taken as the average of the moments of inertia
-	 *    through its center perpendicular to the plane, and the inertia through
-	 *    its center parallel to the plane
+	 * 2. The unitary moment of inertia of one fin is taken as the average
+	 *    of the unitary moments of inertia through its center perpendicular
+	 *    to the plane (Izz/M), and through its center parallel to the plane (Iyy/M)
 	 *    
-	 * 3. If there are multiple fins, the inertia is shifted to the center of the fin
-	 *    set and multiplied by the number of fins.
+	 * 3. If there are multiple fins, the inertia is shifted to the center of the
+	 *    Finset.
 	 */
 	@Override
 	public double getLongitudinalUnitInertia() {
@@ -661,23 +661,28 @@ public abstract class FinSet extends ExternalComponent implements RingInstanceab
 		double w = getLength();
 		double h = getSpan();
 		double w2, h2;
-		
-		if (MathUtil.equals(w * h, 0)) {
+
+		// If either h or w is 0, we punt and treat the fin as square
+		if (MathUtil.equals(h * w, 0)) {
 			w2 = singlePlanformArea;
 			h2 = singlePlanformArea;
 		} else {
 			w2 = w * singlePlanformArea / h;
 			h2 = h * singlePlanformArea / w;
 		}
-		
+
+		// Iyy = h * w^3 / 12, so Iyy/M = w^2 / 12
+		// Izz = h * w * (h^2 + w^2) / 12, so Izz/M = (h^2 + w^2) / 12
+		// (Iyy / M + Izz / M) / 2 = (h^2 + 2 * w^2)/24
 		final double inertia = (h2 + 2 * w2) / 24;
 		
 		if (finCount == 1)
 			return inertia;
-		
-		final double rFront = this.getFinFront().y;
-		
-		return inertia + MathUtil.pow2(MathUtil.safeSqrt(h2) + rFront);
+
+		// recheck (yet again) when I get home.  Need to apply parallel axis theorem
+		// to Izz, but not to Iyy.  Since we're looking at average of Iyy and Izz,
+		// weight parallel axis theorem by 1/2.
+		return inertia + MathUtil.pow2(MathUtil.safeSqrt(h2) / 2 + getBodyRadius()) / 2;
 	}
 	
 	
@@ -685,11 +690,12 @@ public abstract class FinSet extends ExternalComponent implements RingInstanceab
 	 * Return an approximation of the rotational unitary inertia of the fin set.
 	 * The process is the following:
 	 * 
-	 * 1. Approximate the fin with a rectangular fin and calculate the inertia of the
-	 *    rectangular approximate
+	 * 1. Approximate the fin with a rectangular fin and calculate the
+	 *    unitary rotational inertia (Ixx/M) of the rectangular approximation,
+	 *    about the center of the approximated fin.
 	 *    
-	 * 2. If there are multiple fins, shift the inertia center to the fin set center
-	 *    and multiply with the number of fins.
+	 * 2. If there are multiple fins, shift the inertia axis to the center
+	 *    of the Finset
 	 */
 	@Override
 	public double getRotationalUnitInertia() {
@@ -698,23 +704,25 @@ public abstract class FinSet extends ExternalComponent implements RingInstanceab
 		}
 
 		// Approximate fin with a rectangular fin
+		// h2 is square of fin height
 		double w = getLength();
 		double h = getSpan();
-
+		double h2;
+		
+		// If either h or w is 0, punt and treat it as a square fin
 		if (MathUtil.equals(w * h, 0)) {
-			h = MathUtil.safeSqrt(singlePlanformArea);
+			h2 = singlePlanformArea;
 		} else {
-			h = MathUtil.safeSqrt(h * singlePlanformArea/ w);
+			h2 = h * singlePlanformArea / w;
 		}
 
-		final double inertia = h * h / 12;
+		// Ixx = w * h^3 / 12, so Ixx / M = h^2 / 12
+		final double inertia = h2 / 12;
 
 		if (finCount == 1)
 			return inertia;
 
-		final double rFront = this.getFinFront().y;
-
-		return inertia + MathUtil.pow2(h / 2 + rFront);
+		return inertia + MathUtil.pow2(MathUtil.safeSqrt(h2) / 2 + getBodyRadius());
 	}