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In getLongitudinalUnitInertia(), modify so parallel axis theorem is only applied to Izz (yaw), not Iyy (pitch).

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

Add comments to clarify getLongitudinalUnitInertia() and getRotationalUnitInertia()
This commit is contained in:
JoePfeiffer 2020-07-06 19:16:55 -06:00
parent 87c26cccf4
commit 224e0ea3fd
1 changed files with 31 additions and 23 deletions
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54
core/src/net/sf/openrocket/rocketcomponent/FinSet.java
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@ -643,12 +643,12 @@ public abstract class FinSet extends ExternalComponent implements RingInstanceab
* *
* 1. Approximate the fin with a rectangular fin * 1. Approximate the fin with a rectangular fin
* *
* 2. The inertia of one fin is taken as the average of the moments of inertia * 2. The unitary moment of inertia of one fin is taken as the average
* through its center perpendicular to the plane, and the inertia through * of the unitary moments of inertia through its center perpendicular
* its center parallel to the plane * 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 * 3. If there are multiple fins, the inertia is shifted to the center of the
* set and multiplied by the number of fins. * Finset.
*/ */
@Override @Override
public double getLongitudinalUnitInertia() { public double getLongitudinalUnitInertia() {
@ -661,23 +661,28 @@ public abstract class FinSet extends ExternalComponent implements RingInstanceab
double w = getLength(); double w = getLength();
double h = getSpan(); double h = getSpan();
double w2, h2; 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; w2 = singlePlanformArea;
h2 = singlePlanformArea; h2 = singlePlanformArea;
} else { } else {
w2 = w * singlePlanformArea / h; w2 = w * singlePlanformArea / h;
h2 = h * singlePlanformArea / w; 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; final double inertia = (h2 + 2 * w2) / 24;
if (finCount == 1) if (finCount == 1)
return inertia; return inertia;
final double rFront = this.getFinFront().y; // 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,
return inertia + MathUtil.pow2(MathUtil.safeSqrt(h2) + rFront); // 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. * Return an approximation of the rotational unitary inertia of the fin set.
* The process is the following: * The process is the following:
* *
* 1. Approximate the fin with a rectangular fin and calculate the inertia of the * 1. Approximate the fin with a rectangular fin and calculate the
* rectangular approximate * 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 * 2. If there are multiple fins, shift the inertia axis to the center
* and multiply with the number of fins. * of the Finset
*/ */
@Override @Override
public double getRotationalUnitInertia() { public double getRotationalUnitInertia() {
@ -698,23 +704,25 @@ public abstract class FinSet extends ExternalComponent implements RingInstanceab
} }
// Approximate fin with a rectangular fin // Approximate fin with a rectangular fin
// h2 is square of fin height
double w = getLength(); double w = getLength();
double h = getSpan(); 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)) { if (MathUtil.equals(w * h, 0)) {
h = MathUtil.safeSqrt(singlePlanformArea); h2 = singlePlanformArea;
} else { } 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) if (finCount == 1)
return inertia; return inertia;
final double rFront = this.getFinFront().y; return inertia + MathUtil.pow2(MathUtil.safeSqrt(h2) / 2 + getBodyRadius());
return inertia + MathUtil.pow2(h / 2 + rFront);
} }