From a645be80b58599d9e8a7873867592ad80f2cf7fc Mon Sep 17 00:00:00 2001
From: Arjo Chakravarty <arjoc@intrinsic.ai>
Date: Wed, 27 Dec 2023 09:48:10 +0800
Subject: [PATCH] Revert to use of cos^2 in LiftDrag plugin

In response to https://github.com/gazebosim/gz-sim/pull/2189#issuecomment-1869798904

As suggested by @srmainwaring I also renamed cosSweepAngle ->
cos2SweepAngle to prevent further confusion.

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>
---
 src/systems/lift_drag/LiftDrag.cc | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/src/systems/lift_drag/LiftDrag.cc b/src/systems/lift_drag/LiftDrag.cc
index b16766e6a0..bc2afdfdcc 100644
--- a/src/systems/lift_drag/LiftDrag.cc
+++ b/src/systems/lift_drag/LiftDrag.cc
@@ -328,7 +328,7 @@ void LiftDragPrivate::Update(EntityComponentManager &_ecm)
       spanwiseI.Dot(velI), minRatio, maxRatio);
 
   // get cos from trig identity
-  double cosSweepAngle = sqrt(1.0 - sinSweepAngle * sinSweepAngle);
+  double cos2SweepAngle = 1.0 - sinSweepAngle * sinSweepAngle;
   double sweep = std::asin(sinSweepAngle);
 
   // truncate sweep to within +/-90 deg
@@ -390,7 +390,7 @@ void LiftDragPrivate::Update(EntityComponentManager &_ecm)
   {
     cl = (this->cla * this->alphaStall +
           this->claStall * (alpha - this->alphaStall)) *
-         cosSweepAngle;
+         cos2SweepAngle;
     // make sure cl is still great than 0
     cl = std::max(0.0, cl);
   }
@@ -398,12 +398,12 @@ void LiftDragPrivate::Update(EntityComponentManager &_ecm)
   {
     cl = (-this->cla * this->alphaStall +
           this->claStall * (alpha + this->alphaStall))
-         * cosSweepAngle;
+         * cos2SweepAngle;
     // make sure cl is still less than 0
     cl = std::min(0.0, cl);
   }
   else
-    cl = this->cla * alpha * cosSweepAngle;
+    cl = this->cla * alpha * cos2SweepAngle;
 
   // modify cl per control joint value
   if (controlJointPosition && !controlJointPosition->Data().empty())
@@ -421,16 +421,16 @@ void LiftDragPrivate::Update(EntityComponentManager &_ecm)
   {
     cd = (this->cda * this->alphaStall +
           this->cdaStall * (alpha - this->alphaStall))
-         * cosSweepAngle;
+         * cos2SweepAngle;
   }
   else if (alpha < -this->alphaStall)
   {
     cd = (-this->cda * this->alphaStall +
           this->cdaStall * (alpha + this->alphaStall))
-         * cosSweepAngle;
+         * cos2SweepAngle;
   }
   else
-    cd = (this->cda * alpha) * cosSweepAngle;
+    cd = (this->cda * alpha) * cos2SweepAngle;
 
   // make sure drag is positive
   cd = std::fabs(cd);
@@ -444,7 +444,7 @@ void LiftDragPrivate::Update(EntityComponentManager &_ecm)
   {
     cm = (this->cma * this->alphaStall +
           this->cmaStall * (alpha - this->alphaStall))
-         * cosSweepAngle;
+         * cos2SweepAngle;
     // make sure cm is still great than 0
     cm = std::max(0.0, cm);
   }
@@ -452,12 +452,12 @@ void LiftDragPrivate::Update(EntityComponentManager &_ecm)
   {
     cm = (-this->cma * this->alphaStall +
           this->cmaStall * (alpha + this->alphaStall))
-         * cosSweepAngle;
+         * cos2SweepAngle;
     // make sure cm is still less than 0
     cm = std::min(0.0, cm);
   }
   else
-    cm = this->cma * alpha * cosSweepAngle;
+    cm = this->cma * alpha * cos2SweepAngle;
 
   // Take into account the effect of control surface deflection angle to cm
   if (controlJointPosition && !controlJointPosition->Data().empty())