Revert to use of cos^2 in LiftDrag plugin (#2273)

The changes in #2189 have caused a regression for our upstream users.
This fix disables moment calculations by defaulting Cma to zero. It
should help mitigate some of the regression. There is also an [ongoing
discussion](#2189 (comment)) as to whether the cos term should be cos^2 or cos.


Signed-off-by: Arjo Chakravarty <[email protected]>
Signed-off-by: Arjo Chakravarty <[email protected]>
Co-authored-by: Rhys Mainwaring <[email protected]>

src/systems/lift_drag/LiftDrag.cc

@@ -327,8 +327,10 @@ void LiftDragPrivate::Update(EntityComponentManager &_ecm)
   double sinSweepAngle = math::clamp(
       spanwiseI.Dot(velI), minRatio, maxRatio);
 
-  // get cos from trig identity
-  double cosSweepAngle = sqrt(1.0 - sinSweepAngle * sinSweepAngle);
+  // The sweep adjustment depends on the velocity component normal to
+  // the wing leading edge which appears quadratically in the
+  // dynamic pressure, so scale by cos^2 .
+  double cos2SweepAngle = 1.0 - sinSweepAngle * sinSweepAngle;
   double sweep = std::asin(sinSweepAngle);
 
   // truncate sweep to within +/-90 deg
@@ -390,20 +392,20 @@ 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);
   }
   else if (alpha < -this->alphaStall)
   {
     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 +423,16 @@ 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);
@@ -444,20 +446,20 @@ 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);
   }
   else if (alpha < -this->alphaStall)
   {
     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())