From f9d3d0a5a10672154010691e510d2a095e27783f Mon Sep 17 00:00:00 2001
From: Achille Martin <achille.martin@noc.ac.uk>
Date: Thu, 17 Dec 2020 13:29:59 +0000
Subject: [PATCH] Updated the liftdrag two lines model for the drag component
 to make it more realistic

---
 .../uuv_gazebo_plugins/src/LiftDragModel.cc    | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/uuv_gazebo_plugins/uuv_gazebo_plugins/src/LiftDragModel.cc b/uuv_gazebo_plugins/uuv_gazebo_plugins/src/LiftDragModel.cc
index 19196880f..41a447a3b 100644
--- a/uuv_gazebo_plugins/uuv_gazebo_plugins/src/LiftDragModel.cc
+++ b/uuv_gazebo_plugins/uuv_gazebo_plugins/src/LiftDragModel.cc
@@ -247,14 +247,24 @@ ignition::math::Vector3d LiftDragTwoLines::compute(const ignition::math::Vector3
     double sumAlpha = alpha + this->alphaStall;
     cl = -this->cla * this->alphaStall +
         this->cdaStall * sumAlpha;
-    cd = -this->cda * this->alphaStall +
-        this->cdaStall * sumAlpha;
+    cd = this->cda * this->alphaStall +
+        -this->cdaStall * sumAlpha;
   }
   else
   {
     // no stall
-    cd = this->cda * alpha;
-    cl = this->cla * alpha;
+    // angle of attack > -a0
+    if (alpha > 0)
+    {
+        cd = this->cda * alpha;
+        cl = this->cla * alpha;
+    }
+    // angle of attack <= -a0
+    else
+    {
+        cd = -this->cda * alpha;
+        cl = this->cla * alpha;
+    }
   }
 
   double lift = cl*q*this->area;