From: Colin Douglas Howell <colin.d.howell@gmail.com>
Date: Tue, 25 Mar 2014 08:50:21 +0000 (-0700)
Subject: Fix for issue 1423 (YASim miscomputes force vector on Surfaces with nonzero incidence)
X-Git-Url: https://git.mxchange.org/?a=commitdiff_plain;h=aa27b38666320b6826ad6afa102acdcdbb77f5d2;p=flightgear.git

Fix for issue 1423 (YASim miscomputes force vector on Surfaces with nonzero incidence)

Surface::calcForce() now does reverse incidence rotation of force vector properly.
---

diff --git a/src/FDM/YASim/Surface.cpp b/src/FDM/YASim/Surface.cpp
index db977f58a..106de68bc 100644
--- a/src/FDM/YASim/Surface.cpp
+++ b/src/FDM/YASim/Surface.cpp
@@ -225,8 +225,10 @@ void Surface::calcForce(float* v, float rho, float* out, float* torque)
     Math::add3(lwind, out, out);
 
     // Reverse the incidence rotation to get back to surface
-    // coordinates.
-    out[2] -= incidence * out[0];
+    // coordinates. Since out[] is now the force vector and is
+    // roughly parallel with Z, the small-angle approximation
+    // must change its X component.
+    out[0] -= incidence * out[2];
 
     // Convert back to external coordinates
     Math::tmul33(_orient, out, out);