diff --git a/src/FDM/YASim/Surface.cpp b/src/FDM/YASim/Surface.cpp
index 2baa42baa..25d8da489 100644
--- a/src/FDM/YASim/Surface.cpp
+++ b/src/FDM/YASim/Surface.cpp
@@ -228,7 +228,7 @@ void Surface::calcForce(float* v, float rho, float* out, float* torque)
     // 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];
+    out[0] += incidence * out[2];
 
     // Convert back to external coordinates
     Math::tmul33(_orient, out, out);