In GIMP when I skinned for Warbirds, I would use a dark black/brown, more on the black side. Then use the "Wind" distortion feature and drag it the direction of airflow. Depending on the look you are going for, you could smudge it the same direction or use 1 pixel Gaussian blur, and bring the opacity down. It worked pretty good on my G-2
There is some staining around the back of the cowling. It is more noticeable underneath but unfortunately I didn't take closeups before I s*%t- canned Warbirds. Maybe when I get home I can show you what I looks like on the skin. Keep in mind this is a different engine and light source.
