I found a picture of an f-18 with aerodynamic test equipment on board. They had a load of nozzles fitted to places they wanted to study and pumped a liquid through them that stained the body, revealing the airflow. Here's a link to the picture:
http://www.dfrc.nasa.gov/Gallery/Pho...8-0212-002.jpg
I found it elsewhere in the nasa site with a description but can't find it for now.
ollie