NASA's High Angle of Attack Research Vehicle (HARV) program involved the use of a modified Hornet to explore the use of thrust vectoring in the high angle of attack regime. The goal was to achieve better maneuverability in conventional non-V/STOL aircraft, in the hope of giving aircraft designers a better understanding of aerodynamics, flight controls, and airflow at high angles of attack.
The HARV program was a joint effort between NASA's Dryden, Ames, Langley, and Lewis research centers. The Navy loaned a YF-18A (BuNo 160780) to NASA for the tests. It had been serving with the Naval Air Test Center at Patuxent River, Maryland, and had been in storage pending further use. It arrived at NASA's Dryden Flight Research Facility in September of 1985, and was assigned the NASA number of 840.
NASA 840 required 18 months of work to be refurbished. The HARV program began in 1987 with an unmodified aircraft. 840 was finally fit with thrust vectoring equipment in 1991. This consisted of a set of three spoon-shaped paddle-like vanes fitted around each engine's exhaust to provide pitch and yaw forces in those flight regimes where the conventional flight controls tend to lose their effectiveness. In order to shorten the distance that the vanes must be cantilevered, the external exhaust nozzles were removed. This makes supersonic flight impossible, but does not have any effect on the subsonic performance. The flight control computers had to be modified to accommodate the vanes.
The aircraft is equipped with camera pods on the wingtips in lieu of the Sidewinder missiles. These cameras are there to view streams of white smoke that are emitted from the forward fuselage to give information about the airflow patterns. In order to make the smoke trails stand out better, the upper surfaces of the aircraft are painted matte black. In order to provide details about on-surface flow patterns, a special red liquid can be emitted from dozens of tiny holes in the aircraft's nose and filmed as it streams out over the surface of the fuselage.
With the thrust-vectoring vanes, the F-18 HARV has achieved stable flight at angles of attack as high as 70 degrees (previous maximum for conventional F/A-18 was 55 degrees). High roll rates can be achieved at 65-degree angles of attack, whereas controlled rolling was impossible above 40 degrees angle of attack for a conventional F/A-18.