F-18 HARV smoke and tuft flow visualization Project Summary
NASA's Dryden Flight Research Center, Edwards, Calif., used an F-18 Hornet fighter aircraft as its High Angle-of-Attack (Alpha) Research Vehicle (HARV) in a three-phased flight research program lasting from April 1987 until September 1996. The aircraft completed 385 research flights and demonstrated stabilized flight at angles of attack between 65 and 70 degrees using thrust vectoring vanes, a research flight control system, and forebody strakes (hinged structures on the forward side of the fuselage to provide control by interacting with vortices, generated at high angles of attack, to create side forces).
The first phase of "high alpha" flights began in April 1987 and lasted through 1989. It consisted of 101 research flights in the specially instrumented F-18 at angles of attack as high as 55 degrees. During this phase, there were no external modifications to the aircraft, but technicians equipped it with extensive instrumentation. NASA research pilot Einar Enevoldson made the first (functional check) flight on April 2, 1987, and three succeeding flights before turning the piloting duties over to NASA research pilots Bill Dana and Ed Schneider. The purpose of this phase was to obtain experience with aerodynamic measurements at high angles of attack and to develop the flight research techniques needed for this measurement.
Researchers conducted visual studies of the airflow over various parts of the aircraft. Special tracer smoke was released through small ports just forward of the leading edge extensions near the nose; on-board video and still cameras captured images of the smoke as it followed airflow patterns around the aircraft. Also photographed in the airflow were short pieces of yarn (tufts) taped on the aircraft and anti-freeze with dye in it, released onto the aircraft surfaces from hundreds of small orifices around the vehicle's nose.
Researchers used the film and videotape images of the airflow patterns from the smoke, dye, and tufts for a comparison with computer and wind tunnel predictions. Additional data that they obtained included air pressures recorded by sensors located in a 360-degree pattern around the nose and at other locations on the aircraft. Researchers paid particular attention to the location of strong vortices (masses of air in circular motion) that formed off the forebody and wing-body-strake (leading-edge extension or LEX) at high angles of attack and their role in inducing tail buffeting (beating by unsteady flow, gusts, etc.).
Research test pilot Dana Purifoy in front of F-18 Phase Two
Phase Two flights examined the benefits of using vectored thrust to achieve greater maneuverability and control at high angles of attack while continuing the correlation of flight data with wind-tunnel and computational fluid dynamics (CFD) data begun in Phase One. The initial portion of the Phase Two flight program was completed in January 1993.
Phase Two featured major hardware and software modifications to the HARV – a multiaxis thrust-vectoring control system featuring vanes attached to the aft end of the aircraft and a research flight control system. Three paddle-like vanes, made of Inconel®1 metal (a nickel alloy containing chromium and iron that is heat resistant), were mounted around each engine's exhaust. They provided both pitch (up and down) and yaw (right and left) forces to enhance maneuverability when the aerodynamic controls were either unusable or less effective than desired. The engines had the divergent portion of the exhaust nozzles removed to shorten the distance the vanes had to be cantilevered by about two feet. The subsonic performance of the engines, including afterburning, was largely unaffected by the modifications, but supersonic flight was no longer possible – a penalty unique to this experimental project. The thrust vectoring control system added 2,200 pounds to the total weight of the aircraft. In addition, the inclusion of a spin parachute for safety plus an emergency power system and ballast added a further weight penalty of 1,500 pounds. The HARV also carried 419 pounds of other equipment and wiring not directly associated with the thrust-vector-control system.
The research flights began in July 1991 using the thrust vectoring system for control. The system could deflect the exhaust flow from the two turbofan engines to provide enhanced maneuverability and control in areas where conventional aerodynamic controls are ineffective. (These controls included ailerons, rudders, stabilators [stabilizers + elevators – all-movable horizontal stabilizers in the tail area], and leading-edge flaps.) The system resulted in significantly increased maneuverability at moderate angles of attack and some degree of control at angles of attack up to roughly 70 degrees. It also allowed researchers to collect a greater amount of data by remaining at high angles of attack longer than they could have done without it.
The modified flight control computers used a PACE 1750A computer and specially written flight control laws to provide the research flight control capability. These laws commanded the optimum combination of aerodynamic control and vectored thrust to satisfy pilot demand. Program engineers integrated all controls into these flight control laws. The pilot used standard cockpit controls and no special pilot action was required after the system was engaged in flight. In addition to the research flight control computers, pilots also used the original F-18 flight control system both as a backup to the research system and to perform take-offs and landings.
Dryden research pilots Bill Dana and Ed Schneider completed the envelope expansion flights in February 1992. Demonstrated capabilities included stable flight at approximately 70 degrees angle of attack (previous maximum was 55 degrees) and rolling at high rates at 65 degrees angle of attack. Controlled rolling would have been nearly impossible above 35 degrees without vectoring.
Between January 1993 and January 1994 the aircraft was modified with a sophisticated engine inlet pressure measurements system between the inlet entrance and the engine face. This information provided unprecedented understanding of what happens to engine airflow under extreme maneuver conditions. Flights resumed in January 1994 and continued through June 1994, with Ed Schneider and Jim Smolka as the Dryden research pilots, joined for short periods by U.S. Navy pilots. In Phase Two there were a total of 193 flights, including some transition flights to Phase Three.
F-18 HARV Phase Three
The Phase Three effort began in March 1995 to evaluate moveable strakes on both sides of the aircraft's nose to provide yaw control at high angles of attack where conventional rudders became ineffective. These strakes, 4 feet long and 6 inches wide, were hinged on one side and mounted to the forward sides of the fuselage. At low angles of attack, they were folded flush against the aircraft skin. At higher angles of attack, they were extended to interact with the strong vortices generated along the nose and thereby produce large side forces for control. Wind tunnel tests indicated strakes could be as effective at high angles of attack as rudders are at lower angles. Flights with active moveable strakes began in July 1995. The availability of the strakes enabled pilots to employ three separate flight modes. One used thrust vectoring alone. Another used thrust vectoring for longitudinal (pitch) control and a blend of thrust vectoring and strakes for lateral (side-to-side) control. A third mode used thrust vectoring solely for longitudinal control, with strakes alone for controlling lateral motion. These three options were a unique feature of the HARV project. They afforded a great deal of flexibility in research into control power requirements at high angles of attack. They were also a means of achieving detailed investigation of handling qualities at high angles of attack.
The strake project concluded in September 1996 with flight 385 of the HARV program. It yielded a great deal of information about the operation of nose strakes, which were effective in providing control above 35 degrees angle of attack. Phase Three included 109 flights. Besides Schneider and Smolka, the research pilots included Mark Stucky from Dryden, Phil Brown from Langley, and a number of guest pilots from the U.S. Navy and Marine Corps, the Canadian Air Force, the (British) Royal Air Force, McDonnell Douglas, CalSpan and NASA Dryden.