Notes:

Previously in this series of videos, we have looked at two experimental offshoots of popular designs. The first – the F-15 ACTIVE – was a hyper agile, thrust vectoring variant of the F-15B, and saw great success during its testing phase. The next aircraft was the F-16XL – an improved multirole upgrade concept of the F-16, utilising a unique delta wing design, allowing for better speed, agility, fuel capacity, and weapons stores.

In this video we will look at a lesser-known experimental project, known as the F-18 HARV.

BACKGROUND

Since the introduction of the F-16 in 1974, many aircraft designers had developed a renewed interest in high agility to give fighters an edge in dogfighting. Successful aircraft such as the F-15 Eagle underwent modification programs to experiment with such high agility, known as the STOL/MTD program, whilst the F-16 itself would go through a super-manoeuvrability program known as VISTA. A persistent challenge in these programs was aircraft performance at high angles of attack, or AOA.

Designers were intent on overcoming a set of common limitations for fighter pilots and their aircraft. Namely, during dogfights, pilots are often forced to pull into high angles of attack. The further the aircraft is pulled, the more energy is bled off, ultimately resulting in a stall. High angels of attack also cause inconsistent airflow over the surface of the lifting body, meaning that even if the aircraft doesn’t stall, its performance is severely degraded.

In 1985, NASA would formally begin its own program researching high AOA performance. The NASA Langley Research Centre would head a program known as the High Angle of Attack Technology Program, or HATP. Langley would work alongside several other NASA research centres to conceptualise and produce technology which could be used to test higher AOA performance. One of these centres was the NASA Dryden Flight Research Centre. A centre that  would eventually receive the F15 STOL MTD, which it would convert into the F-15 ACTIVE, and the X-31.

As a testbed to study high AOA performance, NASA chose an F-18 Hornet. The production variant of the F-18 demonstrated good AOA performance, and it had no AOA restrictions at normal centre of gravity positions. Thus, it was a good candidate for the project. It would be named the High Alpha Research Vehicle, or H-A-R-V. Researchers decided on a three-phase program, with each phase focusing on new areas of improvement.

Fortunately, a pre-production model of the F-18 had already been delivered to Dryden in October 1984. This aircraft, which arrived on a semitrailer, had been cannibalised by the Navy, who never expected it to fly again. An old aircraft designed before the designation was changed from F-18 to F/A-18, it had been used by the navy because it had a spin chute. They had used it for spin testing and high AOA experiments themselves, before grounding it and using it for parts.

Despite this, the aircraft (Bureau number 160780) would become a project for the Dryden engineers. It lacked over 400 parts and had little information on its existing wiring system.  Thus, researchers had to find substitute parts, take out all wires from the aircraft, and then rewire it.

PHASE ONE

The first phase would begin in April 1987, and would continue until 1989. On April 2nd 1987, NASA research pilot Einar Enevoldson took the HARV for its first functional test flight. Following two more successful flights, he handed the aircraft over to NASA test pilots Bill Dana and Ed Schneider. From here on, the first phase would focus on studying the existing nature of high AOA, and using this data to develop new technology.

Several modifications were added for phase one. A 360 degree air pressure sensor was integrated into the nose, and others across the body. Tracer smoke systems were added at various locations around the aircraft.

Monitoring equipment was set up both onboard and on the ground. Videotape and film cameras were setup to closely monitor airflow using the tracer smoke systems, which released smoke just in front of the leading edge of the wing. Also photographed were pieces of yarn taped around the aircraft to monitor airflow, as well as a system which would release dyed anti-freeze from the aircrafts nose. This coloured anti-freeze would then hit parts of the airframe, further showing where air was impacting the airframe. One phenomenon of interest was the formation of vortices over the leading-edge extensions, and how this produced tail buffeting.

The data collected from this phase was then compared with computer simulations and wind tunnel tests. This data was then used to engineer new tech into the HARV.  Phase one would last for 101 flights.

PHASE TWO

Phase Two would see several major overhauls to the HARV. Beginning with computational fluid dynamics and wind tunnel testing, this phase would look further into the dynamics of high AOA performance and integrate thrust vectoring. By July 1991, the HARV was ready for testing.

The thrust vectoring system had been developed from a heat resistant material known as Inconel 1. The engine nozzles were cut short, and a novel system of three manoeuvrable paddles were used to vector thrust for both yaw and pitch. These paddles weighed 2200 pounds, and did make supersonic flight unfeasible, however subsonic performance remained relatively unchanged.

This system allowed increased agility at medium and high AOA up to 70 degrees. The system carried the added benefit of prolonged flight at this AOA, allowing more data to be collected.

To assist the test pilots, a new flight control system was installed, known as the PACE 1750A, coded with new flight control laws. For takeoff, landings, and in the case of emergencies, the pilots could also switch back to the standard F-18 flight control system.

This second phase, of 193 flights, would end with the integration of new inlet pressure measurement systems. This improvement would take place from January 1993 to January 1994. The integration of this measurement system would deliver important data to researchers. For the first time, it would allow them to measure exactly what was happening inside the engine intakes during these high AOA manoeuvres.

PHASE THREE

The final phase began in March 1995. The F-16 had initially used forward strakes to take advantage of lift created by vortices, allowing it to pull into higher angles of attack while retaining enough lift to stay out of a stall. Under the same principle, the HARV would also attempt to achieve higher angles of attack using strakes. With forebody strakes on the HARV, up to 70-degrees of AOA was achievable.  These strakes, which measured 4 feet long by 6 inches wide, would move depending on angle of attack. In standard flight, they sat flush, whilst at higher AOAs, they would extend to interact with the vortices.

The integration of these strakes would allow the pilot to switch between three flight modes. The first would use thrust vectoring alone. The second used thrust vectoring for pitch, and strakes for lateral control. The third used strakes for lateral control, and vectoring for longitudinal control.

After 109 flights, phase three would come to an end in September 1996. The program had increased the parameters of what was possible, giving important insight into the nature of high AOA performance.

TODAY

After a total of 385 research flights, the HARV was retired shortly after Phase Three ended in September 1996. It now sits on display in the Virginia Air and Space research centre in Hampton, Virginia.