Military


F/A-18A/B Hornet

Making the first flight on 18 November 1978, the F/A-18 and its two-place derivative [subsequently redesignated the F/A-18B] underwent most of their development testing at the Naval Air Test Center at the Patuxent River Naval Air Station in Maryland. Under the new single-site testing concept, the preproduction flight-test program lasted from January 1979 to October 1982. While much attention was focused on development problems, these were largely typical of those in any new program, with their resolution being part of the development process. For the most part, these occurred in the basic aircraft hardware rather than in the digital electronic systems.

Initial results from flight evaluations at Patuxent River in 1979 indicated that the cruise performance of the F/A-18 was significantly below expectations, with a shortfall of about 12 percent in cruise range. The performance deficiency became a weapon for those who sought the termination of the F/A-18 Program. A number of reasons for the poor performance were identified. Modifications to the engines, computer-controlled schedules for the deflection of leading- and trailing-edge flaps, and other changes reduced the cruise range deficit to about 8 percent, but aerodynamic drag remained a problem. Modifications included increasing the wing leading-edge radius, variations in the LEX camber, and filling in the slots in the LEX-fuselage juncture. These changes were implemented on the F/A-18 test aircraft at Patuxent River where they were found to favorably increase the cruise range of the aircraft. The impact of filling in the LEX slot on high-angle-of-attack characteristics was found to be acceptable in additional F/A-18 flight tests.

In 1979, an F/A-18 test aircraft at Patuxent River suddenly and unexpectedly departed controlled flight during a wind-up turn maneuver at high subsonic speeds. None of the baseline wind-tunnel data predicted this characteristic, and the F/A-18 Program was shocked by the event. Following exhaustive wind-tunnel tests in the Full-Scale Tunnel, the wing leading-edge flap deflection was increased from 25 deg to 34 deg at high angles of attack. Following the implementation of this recommendation on the test aircraft (via the flight control computers), no more departures were experienced, and the flap deflection schedule was adopted for production F/A-18's.

During development, two-place trainer versions were added, to be built in limited numbers as TF/A-18s, intermingled with the basic F/As. Minimum changes were made to incorporate the second cockpit, with the two-seat airplanes retaining the ability to perform combat missions.

The original F/A-18A (single seat) and F/A-18B (dual seat) became operational in 1983 replacing Navy and Marine Corps F-4s and A-7s. It quickly became the battle group commander's mainstay because of its capability, versatility and availability. Reliability and ease of maintenance were emphasized in its design, and F/A-18s have consistently flown three times more hours without failure than other Navy tactical aircraft, while requiring half the maintenance time.

The F/A-18 configuration was found to be extremely resistant to spins. (The pilot was required to maintain prospin controls for over 20 sec to promote a spin.) When spins were entered, recovery could be effected very quickly. In the spin tunnel tests, the F/A-18 model demonstrated the best spin recovery characteristics of any modern US fighter (as had the YF-17 configuration). During the limited model tests for spins, the phenomenon known as "falling leaf" was not encountered, but it became a problem in operational usage.

The falling-leaf maneuver originated during World War I as a flight training exercise. In this exercise, pilots intentionally stalled the aircraft and forced a series of incipient spins to the right and left. The aircraft descends as it rocks back and forth, much as a leaf does falling to the ground. In the early 1980's, an unintentional falling-leaf mode surfaced as a severe out-of-control problem during developmental flight tests of the F/A-18A. The out-of-control falling-leaf mode is a highly dynamic mode where the aircraft oscillates so that it is very difficult to reduce angle of attack and recover. The term "alpha hang-up" was used to describe this problem with the F/A-18 and it was a key driver in estab-lishing the aft center of gravity and the maneuvering limits for the aircraft. During early operational use of the F/A-18, the falling-leaf mode was rarely encountered; however, by the early 1990's increasingly aggressive maneuvering had exposed a susceptibility to the falling-leaf mode with numerous incidents and losses of aircraft.

In light of the growing falling-leaf problem on early models of the F/A-18, there was concern that the emerging F/A-18E/F, which was then preparing for developmental flight tests, would have the same problem. Falling-leaf susceptibility was extensively evaluated on the F/A-18E/F during the high-angle-of-attack flight-test program. While the unaugmented aircraft was shown to exhibit the falling-leaf mode, a new control system design was shown to be very effective in suppressing the mode and the falling-leaf problem was considered solved for that aircraft. Largely due to the success of the F/A-18E/F program, the Navy considered retrofitting earlier models of the F/A-18 with the updated control law for the purpose of eliminating the falling-leaf problem.

Interim change #75 to the NATOPS Flight Manual introduced yet another change to F/A-18 Out-of-Control (OOC) flight procedures. The change essentially eliminated specific recovery procedures unique to the "falling leaf" mode. The falling leaf mode is now grouped together with OOC flight departure recovery procedures. This change simplifies NATOPS OOC flight recovery procedures. Alternate recovery procedures for the falling leaf mode have been investigated in the Naval Air Warfare Center engineering simulator. The simulator results suggested that a more rapid recovery from a positive AOA falling leaf might potentially be achieved by applying full aft stick (instead of forward stick or controls released). Strike test pilots attempted to validate this full aft stick recovery method (along with testing the full forward stick recovery which had not previously been flight tested but was adopted after engineering analysis) using an F/A-18B aircraft. . The primary problem with the validation process was making the airplane depart into a recognizable falling leaf OOC flight mode. Throughout the history of the departure demonstration program, over 1,700 departures have been flown without entry into a sustained falling leaf. It is impossible to validate the full aft stick recovery procedure in the airplane without generating repeatable, sustained falling leaf departures. Because of this and concerns that the falling leaf may transition to a full aft stick stall with wing rock, preventing a pilot from recognizing that he was recovered, resulted in full aft stick procedures not being recommended for NATOPS.

The Hornet has been battle tested and has proved itself to be exactly what its designers intended: a highly reliable and versatile strike fighter. The F/A-18 played an important role in the 1986 strikes against Libya. Flying from USS CORAL SEA (CV 43), F/A-18s launched high-speed anti-radiation missiles (HARMs) against Libyan air defense radars and missile sites, effectively silencing them during the attacks on Benghazi facilities.



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