The Precision Aircraft Control Technology YF-4E Program

In order to evaluate the use of computer-controlled fly-by-wire systems to actively control a relaxed stability aircraft, the SFCS YF-4E was further modified under the Air Force Precision Aircraft Control Technology (PACT) program. Movable canard surfaces were mounted
ahead of the wing and above the YF-4E’s inlets. A dual channel electronic fly-by-wire system with two hydraulic systems directed the canard actuators. The canards, along with the capability to manage internal fuel to move the center of gravity of the aircraft aft, effectively reduced the static stability margin to as low as -7.5 percent, that is, fully unstable, in the pitch axis. Relaxing static stability by moving the center of gravity aft reduces trim drag and decreases the downward force that the horizontal tail needs to produce to trim the aircraft. However, as cen­ter of gravity moves aft, an aircraft becomes less and less stable in the pitch axis, leading to a need to provide artificial stability augmentation. A negative static margin means that the aircraft is unstable, it cannot maintain stable flight and will be uncontrollable without artificial stabil­ity augmentation. During its test program, the PACT aircraft was flown 34 times, primarily by McDonnell-Douglas test pilots. The Pact aircraft demonstrated significant performance gains that included an increase in the 4 g maneuvering ceiling of over 4,000 feet (to 50,000 feet) along with an increase in turning radius.[1140] The approach used by the PACT YF-4E to create a relaxed stability research aircraft was soon mirrored by several foreign flight research programs that are discussed in a sep­arate section. In January 1979, the PACT YF-4E aircraft was delivered by Army helicopter from the McDonnell-Douglas factory in St. Louis to the Air Force Museum at Wright-Patterson AFB, OH, for permanent dis­play.[1141] By this time, the Air Force had initiated the Control Configured Vehicle (CCV) F-16 project. This was followed by the Advanced Fighter Technology Integration (AFTI) F-16 program. Those programs would carry forward the fly-by-wire explorations initiated by the PACT YF-4E.