YA-7D DIGITAC

Digital Flight Control for Tactical Aircraft (DIGITAC) was a joint program between the Air Force Flight Dynamics Laboratory (AFFDL) at Wright – Patterson AFB, OH, and the USAF Test Pilot School (TPS) at Edwards AFB. Its purpose was to develop and demonstrate digital flight control technology for potential use in future tactical fighter and attack aircraft, including the feasibility of using digital flight control computer technology to optimize an airplane’s tracking and handling qualities for a full range of weapons delivery tasks. The second prototype LTV YA-7D (USAF serial No. 67-14583) was selected for modification as the DIGITAC testbed by replacing the analog computer of the YA-7D Automated Flight Control System (AFCS) with the DIGITAC digital multimode flight control system that was developed by the AFFDL. The mechanical flight control system in the YA-7D was unchanged and was retained as a backup capability.

The YA-7D’s flight control system was eventually upgraded to DIGITAC II configuration. DIGITAC II used military standard data buses and transferred critical flight control data between individual computers and between computers and remote terminals. The data buses used
were dual channel wire and dual channel fiber optic and were selectable in the cockpit by the pilot to allow him to either fly-by-wire or fly-by­light. Alternately, for flight-test purposes, the pilot was able to imple­ment one wire channel and one fiber optic channel. During early testing, the channel with the multifiber cables (consisting of 210 individual fibers) encountered numerous fiber breakage problems during normal ground maintenance. The multifiber cable design was replaced by sin­gle-fiber cables with tough protective shields, a move that improved data transmission qualities and nearly eliminated breakage issues. The DIGITAC fly-by-light system flew 290 flights during a 3-year period, performing flaw­lessly with virtually no maintenance. It was so reliable that it was used to fly the aircraft on all routine test missions. The system performance and reliability was considered outstanding, with the technical approach assessed as ready for consideration for use in production aircraft.[1208]

The DIGITAC YA-7D provided the TPS with a variable stability tes­tbed aircraft for use in projects involving assessments of advanced air­craft flying qualities. Results obtained from these projects contributed to the flying qualities database in many areas, including degraded-mode flight control cross-coupling, control law design, pro versus adverse yaw studies, and roll-subsistence versus roll-time-delay studies. Under a TPS project known as Have Coupling, the YA-7D DIGITAC aircraft was used to investigate degradation to aircraft handling qualities that would occur in flight when a single pitch control surface (such as one side of the horizontal stabilizer) was damaged or impaired. An asym­metric flight control situation would result when a pure pitch motion was commanded by the pilot, with roll and yaw cross-coupling motions being produced. For the Have Coupling tests, various levels of cross-cou­pling were programmed into the DIGITAC aircraft. The resulting data provided a valuable contribution to the degraded flight control mode handling qualities body of knowledge. This included the interesting finding that with exactly the same amounts of cross-coupling present, pilot ratings of aircraft handling qualities in flight-testing were signif­icantly different compared with those rating obtained in the ground – based simulator.[1209]

The TPS operated the YA-7D DIGITAC aircraft for over 15 years, beginning in 1976. It made significant contributions to advances in flight control technology during investigations involving improved direc­tional control, the effect of depressed roll axis on air-to-air tracking, and airborne verification of computer-simulated flying qualities. The DIGITAC aircraft was used to conduct the first Air Force flight tests of a digital fight control system, and it was also used to flight-test the first fiber-opti­cal fly-by-light DFCS. Other flight-test firsts included the integration of a dynamic gun sight and the flight control system and demonstrations of task-tailored multimode flight control laws.[1210] The DIGITAC YA-7D is now on display at the Air Force Flight Test Center Museum at Edwards AFB.