Spin Prevention: The F-14 Program
Early spin tunnel tests of the F-14 at Langley during the airplane’s early development program indicated the configuration would exhibit a potentially dangerous fast, flat spin and that conventional spin recovery techniques would not be effective for recovery from that spin mode—even with the additional deployment of the maximum-size emergency spin recovery parachute considered feasible by Grumman and the Navy. Outdoor radio-controlled models were quickly readied by NASA for drop-testing from a helicopter at a test site near Langley to evaluate the susceptibility of the F-14 to enter the dangerous spin, and when the drop model results indicated marginal spin resistance, Langley researchers conceived an automatic aileron-to-rudder interconnect (ARI) control system that greatly enhanced the spin resistance of the design. The value of NASA participation in the early high-angle-of-attack assessments of the F-14 benefited from the fact that the same Langley personnel had participated earlier in the development of the flight control system for the F-15, which used a similar approach for enhanced spin resistance. Extensive evaluations of the effectiveness of the ARI concept by NASA and Grumman pilots in the Langley Differential Maneuvering Simulator (DMS) air combat simulator reported a dramatic improvement in high-alpha characteristics.
However, after the ARI system was conceived by Langley and approved for implementation to the F-14 fleet, a new wing leading – edge maneuver flap concept designed by Grumman was also adopted for retrofit production. Initial flight-testing showed that, when combined, the ARI and maneuver-flap concepts resulted in unsatisfactory
pilot-induced oscillations and lateral-directional deficiencies in handling qualities at high angles of attack. Meanwhile, NASA had withdrawn from the program, and Grumman’s modifications to the ARI to fix the deficiencies actually made the F-14 more susceptible to spins. Made aware of the problem, Langley then revisited the ARI concept and, together with Grumman and Navy participation, corrected the problems. Development and refinement of the ARI system for the F-14 continued for several years.
In the mid-1970s, senior Navy leaders were invited to NASA Headquarters for briefings on the latest NASA technologies that might be of benefit to the F-14. When briefed on the effectiveness of the ARI system, a decision was made to conduct flight evaluations of a new updated NASA version of the system. Joint NASA-Grumman-Navy flight-test assessments of the refined concept took place with a modified F-14 at the NASA Dryden Flight Research Center in 1980. Flight tests of the ARI-equipped aircraft included over 100 flights by 9 pilots over a 2-year period during severe high-angle-of-attack maneuvers at speeds up to low supersonic Mach numbers. Results of the activity were very impressive; however, funding constraints and priorities within
the Navy delayed the implementation of the system until an advanced digital flight control system (DFCS) was finally incorporated into fleet airplanes in 1999. The system, designed by a joint GEC-Marconi – Northrop Grumman-Navy team, was essentially a refined version of the concept advanced by Langley over 25 years earlier.
In retrospect, the F-14 experience is a classic example of inadequate followthrough on the technology maturation process for new research concepts. No doubt, if NASA had continued its involvement in the development of the ARI and been tasked to resolve the ARI/maneuver flap issues, the fleet would have benefitted from the concept much earlier.