A QUICK LOOK AT NASA F-14 TOMCAT TESTBEDS

The F-14s had a spin issue that posed problems for its ability to engage successfully in a dogfight

The Grumman F-14 Tomcat was the first of a series of post-Vietnam fighters, followed by the F-15 Eagle, F-16 Fighting Falcon, and F-18 Hornet.

Dubbed “teen fighters” these aircraft were designed for maneuverability in air-to-air combat.

The F-14s had a spin issue that posed problems for its ability to engage successfully in a dogfight, since it tended to depart from controlled flight at the high angles of attack that frequently occur in close-in engagements. Following their initial deployment to the fleet in October 1972, the Navy’s F-14s began to experience out-of-control mishaps. As it turned out, the analog automatic flight-control system on the aircraft had a simple control-law architecture that caused departures from the intended flight path under certain flight conditions. Furthermore, the control system did not provide the pilots full control authority (flight-control-surface deflections) for a recovery from spins and other departures, resulting in the loss of several aircraft and crews.

As reported by Ruby Calzada on Nasa.gov, to overcome this problem, NASA 991, U.S. Navy Tomcat serial number 157991 (which actually was designated “F-14 (1X),” the 1X signifying that it was Grumman’s experimental testbed), was used at Dryden Flight Research Center between 1979 and 1985 in a joint NASA/Navy/Grumman program . This looked at ways of improving the F-14’s handling at high angles of attack, increasing spin resistance, reducing “wing rocking” (i.e. tilting from side to side), and improving aircraft recovery when it departed from controlled flight (e.g., entered a spin).

As it can be seen in the photos NASA 991 had numerous special additions for high-angle-of-attack and spin-recovery research: these included a battery-powered auxiliary power unit, a flight test nose boom, and a special spin recovery system, consisting of forward mounted, hydraulically actuated canards and an emergency spin chute. The NASA/Navy/Grumman team developed new control laws involving what was called an aileron/rudder interconnect (ARI) that succeeded in limiting departures and providing recoveries from spins. The F-14 with the new control laws proved to be “very responsive and maneuverable above 30 degrees angle-of-attack, with no abrupt departure or spin tendencies.”

According to Calzada, among the 212 flights completed for this research project, the F-14 also tested a flush air data system, for gathering data about air speed; provided an updated aeromodel, which was used on Navy F-14 training simulators; created natural laminar flow baseline data for many of NASA’s later laminar flow programs; and tested low altitude, asymmetric thrust.

The program was an unqualified success, but the Navy did not immediately incorporate the new control laws into its F-14s because of insufficient funding. As a result, mishaps with the Tomcats continued. Finally, the Navy contracted with GEC Marconi Avionics of the United Kingdom to incorporate the control laws into a digital flight-control system with minimal changes, and this was deployed on fleet F-14Ds aboard the USS Kitty Hawk and USS Roosevelt in March of 1999, decreasing the danger of out-of-control flight and making powered approaches to carrier landings much safer. Meanwhile, already in 1980 Dryden research pilot Einar Enevoldson had received the NASA Exceptional Service Medal for his contributions as project pilot on the F-14 stall and spin resistance tests.

Instead NASA 834, another U.S. Navy F-14 Tomcat, was used at Dryden in 1986 and 1987 in a program known as the Variable-Sweep Transition Flight Experiment (VSTFE). This program explored laminar flow on variable sweep aircraft at high subsonic speeds. An F-14 aircraft was chosen as the carrier vehicle for the VSTFE program primarily because of its variable-sweep capability, Mach and Reynolds number capability, availability, and favorable wing pressure distribution. The variable sweep outer-panels of the F-14 aircraft were modified with natural laminar flow gloves to provide not only smooth surfaces but also airfoils that can produce a wide range of pressure distributions for which transition location can be determined at various flight conditions and sweep angles. The above photograph shows Glove I (that can be seen installed on the upper surface of the left wing) which was a “cleanup” or smoothing of the basic F-14 wing, while Glove II was designed to provide specific pressure distributions at Mach 0.7.

Laminar flow research continued at Dryden with a research program on the NASA 848 F-16XL, a laminar flow experiment involving a wing-mounted panel with millions of tiny laser cut holes drawing off turbulent boundary layer air with a suction pump.

Source: NASA; Photo credit: NASA