[29] In 1948, an F6F-3 Hellcat was modified by Ames engineers to become the world's first variable stability aircraft. The genesis of this idea followed an investigation (noted previously) into the desired wing dihedral for the Ryan FR-1 Fireball. Three of these aircraft were built, each with a different dihedral angle, to narrow the final design option. This cumbersome and time-consuming approach to a solution inspired William (Bill) Kauffman to develop the concept of a variable stability aircraft. ²⁰ Kauffman had the idea that the basic flight characteristics of such an aircraft could be altered by a stability-augmentation system, so that a wide range of static and dynamic characteristics, representing the flying qualities of a different aircraft, could be safely simulated and evaluated in flight. In the initial design of the F6F-3, the variable stability system altered the effective wing dihedral by deflecting the aircraft's ailerons in response to sideslip. The cockpit control linkage was modified so that the pilot's control stick would not move in response to the aileron deflections commanded by the variable stability system, yet it allowed the pilot to control the roll axis conventionally. Later, modifications to the variable stability system allowed it to command rudder deflections in response to roll rate, yaw rate, and sideslip angle, while roll rate and sideslip were fed into the existing aileron deflection system. Contributions to the development of the variable stability system, along with those of Bill Kauffman, came from G. Allan Smith, an expert in servomechanism design. Reference 64 describes the modifications that were made to develop this variable stability airplane.

Thus configured, the F6F-3 was used for generalized studies of lateral-directional flying qualities criteria and as an in-flight simulator for new aircraft under development. In the latter case, the industry test pilots were able to experience the flying qualities of a new design before its first flight. In a prominent example of the results of these programs, Lockheed was led to incorporate negative dihedral into the design of the F-104 Starfighter after observing the effects on lateral-directional flying qualities of dihedral ranging from positive to negative. This design feature, quite startling and unconventional for most aircraft at that time, was based on a compromise between the expected Dutch roll oscillatory characteristics and the roll response to rudder deflection. Throughout the time the F6F was used as a variable stability aircraft, Rudy Van Dyke, Don Heinle, and Fred Drinkwater were the principal test pilots involved in the programs. Individual members of the Flight Research and Flight Operations Branches, many of whom were associated with the F6F, are shown in figure 67.

Following the development of the F6F, and as high-performance swept-wing jet aircraft came to the fore, evolution of the variable stability concept came about on a series of F-86 aircraft and eventually on an F-100C. To develop lateral-directional flying qualities requirements for the high-performance aircraft of that time, an F-86A and later an F-86E were modified for variable stability. The F-86A's variable-stability system was limited to the yaw axis, whereas that of the F-86E included the roll and yaw axes. Another aircraft, a YF-86D, was used to evaluate longitudinal control system features. Variable stability in this case consisted of changes in control system.....

....feel, such as sensitivity and breakout force, in addition to control system dynamic response. One of the latter investigations of the F-86s involved the determination of minimum allowable stability requirements, a subject of interest subsequently in the design of computer-controlled aircraft. Several engineers led the various programs on these aircraft; among them were Charles Liddell, Walter (Walt) McNeill, Brent Creer, Norman McFadden, Richard (Dick) Vomaske, and Frank Pauli. Results representative of investigations into the desired dihedral effect, lateral damping and oscillatory response characteristics, lateral-directional coupling, longitudinal stability, and longitudinal control system characteristics are contained in references 65-73. All these aircraft that were configured as variable stability vehicles are noted in table 5 and shown in figures 68-71.

During the years that the F6F and F-86s flew as variable stability test beds, a variety of new aircraft designs were simulated in order to investigate their flying qualities for a range of piloting tasks. The new designs included the D-558-II, XF-10F, X-1, B-58, XF-104, XF8U-1, F9F-9, XT-37, B-57D, T-38, and the P6M. They ran the gamut from high-performance fighters to bombers, illustrating the breadth of capability in Ames' stable of variable stability aircraft at that time. Bill Kauffman's genius in the conception and development of these aircraft was widely recognized and appreciated by his peers. His original variable stability system design was granted a U.S. patent in 1955. For his extensive work over the years on in-flight simulation, he was presented the Arthur S. Flemming Award in 1955 as one of 10 outstanding young men in the federal service.

The last high-performance fighter developed with the variable stability capability was the F-100C, the first three-axis (pitch, roll, and yaw) variable stability aircraft at Ames. A team led by John V. Foster (fig. 72) undertook the system design for this aircraft, which is described in reference 74. After serving as an in-flight simulator for one new, highly advanced aircraft in 1960, it was transferred to the Flight Research Center in response to the headquarters directive at that time. It was later returned to Ames, and Walt McNeill then led a program to evaluate direct-lift control as an aid to air-to-air refueling. Jack Ratcliff was in charge of the effort to modify the variable stability system for direct-lift control. Bob Innis and Ron Gerdes both flew as subject pilots and executed actual plug-ins to the drogue on a KC-135 tanker. The program demonstrated the improvement in precision the pilot could achieve in control of the probe engagement with the drogue when using direct-lift control (ref. 75).

The X-14A and B and the CH-47B were also very productive variable stability aircraft that were used to develop V/STOL and rotary-wing aircraft flying qualities criteria. These aircraft are listed in the V/STOL and rotorcraft categories, but their research systems trace their lineage to the original variable stability aircraft.