Monographs in Aerospace History, Number 12




NACA Flight Research on the Vought F8U-1 Airplane


[151] Flight measurements of handling qualities of numerous airplanes had continued through the years following WW 11 at both the Langley and the Ames Laboratories, until about 60 airplanes had been tested by the mid 1950's. The flight research on the Vought F8U-1 (fig. 16.1), an early Navy supersonic fighter airplane, is singled out for special emphasis because it was the last high-speed airplane on which handling-qualities tests were conducted at Langley. In 1958, a decision was made by the NACA Headquarters to conduct all testing of high-speed airplanes at Edwards Air Force Base, California. This decision was based partly on considerations of safety because of the large population of the area surrounding Langley Field and partly to consolidate facilities to save money. From the standpoint of the Flight Research Division at Langley, the change represented a considerable loss of accumulated experience and expertise, as well as instrumentation and evaluation techniques that had been developed over the years. Soon after this decision was made, however, the appearance of Sputnik resulted in the conversion of the NACA to NASA. The dawn of the space age brought many new problems, which resulted in a decline in interest in aeronautical research. Many of the engineers who had worked in my branch on handling qualities left to join the Space Task Group that later moved to the Johnson Space Center in Houston, Texas, to form the nucleus of the Manned Space Flight Program. Other engineers in my branch who remained at Langley were employed in simulation studies of aeronautical and space operations.

The flight tests of the Vought F8U-1 airplane were considered by the Langley Director, Dr. Floyd Thompson, as being especially important. As a result, he requested that frequent reports be written, for his attention, to summarize the progress on the job. I assigned Christopher C. Kraft, Jr., one of my most capable engineers, as project engineer. Donald C. Cheatham, a former Navy pilot, assisted in the many discussions with the Navy William S. Aiken, Jr. of the Loads Division was assigned to aspects of the work involving measurement of structural loads.

The numerous progress reports were originally classified secret, but in 1981 after many years had passed, I had these reports declassified and collected them in a document (ref. 16.1). This collection of progress reports presents an interesting historical account of a typical handling-qualities investigation and shows how this work helped to improve the quality of Air Force and Navy airplanes during the period after WW 11. This document was never published, but copies are available at the NASA Langley library.

Shortly after the beginning of the space program, Christopher C. Kraft, Jr. moved to the Johnson Space Center. He became a Flight....



FIGURE 16.1.

FIGURE 16.1. Vought F8U-1 airplane showing variable-incidence wing in raised position.


...Director in the Apollo Program, and after the retirement of Dr Robert R. Gilruth, became Director of the Johnson Space Center during the latter stages of the Apollo Program. During the development of the Space Shuttle, Donald C. Cheatham also moved to the Johnson Space Center, where he played a prominent role in the development of the Shuttle control system. William S. Aiken, Jr. moved to NASA Headquarters in Washington, where he held various positions, including director of the NASA Aeronautics Program.


Results of the Last NACA Flying Qualities Investigation


The progress reports on the F8U-1 investigation, written mainly by Christopher C. Kraft, Jr., give a good insight into the interaction between the Navy, the Vought Company, and the NACA in developing this fighter airplane. In addition, they catch the spirit of urgency attached to this study because of crashes that occurred in the early development of the airplane. They also record the inevitable delays and difficulties of flight research caused by lack of spare parts, problems with instrumentation, and weather.

The F8U-I airplane was received at Langley in December 1956 as the result of a policy by the Navy to turn over to the NACA an early production model of each of its new fighter airplanes for flight evaluation and research. A novel feature of this airplane was a variable-incidence wing to place the fuselage in a more nearly level attitude for landing on a carrier. An early source of difficulty was inability to move the variable-incidence wing to the down and locked position after takeoff. This deficiency was responsible for the loss of at least one airplane and was a cause of frequent aborting of flight missions. The Aircraft Loads Branch was called into the study. Measurement of the forces in the wing-strut actuator during the retraction and extension cycle provided the information necessary for the company to design a wingstrut actuator with adequate power. In addition, operational sequences were determined to allow satisfactory use of the airplanes with the original strut that were already in service.

The longitudinal stability and control measurements of the airplane showed a decrease in stability at high values of acceleration in maneuvers. This deficiency was emphasized by a crash of an airplane during a flyby at the Vought plant. This crash, like that of the F-84 discussed previously, was a highly visible disaster as the airplane was being demonstrated to a group of Navy test pilots. The cause of the difficulty was determined [153] by quantitative measurements of the deflections in various parts of the control system in accelerated maneuvers. Deflection of the fuselage under load was found to cause a movement of the stabilizer. An up load on the airplane caused a deflection of the stabilizer to a more negative incidence, which was a direction that tended to increase the upward load. The effect was therefore in a direction to destabilize the airplane. Once the cause of the problem was established, the cure was simple. Reversing the position of a link in the elevator control system caused the fuselage bending to stabilize the airplane rather than destabilize it. This modification was retrofitted to all airplanes. Prior to this modification, the low-altitude maneuverability of these airplanes was severely restricted for reasons of safety.

In addition to these important contributions to the safety of the airplane, the studies on the F8U-I provided considerable experience in the general area of handling qualities of airplanes in supersonic flight. The airplane was one of the first to be fitted with roll and yaw dampers. The flight tests provided information of interest in the required stabilizing effects of these systems.

Another novel feature of the control system was a nonlinear linkage between the stick motion and the stabilizer deflection. The nonlinearity was such that the stabilizer motion was zero for small stick deflections about neutral, but increased with increasing slope as the stabilizer deflection increased. This feature was intended to offset the increased effectiveness of the stabilizer with increasing air speed. At high values of speed, small stabilizer deflections are usually used because they produce large loads on the airplane. The small sensitivity of the control stick at small deflections was intended to allow larger stick deflections in this regime. These larger stick deflections give the pilot more accurate control of acceleration. At low speeds, where the stabilizer effectiveness is decreased, the control-stick movement, and hence sensitivity, was increased. This feature appears logical. A problem arises, however, in that when the pilot has become accustomed to flying with gentle maneuvers for a period of time, he will be surprised by the large increase in control sensitivity in a large-amplitude maneuver. Some nonlinearity in the control-stick gearing may be beneficial, but the amount used on the F8U-I, which caused the stick sensitivity to approach zero at zero deflection, was found to be excessive. The results of the studies of the flying qualities of the F8U-I are given in a memorandum report (ref. 16.2).