and wing design. Supercritical wing research illustrates the principal phases of the program. On a conventional airfoil the shock, that forms as the speed of the air flow over the airfoil surface approaches the speed of sound, is located near the midpoint of the airfoil chord. The pressure rise across the shock causes the flow to separate from the surface behind the shock. The flow separation causes an abrupt drag increase and aircraft buffeting. Through newly developed concepts of airfoil shaping it is possible to move the shock back to near the trailing edge, thus reducing the area of the surface exposed to separated flow. As a result, the abrupt drag rise is delayed to a higher aircraft speed and buffet is significantly alleviated. The middle sketch on the chart illustrates a wind tunnel model of a 3-dimensional wing mounted on a fuselage. Tests of this model have verified that the potential aerodynamic improvements indicated by the earlier analyses and 2-dimensional airfoil tests extend to the 3-dimensional wing-body combination. The results of the wind tunnel studies have shown such promise that it is planned to take this supercritical wing research into the flight test phase to test the concept in the real and dynamic environment of full-scale flight. Consequently, in parallel with a definitive ground-based effort a program to modify and flight test an existing aircraft has been initiated to (1) verify in practice the higher operating speeds and buffet-free maneuverability promised by the new airfoil, (2) evaluate the aerodynamic characteristics under actual operational conditions, and (3) determine the sensitivity of the new airfoil to realistic construction tolerances and deformations under maneuvering flight loads. The airplane chosen is an F-8 fighter aircraft (provided by the U.S. Navy) which will be modified to incorporate a high-aspect-ratio sweptback supercritical wing. The modified vehicle, sketched at the bottom of the chart, will be instrumented and flight tested during fiscal year 1970. For military fighter aircraft buffeting or airflow separation at transonic speeds significantly limits airplane maneuverability and imposes excessive structural loads. As indicated previously the use of a supercritical airfoil section may greatly increase the buffet-free maneuverability of a fighter aircraft. This is illustrated in the next chart, Effect of Airfoil Section on Buffet (Figure NASA RA 69-898). The lower curve represents the buffet onset boundary as a fur.ction of Mach number for a conventional airfoil. Operation below this curve is buffet free and the further an aircraft penetrates above this boundary the more intense the buffet becomes. As shown the buffet onset boundary for the supercritical wing should represent a dramatic improvement and largely alleviate buffet as a limiting factor to aircraft maneuvers. In addition to this research on wing sections, a comprehensive continuing program is directed to gaining a better understanding of the structural loads due to buffeting and the buildup of buffet intensity for flight conditions above the buffet boundary. This program is illustrated in the next chart, Buffet Research (Figure NASA RD 68-16191). An F-111A aircraft is being used for the study. Measurements of the buffeting loads on the wing and tail are being made. This program will also include verification and demonstration in flight of the favorable effects of flaps in alleviating buffeting. The F-111A is particularly well suited to buffet research since it may be flown with the wing position at various angles of sweep. This program, which will be carried out at the Flight Research Center, is being supported and complemented by wind tunnel studies at the Langley Research Center and the Ames Research Center. The tests at Ames involve pressure measurements on a rigid model to determine the unsteady aerodynamic forces which produce buffeting. The tests at Langley involve loads measurements on a flexible model in which the structural characteristics of the airplane are dynamically scaled. Successful correlation of the results of the wind tunnel tests of the rigid and flexible models with flight test measurements will lead to wind tunnel test techniques to permit prediction of buffet characteristics from small scale tests. Loads and structures A year ago a new structural concept of bonding boron filament-epoxy tape to aluminum alloys was described and the potential weight saving for a typical aircraft structure was shown. Effort on this concept has continued and has been expanded to include titanium alloys. We can now report on the progress of this research. The weight of various materials relative to an aluminum alloy that is commonly used in aircraft structures today is shown in the next chart, Structural Effectiveness of Composite Materials (Figure NASA RA 69-897). The relative weights shown are for equal compressive yield strength. Aluminum alloy is shown at a value of 1.0 by the bar on the left. Next to it is shown the relative weight of a titanium alloy. The titanium alloy offers a weight advantage over the aluminum alloy being only about 91% of the weight of aluminum for the same compressive yield strength. The aluminum and titanium alloys reinforced with boron-epoxy tape afford a substantial weight saving as shown by the two bars labeled "Filament Reinforced Metals." Also shown for comparison is the relative weight of a boron-epoxy composite alone. Although the aluminum and titanium alloys reinforced with boron-epoxy tape do not promise as much weight saving as the boron-epoxy structure it is an efficient way of capitalizing on the high strength and stiffness of boron filaments at much reduced cost. The design concept being pursued in this work is the application of the high-strength boron fibers at the location of maximum stress in the structure thus using the material in the most efficient manner. For example, the costly boron filaments constitute only 25% of the total weight of the structural specimens which combine boron with metal alloys as the basic material. In the next chart, Aluminum Reinforced with Boron (Figure NASA RA 69-896) are shown data from tests of an aluminum tube to which boron-epoxy tape has been bonded to the surface. The curves on the right show the stress or load per square inch as a function of the material deformation as the loading is increased. The point on the curves at which the rate of deformation with loading changes is called the yield point of the material (approximately 38,000 lbs. per sq. in. for the aluminum specimen). Further increase in the loading beyond this point causes the material to exceed its elastic limit and it will be permanently deformed. As illustrated in the chart the boron reinforced aluminum tube sustained a load (approximately 126,000 lbs. per sq. in.) over three times that of the plain aluminum tube, before reaching its yield point. The two tubes in these tests were of equal weight. Shown in the next chart, Fatigue Tests of Aluminum Reinforced with Boron (Figure NASA RA 69-895) are results of tests to determine the fatigue characteristics and the rate of crack propagation of boron reinforced metals under reSTRUCTURAL EFFECTIVENESS OF COMPOSITE MATERIALS peated or cyclic loads. The test article sketched on the left of the chart was prepared with an initial crack and then subjected to cyclic loading as indicated by the arrows at the ends of the sketched specimen. The test results are shown by the curves on the right where the crack length is plotted on the abscissa versus the number of loading cycles. Also shown is the width of the test specimen for comparison. For the unreinforced aluminum specimen (the lower curve) the crack propagated across the specimen until complete failure occurred when the crack length was about 40% of the specimen width. The specimen reinforced with boron filament tape exhibited slower crack growth and complete failure did not occur until the crack had propagated nearly all the way across the aluminum. Further research is planned to establish more completely the fatigue behavior of this structural concept but, based on the results to date, it appears that this reinforcement concept could provide a method for repairing damaged structures or for increasing the service life of structural parts which may be critical in fatigue. This research to exploit the potential inherent in this structural concept for aluminum and titanium structures will be continued and expanded in FY 1970. The next chart, Test Panels of Typical Aircraft Structures Reinformed with Boron Tape (Figure NASA RA 69-894) illustrates some of the typical components which will be tested and evaluated to develop the engineering design and fabrication techniques essential to the utilization of this concept in aircraft. As illustrated on the left of the chart a variety of stiffened panels, reinforced with boron filaments will be constructed and tested. This will be followed by the fabrication and testing of fuselage sections which incorporate the best designs derived from the small panel tests. Operating environment This area of research is concerned with problems associated with the runway and flight environment, aircraft safety and noise, and flight instrumentation. The programs include theoretical analyses and laboratory and flight test experiments. The results of this research provide the technology for safer and quieter aircraft operations and basic environmental design data. Last year we discussed laboratory work, conducted under contract by the Cornell Aeronautical Laboratory, to obtain both a better understanding of warm fog properties and a means to modify it, to enable better utilization of airports and to increase safety. Based on laboratory experimental results obtained at the Cornell Aeronautical Laboratory, field tests were run this past TEST PANELS OF TYPICAL AIRCRAFT STRUCTURES |