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Aviation History
1964
1964 - 1918.PDF
FLIGHT International, 25 June 1964 1061 B-70 .". . HIGH TEMPNO INSULATION MAXIMUM STIFFNESSLOW LOADS. HIGH TEMP INSULATION FUEL HIGH LOADS HIGH TEMPNO INSULATION MAXIMUM STIFFNESSHIGH LOADS XB-70A airframe materials, design criteria and external skin temper- atures in degrees Fahrenheit 600 HIGH TEMPCOOLED PRESSURIZED CABIN NO FUEL LOW LOADSMAXIMUM STIFFNESS 675 HIGH INTERNAL TEMPNO INSULATION NO FUEL HIGH LOADS TITANIUM-CONVENTIONAL STRUCTURE STEEL SANDWICH (PH 15-7 Mo) H-ll STEEL-CONVENTIONAL STRUCTURE I I N.CKEL BASE ALLOy-CONVENTWNAL ffiU RADOME I I Vibran construction. The remainder of the structure, totalling 69 per cent of the air-vehicle weight, is of PH15-7Mo brazed steel honeycomb sandwich, with panels welded together. Insulation for the fuel, carried in 11 integral tanks, is largely provided by the honeycomb itself, with the fuel as the primary heat sink, supple- mented with water, as described later. Airframe structural loads were obtained on all anticipated flight conditions, reflecting all combinations of altitude, speed, manoeuvre, gust and load factors. All potential conditions were surveyed, including the various wing- tip, nose-ramp, and e.g. positions, at numerous weight conditions, 15 Mach numbers and 14 altitudes. Of these numerous possible combinations, approximately 250 potentially critical design flight conditions were investigated in detail. Camber-type deflections of the thin delta wing introduced a significant new factor into the aeroelastic effects. To evaluate these a portrayal of the structural deflections under load was required, and was calculated as influence coefficients. Such data were calcu- lated for all structural components of the aircraft, including the effect of wing-tip interaction, which were then tied together to represent deflection characteristics of the complete airframe. In addition to these structural coefficients, aerodynamic influence coefficients were necessary because of the slope-to-wind pressure changes on various sections of the structure, and the requirements of aircraft balance under all forces, both static and dynamic. To accomplish this, the delta wing was diced into 95 grids. For the aeroelastic effects, a direct converging solution was obtained, requiring no iteration, with the resulting incremental flexibility effect superimposed upon the rigid-body data. To select the critical design conditions a parametric form of load analysis was used with cross-plotting of the effects of speed, altitude, load factor, e.g. position, weight, gusts and manoeuvres. Critical loads were then obtained for each component. For critical ground conditions, the Mil Spec 5700 and ANC-2 criteria were used, with a maximum sink rate of 8ft/sec. Because of the possible natural-frequency interaction between the long, over- hanging, flexible fuselage and the landing gear, a dynamic analysis was also required for the ground conditions. Both an IBM digital and analogue programme were used, with portions of the fuselage consequently being designed to stiffness requirements rather than strength. The programme has eight degrees of freedom including five structural modes, and includes the non-linear undercarriage effects, plus the loss of lift at high angle of attack let-down. In addition to the forward fuselage, sections of the vertical and canard surfaces are also designed to meet stiffness criteria. The requirements for these aerodynamic surfaces are established to preclude high-q flutter. The structural support areas for both the nose and the main gear are critical for ground conditions: spin-up, turning and braked-roll. The remaining portions of the airframe, representing most of the major structural components, are designed for strength, and are critical for various conditions generally in the transonic, high-q flight regime. Fortunately, maximum temperature and maximum load do not occur simultaneously except in the duct area and in the engine compartment. The design of many details was determined by requirements to prevent panel flutter and, in the vicinity of the engine, by the acoustic environment. Elastic Analysis Because of the relatively flexible structural configuration resulting from the large size of the XB-70, the high material allowables used, the low load factors and also the inte- grated complex assembly, an elastic analysis was considered man- datory in order to determine the distributions of internal loads and the structural influence coefficients. Moreover, the multitude of potentially critical conditions which had to be investigated made high-speed computation necessary. The airframe was therefore divided into five sections: the forward fuselage, noteworthy for complex, rapidly changing sections, the windshield and cutouts for hatches and doors; the horizontal canard surface, with t/c ratio less than a table knife; the vertical surfaces, pivoting about a slanted axis; the folding wing tip, almost as large as the B-58 wing and including the complexities of wing-fold interaction; and finally the main integrated-fuselage (engine box) and fixed wing. All com- ponents are tied to this latter section, the elastic analysis of which numbers approximately 1,600 redundants; approximately 2,300 redundants are used for the entire airframe. Steps involved in the elastic analysis are the idealization of the structure, a redundant analysis of each section for the unknown internal loads, and finally the determination of the unknowns on the basis of the minimum energy distribution of internal loads. As an adjunct to the internal loads programme the structural- deflection data, or influence coefficients, are also obtained. These data are used for the aeroelastic load distributions, aerostability calculations and flutter and vibration structural analysis. Because of the importance of accuracy here also, numerous checks were written into the programme, and the calculated results carefully evaluated to ensure validity. Influence coefficients were obtained for all components for a total of 254 points: for tips in various positions, flaps up and down, skins buckled and unbuckled, and for both the e.g. and wing-apex points. These coefficients were required to have the mathematical consistency of real structure; and one set of influence coefficients alone amounted to 1,500,000 numbers, each to eight significant figures. Stress Analysis Following the determination of all internal loads, stress analysis proceeded in the conventional manner. Particular importance attached to the stability equation; "b/t" (spacing divided by thickness) played a major role, for in the pursuit of structural efficiency it led to brazed steel honeycomb sandwich and corrugated spar web configurations. Development of corrugated spars and ribs was likewise essential in obtaining a minimum-weight con- figuration for shear webs and panel-supporting structure. These have been used extensively throughout the airframe, the materials being both titanium and steel. The relationship of corrugation thickness and radius were based upon semi-empirical requirements to prevent both local and general instability. Rounding out the structural analysis in conventional manner were the monocoque construction in the forward fuselage and the many fittings involving H-ll tool steel and AM-355, PH and other steels. This stress analysis involved approximately 10,000 drawings, representing approximately 120,0001b of structure, each designed as closely as possible for an optimum compromise between integrity, stiffness and minimum weight.
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