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Analytical Investigation of Medium STOL Transport Structural Concepts: Volume II - Isogrid Fuselage Study

Report Number: AFFDL TR 74-109 Volume II
Author: Adkisson, R. E.
Author: Deneff, G. V.
Corporate Author: Douglas Aircraft Company, McDonnell Douglas Corporation
Corporate Report Number: MDC-J6625A
Laboratory: Air Force Flight Dynamics Laboratory
Date of Publication: 1974-08
Pages: 123
Contract: F33615-73-C-3049
Project: 1368
Task: 0212
AD Number: ADA006157
Original AD Number: ADA006157

Results of a study program to evaluate application of the isogrid structure concept to a medium STOL transport aircraft are presented. Isogrid is an integrally stiffened panel concept incorporating a triangular arrangement of the stiffening material which has been used successfully on space vehicle structure. The fuselage shell structure of the projected C-15 production airplane is used as the study (and baseline) component. The isogrid concept is evaluated for structural integrity, weight, manufacturing methods, applicability of NDI methods, production and life cycle costs, and aircraft performance payoffs. Structural integrity analyses of both the isogrid and the baseline concepts are based on a common set of requirements for ultimate strength, fatigue, and damage tolerance. Because of generally lower stress levels and a general absence of rivet and bolt holes in basic isogrid structure, fatigue and damage tolerance are of reduced criticality relative to baseline structure.

Aluminum materials (7475 plate selected) are the best choice for minimum production cost and weight for isogrid. The isogrid concept, as applied to the C-15 fuselage, however, is shown to be penalized in cost and weight by the following adverse configuration characteristics: (1) high wing and fuselage mounted landing gear which require heavy supporting frames; (2) significant areas of non-circular fuselage section which also require additional frames; (3) significant fuselage areas of double contour shape which result in increased forming costs; and, (4) low panel loadings which result in minimum gage machining constraints. The isogrid fuselage shell is approximately six percent heavier and 65 percent costlier to produce on a participating structure basis. Cost estimates are based on a 'bottom-up' detailed analysis approach for labor and materials. Applications of isogrid to other structural components on an engineering judgment basis are also considered.

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