Structural Concepts Study of Non-circular Fuselage Configurations

Vivek Mukhopadhyay*

Systems Analysis Branch,
Aeronautical Systems Analysis Division NASA Langley Research Center, Hampton, VA

ABSTRACT

A preliminary study of structural concepts for noncircular fuselage configurations is presented. For an unconventional flying-wing type aircraft, in which the fuselage is inside the wing, multiple fuselage bays with non-circular sections need to be considered. In a conventional circular fuselage section, internal pressure is carried efficiently by a thin skin via hoop tension. If the section is non-circular, internal pressure loads also induce large bending stresses. The structure must also withstand additional bending and compression loads from aerodynamic and gravitational forces. Flat and vaulted shell structural configurations for such an unconventional, non-circular pressurized fuselage of a large flying-wing were studied. A deep honeycomb sandwich-shell and a ribbed double-wall shell construction were considered. Combinations of these structural concepts were analyzed using both analytical and simple finite element models of isolated sections for a comparative conceptual study. Weight, stress, and deflection results were compared to identify a suitable configuration for detailed analyses. The flat sandwichshell concept was found preferable to the vaulted shell concept due to its superior buckling stiffness. Vaulted double-skin ribbed shell configurations were found to be superior due to their weight savings, load diffusion, and fail-safe features. The vaulted double-skin ribbed shell structure concept was also analyzed for an integrated wing-fuselage finite element model. Additional problem areas such as wing-fuselage junction and pressure-bearing spar were identified.

1. Introduction

Fuselage configuration and structural design of a very large transport aircraft is a major task. For such a

design, extensive experience and database exists1 for conventional aircraft. For an unconventional aircraft such as a flying-wing, in which the fuselage is part of the wing, partially circular or non-circular sections need to be considered. Efficient structural design of such an unconventional non-circular pressurized fuselage imposes a special challenge since extensive experience and databases do not exist. Moreover, during the conceptual design stage, loading conditions, material properties, detailed configuration and structural --------------------------------------------------------------- * Associate Fellow, AIAA

dimensions of the fuselage are usually not known. Hence, in this preliminary structural analysis stage, it is often convenient to assume a representative configuration with appropriate dimension and undertake a comparative study using simple finite

element modeling and analysis2 for a typical set of loads and material properties. Although this procedure may initially lead to a conservative design, it often provides guidance towards a better configuration through comparative weight and stress analysis, and could be used to identify new concepts which may impose less weight penalty to this unconventional construction in an otherwise efficient flying wing with aerodynamic advantages.

First, basic design issues of partially circular and non circular load-bearing pressure vessels were discussed. Based on their relative advantages and disadvantages, two non-circular pressure vessel concepts were selected for analysis: a) a flat sandwich shell concept and b) a vaulted sandwich shell concept, both with a honeycomb core. Simple structural finite element models (FEM) were used for displacement and stress analysis, followed by a limited sizing study. Next, an alternate concept was investigated for weight reduction. In this concept, the honeycomb was replaced by a double-skin shell with vertical spanwise and chordwise rib stiffeners. Both the flat and vaulted double skin shell construction were analyzed and compared. Finally, one integrated cantilever wing-fuselage configuration using the vaulted ribbed shell concept was analyzed.

2. Basic Issues

For a very large conventional subsonic transport, a scaled-up fuselage with a double-deck circular cross section is often considered as shown in Fig. 1, where the shaded area is the pressurized section. When the cross section of a fuselage is circular, the internal pressure is resisted efficiently by the thin skin via hoop tension. The hoop stress, based on force equilibrium, is given by s=pr.D/2t, where pr is the internal pressure, D is the diameter and t is the skin thickness. Any buckling instability is prevented by stiffening the thin skin with frames and stringers. The fuselage end pressure and concentrated loads from landing gear, engine and wing mount are carried by heavy bulkheads.