Vibroacoustic Analysis and Optimization of an Aft-Fuselage Mounted Twin-Engine Aircraft

Abstract

Aft-fuselage mounted twin-engine style aircraft dominate the business jet industry. This aircraft provides a decreased transmission path for structure-borne engine vibrations when compared to wing-mounted aircraft found predominantly in the commercial aircraft industry. Despite the widespread use of aft-fuselage mounted twin-engine aircraft, the literature investigating the vibroacoustic properties of this aircraft is sparse. This work aims to generate a thorough understanding of the structure-borne interior cabin noise for an aft-fuselage mounted twin-engine aircraft such that future aircraft design can mitigate cabin noise. Modifications to key fuselage components were performed, decreasing the modal density in the engine excitation frequency range. A novel model updating technique was introduced to the literature, allowing for refinement of both the experimental and computational data sets. The coordinate orthogonality check method was implemented to eliminate erroneous degrees of freedom from the experimental data set. A multi-objective model updating optimization technique using the pseudo-orthogonality check and the natural frequency difference between the computational model and the refined experimental model as objective functions was used to converge on the most accurate representation of the physical system. A novel substructuring technique was introduced into the literature, verifying the correlation between local and global fuselage modes, allowing for fuselage components to be analyzed in isolation with confidence the results will remain consistent when the component is assembled as part of a complete fuselage. A complete vibroacoustic analysis of the fuselage was performed, investigating the modal parameters, harmonic response to engine excitation, and the interior cabin sound pressure level. Two aircraft designs were considered: a single and a double bulkhead aft-fuselage mounted twin-engine aircraft. A new method of pressurization was introduced that decreased the peak displacement values of the pressurized bulkhead by a factor of two. Finally, the first successful topology optimization of a pre-stiffened aircraft bulkhead was performed, with the refined design reducing stiffener mass and eliminating all natural frequencies within 5% of the critical frequency plus a safety factor of two.