Multidisciplinary Design and Optimization of the Silent Aircraft A. Diedrich, J. Hileman, D. Tan, K. Willcox * , Z. Spakovszky Department of Aeronautics and Astronautics Massachusetts Institute of Technology A “silent † aircraft” is defined to be an aircraft that, in a typical urban area, is inaudi- ble outside of the airport boundary. This paper describes the creation, implementation, and use of an integrated design tool to predict and optimize the performance and costs associated with producing a novel, commercial aircraft design with a step change in noise reduction. The silent aircraft uses a highly integrated configuration where a quiet propul- sion system is embedded in a Blended-Wing-Body type airframe. This allows the shielding of forward radiated engine noise and the extensive use of acoustic liners. Multidisciplinary aircraft design models, which use a combination of simple physics and empirical relations, are adapted for the silent aircraft configuration. These models are used in conjunction with a multidisciplinary planform optimization capability. The resulting silent aircraft design is assessed in terms of performance and acoustic signature. Significant component noise reductions can be achieved with a design that has a fuel burn competitive with next-generation commercial aircraft. Barriers to achieving the aggressive noise goal of the Silent Aircraft Initiative and the associated required technology developments are described. Introduction Designing for noise is a highly integrated problem that must take into account engine and airframe de- sign, aircraft operation, airline economics, and noise generation. This research targets the design of a novel aircraft with a radical reduction in noise – an aircraft that, in a typical urban area, is inaudible outside of the airport boundary. The aircraft design and assess- ment framework described in this paper places noise as the primary design goal, by bringing together mul- tidisciplinary design tools, noise assessment tools, and innovative concepts such as a more closely integrated airframe and propulsion system. Multidisciplinary design optimization (MDO) pro- vides a formal framework which simultaneously consid- ers the effects of different disciplines and their interac- tions. Exploration of the design space via optimization algorithms allows high-level design decisions and the quantitative assessment of trade-offs. Studies using MDO to aid in conceptual aircraft design have been made in numerous areas with considerable success. 1–4 Venter and Sobieszczanski-Sobieski showed that tra- ditional metrics like aircraft range can be extended through variations that might otherwise seem only loosely coupled. 5 In their study of transport wing opti- mization, optimal range was achieved not only through overall wing shape changes, as would be expected, but also by a choice of construction technique whereby the internal spar was removed while the use of skin * Corresponding author. 77 Massachusetts Ave. 37-447, Cambridge, MA 02139. Email: kwillcox@mit.edu † “Silent” in this context does not refer to absence of acoustic sources. stiffeners was incorporated. This exploitation of the interplay between disciplines (here, aerodynamics and structures) is typical of MDO in conceptual design. Another objective that is often minimized in air- craft MDO is maximum takeoff weight (MTOW). By minimizing MTOW, designers hope to produce an inherently inexpensive aircraft by producing a small aircraft, while incorporating the fuel weight into the objective as part of the MTOW, so that both the acquisition costs and the operational costs are low. This technique produces particularly impressive re- sults when the coupling of the disciplines in an air- frame are strong, as in Boeing’s Blended-Wing-Body (BWB) concept. 6, 7 Wakayama demonstrated that MDO codes can be used to both balance and reduce the MTOW of an aircraft simultaneously by exploiting geometric changes to the airframe. 4 More recently, environmental considerations have been included in MDO-based design approaches. An- toine et al. applied multidisciplinary optimization to determine the extent to which noise can be traded against other performance measures. 8–10 This work showed that, of the different figures of merit that were optimized (takeoff weight, operating cost, noise, ni- trous oxide emissions, and fuel burn), optimization for noise required the greatest concessions in the other potential objectives. For conventional tube-and-wing aircraft with 2020 technology levels, in order to achieve a cumulative 15 EPNdB decrease in total certification noise, operating costs rose 26%, MTOW rose 27%, fuel load rose 17%, and NO x emissions rose 33% relative to the aircraft designed for minimum operating costs. The extremely high cost to reduce noise is indica- 1 of 12 American Institute of Aeronautics and Astronautics Paper 2006-1323 44th AIAA Aerospace Sciences Meeting and Exhibit 9 - 12 January 2006, Reno, Nevada AIAA 2006-1323 Copyright © 2006 by A. Diedrich, J. Hileman, D. Tan, K. Willcox, Z. Spakovszky. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.