The Role of Constraints in the MDO of a Cantilever and Strut-B raced Wing Transonic Commercial Transport Aircraft

Abstract

This study examines the role of different design constraints applied to the multidisciplinary design optimization of a strut-braced wing (SBW) transonic passenger aircraft. Four different configurations are examined: the reference cantilever wing aircraft, a fuselage mounted engine SBW, a wing mounted engine SBW, and a wingtip mounted engine SBW. The mission profile was for 325-passengers, Mach 0.85 and a 7500 nautical mile range with a 500 nautical mile reserve. The sensitivity of the designs with respect to the individual design constraints was calculated using Lagrange multipliers. A design space visualization technique was also used to gain insight into the role of the different constraints in determining the design configuration. This design visualization technique uses a classic ‘thumbprint’ plot to represent the design space. As expected, all the designs are very sensitive to the range constraint. The designs are also sensitive to the field performance constraints. The design visualization revealed that the second segment climb gradient constraint was a limiting factor in all the designs. It was also found that the wing mounted engines SBW and tip mounted engines SBW designs are more constrained than the cantilever wing optimum and fuselage mounted engines SBW designs. INTRODUCTION Transonic passenger transport aircraft designs over the past 50 years have utilized the same general configuration, the cantilever low wing concept. Keeping the general layout the same, advances in this concept have relied on advances in individual technologies, such as better engines, airfoil designs, high lift devices and control system alternatives. It is quite unlikely that major improvements in performance will occur if new design configurations are not considered in the transonic passenger transport aircraft industry. One such design configuration is the strut-braced wing design concept. Although this design configuration is common among small general aviation aircraft, it is rare in the large passenger transport arena. The idea of using a truss-braced wing configuration for a transonic transport originated with Werner Phenninger [1] at Northrop in the early 1950s. The strut-braced wing (SBW) concept can be considered a subset of the trussbraced wing configuration. Other SBW aircraft investigations followed Phenninger’s work, notably by Kulfan et al. [2] and Park [3] in 1978. Turriziani et al. [4] considered the advantages of the strut-braced wing concept on a subsonic business jet with an aspect ratio of 25. He found that the strut-braced wing concept achieved approximately 20% in fuel savings compared to a similar cantilevered subsonic business jet.