Rakesh K. Kapania

Conceptual Design Studies of a Strut-Braced Wing Transonic Transport

Recent transonic airliner designs have generally converged upon a common cantilever low-wing configuration. It is unlikely that further large strides in performance are possible without a significant departure from the present design paradigm. One such alternative configuration is the strut-braced wing (SBW), which uses a strut for wing-bending load alleviation, allowing increased aspect ratio and reduced wing thickness to increase the lift to drag ratio. The thinner wing has less transonic wave drag, permitting the wing to unsweep for increased areas of natural laminar flow and further structural weight savings. High aerodynamic efficiency translates into smaller, quieter, less expensive engines and less pollution. A multidisciplinary design optimization (MDO) approach is essential to realize the full potential of this synergistic configuration caused by the strong interdependence of structures, aerodynamics, and propulsion. NASA defined a need for a 325-passenger transport capable of flying 7500 n miles at Mach 0.85 for a 2010 service entry date

Design Optimization of a Truss-Braced-Wing Transonic Transport Aircraft

the conventional cantilever configuration. One comparison produces a reduction of 45% in the fuel consumption while decreasing the minimum takeoff gross weight by 15%. For a second comparison, the fuel weight is reduced by 33% with a decreased minimum takeoff gross weight of 19%. Very attractive vehicle performance can be achieved without the necessity of decreasing cruise Mach number. The results also indicate that a truss-braced wing has a greater potential for improved aerodynamic performance than other innovative aircraft configurations. Further studieswillconsidertheinclusionofmorecomplextrusstopologiesandotherinnovativetechnologiesthatarejudged to be synergistic with truss-braced-wing configurations.

Multidisciplinary Design Optimization of a Transonic Commercial Transport with Strut-Braced Wing

The multidisciplinary design optimization of a strut-braced wing (SBW) aircraft and its benee ts relative to a conventional cantilever wing cone guration are presented. The multidisciplinary design team is divided into aerodynamics, structures, aeroelasticity, and the synthesis of the various disciplines. The aerodynamic analysis uses simple models for induced drag, wave drag, parasite drag, and interference drag. The interference drag model is based on detailed computational e uid dynamics analyses of various wing ‐strut intersections. The wing structural weight is calculated using a newly developed wing bending material weight routine that accounts for the special nature of SBWs. The other components of the aircraft weight are calculated using a combination of NASA’ s e ight optimization system and Lockheed Martin aeronautical systems formulas. The SBW and cantilever wing cone gurations are optimized using design optimization tools (DOT) software. Ofe ine NASTRAN aeroelastic analysis results indicate that the e utter speed is higher than the design requirement. The minimum take-off gross weight SBW aircraft showed a 9.3% advantage over the corresponding cantilever aircraft design. The minimum fuel weight SBW aircraft showed a 12.2% fuel weight advantage over a similar cantilever aircraft design.

Structural Wing Sizing for Multidisciplinary Design Optimization of a Strut-Braced Wing

A structural and aeroelastic model for wing sizing and weight calculation of a strut-braced wing is described. The wing weight is calculated using a newly developed analysis accounting for the special nature of strut-braced wings. A specially developed aeroelastic model enables one to consider wing flexibility and spanwise redistribution of the aerodynamic loads during in-flight maneuvers. The structural model uses a hexagonal wing-box featuring skin panels, stringers, and spar caps, whereas the aerodynamics part employs a linearized transonic vortex lattice method. Thus, the wing weight may be calculated from the rigid or flexible wing spanload. The calculations reveal the significant influence of the strut on the bending material weight of the wing. The strut enables one to design a wing featuring thin airfoils without weight penalty. It also influences the spanwise redistribution of the aerodynamic loads and the resulting deformations. Increased weight savings are possible by iterative resizing of the wing structure using the actual design loads. As an advantage over the cantilever wing, the twist moment caused by the strut force results in increased load alleviation, leading to further structural weight savings.

Transport Weight Reduction through MDO: The Strut-Braced Wing Transonic Transport

Multidisciplinary Design Optimization (MDO) has been used to investigate the use of anew concept for a transonic transport, the strut-braced wing. The incorporation of a strutinto more traditional transonic transport concepts required the application of computationaldesign techniques that had been developed at Virginia Tech over the previous decade.Formalized MDO methods were required to reveal the benefits of the tightly coupledinteraction between the wing structural weight and the aerodynamic performance. Toperform this study, a suite of approximate analysis tools was assembled into a complete,conceptual-level MDO code. A typical mission of the Boeing 777-200IGW was chosen as thedesign mission profile. Several single-strut configurations were optimized for minimumtakeoff gross weight, with the best single-strut configuration showing a nearly20% reductionin takeoff gross weight, a 29% reduction in fuel weight, a 28% increase in the lift-to-dragratio, and a 41% increase in seat-miles per gallon relative to a comparable cantileverconfiguration. The use of aeroelastic tailoring in the design illustrated ways to obtain furtherbenefits. The paper synthesizes the results of the five-year effort, and concludes with adiscussion of the effects various constraints have on the design, and lessons learned oncomputational design during the project.

Progress Towards Multidisciplinary Design Optimization of Truss Braced Wing Aircraft with Flutter Constraints

on multidisciplinary design optimization (MDO) of truss-braced wing airplanes. The primary focus has been to include utter constraints for structural sizing of the wing. The structural sizing uses a gradient-based optimization procedure along with an analytically calculated response function sensitivity with respect to the thickness design variables. It is shown that using the updated routine leads to lower structural mass in comparison with the fully-stressed structural design procedure used in the previous MDO studies. The primary reasons for the lower mass is that inertial weight relief due to secondary structure is now included in the sizing process, and the buckling analysis is now based on a linearized eigenvalue problem, as opposed to a simple beam Euler buckling criteria used for the previous study which was signicantly conservative. However, the results show that for a wing with lower mass the utter constraint becomes active for both strut-braced and truss-braced wing congurations. Hence, it is important to include those in the MDO studies to maintain feasibility of designs. Two challenges encountered during the process of including structural optimization with the utter constraint within the system-level MDO architecture are discussed along with the strategies devised to overcome them: convergence of structural optimization and the resulting numerical noise. A response surface methodology is used to integrate the structural optimization and system-level MDO and some initial results for the design of a truss-braced wing transonic transport airplane for minimum fuel consumption and emissions are presented.

Multidisciplinary Design Optimization of a Truss Braced Wing Aircraft

†‡ § ¶ This paper focuses on establishing the po tential benefits of a truss-braced wing aircraft configuration as compared to the current generation cantilever wing aircraft, as well as a strut-braced wing. Mu ltidisciplinary Design Optimization is used to design aircraft with three diffe rent wing configurations with increasing complexity of structural topology: cantilever, 1-member truss (strut) and 3-member truss. Three different objective functions are studied: minimum takeoff gross weight, minimum fuel consumption and emissions, and maximum lift to drag ratio. The results show the significant advantage of strut and simple truss configurations over the conventional cantilever configuration. They also indicate that a truss-braced wing has a greater potential for improved aerodynamic performance than has been reported for other innovative aircraft configurations. In addition, the comparison between the various design objective functions shed light on their effect on the resulting configurat ions. Some initial aeroelastic analyses are then presented and discussed. Further studies will consider the inclusion of more complex truss topologies and other innovative technologies which are judged to be synergistic with truss-braced wing configurations.

PASSIVE LOAD ALLEVIATION IN THE DESIGN OF A STRUT-BRACED WING TRANSONIC TRANSPORT AIRCRAFT

This paper describes the multidisciplinary design optimization (MDO) of a transonic strutbraced wing aircraft. The optimization considers aeroelastic deformations of the wing and passive load alleviation. The calculations reveal that the strut twist moment provides substantial load alleviation and significant reductions in structural wing weight. To benefit from the potential of appl ying passive load alleviation during preliminary aircraft design, a flexible wing sizing module has been linked to the MDO design tool to optimize the design of three different strut-braced wing aircraft configurations featuring fuselage mounted engines, underwing mounted engines, and wingtip mounted engines.

EFFECT OF COMPRESSIVE FORCE ON FLUTTER SPEED OF A STRUT-BRACED WING

Previous investigations on a strut-braced wing analysis revealed that aeroelasticity plays an important role on the strut-braced wing design. The investigations showed that the location of the strut support on the wing affects the wing deformation, aerodynamic load redistribution and flutter speed. Another study on the buckling analysis of the strut-braced wing indicated also that the wing stiffness may have a significantly lower stiffness due to the compressive force due to the strut during a positive flight load maneuver. In the present work, a further investigation on the effect of compressive force on flutter and divergence speeds of the strut-braced wing is presented. To reduce the computational time, an efficient non-uniform beam finite element model is used for the wing structure, and a modified doublet lattice method is used for the unsteady aerodynamic calculations. Variation of several parameters, including strut location along the wing spanwise and chordwise directions, were investigated. To calculate the compressive force, a trim analysis was performed for each variation of these parameters. Comparison between the flutter boundary and flight envelope of the present strut braced wing design indicates that the influence of the compressive force on flutter speed is significant when the strut is placed near the wing tip.