Virginia Tech Truss-Braced Wing Studies J.A. Schetz and R.K. Kapania and TBW Group at VT Multidisciplinary Analysis and Design Center for Advanced Vehicles, Virginia Polytechnic Institute and State University and Collaborators at Georgia Tech, Univ. Florida & UT Arlington and Vivek Mukhopadhyay (NASA) and B. Grossman (NIA) 1

Goal of the Research Use Multidisciplinary Design Optimization (MDO) to explore the potential for large improvements in long- and medium-range transonic, transport aircraft performance by employing truss-braced wings (TBW) combined with other synergistic advanced technologies. Ground Rules for VT Studies: Mach 0.85 cruise All-metal airplanes GE90 type engines Focus on truss benefits

Original Pfenninger Vision Fuselage profile For low wetted area Wing tip for vortex control Large span wing to reduce induced drag Optimized truss support to reduce wing weight- Reduce interference drag Thin wing at root for laminar flow Pfenninger, W., “Laminar Flow Control Laminarization,” AGARD Report 654, “Special Course on Concepts for Drag Reduction” , 1977

Main Mission (“777 ER”) 11,0 0 F T /O Fi eld L e ngth 773 0 N MI R ange C l i mb Ma c h 0.85 Cr u se 140 K not s Ap p ro a ch Sp ed 350 NM I er v L DG F ie d Lengt h Use MDO to design 305-passenger, 7730 nmi range, Mach 0.85 transport aircraft of Cantilever, Strut-Braced-Wing (SBW), and Truss-Braced Wing (TBW) configurations

MDO Design Environment Design Environment N^2 Diagram ModelCenter Environment Design Environment Block Diagram Baseline Design Parametric Geometry Propulsion Aerodynamics Optimizer Fuel Loading TOGW Convergence Structural Optimization Performance, Cost Function, Constraints Weight Estimation

Propulsion Model Dimensions and weights Performance: Simplified VT model similar to Mattingly Elements of Propulsion: Gas Turbines And Rockets Performance: 1. Simplified model 2.NASA fixed deck “GE-90-like” 3.NPSS or Reduced-order NPSS from GT M=0.85

Aerodynamic Model Calculates: Drag breakdown models: Aerodynamic drag Aerodynamic loading (input for structural design module) Drag breakdown models: Induced drag based on Trefftz plane model Friction/Profile drag based on semi-empirical methods Wave drag based on the Korn Equation Interference drag based on literature and response surfaces from offline CFD

Structural Design Requirements Total of 17 cases: 2.5 g 100% / 50% fuel -1 g 100% / 50% fuel 2 g taxi bump 12 gust cases, 50% 100% fuel, various altitudes Motivated by low wing loading MDO designs Simplified discrete gust modeling Using gust alleviation factor Designs evaluated for flutter performance post MDO Flutter Envelope Mach Altitude (x103 ft) Vc

Structural Design Methodology Estimation of load carrying structural weight Bending and shear material Structural optimization Finite element analysis Stress, displacement and buckling constraints Flutter constraints with geometric stiffness influence Structural response surface model used in MDO t1=(t/c) ·c /2 t0 cst z x t2 B C A D Offline RSM generation Latin-Hypercube Sampling Kriging Surrogate Model Design Variables Wing Structural Weight Estimation Evaluate Response Surface Response Surface Model Wing Weight

TBW Weight Estimation Detailed physics based wing system structural weight estimation In-house tool optimizes for bending and shear material weight Other components: FLOPS: secondary weight Folding wing penalty Fuselage pressurization penalty

Performance Constraints Range ≥ 7730 [NM] + 350 [NM] (reserve) Initial Cruise ROC ≥ 500 [ft/min] Max. cl (2-D) ≤ 0.8 Available fuel volume ≥ required fuel volume 2nd segment climb gradient (TO) ≥ 2.4% (FAR) Missed approach climb gradient ≥ 2.1% (FAR) Approach velocity ≤ 132.5 [kn.] Balanced field length (TO & Land.) ≤ 11,000 [ft] Cruise altitude ≤ 48,000 [ft]

Truss Topology Optimization Study Triangular Loading Two-dimensional analysis Buckling not included Single load case 15% Volume Fraction Fewer Members Larger tip deflection Larger strain energy All designs with same volume fraction have same mass

Muldisciplinary Design Optimization Study Cost functions: Minimum fuel/emissions and TOGW Configurations Cantilever Strut-Braced wing (SBW) Single Jury TBW 2-Jury TBW 3-Jury TBW Aggressive laminar flow Aggressive junction fairing Fuselage riblets

Minimum Fuel/Emissions Design Study Active Constraints range, deflection range, fuel range,clmax range,clmax, Vapproach range,clmax, Vapproach

Minimum Fuel/Emissions Design Study B777: 183 -33% B777: 20 +80% B777: 512 -8% B777: 10 +160% B777: 71 +11% B777: 106 +70%

Minimum TOGW Design Study Active Constraints range, initial cruise rate of climb, fuel range, balanced field length, Vapproach range range, initial cruise rate of climb, Vapproach, clmax range

Minimum TOGW Design Study B777: 183 -26% B777: 20 +50% B777: 512 -10% B777: 10 +80% B777: 71 -17% B777: 106 +40%

Comparison of Designs: Min. Fuel and Min. TOGW B777: 183 B777: 20 B777: 512 B777: 10 B777: 71 B777: 106

1-Jury TBW Configurations Minimum Fuel/Emissions Minimum TOGW

MDO Configurations: Drag Breakdown Minimum Fuel Minimum TOGW All minimum fuel configurations cruise altitude is between 46,000 to 48,000 ft Increasing number of members reduces induced drag and increases profile drag Additional surface area from more members reduces system benefit Fuselage drag reduction is needed.

Flutter Boundary of TBW Airplane Designs Minimum TOGW Minimum Fuel 3-Jury SBW 3-Jury 2-Jury 1-Jury SBW 2-Jury 1-Jury Vc Altitude (x103 ft) Altitude (x103 ft) Vc Flutter Envelope Flutter Envelope Mach Mach Flutter Mach numbers for 100% fuel at 2.5g pull-up maneuver Flutter margin reduces with increasing number of members due to higher span Passive and active control measures under investigation Passive methods Ballast mass TBW geometry modification: parametric study Aeroservoelasticity

Flutter Ballast Mass Study: Ballast Mass is 2% of Wing Mass SBW Flutter speed: VF=588 fps, MF=0.526; 600 lb Ballast mass Best improvement of 1.1% with mass at 36% span, 98% chord Very low sensitivity to ballast mass location

Flutter Ballast Mass Study: Ballast Mass from 2% to 8% Very low sensitivity to size and location of ballast mass

Truss-Braced Wing Geometry Parametric Study Influence of selected geometric parameters on aeroelastic performance of TBW Strut-sweep (ΛS), Wing-strut span intersection (η) SBW, TBW 1-jury, TBW 2-jury, TBW 3-jury Same cross-sectional dimensions for each configuration Chord, t/c ratio Values correspond to TBW 1-jury configuration (from MDO) Each configuration sized for same requirements

Comparison of TBW Configurations (η=55%, b/2=175 ft, ΛW =10°) Addition of jury strut members Reduces wing weight Largest reduction (21%) from SBW to TBW 1-jury TBW configurations have similar flutter boundary Low sensitivity to strut-sweep TBW 1-jury and 2-jury offer 19% increment in flutter boundary with 14% higher weight TBW 3-jury offers 49% increment in flutter boundary with 20% higher weight

Comparison of TBW Configurations (η=70%, b/2=175 ft, ΛW =10°) Addition of jury strut members Reduces wing weight Largest reduction (14%) from SBW to TBW 1-jury TBW configurations show strong sensitivity to strut-sweep Significant flutter boundary increment from SBW TBW 1-jury and 2-jury have similar weight and flutter boundary 33% increment in flutter boundary with 20% higher weight TBW 3-jury offers 75% increment in flutter boundary with 8% higher weight

SBW Flutter modes (sea-level, b/2=175 ft, ΛW =10°)

TBW 3-jury Flutter modes (sea-level, b/2=175 ft, ΛW =10°)

Conclusions and Future Work TBW airplane configurations offer significant performance benefits Higher span increases weight and reduces flutter speed Outboard wing-strut intersection location Increases wing weight in present study due to active buckling Increases flutter speed for TBW configurations Larger difference in wing- & strut-sweep could be used to help flutter performance Flutter speed sensitivity to strut-sweep increases with spanwise intersection location Airplane MDO would show multidisciplinary influence Large benefit in wing weight reduction and flutter boundary increment from SBW and TBW configurations TBW 3-jury offers highest benefit in flutter performance Ongoing efforts and future work Active control techniques Body-freedom flutter and nonlinear aeroelasticity

Backup Slides

Minimum Fuel/Emissions Design Study B777: 183 -33% B777: 20 +80% B777: 4340 +18% B777: 512 -8% B777: 10 +160% B777: 106 +70% B777: 71 +11%

Minimum TOGW Design Study B777: 183 -26% B777: 20 +80% B777: 4340 +12% B777: 512 -10% B777: 10 +80% B777: 71 -17% B777: 106 +40%

Comparison of Designs: Min. Fuel and Min. TOGW B777: 183 B777: 4340 B777: 20 B777: 512 B777: 10 B777: 106 B777: 71

Minimum Fuel/Emissions Design: 1-Jury TBW Buckling and Flutter Mode Shapes Buckling Mode: 2g taxi-bump Buckling factor = 1.0 Buckling Mode: 2.5g pull up Buckling factor = 1.8 Global wing buckling mode for 2.5g pull-up Strut buckling mode for 2g taxi-bump Flutter mode: combination of 3 modes: two bending + torsion Flutter Mode: 2.5g, 100% fuel, Sea-level Vf = 300 fps, 1.7 Hz, reduced freq. = 0.32