1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. Performance Constraints
Range ≥ 7730 [NM] [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 ≤ [kn.]
Balanced field length (TO & Land.) ≤ 11,000 [ft]
Cruise altitude ≤ 48,000 [ft]

12. 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

13. 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

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

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

16. 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

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

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

19. 1-Jury TBW Configurations
Minimum Fuel/Emissions
Minimum TOGW

20. 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.

21. 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

22. 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

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

24. 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

25. 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

26. 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

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

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

29. 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

30. Backup Slides

31. 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%

32. 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%

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

34. 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