Virginia Tech Truss-Braced Wing Studies

Multidisciplinary Analysis and Design Center for Advanced Vehicles 1

Virginia Tech Truss-Braced Wing Studies

1

J.A. Schetz and R.K. Kapaniaand 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 Arlingtonand

Vivek Mukhopadhyay (NASA) and B. Grossman (NIA)


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 cruiseAll-metal airplanesGE90 type enginesFocus on truss benefits


Original Pfenninger Vision

Large span wing to reduce induced drag

Thin wing at root for laminar flow

Fuselage profileFor low wetted area

Optimized truss support to reduce wing weight-

Reduce interference drag

Wing tip for vortex control

Pfenninger, W., “Laminar Flow Control Laminarization,” AGARD Report 654, “Special Course on Concepts for Drag Reduction” , 1977


Main Mission (“777 ER”)

• 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

11,000 FTT/O Field Length

7730 NMI Range

Climb

Mach 0.85 Cruise

140 KnotsApproachSpeed

Mach 0.85

350 NMIReserve Range

11,000 FT LDGField Length


MDO Design Environment

Weight Estimation

TOGW Convergence

Optimizer

Performance,Cost Function,

Constraints

Parametric Geometry

Baseline Design

Propulsion

Aerodynamics

Structural Optimization

Fuel Loading

Design Environment N^2 Diagram ModelCenter Environment

Design Environment Block Diagram


Propulsion Model

• Dimensions and weights– Simplified VT model similar to Mattingly

Elements of Propulsion: Gas Turbines And Rockets• Performance:

1. Simplified model2.NASA fixed deck

• “GE-90-like”3.NPSS or Reduced-order NPSS from GT

0.55

0.555

0.56

0.565

0.57

0.575

0.58

20 25 30 35 40 45 50Altitude[kft]

TSFC

[lbm

/hr/l

b]

Fixed Deck, 100% ThrottleSimplified, Tmax=75 klb

M=0.85


Aerodynamic Model• Calculates:

– 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

FlutterEnvelope

Mach

Altit

ude

(x10

3 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

xt2

BC

AD

Wing Structural Weight Estimation

Evaluate Response Surface

Design Variables

Wing Weight

Response Surface Model

Offline RSM generationLatin-Hypercube SamplingKriging Surrogate Model


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 MembersLarger tip deflectionLarger 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 StudyB777: 183

-33%B777: 20

+80%

B777: 512-8%

B777: 10

+160%

B777: 71+11%

B777: 106

+70%


Minimum TOGW Design Study

Active Constraints

range, balanced field length, Vapproach

range range, initial cruise rate of climb, Vapproach, clmax

range, initial cruise rate of climb, fuel

range


Minimum TOGW Design StudyB777: 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: 71B777: 106


1-Jury TBW Configurations

Minimum Fuel/Emissions Minimum TOGW


MDO Configurations: Drag BreakdownMinimum 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.


SB

W

2-Ju

ryVc

FlutterEnvelope

1-Ju

ry

3-Ju

ry

• 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

Mach

Altit

ude

(x10

3 ft)

Minimum FuelMinimum TOGW

SB

W

2-Ju

ry

Vc

FlutterEnvelope

1-Ju

ry

3-Ju

ry

Mach

Altit

ude

(x10

3 ft)

Flutter Boundary of TBW Airplane Designs

Flutter Mach numbers for 100% fuel at 2.5g pull-up maneuver


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 StudyB777: 183

-33% B777: 20

+80%

B777: 4340+18%

B777: 512-8%

B777: 10

+160%

B777: 71

+11%

B777: 106

+70%


Minimum TOGW Design StudyB777: 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: 20

B777: 4340

B777: 512

B777: 10

B777: 71

B777: 106


Minimum Fuel/Emissions Design: 1-Jury TBW Buckling and Flutter Mode Shapes

Buckling Mode: 2.5g pull up Buckling factor = 1.8

Buckling Mode: 2g taxi-bump Buckling factor = 1.0

Flutter Mode: 2.5g, 100% fuel, Sea-levelVf = 300 fps, 1.7 Hz, reduced freq. = 0.32

• 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

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Virginia Tech Truss-Braced Wing Studies