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)
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Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Goal of the Research
• Use Multidisciplinary Design Optimization (MDO) to
explore the potential for large improvements in longand 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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Original Pfenninger Vision
Fuselage profile
For low wetted area
Wing tip for
vortex control
Large span wing to
reduce induced drag
Thin wing at root
for laminar flow
Optimized truss support to
reduce wing weightReduce interference drag
Pfenninger, W., “Laminar Flow Control Laminarization,” AGARD Report 654, “Special Course on Concepts for Drag
Reduction” , 1977
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Main Mission (“777 ER”)
Mach 0.85 Cruise
Mach 0.85
140 Knots
Approach
Speed
Climb
11,000 FT
T/O Field Length
7730 NMI Range
350 NMI
11,000 FT LDG
Reserve Range
Field Length
• 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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
Performance,
Cost Function,
Constraints
Structural
Optimization
Weight
Estimation
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Propulsion Model
• Dimensions and weights
– Simplified VT model similar to Mattingly
Elements of Propulsion: Gas Turbines And Rockets
• Performance:
0.58
Fixed Deck, 100% Throttle
Simplified, Tmax=75 klb
0.575
TSFC[lbm/hr/lb]
1. Simplified model
2.NASA fixed deck
• “GE-90-like”
3.NPSS or Reduced-order
NPSS from GT
0.57
M=0.85
0.565
0.56
0.555
0.55
20
25
30
35
40
Altitude[kft]
45
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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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
40
35
Altitude (x103 ft)
•
•
•
•
Vc
30
25
20
Flutter
Envelope
15
10
5
0
0
0.5
1
1.5
Mach
• Designs evaluated for flutter performance
post MDO
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Structural Design Methodology
• Estimation of load carrying
structural weight
z
– Bending and shear material
– Structural optimization
t0
t1=(t/c) ·c
/2 B
t2
A
D
C
cst
• Finite element analysis
– Stress, displacement and buckling
constraints
– Flutter constraints with geometric
stiffness influence
• Structural response surface
model used in MDO
x
Offline RSM generation
Latin-Hypercube Sampling
Kriging Surrogate Model
Design Variables
Wing Structural
Weight Estimation
Evaluate Response
Surface
Response
Surface Model
Wing Weight
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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]
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Continuum results – triangular loading, structural length control (SHPM), TVF=15%
Truss Topology Optimization Study
a)
•
•
•
•
b)
Triangular Loading
Two-dimensional analysis
Buckling not included
Single load case
15% Volume Fraction
c)
d)
Fewer Members
Larger tip deflection
Larger strain energy
e)
f)
g)
• All designs with same volume fraction have same mass
Figure 4. Solid projection continuum results for triangular load, TVF=15%
Table 1. Solid Projection – Response Magnitudes (Triangular load: 250 kips * -2.5g * 1.5sf)
Min radius (ft)
rmin
0.50
Fig
(a)
Strain Energy
SE*
2877
Tip Deflection
² tip*
14.2
Normalized
Strain Energy
1.000
Normalized
Tip Deflection
1.000
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Minimum Fuel/Emissions Design Study
Active Constraints
range, deflection
range,clmax
range,clmax,
Vapproach
range, fuel
range,clmax,
Vapproach
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Minimum Fuel/Emissions Design Study
B777: 183
40
170
150
130
10
90
0
Cant. SBW 1-Jury 2-jury 3-jury
Cant. SBW 1-Jury 2-Jury 3-Jury
40
550
30
B777: 512
AR
TOGW, klb
600
-8%
450
20
B777: 10
0
Cant. SBW 1-Jury 2-jury 3-jury
Cant. SBW 1-Jury 2-Jury 3-Jury
140
200
120
100
+11%
B777: 71
60
Half Span, ft
Wing Weight, klb
+160%
10
400
80
B777: 20
20
110
500
+80%
30
-33%
L/D
Fuel Weight, klb
190
+70%
150
B777: 106
100
50
0
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant. SBW 1-Jury 2-Jury 3-Jury
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Minimum TOGW Design Study
Active Constraints
range, balanced
field length,
Vapproach
range
range, initial
cruise rate of
climb, fuel
range, initial
cruise rate of
climb, Vapproach,
clmax
range
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Minimum TOGW Design Study
B777: 183
150
40
-26%
100
10
0
0
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant. SBW 1-Jury 2-Jury 3-Jury
20
-10%
AR
TOGW, klb
25
B777: 512
500
480
460
420
0
Cant. SBW 1-Jury 2-Jury 3-Jury
200
B777: 71
Half Span, ft
Wing Weight, klb
Cant. SBW 1-Jury 2-Jury 3-Jury
60
40
20
0
B777: 10
10
5
-17%
+80%
15
440
80
B777: 20
20
50
520
+50%
30
L/D
Fuel Weight, klb
200
150
+40%
B777: 106
100
50
0
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant. SBW 1-Jury 2-Jury 3-Jury
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Comparison of Designs: Min. Fuel and Min. TOGW
B777: 183
170
150
L/D
Fuel Weight, klb
190
130
110
90
40
35
30
25
20
15
10
B777: 20
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant.
40
550
30
B777: 512
AR
TOGW, klb
600
500
450
20
B777: 10
10
400
0
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant.
SBW 1-Jury 2-Jury 3-Jury
200
100
B777: 71
50
0
Half Span, ft
150
Wing Weight, klb
SBW 1-Jury 2-Jury 3-Jury
150
B777: 106
100
50
0
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant. SBW 1-Jury 2-Jury 3-Jury
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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1-Jury TBW Configurations
Minimum Fuel/Emissions
Minimum TOGW
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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MDO Configurations: Drag Breakdown
Minimum Fuel
15000
Drag, lb
12000
Interference
9000
Wave
6000
Friction
3000
Induced
Drag Component
100%
80%
60%
40%
20%
0%
0
Cant.
SBW 1-Jury 2-Jury 3-Jury
Cant.
SBW
1-Jury
Cant.
SBW
1-Jury
2-Jury
3-Jury
Minimum TOGW
Drag Component
15000
Drag, lb
12000
9000
6000
3000
0
Cant.
-
SBW
1-Jury 2-Jury 3-Jury
100%
80%
60%
40%
20%
0%
2-Jury
3-Jury
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.
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Flutter Boundary of TBW Airplane Designs
Vc
Flutter
Envelope
0
0.2
0.4
0.6
Mach
0.8
1
1.2
SBW
1-Jury
40
35
30
25
20
15
10
5
0
3-Jury
Altitude (x103 ft)
SBW
1-Jury
2-Jury
3-Jury
Altitude (x103 ft)
40
35
30
25
20
15
10
5
0
2-Jury
Minimum Fuel
Minimum TOGW
Vc
Flutter
Envelope
0
0.5
Flutter Mach numbers for 100% fuel at 2.5g pull-up maneuver
1
Mach
• 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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Flutter Ballast Mass Study:
Ballast Mass from 2% to 8%
• Very low sensitivity to size and location of ballast mass
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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SBW Flutter modes
(sea-level, b/2=175 ft, ΛW =10°)
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TBW 3-jury Flutter modes
(sea-level, b/2=175 ft, ΛW =10°)
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Backup Slides
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Minimum Fuel/Emissions Design Study
170
130
110
10
90
0
Cant. SBW 1-Jury 2-jury 3-jury
20
10
400
0
200
120
100
+11%
B777:
71
Cant. SBW 1-Jury 2-Jury 3-Jury
Half Span, ft
140
Wing Weight, klb
0.7
+160%
B777:
10
150
B777:
106
50
0
Cant. SBW 1-Jury 2-Jury 3-Jury
0.5
0.4
Cant. SBW 1-Jury2-Jury3-Jury
+70%
100
0.6
0.3
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant. SBW 1-Jury 2-jury 3-jury
60
Cant. SBW 1-Jury2-Jury3-Jury
Strut Intersection
Ratio
AR
TOGW, klb
-8%
450
80
0
30
B777: 512
B777: 4340
2000
40
550
+18%
4000
Cant. SBW 1-Jury 2-Jury 3-Jury
600
500
B777:
20
20
6000
Specific Wing Weight
lb/ft
150
8000
+80%
30
-33%
L/D
Fuel Weight, klb
40
Wing Area, ft^2
B777: 183
190
450
400
350
300
250
200
150
Cant. SBW 1-Jury2-Jury3-Jury
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Minimum TOGW Design Study
B777: 183
-26%
100
10
0
0
AR
TOGW, klb
20
460
420
0
60
40
20
0
150
+40%
B777: 106
100
50
0
Cant. SBW 1-Jury 2-Jury 3-Jury
0.52
0.5
0.48
B777: 71 200
Half Span, ft
Wing Weight, klb
-17%
B777: 10
0.54
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant. SBW 1-Jury 2-Jury 3-Jury
80
+80%
10
5
Cant. SBW 1-Jury2-Jury3-Jury
0.56
15
440
B777:
4340
4000
25
500
+12%
4500
Cant. SBW 1-Jury 2-Jury 3-Jury
B777: 512
-10%
5000
Strut Intersection
Ratio
Cant. SBW 1-Jury 2-Jury 3-Jury
480
B777: 20
20
50
520
+80%
30
Cant. SBW 1-Jury 2-Jury 3-Jury
Cant. SBW 1-Jury2-Jury3-Jury
Specific Wing Weight
lb/ft
150
5500
Wing Area, ft^2
40
L/D
Fuel Weight, klb
200
350
300
250
200
150
Cant. SBW 1-Jury 2-Jury 3-Jury
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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Comparison of Designs: Min. Fuel and Min. TOGW
170
150
130
110
90
40
35
30
25
20
15
10
Cant. SBW 1-Jury 2-Jury 3-Jury
2000
0
SBW 1-Jury 2-Jury 3-Jury
Cant. SBW 1-Jury 2-Jury 3-Jury
0.8
20
B777:
10
450
10
400
0
Cant. SBW 1-Jury 2-Jury 3-Jury
0
Half Span, ft
B777:
71
0.2
Cant. SBW 1-Jury 2-Jury 3-Jury
150
B777:
106
100
50
0
Cant. SBW 1-Jury 2-Jury 3-Jury
0.4
SBW 1-Jury 2-Jury 3-Jury
200
100
0.6
0
Cant.
150
Strut Intersection
Ratio
30
B777: 512
500
B777: 4340
4000
Cant. SBW 1-Jury 2-Jury 3-Jury
Specific Wing Weight
lb/ft
550
50
6000
40
AR
TOGW, klb
B777:
20
Cant.
600
Wing Weight, klb
8000
Wing Area, ft^2
B777: 183
L/D
Fuel Weight, klb
190
450
400
350
300
250
200
150
Cant. SBW 1-Jury 2-Jury 3-Jury
Multidisciplinary Analysis and Design Center for Advanced Vehicles
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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
• 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
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