MAD Center Advisory Board Meeting

1

• MAD Center Advisory Board Meeting
• November 13, 1998

A Structural and Aerodynamic Investigation of a
Strut-Braced Wing Transport Aircraft Concept
2
Overview and Team Composition

• Aerodynamics and MDO
• John Gundlach IV
• Andy Ko
• Structures
• Amir Naghshineh-Pour
• Dr. Frank H. Gern
• Aeroelasticity
• Erwin Sulaeman
• CFD and Interference Drag
• Philippe-Andre Tetrault

• Faculty Members
• Dr. B. Grossman,
• Dr. R.K. Kapania
• Dr. W.H.Mason
• Dr. J.A. Schetz
• Dr. R.T. Haftka (University of Florida)

3
VPI Strut-Braced Wing Studies

• Dennis Bushnell Challenges the VPI MAD Center
• Perform a MDO study of a strut-braced wing
• Consider natural laminar flow
• Consider tip mounted engines
• Design cruise at M 0.85
• Lockheed Martin Work
• Advanced transport study
• Industry experts add realism to design studies
• Airline Acceptance Issues
• Certification Issues
• Mutually Beneficial Interactions

4
VPI Strut-Braced Wing Studies

• Technology Integration Objectives
• High aspect ratio and small t/c via strut bracing
• Laminar flow via low sweep
• Engine Integration
• Special Challenges
• Wing-strut interference drag
• CFD Design
• Engine-out condition
• Circulation control on vertical tail
• Structures
• Strut buckling requires innovative bi-linear
  strut stiffness
• Flutter, load alleviation and active control

5
Strut-Braced Wing Advantages

• The strut increases the structural efficiency of
  the wing
• Wing t/c reduced without a weight penalty
• Lower weight and increased span reduce induced
  drag
• Reduced t/c allows less sweep without wave drag
  penalty
• Parasite drag is reduced via increased laminar
  flow
• Un-sweeping the wing reduces cross-flow
  instability
• Higher aspect ratio means smaller chords and
  smaller Re

6
Design Mission

• Two GE-90 Class Engines
• 325 Passengers

7
1995 Baseline Configuration

• Length 224.3 ft.
• Span 220.2 ft.
• Wing Area 5,584.3 ft.2
• H-Tail Area 1,410.0 ft.2
• V-Tail Area 778.8 ft.2
• Thrust (per ENG.) 97,540 lb.
• Empty Weight 333,055 lb.
• Operating Weight 350,834 lb.
• Zero Fuel Weight 419,084 lb.
• Ramp Weight 714,499 lb.

1995 Baseline Configuration
8
2010 Baseline Configuration
Length 221.6 ft. Span 215.5
ft. Wing Area 4,672.0 ft.2 H-Tail Area
857.0 ft.2 V-Tail Area 637.8 ft.2 Thrust
(per ENG) 76,529 lb. Empty Weight 272,279
lb. Operating Weight 289,907 lb. Zero Fuel Weight
358,157 lb. Ramp Weight 568,134 lb.
2010 Baseline Configuration
9
Optimized Strut-Braced Wing Concept
Length 241.3 ft. Span 216.9
ft. Wing Area 4,237.3 ft.2 H-Tail Area
859.8 ft.2 V-Tail Area 779.7 ft.2 Thrust
(per ENG) 62,662 lb. Empty Weight 249,670
lb. Operating Weight 267,350 lb. Zero Fuel Weight
335,600 lb. Ramp Weight 504,835 lb.
2010 Strut-Braced Wing
10
Description of the MDO Process
Baseline Design
Induced Drag
Initial Design Variables
Updated Design Variables
Geometry Definition
Friction and Form Drag
Structural Optimization
Offline Aeroelasticity
Aerodynamics
Wave Drag
Drag
Interference Drag
Propulsion
Weight
Performance Evaluation
Offline CFD Analysis
Stability and Control
Objective Function, Constraints
Optimizer
11
MDO Problem Statement

• Objective minimize ramp weight
• Design variables (19)
• Wing
• Sweep
• Span
• Thickness-to-Chord (3)
• Root Tip Chord
• Strut
• Sweep
• Chord
• Thickness-to-Chord
• Offset Distance
• Wing-Strut Attachment

12
MDO Constraints

• Range 7,500 NM
• Reserve 500 NM
• Balanced field length lt 11,000 ft
• 2nd segment climb gradient gt 2.4
• Approach speed lt 140 KT
• Missed approach climb gradient gt 2.1
• Fuel volume ratio gt1.0 (Fuel In Wing Only)
• Initial cruise altitude ROC gt 500 ft/min
• Tail volume coefficients or Cn constraint
• Wingtip deflection
• Section Cl at cruise limited to prevent shock
  stall (0.8)

13
Propulsion Technical Results

• Empirical engine performance formula matches
  within 1 of GE-90 tabulated engine deck
• Projected SFC reduction of 3 by year 2010
• Projected thrust growth limit of GE 90 class
  engines is 110,000 lb.

14
Aerodynamics Drag Build-up

• Modified and calibrated skin friction and form
  drag to agree with LMAS methods
• Added form factors from LMAS Modular Drag
  (MODRAG) program
• Modified wetted area calculations to agree with
  ACAD results
• Calibrated airfoil technology factor in Korn
  equation to agree with EDET wave drag
• Induced drag based on optimized spanload Trefftz
  Plane analysis
• Interference drag from CFD (wing-strut) and
  Hoerner (strut-fuselage)
• Lift-to-drag ratios agree closely

15
Stability Control

• Vertical Tail
• Roskam/DATCOM methods find maximum available
  yawing moment
• Vertical tail initially sized by tail volume
  coefficient method
• Vertical tail scaling factor applied if yawing
  moment constraint violated
• Horizontal Tail
• Tail volume coefficient method

16
Sensitivity StudyTechnology Waterfall
1995 Technology SBW TOGW623,430 lb.
Natural Laminar Flow Riblets Active Load
Management All Moving Control Surfaces Composite
Wing And Tails Integrally Stiffened Fuselage
Skins Integrated Modular Flight
Controls Fly-By-Light, Power-By-Wire Simple
High-Lift Adv. Flight Mgt. Systems (Avionics)
Propulsion SFC and Engine Systems
NLF D TOGW - 5.49
AERO D TOGW - 5.13
AIRFRAME D TOGW - 8.98
-118,595 lb. (-19)
SYSTEMS D TOGW - 1.51
PROPULSION D TOGW - 2.67
2010 Technology SBW TOGW 504,835 lb.
17
Structures Objectives

• Development of a new methodology to quantify wing
  and strut structural weights on strut-braced
  wings
• Common wing weight calculation models (e.g.
  FLOPS) are not accurate enough for strut-braced
  wings
• Single strut configurations piecewise linear
  beam theory
• Design critical load conditions
• Sizing for drag reducing strut-to-wing offset
  member
• Wing weight calculation procedure
• Industrial scale wing sizing model (provided by
  Lockheed Martin Aeronautical Systems, Bob
  Olliffe)
• Perform a preliminary aeroelastic analysis

18
Structures Assumptions

• Piecewise linear beam theory
• Idealized wing box (double plate model)

• Wing materials
• State-of-the-art aluminum alloys
• Composite materials and concepts (weight
  technology factors applied to attain proper
  sizing)
• Critical load conditions
• 2.5g maneuver
• -1.0g pushover
• -2.0g taxi bump
• Ground strike deflection constraint for taxi bump
  condition

19
Structures Assumptions

• Strut design parameters
• Active only in tension
• Inactive in compression to avoid strut buckling
• Telescoping sleeve mechanism (damper)
• To achieve optimum strut force, strut engages at
  a certain positive load factor
• Airfoil-shaped cross-section to attain acceptable
  airflow characteristics
• At 2.5gs, strut force and wing-strut
  intersection location are determined by design
  optimization to achieve minimum weight

20
Strut-Fuselage Attachment

• Strut damping system with telescoping sleeve
• prevents strut buckling
• prevents sharp initiation of tension loads
• prevents rapid, dynamic loading of strut
• Structural synergy with main landing gear frames
• Active damping system
• prevent strut flutter

21
Wing-Strut Attachment

• Vertical strut offset to reduce wing/strut
  interference drag
• Combined tension/bending loading of the offset
  member
• Significant bending loads at wing attachment
• MDO used to optimize
• offset length
• strut force
• wing/strut intersection

22
Vertical Strut Offset

• Two conflicting design requirements
• minimum strut offset ? reduced loading and weight
• maximum strut offset ? minimize wing/strut
  interference drag
• Method developed to size offset member
• Perform full optimization for
• offset length
• strut force
• wing/strut intersection

23
Vertical Strut Offset
Influence of Vertical Offset on Strut
Weight (Fixed Strut Junction and Strut Force)
Weight Changes due to Strut Offset (Full
Optimization)
24
Structures Technical Results
Bending Moment Distributions
2.5E07
2.0E07
2.5G Maneuver
-1.0G Pushover
1.5E07
-2.0G Taxi Bump
1.0E07
5.0E06
Bending Moment (Ft-Lb)
0.0E00
0
20
40
60
80
100
120
-5.0E06
-1.0E07
-1.5E07
-2.0E07
Wing Half Span (Ft)
25
Mass Properties

• Wing bending weight calculated from panel
  thickness results

26
Mass Properties

• Wing bending, strut, and offset weight module for
  MDO tool

Technology factor 0.8 Non-optimum factor 1.1
27
Mass Properties

• Detailed weight breakdown by material and
  component for costing purposes

28
Hexagonal Wing Box

• Industrial scale wing sizing model
• High degree of accuracy (based on LMAS experience
  in wing sizing)
• Wing box geometry variable in spanwise direction
• Optimized area/thickness relationships for
• spar webs and spar caps
• stringers
• wing box skin
• Minimum gauge and stress cutoffs can be
  accurately applied
• Validated as a stand-alone version

29
Hexagonal Wing BoxOngoing Activities

• Incorporation of hexagonal wing box model into
  the strut-braced wing MDO code
• Accurate calculation of wing box shear weight
• Spanwise variation of aeroelastic twist
• Incorporation of torsional stiffness into wing
  weight estimation ? aeroelastic constraints
• flutter speed
• divergence speed
• static aeroelastic response
• aileron reversal
• load alleviation

30
Hexagonal Wing BoxOngoing Activities

• Consideration of chordwise strut offset
• influencing aeroelastic twist and lift
  distribution
• increase of
• flutter speed
• divergence speed
• decrease of wash-out effect
• aeroelastic tailoring without employment of
  composites
• Realistic element sizing ? Input for detailed FEM
  analysis

31
Structures Wing Model
Double plate model used in Wing.f
Hexagonal model with optimized thickness/area
relationships
32
Aeroelastic Analysis

• Structural Finite Element Modeling
• Based on Hexagonal Section Model
• Structural Dynamics
• Flutter and Divergence
• Use NASTRAN code
• Investigate the Effect of the Strut on the Wing
  Aeroelastic Behavior
• Variation on the strut stiffness
• Variation on the position of the wing-strut
  junction
• Variation on the position of the fuselage-strut
  junction
• Fuel mass effect

33
Wing Vibration Modes
1st bending mode, 1.72 Hz.
1st torsion mode, 11.40 Hz.
34
Vflutter for the Wing-Strut Configuration

• Preliminary results for the wing-strut
  configuration
• No aeroelastic optimization was performed
• Doublet-Lattice method with compressibility
  correction
• NASTRAN, PK method

35
Variation of the Strut Position at Fuselage

• Basic condition ( full fuel, sea level, the
  strut-wing junction at the wing break)
• The strut root position is measured from the
  current strut position
• The divergence speed decreases as the strut
  position moved forward
• The flutter speed is more critical than the
  divergence speed

vdivergence
vflutter
36
Spanwise Variation of the Wing-Strut Junction

• Basic condition ( full fuel, sea level, the
  strut-wing junction at the wing break)
• The FS and RS positions of the struts are
  varied spanwisely
• Note that the change in the spanwise position of
  the strut would change the wing stiffness. The
  calculation here neglects such changes in the
  stiffness.

y
37
Chordwise Variation of the Wing-Strut Junction

2
3
4
1

• The lowest Vflutter is if the junction
  concentrated at the rear spar (Model 4)
• The junction model 1 and 2 give higher Vflutter
• The flutter calculation is based on the wing and
  strut with fuel mass configuration at sea level

RS
FS

38
Stiffness Variation of the Strut

FS
RS
Percentage of the strut loads connected to the
front spar and rear spar

• Basic condition ( full fuel, sea level, the
  strut-wing junction at the wing break)
• The highest Vflutter is for the strut
  configuration Afs Ars 80 20

39
Flutter Boundary

• Flutter Speeds of the SF-Opt-1 Wing at true
  air-speed flight
• Nominal case Wing-strut
• configuration for zero/full fuel
• conditions
• Failure Case Wing without strut for zero
  / full fuel conditions
• The most critical case is the wing without
  strut at zero fuel condition
• No aeroelastic optimization was performed.
  Further work will include the effect of the
  transonic dip correction

40
Future Work

• Nonlinear unsteady transonic aerodynamic
  correction
• Aeroelastic optimization
• Axial-flexural coupling effect in wing flutter
  and structural vibration
• Aeroelastic analysis for wing with an arch-shaped
  strut configuration
• Nonlinear strut modeling for inactive compression
  case
• T-tail and wing-fuselage-tail flutter

41
CFD and Interference Drag Analysis

• CFD Analyses on Unstructured Grids
• Grid Generator VGRIDns
• Flow Solver USM3D
• Transonic Flow M0.85
• Helpful Interactions
• Lockheed-Martin
• Dr. Pradeep Raj
• NASA Langley
• Dr. Neal Frink
• Dr. W. Kyle Anderson
• Dr. Shahyar Pirzadeh
• Javier Garriz

42
Objectives of the Study

• Design the wing with a twist distribution for an
  optimized strut-braced wing geometry (SS5 Design)
• Evaluate the aerodynamic benefits of an
  arch-shaped strut compared to a straight strut
• Obtain the drag penalty compared to the clean
  wing case
• Study the effect of the arch radius on the
  interference drag
• Determine a parametric relationship between the
  interference drag and the strut geometry by
  performing CFD analyses

43
Wing AloneGrid Refinement Study
Normal Grid 613,000 Volume Cells
Improved Grid 787,000 Volume Cells
CL 0.5186 CD 113.6?10-4 CM -0.6880
CL 0.5187 CD 114.3?10-4 CM -0.6879
44
Advantages of an Arch Compared to a Straight Strut

• Two alternatives for the strut design
• The arch-braced wing design provides means to
  reduce the interference drag
• Intersection of the wing and the strut at 90o
• Increased distance between the wing lower surface
  and the strut upper surface

Wing Straight Strut
Wing Arch Strut
45
Results for the Straight Strut Design

• Adding a straight strut to the wing has two main
  effects compared to the wing alone
• Dramatic increase of the drag 39
• Reduction of the lift at same angle of attack
  -27

?CD is the drag increment compared to the wing
alone. It includes the drag of the strut, the
wing-strut intersection as well as the
strut-fuselage effect.
46
Improvements to the Straight Strut Design

• The pressure distributions show a big effect of
  the strut near the wing-strut intersection
• This effect could be alleviated by the shape
  design of the wing and the strut
• Even by carefully adding twist to the strut near
  the junction, the drag penalty is still very
  important

47
Analysis of the Arch-Braced Wing Design

• An arch-shaped strut was added to the wing
• The radius of the arch was varied from 1 ft to 4
  ft
• The strut was rotated about its leading edge to
  reduce the incidence to zero (structural reasons)
• The disruption of the flow field due to the arch
  is limited to the region close to the junction

Arch radius
48
Mach Number Contours on the Arch-Braced Wing
Wing Upper Surface
Wing and Strut Lower Surface
View with strut removed
49
Effect of the Arch Radius on the Lift Distribution

• The arch-braced wing provides about the same lift
  as the wing alone at the same angle of attack

50
Drag Variation Compared to the Clean Wing

• For an arch radius of 3 ft, the interference drag
  is 50 less than for a 1-ft radius

51
Interference Drag in Transonic Flow

• In subsonic flow, Hoerner 1965 studied the
  interference drag of strut-wall intersections
• Effect of the lateral and longitudinal angles
• Variation of the thickness of the strut
• Nothing equivalent exists in the literature for
  transonic flow
• We want to use CFD to study the effect of the
  strut angles and geometry on the interference
  drag

52
Method to Determine the Interference Drag

• Calculate the drag of two 2-D airfoil sections
  (5 and 10 thick) with Navier-Stokes solver
• Run Navier-Stokes calculations for 3-D struts
  with the 5 and 10 thick airfoil sections
• Vary the angle ? of the strut with the wall
• Calculate the drag of each arrangement (CD tot)
• Transform the 2-D drag to a 3-D equivalent (CD
  2D)
• Interference drag CD interf CD tot - CD 2D

53
Preliminary Results forNACA 64A- Airfoil Family

• Unstructured 2-D grids AFLR2 and FUN2D
• Freestream conditions M0.85, ?0o

NACA 64A005
NACA 64A010
54
Future Work

• Wing and Strut Weight Analysis with Hexagonal
  Wing-Box Model
• Strut Interference Drag with CFD analyses
• Estimate the Benefits of Tip-Mounted Engines
• Consequences of Aerodynamic Loads on the Strut
• Detailed Aeroelastic Studies with Composites
• Improved Structural Analysis of the Strut-Wing
  Intersection
• Landing-Gear Pod Drag Analysis with CFD

55
Image Rendering and Rapid Prototyping

• On-Screen Visualization
• Fortran subroutine creates DXF file
• AutoCAD is used to create rendered images
• Rapid Prototyping
• A solid model is created in I-DEAS
• Fused Deposition Modeling is used to create a
  plastic model