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New winglet for LS6
Winglet aerodynamic design experience
&
Corresponding RANS analysis using TAU (DLR)
Matthieu Scherrer & Stefan Melber-Wilkending (DLR)
Introduction Winglet for LS6 ? Physical aspects at stake for winglet design
Winglet aerodynamic design Winglet concept Parametrical study Final design
Back to back analysis of winglets vs plain tips with TAU-RANS (DLR) Presentation of the computations Local analysis Computed drag & lift analysis
Conclusion
Contents
Introduction
Winglet is a must on all last sailplane Benefits in competition proved over the years,
though not fully catched in theory
LS6a/b Probably the most optimised 15m with plain tips.
Wonderful handling & confort
Existing retrofits LS6c/WL factory WL (similar to LS8)
Piontowsky design for LS6a version (similar to factory LS6-18W)
Darlington design for LS8
Winglets for LS6 ?
Aerodynamic aspects to be considered in winglet design
Expected performance benefit of winglet is small Sailplane are already high AR, low induced drag ships
Even “side effects” to be considered
List of items to be considered in winglet design Effect on induced drag (equivalent AR)
& Additional wetted area
Multipoint design
Effect of winglet on wing tip transition pattern
Trimming drag & winglets
It is reported that re-laminarization due to winglet plays an important role in the benefit on the speed polar Principle sketch
Can CFD catch this phenomenon ?
Change in wing tip transition pattern
tr
Tail loading affects the sailplane wake, hence the induced drag Detectable effect on the benefit brought by winglet
Trimming drag
Wingtlet benefit DCD: -2.21% Wingtlet benefit DCD: -1.72%
Winglet design
Winglet concept
Two extrem sort of existing winglets
Big “shovel” Robustness to sideslip
Plain tip + winglet blade High efficiency winglet
It was choosen to have a high AR blade plus a large transition region
Design parameters
Height/span Retrofit limitation
Chords Reynolds vs wetted area
Sweep angles
Local loading
Dihedrals
Blade angle for wing flexion Transition radius
Twist / toes
Local loading ->3DOF to be optimised
-0.5
-0.25
0
0.25
0.5
0.75
1
1.25
1.5
6.000 6.500 7.000 7.500 8.000
-0.5
-0.25
0
0.25
0.5
0.75
1
1.25
1.5
6.000 6.500 7.000 7.500 8.000
Custom airfoil design Derived from model glider experience
Forward pressure recovery gradient for robustness to separation
Main winglet airfoil design
CL=0.6
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0 0.25 0.5 0.75 1
x/c
Cp
PSU94097 (ref)
MS-Wglt
Parametric study
4DOF study Geometric : Outer wing twist, winglet twist & toe
Aerodynamic : CL (0.2, 0.6, 1.2)
Use of rapid – relatively simple tool Induced drag is computed without wake balancing (AVL)
Viscous drag is locally evaluated along span as function of Cl, based on 2D polars.
Drag figures increment computed with trimming constraint through elevator deflection, for a mid CG.
saiplanesaiplanesaiplanewinglet CLCDCLCDCLCD sailplane trimmed
wingletw/o
sailplane trimmed
wingletw D
Raw results from parametric study
Evolution of winglet benefit vs 4DOF (3 angle & CL)
CL=0.2
CL=0.6
CL=1.2
Multipoint approach
Merit figure to be optimised : weighted polar, according to flight template theory.
“Envelope Flight template”
0 0.5 1 1.5CL
FT
(CL
)
Enveloppe Flight Template
Represents a statistically relevant program of a typical cross country flight. It is used for defining a multipoint cost function :
LLt
L
LD
L
D dCCfC
CC
C
C
range C 2/32/3L
)()(
Ref paper ostiv
Parametric study with multipoint approach
Merit figure to be optimised : weighted polar, according to flight template theory.
Target wing loading approach
Local load along span Local Cl should not exceed the max lift of airfoil
Helps at selecting toe/twist combination among quasi equivalent solutions
Local lift distributions
0
0.25
0.5
0.75
1
1.25
5.5 6 6.5 7 7.5 8
Y
Cl
Local Cl, Ref LS6 (no WL), CL=0.2Local Cl, Ref LS6 (no WL), CL=0.6Local Cl, Ref LS6 (no WL), CL=1.2Local Cl, LS6WL, CL=0.2Local Cl, LS6WL, CL=0.6Local Cl, LS6WL, CL=1.2Local Cl, LS6WL final tw ist/toe, CL=0.2Local Cl, LS6WL final tw ist/toe, CL=0.6Local Cl, LS6WL final tw ist/toe, CL=1.2
Rational for selecting winglets settings
The influence of the toe/twist angle seems low within a quite wide range (for the prediction method used) Typical “flat optimum range : +/-2deg
This is not in line with reported experience in WL design
This must somehow be filtered by some other criteria Local CL available from AVL
Evidence of risk of overloading in the transition region
Practical conclusion Once the winglet exists, most of its effects on induced drag is
settled (i.e. twist optimisation for Cdi is exagerated refinement)
Winglet setting should be sized for controlling viscous drag in extreme cases : intrados laminarity at low CL & extrados corner separation at high CL.
Final winglet geometry
Twist & toe Risk mitigation vs transition region loading
Not absolute optimal for AVL, local minimum
CAD for meshing & milling purpose (Flybiwo)
Swetted/Sref=1
Non-performance sizing aspects
Wing root bending moment
Handling quality
Flutter Measurement of sailplane modes sensitivity to additional masses
planned
Analysis of winglets vs plain tips using TAU-RANS (DLR)
Computation presentation Reference wing winth plain tips
Effect of winglets
CFD for sailplane purpose
Heavy CFD is rapidly developping for aerospace industry
TAU is an unstructured RANS code from DLR, that is used daily at Airbus
Can those developments give interesting results for sailplane design ? A glider is not an airliner !
Ingredient for this test case : 3D aero, laminarity
Geometry & mesh
Half a wing was meshed (symmetrical cases) Around 7.5 E6 points in the meshes
Setting up of computations
A list of 7 points along the speed polar were computed
Computation attempted for 4 conditions (7x4=28 cases) :
Winglet & plain tip geometry
Full turbulent flow & with transition
A list of 7 points along the speed polar were computed Turbulence model : K-w-SST
Transition : Tolmienn-Schlichtting waves & laminar bubble detection (N=11.5)
Elements of computation performance : 1 case = 80h on 40 cores
case CL V (m/s)
1 0.2 56.6
2 0.6 32.7
3 0.8 28.3
4 1.2 23.1
5 1.259 22.6
6 1.308 22.2
7 1.353 21.8
Analysis of winglets vs plain tips using TAU-RANS (DLR)
Computation presentation Reference wing winth plain tips
Effect of winglets
TAU for plain wing tip
Local Cp & transition comparison Comparison with MIAReX extended lifting line
Drag polar comparison Comparison with MIAReX computation & IDAFLIEG measurement
Evaluation of aerodynamic coefficients through two methods : Wing surface integration & far-field analysis
TAU for plain wing tip
Local Cp & transition comparison y/b=0 Comparison with MIAReX extended
lifting line, same CL
Comment General shape reproduced
No laminar bubble modelling for
TAU (with transition computation)
Undersurface at TE not behaving similarly (lam bubble)
Transition prediction differs
TAU for plain wing tip
Local Cp & transition comparison y/b= 0.28 & 0.53 Comparison with MIAReX extended lifting line
TAU for plain wing tip
Local Cp & transition comparison y/b=0.75 & 0.91 Comparison with MIAReX extended lifting line
TAU for plain wing tip
Comparion of transition predictions for CL=0.6 Comparison with MIAReX computation : more fwd transition line
Same definition of Transition point ?
Compatible with comparison of drag values (next slide)
Transition prediction for original wing
TAU vs MIAReX
-0.5
0
0.5
1
1.5
0 1 2 3 4 5 6 7 8y (m)
X T
r
Xtr Miarex - Upper
Xtr Miarex - Lower
Xtr Tau - Upper
Xtr Tau - Lower
TAU for plain wing tip
Results for aerodynamic coefficients
Conclusion : Absolute value from TAU cannot be used with confidence. Back to back analysis necessary :construction of an increment of
performance
Comparison of drag polars
Original wing - Total drag
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 0.055
CD
CL
A/C CD from IDAflieg speed polar
Cxtot - Miarex
CD tot NF Turb - Tau
CD tot NF Lam - Tau
CD tot FF Turb - Tau
CD tot FF Lam - Tau
0. Two references: IDAFLIEG. Polar MIAReX computation (lifting line)
1. Full turbulent result - More drag for the wing alone than for
sailplane as measured by IDAFLIEG. - Difference FarField/Nearfield
1
2. Results with transition - Farfield analysis shows result more
compatible with IDAFLIEG
2
Analysis of winglets vs plain tips using TAU-RANS (DLR)
Computation presentation Reference wing winth plain tips
Effect of winglets
Effect of Winglet in the flowfield
It is possible to “explore” the flowfield in the wake of the wing & winglet
Winglet & wing tip pressure field
Three station where considered to analyse local pressure features
Cut 1
Cut 2
Cut 3
Pressure line near tip for CL=0.2
-1.25
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0 0.25 0.5 0.75 1
x/c
Cp
Plain tip cut 1WL Cut 1WL Cut 2WL Cut 3
Evolution of wing tip pressure field
CL=0.2, V=56.6m/s
Full turbulent solution
Pressure line near tip for CL=0.6
-1.25
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0 0.25 0.5 0.75 1
x/c
Cp
Plain tip cut 1WL Cut 1WL Cut 2WL Cut 3
Winglet & wing tip pressure field
CL=0.6, V=32.7m/s
Full turbulent solution
Pressure line near tip for CL=0.8
-1.5
-1.25
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0 0.25 0.5 0.75 1
x/c
Cp
Plain tip cut 1WL Cut 1WL Cut 2WL Cut 3
Winglet & wing tip pressure field
CL=0.8, V=28.3m/s
Full turbulent solution
Pressure line near tip for CL=1.2
-2.5
-2.25
-2
-1.75
-1.5
-1.25
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0 0.25 0.5 0.75 1x/c
Cp
Plain tip cut 1WL Cut 1WL Cut 2WL Cut 3
Winglet & wing tip pressure field
CL=1.2, V=23.1m/s
Full turbulent solution
Winglet & transition line
Effect of winglet on transion line is represented by CFD
Delay the transition by up to 8% on pressure side
Phenomenon damped over 4% of span
Transition prediction with & w/o winglet
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
6.8 6.9 7 7.1 7.2 7.3 7.4 7.5 7.6
y (m)
X T
r
Xtr Tau - Upper
Xtr Tau - Lower
Tau WL - Upper
Tau WL - Lower
Transition prediction with & w/o winglet
0%
10%
20%
30%
40%
50%
60%
70%
6.8 6.9 7 7.1 7.2 7.3 7.4 7.5
y (m)
X T
r
Xtr Tau - Upper
Xtr Tau - Lower
Tau WL - Upper
Tau WL - Lower
Winglet benefit evaluation
Drag modification brought by winglet
NB : Reliable winglet computation with transition : only CL=0.2 & 0.9, higher CL extrapolated
Effect of winglets
Drag benefit
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-0.0015 -0.001 -0.0005 0 0.0005
DCD
CL
DCD winglet (full turb)
DCD winglet (Transition computation)
Effect of winglets
Relative drag benefit
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-4.00% -3.00% -2.00% -1.00% 0.00% 1.00% 2.00% 3.00%
DCD
CL
DCD winglet (full turb)
DCD winglet (Transition computation)
Winglet benefit evaluation
Pilot perspective
NB : Reliable winglet computation with transition : only CL=0.2 & 0.9, higher CL extrapolated
Effect of winglets - Pilot perspective
-1
-0.5
0
0.5
1
1.5
2
50 100 150 200 250
V (kph) @ 33kg/m²
D L
/D
Increment of L/D due to winglet(full turb)Increment of L/D due to winglet(Transition computation)
0.5
0.75
1
1.25
1.5
70 90 110 130 150 170
V (kph) @ 33kg/m²
Vz (
m/s
)
IDAFLIEG
WL with benefit (full turbulent computation)
WL with benefit (computation with transition effect)
Conclusion
Conclusion
Design of new winglet for LS6 A concept as starting point
Simple tools & pragmatic approach for angle setting
Winglet analysis Use of available method, over a quite large number of
computation cases
Understanding of local aerodynamic of winglet
Documented evaluation of performance modification
Next step : let’s fly those winglets !
Special thanks
DLR (TAU code)
Flybiwo (CAD & milling)
Have nice flights !
Questions ?