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1 International Conference– ICAUV-2009, Bangalore, India
Development of High-Lift, Mild-StallLow Reynolds Number Airfoils
Alexander Nagel, Yonatan Klein and Misha Shepshelovich
Engineering Center, Israel Aerospace Industries
2 International Conference– ICAUV-2009, Bangalore, India
Development of High-Lift, Low Reynolds Numbers Airfoils
Motivation
development of Mini and light tactical UAV is a reason for renewed interestin aerodynamics of low Reynolds number airfoils
high-lift wings sections are especially advantageous for this development because of their potential to reduce the size of air vehicles, improve their endurance performance, enhance take-off and landing characteristics and ensure the flight at very low airspeeds
the major difficulty associated with design of high-lift, low Reynolds number airfoils is attributed to formation of laminar separation bubble, its burst at high lift coefficients and development of unacceptable abrupt stall pattern
until appropriate solution is found for the treatment of this problem, high-lift, low Reynolds number airfoils are of a limited value for development of small UAV operating at domain of low airspeeds
3 International Conference– ICAUV-2009, Bangalore, India
Mini Truck – High-Lift Mini UAV/airfoil MTD-120MW=3kg, span=1.2m, CLmax~2.3
flight testing stage - implementation of two-element, low Reynolds number airfoils
4 International Conference– ICAUV-2009, Bangalore, India
Mini Truck wing section - airfoil MTD-120Mmission adaptive geometry, (t/c)max=11%
cruise, loitering flight - δflap = 0cruise, loitering flight - δflap = 0 deg
Vmax- δflap = -10Vmax- δflap = -10 deglanding - δflap = +15landing - δflap = +15 deg
aileron - δflap = ± 15aileron - δflap = ± 15 deg airbrake - δflap = +75airbrake - δflap = +75 deg
5 International Conference– ICAUV-2009, Bangalore, India
Smooth airfoil MTD-120M - TAU WT testAbrupt stall pattern, Re=200K, δflap=0
hysteresis test
5 0 5 10 15 20
Cl
-5 0 5 10 15 20
Cl
MSES code
WT test α
0. 0
0. 5
1. 0
1. 5
2. 0
2. 5
0. 0
0. 5
1. 0
1. 5
2. 0
2. 5
design point
burst of laminarseparation bubble
test-theory comparison
0 .0
0 .5
1 .0
1 .5
2 .0
2 .5
A
C
D
- 5 0 5 10 15 20
α
Cl
B
6 International Conference– ICAUV-2009, Bangalore, India
Smooth airfoil MTD-120, Re=200K, δflap =0hysteresis test – the burst of laminar separation bubble
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.00.0 0.2 0.4 0.6 0.8 1.0x/c
Cp
14
15
-4.0
-3.0
-2.0
-1.0
0.0
1.00.0 0.2 0.4 0.6 0.8 1.0x/c
Cp
10
8
α point
A
B
steep adversepressure gradient
laminar separation the burst of laminar bubble
α point
C
D
7 International Conference– ICAUV-2009, Bangalore, India
Smooth airfoil MTD-120M - TAU WT testAbrupt stall pattern, Re=120K, δflap=0
the burst of laminar bubble-4.0
-3.0
-2.0
-1.0
0.0
1.00.0 0.2 0.4 0.6 0.8 1.0x/c
Cp
10
12
α
test-theory comparison
0.5
1.0
1.5
2.0
-5 0 5 10 15
α
Cl
MSES code
WT test
burst of laminarseparation bubble
8 International Conference– ICAUV-2009, Bangalore, India
Smooth airfoil MTD-120, Re=120Kformation of laminar separation bubble at low Reynolds numbers
-3.0
-2.0
-1.0
0.0
1.00.0 0.2 0.4 0.6 0.8 1.0x/c
Cp
separation of laminarboundary layer
reattachment of turbulentboundary layer
laminar - turbulenttransition laminar separation
bubble
steep adversepressure gradient
TAU WT test – Cl=1.6
laminar separation bubble
9 International Conference– ICAUV-2009, Bangalore, India
Laminar separation bubble
formation of laminar separation bubble is a dominant physical phenomena in aerodynamics of low Reynolds number airfoils
aerodynamic characteristics of low Reynolds number airfoils are dependent on formation, location and size of laminar separation bubble
the burst of laminar bubble produces unacceptable abrupt stall characteristics, followed by development of hysteresis phenomena
at low Reynolds numbers, transition control technique (rough surface)is mandatory for control of the size of laminar bubble
intensive WT testing is required for evaluation of aerodynamiccharacteristics of low Reynolds number airfoils and for substantiation of transition control in the wide range of lift coefficients
10 International Conference– ICAUV-2009, Bangalore, India
New Generation of High-Lift, Mild Stall UAV wingswith stall, post stall flight capabilities
MS-SA wing - US and Israel Patent ApplicationsSA-MS wing - US and Israel Patent Applications
1.0
1.5
2.0
2.5
3.0
0 5 10 15 20 25
α
CL
MS-SA wing
Advantages of mild-stall wings:
• flight safety considerations• elimination of speed safety margin• extension of usable lift up to CLmax
• flight at stall airspeeds• improved take-off/landing• landing at stall option• improved endurance• increased glide angles• reduced sensitivity to contamination
advanced wing concepts – lift characteristics
SA-MS wing
flight proven
11 International Conference– ICAUV-2009, Bangalore, India
Geometry of prototype SA-MS airfoil versus conventional two-element wing section
high-lift, mild stall SA-MS airfoil conventional two-element airfoil
MS-ramp
main body / upper surface - distribution of local radius
1/rlocal
12 International Conference– ICAUV-2009, Bangalore, India
High-Lift, Mild-Stall SA-MS airfoil mission adaptive geometry, (t/c)max=18%
cruise, loitering, δflap = 0
high-lift flight, δflap = +20 decambering, δflap = -10
airbrake, δflap = +75aileronδail
-20
+20
13 International Conference– ICAUV-2009, Bangalore, India
High-Lift, Mild-Stall SA-MS airfoil TAU WT test, smooth airfoil
Re=300KRe=500K
Cl
0.0
0.5
1.0
1.5
2.0
2.5
-5 0 5 10 15 20 25
α
10
0δflap (deg)
0.0
0.5
1.0
1.5
2.0
2.5
-5 0 5 10 15 20 25
10
0
δflap (deg)
α
Cl
14 International Conference– ICAUV-2009, Bangalore, India
SA-MS airfoil - pressure distributionsTAU WT test, clean airfoil, Re=300K, δflap=0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
α=14, Cl=2.26Cp
x/c
α=18, Cl=2.34
development of separation region on MS-ramp
Cp
x/c
start of flow separation on MS-ramp
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.0
Cp α=22, Cl=2.28 α=25, Cl=2.09
flow separation upstream of MS-ramp
Cp
x/c
fully separated MS-ramp trailing edge separation
x/c
15 International Conference– ICAUV-2009, Bangalore, India
SA-MS airfoil - elimination of speed safety margin
lift curves, Re=1M, MSESairfoil SA/MS-18/1.0, flap 25%C
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-15 -10 -5 0 5 10 15 20α
Cl
1.2Vstall
usable lift
δfl = +40
δfl = +20DSA
SA-MS
-15
+20
airfoil DSA, flap 40%C
-10
+40
rigid connection
16 International Conference– ICAUV-2009, Bangalore, India
Development of high-lift, mild-stall low Reynolds number airfoils
Combination of MS-ramp concept and transition control methodology
Technical objectives:
• delay of the burst of laminar bubble• elimination of hysteresis phenomena• elimination of speed safety margin• extension of usable lift up to CLmax
• safety considerations at low airspeeds
Technical activities:• SA-MS airfoil - WT test at low Reynolds numbers• application of transition control methodology
17 International Conference– ICAUV-2009, Bangalore, India
SA-MS airfoil - transition control effectTAU WT test, δflap=0
clean airfoil
0.0
0.5
1.0
1.5
2.0
2.5
-5 0 5 10 15 20 25
Cl
α
200
150
Re (K)
roughness effect - Re=150K
Cl
α0.0
0.5
1.0
1.5
2.0
2.5
-5 0 5 10 15 20 25
cleanrough
18 International Conference– ICAUV-2009, Bangalore, India
Smooth SA-MS airfoil - abrupt stall patternTAU WT test, Re=200K
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.00.0 0.5 1.0x/c
Cpα = 20˚, Cl = 2.26
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.00.0 0.5 1.0x/c
Cpα = 21
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
1.00.0 0.5 1.0x/c
Cpα = 18˚, Cl = 2.23
-4.0
-3.0
-2.0
-1.0
0.0
1.00.0 0.5 1.0x/c
Cpα = 14˚, Cl = 2.16
19 International Conference– ICAUV-2009, Bangalore, India
Rough SA-MS airfoil - pressure distributionsTAU WT test, Re=150K
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0x/c
Cp-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0x/c
Cp α = 9˚, Cl = 1.79 α = 12˚, Cl = 2.00
roughness strips
-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
α = 16˚, Cl = 2.15
flow separationon MS-ramp
-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
α = 18˚, Cl = 2.15
development of separation region on MS-ramp
20 International Conference– ICAUV-2009, Bangalore, India
Rough SA-MS airfoil - pressure distributionsTAU WT test, Re=150K
-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
α = 20˚, Cl = 2.15
fully separated MS-ramp
-5
-4
-3
-2
-1
0
10 .0 0 .2 0 .4 0 .6 0 .8 1 .0
α = 23˚, Cl = 2.10 ,
flow separationupstream of MS-ramp
0.0 0.2 0.4 0.6 0.8 1.0
-5
-4
-3
-2
-1
0
1
α = 25˚
flow separationupstream of MS-ramp
-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
α = 24˚ , Cl = 1.91
roughness strips
21 International Conference– ICAUV-2009, Bangalore, India
SA-MS airfoil - roughness effectTAU WT test, Re=150K, δflap=0
the burst of laminar bubbleformation of large bubble-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0x/c
Cp
cleanrough
roughness - main body
α=12-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0x/c
Cp
clean
rough
roughness - main body
α=9
22 International Conference– ICAUV-2009, Bangalore, India
Rough SA-MS airfoil at low Reynolds numbersTAU WT test, δflap=0
location of laminar bubble, Re=150Klift curves at low Re numbers
0.0
0.5
1.0
1.5
2.0
2.5
0 5 10 15 20 25
150
120
100
80
Re (K)
α
Cl
laminar bubbleclose to 2nd strip
-5
-4
-3
-2
-1
0.0 0.1 0.2 0.3
α = 20˚α = 23˚α = 24˚
23 International Conference– ICAUV-2009, Bangalore, India
Rough SA-MS airfoil - pressure distributions TAU WT test, Re=120K, δflap=0
-5
-4
-3
-2
-10.0 0.1 0.2 0.3 0.4
x/c
Cp Cl = 2.11 , α = 22˚-5
-4
-3
-2
-10.0 0.1 0.2 0.3 0.4
x/c
Cp Cl = 2.15 , α = 20˚
-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
x/c
Cp Cl = 2.15 , α = 21˚-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
x/c
Cp
22 23
α
roughness strips
24 International Conference– ICAUV-2009, Bangalore, India
Rough SA-MS airfoil - pressure distributions TAU WT test, Re=100K, δflap=0
-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
x/c
Cp-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
x/c
Cp
-5
-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
x/c
Cp-4
-3
-2
-1
0
10.0 0.2 0.4 0.6 0.8 1.0
x/c
Cp Cl = 1.99 α =16˚
Cl = 1.96 , α = 19˚ α = 20˚
Cl = 1.99 , α = 18˚
roughness strips
25 International Conference– ICAUV-2009, Bangalore, India
Mini Truck - wing options / rough airfoils, δflap= 0airfoil MTD-120 SA-MS airfoil
0.0
0.5
1.0
1.5
2.0
2.5
-5 0 5 10 15 20 25
lift curves - Re=120K
SA-MS airfoil
Cl
α
MTD-120 airfoil
0.0
0.5
1.0
1.5
2.0
2.5
-5 0 5 10 15 20 25
Cl
α
lift curves - Re=150K
burst of laminarseparation bubble
26 International Conference– ICAUV-2009, Bangalore, India
Conclusions
combination of MS-ramp and transition control provides acceptable high-lift characteristics at the studied range of low Reynolds numbers
the burst of laminar separation bubble and development of hysteresisphenomena were delayed to high post-stall angles of attack
for SA-MS wing, the speed safety margin may be eliminated, allowing extension of usable lift up to the maximum lift
the concept allows safe operation of UAV at post-stall angles of attack
for continuation effort, the thickness of the airfoil should be adjusted to the values that are typical for low Reynolds numbers applications
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