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