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Aero India 2013 International SeminarBangalore, India, 04-06 February 2013

Wings for UAV Based on High-Lift Airfoils

Dr. Nagel A., Aerodynamic Research Engineer, IAI, Israel,[email protected]

Abstract

Wing design for tactical UAV applications requires a special approach which is differentfrom man-operated airplanes. The maximum lift is a primary factor to achieve high endurancefactor values and short take-off and landing distances. The permanent design problem for UAVwings design is engineering compromise between achieving maximum lift, reducing dragpenalty for high speed flight and providing flight envelope capabilities from low-lift maximumspeed flight up to stall lift levels. Speed safety margins and acceptable stall characteristics havetoo to be taken in attention.

The relatively small UAV size and low level of flight speed which correspond with lowReynolds numbers bring specific design difficulties. While classic thin single-element airfoils

lift increment is limited, the high-lift potential is based on multi-element airfoils, in particulartwo-element. The special feature of two-element airfoils is extremely low local Reynoldsnumbers on second element (flap section) providing danger of flow separation, especially fordeflected flap. The rear part of main element must provide enough length for flow recovery afterlaminar-turbulent transition, hence limiting airfoil laminar capabilities. The engineering

optimum in low Reynolds numbers airfoil design is search of compromise between the abovementioned parameters. The different approaches of two-element airfoil design are presented the airfoils with permanently opened slot and airfoils with retracted flap were designed andexperimentally tested. Advantages and drawbacks of proposed two-element design concepts areanalyzed. The computational methods and comparison with experimental wind tunnel results are

performed for both design concepts.The present paper not only addresses airfoil design, but also the complex 3-D high-lift wings

design development potential for low Reynolds number application. The attempt to analyzecomplex wing that combine two different two-element airfoil concepts is presented. UAVconfigurations employing high-lift low Reynolds wings are presented to demonstrate high-liftflight concept advantages.

The presentation partially includes the results of joint ADE-IAI project "High Lift Wing

Design Technology".

Key Words: airfoil, wing, UAV, aerodynamics
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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2.Nomenclature

Cl airfoil lift coefficient

CL aircraft lift coefficient

Cd airfoil drag coefficient

CD aircraft drag coefficient angle of attack

flap flap deflection

Re Reynolds number

AR aspect ratio

NLF natural laminar flow

IAI Israel Aerospace Industries

ADE Aeronautical Development EstablishmentUAV Unmanned air vehicle

WT wind tunnel

3. Introduction

Wing design for UAV applications requires a special approach which is different from

conventional man-operated airplanes. The main goal of operational UAV is endurance flight atrelatively low speed. The aerodynamic endurance for propeller driven aircraft is proportional tothe factor of CL

1.5/CD [ref. 1], where the exponent indicates dominant role of lift factor for

maximum endurance achievement. Therefore, the use of high lift airfoil is beneficial, eventhough the high-lift capabilities could be accompanied by a drag penalty. The design problemfor high lift airfoils design is engineering compromise between achieving maximum lift,

reducing drag penalty and providing full flight envelope. The last requirement means acceptabledrag level at high speed (low lift coefficient). Additional desirable feature is good airfoil stall

characteristic.For low speed applications, the main parameter that characterizes the air flow condition is

Reynolds number (Re=VL) [ref 1]. A UAV with relatively small size and low flight speed

has reduced Reynolds numbers on the wing (and tail). Reynolds numbers in the range of 300Kor less bring specific design issues. The first issue is the maximum lift capability whichtraditionally was considered difficult for low Reynolds number [Ref. 1-3]. The second issue isthe drag level of the airfoil which increases due to higher friction coefficients and due to largersize of laminar separation bubble. A large laminar separation bubble causes drag increment andmay lead to risk of bubble burst. Rear part of airfoil must provide enough length for flowrecovery after laminar separation bubble, hence limiting the length available for laminar flow onthe forward airfoil portion. The engineering optimum in low Reynolds numbers airfoil design isa compromise between the various mentioned parameters.

Reynolds number Re=300K was chosen for present work analysis as typical value for

small/medium size UAV wings, and easy accessible for WT test evaluation.

4. High-lift flight conceptContrary to general view, usually operational UAV have a rather high drag due to external

payloads that are installed permanently thus increasing significantly the drag as compared to aclean configuration. The tendency to reduce cost and simplify production may result in items

like non-retractable landing gears, not-clean external payloads contours and not-elegantstructures and installations. The result is usually high parasitic drag. Hence to increase theairplane flight endurance, defined by aerodynamic endurance factor (CL

1.5/CD) [ref 1-3] forpropeller-driven aircrafts, the importance of high lift wing is even more dominant. Of course,the potential for drag reduction of wing and other components should not be neglected, as itassists both in loiter and even more in other flight regimes. The usage of high CL in theendurance formula is limited by several factors. First it is bounded by the wing maximum liftcapability. The airfoil maximum lift is the major factor determining wing maximum lift while

other factors like twist and taper ratio have secondary role. Second factor is a stall safety marginthat is required in regulations. Maximum usable lift may also be limited due to issues of stall


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characteristics. Thus, it is also important to design for good stall characteristics to avoidadditional restrictions on maximum usable lift.

The advantage of high-lift long endurance concept is illustrated in fig. 1 for the case of smallUAV (characterized by low Reynolds loitering flight speed) with assumed parasite dragcoefficient of about 450 drag counts [ref 5].

The concept of high lift needs not only high-lift airfoil implementation, but appropriatethree-dimensional wing design. The Aspect Ratio parameter is important for long-enduranceflight and not less significant than wing section maximum lift. Aspect ratio effect is illustratedschematically in fig. 2 for the case of Panther-class small UAV. The choice of wing area is also

very important as it enters directly into the endurance calculation ( wingS ), but also affectsflight speed and local Reynolds numbers on the wing [ref. 1-3].

The designer has additional means to affect the performance. Several of them are

configuration related like engine characteristics, fuel and weight/structure aspects.Low Reynolds numbers on wings are characteristic of the small UAV. This poses additional

aerodynamics considerations to wing geometry and sizing. The low local Reynolds number,especially on tapered wings in the tip area, may cause premature stall, increase of drag and

degradation of control power. This results from aerodynamic phenomena that cause increase ofseparation bubble size and increase risk of its burst with ensuing flow separation. Danger oftrailing and leading flow separation is increased as well.

The drag penalty caused by outboard wing reduced Reynolds number could exceed theinduced drag gain from wing taper ratio. So the choice of local chord length is very important.

5. High-lift airfoil features.

The main design features to achieve high lift level are airfoil front part leading edgebluntness and high camber shape. Blunt leading edge means not only large leading edge radiusitself, but continuous surface convex up to maximum thickness section. The goal of front partbluntness is air flow acceleration and prevention of suction peak development. This principalapproach is actual as for single as for two-element airfoils, with clarification that for two-element airfoils the concept apply both to main element and to flap section. The design problemfor high lift airfoils design is engineering compromise between achieving maximum lift andreducing drag penalty for primary design points.

Single-element airfoils, even specially designed, have limitation of achievable maximum liftfor corresponding Reynolds numbers. The way to achieve higher lift level is implementation oftwo-element airfoils. Two-element airfoils are the particular case of more global class of multi-element airfoils. Maximum lift of two-element airfoil is significantly higher than that of singleairfoils for the same flow conditions because of air flow acceleration through slot channel andinteraction with upper boundary layer coming from the main element. Design of two-elementairfoils has to take into account additional parameters, which are not present for single elementairfoils: horizontal overlap, vertical gap between flap and main body, and hinge point location.

The design must provide horizontal overlap in order to achieve reliable flow interaction. Thehinge point location has to provide acceptable gap and overlap ranges for all flap deflections.The important special task for two-element design is slot channel geometry. The channel has tobe built as flow accelerator, to provide effective flow inclination on the second element.

The two main features define drag level for low Reynolds airfoils are: level of laminarity(laminar part flow before turbulent transition) and the size of laminar separation bubble.

Increasing the laminar separation bubble causes lift increment but on the other side threat ofbubble burst. The drag always increases proportionally to separation bubble size. The drag,caused by laminar separation bubble, usually non-significant for large-scale aircraft, became thedominant factor for low-Reynolds applications. The lead to achieve maximum lift requiresincreased airfoil thickness that always contraries with drag reduction requirement, especially for

low Reynolds numbers. Rear part of airfoil must provide enough length for flow recovery after

laminar-turbulent transition, hence limiting airfoil laminar capabilities. For two-element airfoilsthe special issue is extremely low local Reynolds numbers on second element (flap section)


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providing danger of flow separation, especially for deflected flap. The engineering optimum inlow Reynolds numbers airfoil design is search of compromise between the various mentionedparameters.

6. High-lift airfoils design

Three different concepts high-lift of high-lift airfoil design are discussed in present work. All

presented airfoils were designed during joined ADE-IAI program and tested in wind tunnel inIIT, Kanpur. Low-speed wind tunnel facility and experiment methodology are described indetails in references 6-7.

The conventional way is to design a single-element airfoil. Airfoil B-19 is a single elementand it was designed during present work. The airfoil geometry is schematically shown on fig. 3.

It's lift characteristics are presented on fig. 4 for NLF flow condition Re=300K. MSESnumerical simulation in comparison with WT test results are available and demonstrated good

test-theory agreement.Modification of trailing edge geometry was proposed and tested. Wedge trailing edge was

proposed as simplified version of DTE (divergent trailing edge) concept refs. [9]. The wedge

trailing edge geometry is presented on fig. 5. A comparison of lift curve between calculationand WT tests is presented on fig. 6, for the case of clean airfoil. Fig. 6 shows that modified

trailing edge provides a lift increment of magnitude about Cl~0.15. Though this is aconsiderable maximum lift increase, it is still restricted, and to achieve higher values anotherconcept of two-element airfoil should be considered.

Two different concepts of two-element airfoil design were evaluated during present work:slotted airfoil with permanently opened slot and airfoil with retractable flap.

The first concept is two-element slotted airfoil with permanently opened slot, as shown onfig. 7, refs.[5,8]. This concept was already effectively applied for low-Reynolds UAVapplications as described in reference 5. The advantage of slotted two-element airfoil concept isthe built-in possibility of flap deflection providing camber shape adaptation to missionrequirement. The known disadvantage of this concept is increased airfoil drag. The increased

drag, caused by opened slot, that can have small effect for high-lift low-speed loitering flight,becomes significantly undesirable factor for high speed, low-lift flight. Presented on fig. 7airfoil TE-300 was designed for ADE-IAI join work and tested in Kanpur WT facility. Design

Reynolds number was Re=300K, the main design tool was MSES. Airfoil TE-300 employs zeroflap deflection (nominal flap setting) for loitering flight. The flap negative deflection position

(approx. -10 degrees) provides decambering option meaning option for drag reduction at low-lift high speed flight. Positive flap deflections provide maximum lift increment, for take-off orlanding. High positive flap deflections on the order of 60 degrees can provide drag for air brakeoption.

TE-300 nominal setting provides aerodynamically effective positive and negative deflections

up to 20 degrees (symmetrical deflection for both directions) that can be used for roll control.The hinge point location was chosen to provide suitable gap and overlap range for the whole

range of deflections. Another consideration for hinge location was to provide adequate hingemoments. The hinge point is located forward of flap's neutral point. The relative motionbetween main body and flap leading edge provides effective flow inclination on the flap.

The lift characteristics of TE-300 airfoil for Re=300K and NLF conditions are presented onfig. 8, both for for MSES numerical simulations (fig. 8a) and for WT tests results (fig. 8b), for

flap deflections range -20+20 deg. The design provided good stall characteristics. There is nohysteresis loop for nominal and for positive flap deflections (fig. 9). Airfoil two-elementconcept allows in this case an increase of maximum lift up to level Cl~2.5 for deflected flap andCl~2.3 for nominal flap position. Availability of flap deflections to negative and positivedirections provides full flight envelope with acceptable drag levels. This result could not beachieved by conventional single-element airfoil.

The second concept is two-element airfoil with retractable flap. The airfoil whichwas designed through this concept and tested in WT we named FH-300. The second


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element (flap) provides only positive deflections with a fixed hinge location. The flap

can not be used as an aileron wing section. The main advantage of this concept is higher

maximum lift than for the 2 element slotted airfoil.. Another advantage is lower drag at

low lift coefficients because the flap can be retracted to zero deflection. In this position

the airfoil provides drag level similar to single-element airfoil in contrary toconventional two-element airfoil with permanently opened slot.

The drawback of this concept is high hinge moment. The production complexity is

similar to a permanently slotted two-element airfoil.

This concept is similar to wide use of Fowler flap on commercial aircrafts, where the

second element deflects and moves backward. The reason of not using the Fowler flap

option in present work is its higher production and installation complexity. The

complexity of production and installation is especially problematic for small low-cost

UAV.

The FH-300 airfoil geometry is presented in fig. 10.

Airfoil was designed with flap chord 30% of global chord. The flap chord length is a

compromise between the high lift requirement and the room required for flow recoveryon main element. The airfoil with fixed hinge concept is designed only for two flap

positions: the retracted flap and opened (deflected flap). Airfoil with retracted flap looks

(and works) as single-element airfoil. The flap deflection provides increment of camber

shape and at the same time also chord increment (fig. 10). The special problem of fixed

hinge point concept is hinge point location. Hinge point is located forward the flap

leading edge and far downward the lower surface. The downward location of the hinge

point increases actuator arm size and creates additional drag. The hinge axis forward the

leading edge causes high hinge moment that requires additional actuator power. These

disadvantages were taken into consideration for the fixed hinge concept.

Airfoil FH-300 design Reynolds number was Re=300K. Lift computations for NLF

flow and Re=300K conditions are presented on fig. 11 for retracted and deflected flap

positions. The airfoil was tested in WT in Kanpur, India. Test-theory comparisons are

presented for retracted flap on fig. 12 and for deflected flap on fig. 13, respectively.

Lift curves are compared on fig. 12 and drag polars on fig. 13 respectively. Fig. 13

illustrates the achievement of high maximum lift of fixed hinge airfoil. The comparison

with slotted two-element airfoil and single-element airfoil is presented on fig. 14, where

the fixed hinge airfoil maximum lift is the highest. The MSES-calculated drag polars are

compared on fig. 15 for NLF flow Re=300K conditions. The drag of fixed hinge airfoil

is lower than slotted airfoil for positively deflected flap as shown on fig. 15a. For high

speed flight (low lift) FH-300 airfoil demonstrates drag level close to singleelement

airfoil B-19, and less than for slotted two-element airfoil TE-300, not only for nominalsetting but even for decambering option flap deflection (fig. 15b).

7. High-lift wing concept

Previous chapters described two-dimensional airfoils design and analysis. Presented high-lift

airfoils belong to one family with similar front part geometry. This fact facilitates to use thedesigned airfoils as wing sections for UAV wing. The wing based on single-element airfoil

could provide limited lift capabilities. Wing based on full-span slotted two-element airfoilconfirmed long-endurance advantages (the main well-known sample is Heron UAV).Nevertheless, wing based on slotted airfoil may have some drawbacks especially for low-lifthigh speed flight.

Implementation of fixed hinge airfoil as UAV wing section requires several considerations.

The fixed hinge high-lift airfoil does not have option for negative deflections, hence couldn't beused as aileron section. To satisfy for aircraft requirements a complex high-lift wing design is


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proposed and schematically shown on fig. 16. The inboard wing (flap sections) is based on fixedhinge two-element airfoil and the outboard wing (aileron sections) is based on slotted two-element airfoil. The single-element airfoil with similar front part geometry will be used likewing tip to complement the proposed airfoils.

Proposed wing provides not only high maximum lift, but may reduce drag for cruise flight

flap flight, by using retracted flap position. High level of sectional maximum lift on outboardwing provides full control for all flight conditions. Low drag for cruise speed flight providesincreased mission range, while high-lift characteristics provide STOL capabilities.

8. Summary

- high-lift airfoils of different concepts were designed during joined ADE-IAI work- the airfoils representing each design concept were tested in wind tunnel and confirmed high-lift capabilities- the advantage of high-lift concept for UAV wing design was evaluated- different approaches for two-element airfoils are presented- complex high-lift wing concept based on the designed high-lift two-element airfoils is

proposed9. Acknowledgment

The author would like to thank prof. Kamal Poddar (IIT, Kanpur) for active contribution andrecommendations during the airfoil wind tunnel testing. The staff of Low Speed Aerodynamics

lab and National Wind Tunnel Facility (Indian Institute of Technology, Kanpur) is gratefullyacknowledgedfor technical support and assistance.

10. References.

1. Anderson, J., D., "Fundamentals of Aerodynamics", 3rd edition, McGraw-Hill Series inAeronautical and Aerospace Engineering, NY, 2001

2. Abbott, I.H. and Von Doenhoff, A.E., Theory of Wing Sections, Dover Publications Inc. , N.Y.,1959.

3. Torenbeek, E., "Synthesis of Subsonic Airplane Design", Delft University Press, Delft,Netherlands, 1982.

4. Hoerner, S., F., and Borst, H., B., "Fluid Dynamic Lift" Hoerner fluid Dynamics, N.J., US, 1975.5. Nagel A., "High-Lift Low Reynolds Wings for UAV - development potential", 5 th symposium on

Applied aerodynamics and design of Aerospace Vehicles, Bangalore, India, 16-18 November 2011

6. Poddar, K., and Sharma, D.M., "Investigations on Quasi-Steady Characteristics for an AirfoilOscillating at Low Reduced Frequencies", International Journal of aerospace Engineering

Volume 2010, 2010.7. Poddar, K., and Sharma, D.M., "Experimental Investigations of laminar separation Bubble for a

Flow Past an airfoil",Proceeding of the ASME Turbo-Expo Conference, Glasgow, UK, June 2010.

8. Sankar G., Bhavneet, Bauminger S., Nagel A., "High-Lift technology for low Reynolds UAVairfoils design", 5

thsymposium on Applied aerodynamics and design of Aerospace Vehicles,

Bangalore, India, 16-18 November 20119. Selvaraj K., Sankar G., Bhavneet, K. Poddar, Bauminger S., Nagel A., "High-lift Airfoil for Small

UAV Wing", 52nd

Israel Annual Conference on Aerospace Sciences, February 29-March 1, 2012.

10. Nagel, A., Levy, D.E. and Shepshelovich, M., Conceptual Aerodynamic Evaluation ofMini/Micro UAV, 44

thAIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, 9-12 January

2006.

11. Nagel, A., Klein, Y., and Shepshelovich, M., Development of High-Lift Low Reynolds NumberAirfoils", International Conference on Autonomous Unmanned Vehicles , Bangalore, India, 3-4

April 2009.

12. Nagel, A., and Shepshelovich, M., Development of High-Lift UAV Wings, 24th AIAA AppliedAerodynamics Conference, San Francisco, Ca, 5-8 June 2006.

13. Nagel, A. and Shepshelovich, M., "Wings for Aircrafts", US Patent Application, No. 11/802,139(21.05.2007)


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Fig. 1. High-lift flight concept for operational UAV. Fig. 2. Aspect ratio effect on high-lift flight

Fig. 3. Airfoil B-19 Fig. 4. Airfoil B-19 lift characteristics

Fig. 5. Airfoil B-19 wedge TE Fig. 6. Airfoil B-19 wedge flap lift increment

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

-5 0 5 10 15 20 25 30

Cl

WT

MSES

Re=300K

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-5 0 5 10 15 20 25 30

Cl

WT clean

WT wedge

MSES clean

MSES wedge

Re=300K

10

15

20

25

30

35

0.5 1.0 1.5 2.0 2.5 3.0

CD0=440

CD0=200

CD0=450ctsoperational UAV

CD0=200ctsclean configuration

AR=20

CL

Endurancefac

tor

10

15

20

25

0.5 1.0 1.5 2.0 2.5 3.0

AR=20

AR=10

AR=20

AR=10

CL

Endurancefactor


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Fig. 7. Slotted airfoil TE-300

a) MSES calculations b) WT test results

Fig. 8. Two-element slotted airfoil TE-300 calculated and experimental lift curves

Fig. 7. Airfoil TE-300 lift

a) nominal flap position b) flap positive deflection +15 deg

Fig. 9. Airfoil TE-300 hysteresis effect

Fig. 10. Airfoil FH-300 with fixed hinge retractable flap.

hin e oint

0.5

1.0

1.5

2.0

2.5

0 5 10 15 20 25 30

Cl

acsent

descent

flap retracted,

WT, Re=300K

0.5

1.0

1.5

2.0

2.5

0 5 10 15 20 25 30

Cl

acsent

descent

flap deflected +15 deg,

WT, Re=300K

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-5 0 5 10 15 20

Cl

+20

+10

0

-10

-20

MSES, Re=300K

flap

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-5 0 5 10 15 20

Cl +20

+10

0

-10

-20

WT, Re=300K

flap


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Fig. 11. Airfoil FH-300 lift, MSES Fig. 12. FH-300 test-theory comparison

for retracted flap position

Fig. 13. Airfoil FH-300 test-theory comparison Fig. 14. Airfoils lift comparison.for deflected flap position

Fig. 11. Airfoil FH-300 with deflected flap

a) Deflected flap position b) Retracted flap positionFig. 15. Airfoils drag comparison, MSES

0.0

0.5

1.0

1.5

2.0

-5 0 5 10 15 20

Cl

WT

MSES

flap retracted,

Re=300K

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-5 0 5 10 15 20

Cl

FH300 flap +15

TE300 flap +15

B-19

MSES, Re=300K

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-5 0 5 10 15 20

Cl

flap deflected +15

flap retracted

MSES, Re=300K

0.0

0.5

1.0

1.5

2.0

0.010 0.015 0.020 0.025

Cd

Cl

FH-300 flap retracted

B-19

TE-300 flap -10

MSES, Re=300K

0.0

0.5

1.0

1.5

2.0

2.5

0.010 0.015 0.020 0.025 0.030

Cd

Cl

FH300 flap +15

TE300 flap +15

TE-300 flap 0

MSES, Re=300K

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-5 0 5 10 15 20

Cl

WT

MSES

flap deflected +15 deg,Re=300K


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Fig. 16. High-lift wing principal scheme.

Inboard wing:

fixed-hinge

two-element airfoil

Outboard wing:

two-element slotted

airfoil

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