[american institute of aeronautics and astronautics 2000 world aviation conference - san...

(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

AOO-45043 2000-01-5572

Realization of a Shapevariable Fowler Flapfor Transport Aircraft

Christian Anhalt, Elmar Breitbach and Delf SachauGerman Aerospace Center, Institute of Structural Mechanics

2000 World Aviation ConferenceOctober 10-12, 2000

San Diego, CA

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2000-01-5572

Realization of a Shapevariable FowlerFlap for Transport Aircraft

Christian Anhalt, Elmar Breitbach and Delf SachauGerman Aerospace Center, Institute of Structural Mechanics

Copyright 2000 by German Aerospace Center. Published by SAE International, and the American Institute of Aeronauticsand Astronautics, Inc. with permission.

ABSTRACT

This paper presents a part of the majorR&D project "Adaptive Wing (ADIF)" of thepartners German Aerospace Center (DLR),DaimlerChrysler Research, and Daimler-Chrysler Aerospace Airbus. Using as anexample the Airbus A330/340 fowler flap, theobjective of this project is to show, how the

high lift behavior of modern transport aircraftcan be improved by intelligent structural con-cepts, without negative effects on cruiseflight. Even an improvement in transonicflight is possible. To reach this aim, a combi-nation of passive and active measures isnecessary, as the following paper shows.

INTRODUCTION

Wings of modern transport aircraft areoptimized for cruise flight. They have tran-sonic wing profiles with the best lift/drag ratioat cruise speed. In the low speed range thisprofiles are rather inefficient. Therefore, therequired additional lift for take off and landinghas to be achieved by using high lift devices,such as slats at the wing leading edge andflaps at the trailing edge [1]. In cruise con-figurations slats and flaps are retracted. Aspecial type of a high lift device is the fowlerflap, used in the Airbus A330/340. The liftincreasing effect of this flap is based on en-larging the wing area, increasing the airfoilcamber, and accelerating the airflow withinthe gap between wing and extended flap(Fig.1).However, the fowler flap used at the AirbusA340 has some disadvantages. Due to theflexibility of the carbon fiber composites, ofwhich the flap skin is made, the trailing edgeof the flap deforms when air loads are ap-plied. These deformations lead to higherdrag rates during cruise flight, resulting in adecreased efficiency of the airplane.

Another disadvantage is the necessity ofthree flaptracks, which means higher weight,higher complexity, and higher costs, com-pared to a two track support. The middletrack is used to force the elastic line of theflap into the same shape as the elastic line ofthe wing in this region. Matching elastic linesare necessary to reach proper shape of thegap between wing and flap. The accelerationof the airflow inside the gap is based on thenozzle effect. If the gap is too wide, there willbe no adequate acceleration. On the otherhand, if the gap is too narrow, it will beplugged up with boundary layer, hamperingair transition from the lower side of the wingthrough the gap to the upper side. In bothcases there is no adequate energy increasein airflow on the upper side of the wing, en-tailing flow separation. The lift decrease dueto separation can not be accepted in the verycritical situations, especially during take-offand landing. This means, that the size of thegap is crucial for the efficiency of the flap, i.e. for the overall high lift behavior of the air-craft.


Figure 1: Airfoil with extended slat and flap

OBJECTIVES OF THE PROJECT

The focus under the topic "Shapevari-able Fowler Flap" is the decrease of drag,complexity, weight and costs. To realize this,the flap track number at the outer flap of theA340 will be reduced from three to two.Therefore, a totally novel design has beenchosen, aiming a considerably higher stiff-ness in spanwise direction. In the course ofthis re-design it is possible to gain a muchbetter control of the gap shape, thus im-proving the high lift behavior. To reach thisaim, passive means such as optimization ofthe stiffness distribution and track positions,pre-deformation, and aeroelastic tailoring as

well as active means like the integration ofan actuator system into the flap is necessary.One positive effect of the higher flap stiffnessis the decrease in deformation at the flaptrailing edge during cruise flight, leading tolower drag. With a structurally integratedactuator system it is also possible to reducevibrations, induced by external loads. Fur-thermore, the additional mechanical loadingof the wing due to the middle flap track dis-appears and the desired elastic line of theflap is accomplished with flap-internal con-straints.

RE-DESIGN OF THE FOWLER FLAP

The re-design of the Airbus A340 fowlerflap, supported by two flap tracks is per-formed by the following steps:

1. Identification of the crucial loadcases.

2. Determination of the elastic line ofthe wing as a reference for the elas-tic line of the flap.

3. Determination of an optimal trackpositioning for minimal flap deforma-tion, using a simple beam model.

4. Calculation of the required stiffnessdistribution leading to the desiredflap deformation with applied airloads.

5. Design of a structurally integratedactuator allowing an adaptivematching of the elastic lines.

6. Transfer of the stiffness distribution,as derived from the beam model, to


the real flap, using a parametric finiteelement model of the fowler flap, in-cluding spars, ribs, stringers, andtailored skin. Pre-deformation, aero-elastic tailoring and the integratedactuator system are taken into con-sideration.

7. Detailed design of critical parts, e.g.the spar/actuator interface with anextremely high stiffness.

8. Examination of the dynamic behaviorof the flap with integrated actuator.

To show, how the re-design actually is con-ducted, all the above mentioned steps aredescribed below in detail.

A variety of some hundred load cases haveto be considered in the design phase. Forthe preliminary design of the two-track-flapconfiguration it is both, impossible and un-necessary to consider all of them. Instead,only four dimensioning load cases were cho-sen [2].

Load case RS1 is defined by cruise loadswith the flap retracted. This load case is cru-cial for the cruise flight stiffness design. Another load case with the flap retracted is

defined by the recovery loads after an emer-gency dive (R2). This load case, with thehighest possible air loads is used for thestrength design of the flap. At the remainingtwo load cases S17 (take-off) and S32(landing) the flap is extended with 17° and32°, respectively. At these load cases thestiffness design, crucial for the gap shape, isperformed. These four load cases are suffi-cient to define the main part of a preliminaryload envelope.

The elastic line of the wing is the referencefor the calculation of the flap stiffness. Bothelastic lines have to match tightly in order toguarantee a constant gap size. With a knownreference line and known air loads it is pos-sible to calculate the necessary flap stiffnessdistribution for all four load cases. The elasticline of the wing (reference line) is calculatedwith local deformations at the flap tracks andthe spoiler hinge line. There is a bending-torsion-coupling due to the wing sweep (Fig.2). This coupling leads to the necessity ofaligning the flap stiffness axis with a certainangle against the spanwise direction, knownas aeroelastic tailoring [3,4,5]. With thismeasure it is possible to accomplish, that nogaps arise between inner and outer flap orbetween flap and aileron.

Figure 2: Elastic line of the wing


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Figure 3: Elastic lines of wing and flap for all four load cases

Fig. 3 shows the elastic lines of wingand flap for all four load cases considered. Itis clearly visible, that there is a tight matchbetween the wings elastic line and the elasticline of the flap, especially when the flap isextended (S17 and S32). The match in RS1and R2 (retracted flap) has not to be ex-tremely tight, because there are so-calledcruise rollers at both ends of the flap. Theymake sure, that there are no gaps betweeninner and outer flap or outer flap and aileron.The chosen track positions are at 2.9 m and8.1 m for a flap span of 10.7 m.

The determination of the best track po-sitions is extremely important for the defor-

mation of the flap, as shown in Fig. 4. Themaximum deflection (or actually inverse de-flection) of the flap for all possible flap trackpositions for one load case, displayed in thisfigure, has been calculated. Between thebest and the worst position is a difference ofapproximately 3500% in flap deformation,visible at the vertical axis. The left hand hori-zontal axis shows the position of the left flaptrack, the right hand horizontal axis the posi-tion of the right one. There is a peak at 2.9 mand 8.1 m in the inverse deflection display,indicating minimal deformation of the flap.


Figure 4: Optimization of the flap track positions

The integration matrix method applied to abeam model of the flap has been used forsteps 3, 4, and 5 of the re-design. There aresome advantages of this method, comparedto a complex FE-model [6]. First of all, thecalculation time is much smaller, makingparameter studies more efficient. Thus, loopprogramming for a stiffness optimizationbecomes very simple. A further big advan-tage of this method is the possibility to simplyintegrate an actuator into the model. It ispossible to load the beam either with a single

actuator force or an equivalent active bend-ing moment distribution. That makes it easyto show, how an actuator can influence theelastic line of the flap.

With the determined optimal track posi-tions for all four load cases is it possible tocalculate the required stiffness distributionsin order to achieve the desired elastic lines.As an example, the necessary stiffness dis-tribution for one load case is shown in Fig.5.

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Figure 5: Spanwise stiffness distribution of the flap


The green line indicates the original stiffnessof the flap with three tracks. Determining anaverage stiffness level from the green lineand adding the additional stiffness of the tworemaining tracks gives the blue line distribu-tion. The red line indicates the necessarystiffness distribution by scaling up the blueline distribution, until the deflection of the flapis in the desired range.

There is a difference between the fourstiffness distributions, depending on differentloads and reference lines. Therefore, thechosen passive flap stiffness can only be acompromise with special emphasis on thetwo extended flap load cases.The adaptation between the elastic lines atdifferent load cases can only be achievedwith an active adaptation of the stiffnessdistribution. Therefore, an actuator system

has to be integrated into the main load pathof the structure. Two different types of ac-tuator systems are possible, a discrete ac-tuator system for a discrete active momentdistribution or a distributed actuator system[7]. Both types have advantages and disad-vantages.A distributed actuator may be integrated intothe skin of the flap. The advantage is, that nospecial load introduction devices are neces-sary. This type of actuation is the most effi-cient one. Unfortunately there are some criti-cal disadvantages: Present array-actuators(e.g. PVDF) are not able to withstand theapplied loads (forces, moments, and tem-peratures) and can not be activated to suffi-ciently large deformations. Another problemis the integration into the skin without forcingdelaminations.

titanium insert

Figure 6: Integration of the actuator into the spar

On the other hand, the discrete actuationneeds special devices to redistribute discreteloads continuously into the structure. Suchan interface has to be extremely stiff, other-wise the stroke of the actuator is made ineffi-cient due to the structural flexibility. There isa big variety of discrete actuators, for exam-ple piezo stacks, shape memory alloys oreven hydraulic systems. For the best effi-ciency discrete actuators have to be inte-grated into the flap spar, because of its highstiffness concentration. The other structuralelements like ribs, stringers, and the skinremain passive elements.

A concept for the integration of an actuator isshown in fig. 6. The spar has an extremelynarrow gap on the upper side (compressionside). The gap is reinforced with a titaniuminsert for a smoother load distribution into thespar. When air loads are applied, both sidesof the gap are pressed together. In this case,the spar acts like having no gap.The remaining cross section of the spar mustbe able to withstand all occurring negativeloads, e.g. landing shocks or gust loads. Astopper on the upper side of the spar guar-antees, that in these cases no unacceptablelarge deformations occur. Due to the low


negative loads, compared with the positivedesign loads, the stopper is only a safetydevice. Under normal circumstances it willnot be used.With the integration matrix method is it pos-sible to calculate the deformation of the flapunder active forces and moments. Prelimi-nary estimations have shown, that the weightof a piezoceramic stack actuator is approxi-mately 55 kg per flap, compared with 140 kgweight decrease of the two flap track con-struction.Another discrete actuator, actually a combi-nation of a distributed and discrete actuatoris used in a demonstrator of the GermanAerospace Center. Wires, made of shapememory alloy (SMA) are integrated into thetension side of the spar. When the flap isdeformed by air loads, the SMA wires as wellas the lower part of the flap are stretched.Heating the wires leads to a contraction ofthem, resulting in a re-deformation of theentire flap. In this way it is possible to adapt

the flaps deformation to the different loadcases. Due to the integration of the SMAwires into the spar it is not necessary to havea certain load distribution device as neces-sary for the piezo actuator.

All the above mentioned calculationswere performed with a beam model and theintegration matrix method, resulting in therequired stiffness distributions. To realize thisconcept in a real flap structure a parametricfinite element model with spars, ribs, string-ers, and tailored skin has to be designed(Fig. 7). The dimensions of the structuralelements have to be varied, in order to findthe solution with the lowest possible weight.The actuator can be modeled as a substi-tute-stiffness. By means of an FE-model it isalso possible to find the optimal fiber anglefor the bending torsion coupling, in order toprevent gaps at the inner and outer side ofthe flap.

Figure 7: Finite element model of the fowler flap

After finishing the global flap designdetails have to be examined by using sub-models. One of the most difficult problems isthe above mentioned load interface betweenspar and the discrete actuator. The spar hasto be very stiff in order to provide an efficientstructural integration, on the other hand thespar must have a certain flexibility to be ableto be deformed by the actuator [8].Another difficult detail is due to the connec-tion of the tracks and the flap itself. All forcesbetween wing and flap are carried by thetracks. These forces are very high and haveto be continuously distributed into the flap

without causing and damage. Therefore thetracks have to be modeled, too.

In case piezoceramic stack actuators orhydraulic systems are used to statically de-form the flap in the desired way, the dynamicproperties of the flap have necessarily to bedetermined. Although it was not a primaryobjective of the re-design, a vibration andflutter control will be necessary for futureaircraft. At the moment no critical vibrationproblems have been identified, but with in-creasing size and flexibility of future flapsystems (A3XX) this problem will becomecrucial. The influence of an actuator on the


dynamic behavior of the flap can efficientlybe determined by means of a modal formu-lation, based on a thorough modal analysis.The resulting modal equations are the basicsfor a subsequent Harmonic Response

Analysis. The reaction of the flap structure tothe actuators stimulation can easily be de-termined through such a modal predictionconcept.

PROBLEMS TO BE SOLVED

There are several problems that have tobe solved and have already been solvedduring the re-design of the flap. The followinglist shows the most important problems,without claiming to be complete.

The optimization of the track positionsand the stiffness distribution depends ona big variety of independent variables.Therefore, it is very difficult to find an ab-solute optimum.

The transformation of the beam modelbased stiffness distribution to a real flapoffers a wide choice of structural options.Main emphasis has to be placed on thesolution with minimum structural weight.

There must be a reliable pre-informationon the gapsize, in order to know how theflap has to be deformed by an actuatorsystem. Two ways are possible, either bya sensor system and a real time control-ler, or with measured information charts,depending on the angle of attack, speedand actual weight of the aircraft. If possi-ble, the second way should be favoreddue to lower costs and higher reliability.

The spar/actuator interface must have anextremely high stiffness, in order to pro-vide a good actuator performance.

The fail safe concept has to be fulfilled,even if the actuator fails. Otherwise, itwill be not possible to achieve the FAA-or JAA - certification for the aircraft.

If piezoceramic stacks or shape memoryalloys are used, the power supply canbecome problematic due to the specialenergy requirement of these actuators.The usage of hydraulic actuators doesnot bear any problem, because of theavailability of hydraulic systems in trans-port aircraft.

The accessibility and maintainability ofthe active components must be guaran-teed, otherwise these elements have tobe maintenance-free. The service life ofactuators or at least the TBO (time be-tween overhaul) has to be as long as thenormal maintenance interval of the flapmechanism.

The major problem is the price of anactive fowler flap. All efforts have beenmade to increase the overall aircraft effi-ciency. If the profit achieved by thesemeans is lower than their costs (initialand maintenance costs), it is not reason-able to change the flap design.

CONCLUSIONS

The paper presented describes an ap-proach to save one flaptrack of a fowler flap,for the example of the Airbus A340. Thisrequires a totally new flap design in order tofulfill the aerodynamic demands. In thecourse of this re-design, a configuration canbe found that improves the high lift perform-ance as well as the cruise flight efficiency.These improvements are the result of asmart combination of passive and active

means. The passive re-design must reach ahigh level of reliability, so that the activeparts of the flap are only necessary for afine-tuning. Therefore, a combination of op-timal track positioning, reasonable stiffnessdistributions (including aeroelastic tailoring),pre-deformation of the flap, and integration ofactuators is necessary. First results showthat the desired objectives are achievable.


ACKNOWLEDGEMENTS

The present investigation was carried Aerospace Airbus as part of the major re-out by the Department of Adaptronics of the search & development project ADIF (Adap-DLR Institute of Structural Mechanics in co- tiver Fluegel - Adaptive Wing),operation with the partner DaimlerChrysler

REFERENCES

1. Roskam, J.; "Airplane Design Part I-VIH"; DAR Corporation, Lawrence, Kansas, USA; 1996

2. "Report No. 27 F021 K7140 C02, Deflections of Wingbox for Movable Parts at Trailing Edge,Airbus A340-300"; Deutsche Aerospace Airbus; 1994

3. Niu, M.; "Composite Airframe Structures"; Hong Kong Conmilit Press; Hong Kong; ISBN 962-7128-06-6; 1996

4. Niu, M.; "Airframe Structural Design"; Hong Kong Conmilit Press; Hong Kong; ISBN 962-7128-09-0; 1988

5. Moser, K.; "Faser-Kunststoff-Verbund"; VDI-Verlag GmbH; Dusseldorf; ISBN 3-18-401187-9;1992

6. Biiter, A.; "Untersuchung adaptiver Konzepte zur Reduktion von Hubschraubervibrationen,zur Minderung des Hubschrauberlarms und zur Steigerung der aerodynamischen Effizienz";Dissertation RWTH Aachen; 1998

7. Barrett, R.; "Introduction to Adaptive Aerostructures"; Conference at the University of Stutt-gart, Germany ; 1998

8. Breitbach, E.; "Vorlesungsskript Adaptronik"; Technische Universitat Braunschweig; 1998

CONTACT

German Aerospace Center (DLR)

Institute of Structural MechanicsLilienthalplatz 7

38108 BraunschweigGermany

Dipl.-lng. C. Anhalt Prof. Dr.-lng. E. Breitbach Dr.-lng. D. Sachau

fon +49(531)295-2354 fon +49(531)295-2300 fon +49(531)295-2316fax +49(531)295-2876 fax +49(531)295-2875 fax +49(531)295-2876e-mail [email protected] e-mail [email protected] e-mail [email protected]

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