Morphing Conceptual Design – A320 Vertical Tail

Joao Antonio Bastos Brandao Moreira Figueiredojoaoantoniofigueiredo@tecnico.ulisboa.pt

Instituto Superior Tecnico, Universidade de Lisboa, Portugal

November 2018

Abstract

In the last decades, flight transportation has grown and environmental requirements have becomemore stringent. This challenges the aerospace industry to design more efficient aircrafts with lowerfuel consumption per passenger. An improvement opportunity was identified on the Vertical Tail ofthe AIRBUS A320 where a morphing design of the rudder could contribute towards these objectives.Several morphing concepts exist based on state-of-art materials and technologies but just a few couldbe certified and implemented on a civil aircrafts. A group of morphing rudder solutions was createdand a trade-off analysis performed. A conceptual design based on a composite bended skin was selectedfor further investigation. The large displacements involved in this morphing movement induce highstrains in the material and further analyses should be performed to check the feasibility of this design.For this, the skin was modelled and studied in detail with 2D FEM Strip Models. The nature ofthe problem implies a Non-Linear Geometric FEM analysis. The morphed shapes obtained with theanalysis were compared to the target ones. Furthermore, the most important allowables were checkedfor the composite skin under bending and air loading. The locations with highest strains were identifiedas well as the structure characteristic behaviour. An optimization process was done in order to reducethe structural weight. This research proves that current AIRBUS qualified materials can achievemorphing, in the case of the A320 VTP, and contributes to a better understanding on the behaviour ofbended composite plates.Keywords: Morphing, Morphing Trailing Edge, Variable Camber, Morphing Structures, VerticalStabilizer, Morphing Rudder

Glossary

FEM Finite Element MethodLIP Load Introduction PointMSL Morphing Starting LineMSP Morphing Starting PointVTP Vertical Tail Plane

1. Introduction

This document condensates the most importanttopics and conclusions from the Master Thesis:”Morphing Conceptual Design – A320 VerticalTail” and gives a first insight on the design of aMorphing Rudder. A full thesis reading is requiredto have a better understanding on the involved phe-nomena, engineering assumptions and results. Fig-ure 1 shows the current A320 Vertical Fin. ThisMaster Thesis Research was done within AIRBUSOperations GmbH, in Bremen, Germany.

Figure 1: Airbus A320 Vertical Tail[2]

2. Motivation

An improvement opportunity was identified on aconventional Vertical Tail Plane (VTP) design. Themorphing capabilities could theoretically increasethe aerodynamic efficiency of this component andconsequently reducing the fuel burn and flying costper passenger per kilometre.

A new morphing configuration would increase

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the aerodynamic efficiency of the rudder, avoidingsharp corners and delaying flow separation (see Fig-ure 2).

Figure 2: Flow behaviour difference in a sharp andround corner

Thus, a smaller vertical tail would provide thesame lift needed to control and stabilize the air-craft laterally. If the VTP size is reduced, the wetarea in contact with the exterior flow will decrease,reducing the drag in cruise, comparing to the cur-rent configuration. At least three scientific articles( [1], [4] and [5]) concluded that this technologycould lead to real improvements (around 15%) inthe rudder aerodynamic efficiency. One of the maingoals of this research is to create a structure andsystem that could be industrially implemented inmaximum 5 years.

3. Morphing Concepts and Technologies

The selection of a specific morphing concept, able toachieve the aerodynamic goals, is a complex processand dependents on several interconnected variables.In order to select a concept with a high probabil-ity of success, an appropriate trade-off analysis wasperformed, taking into account the most significantparameters:

� Design Maturity / Time to entry into service

� Mechanism / Actuation Simplicity

� Manufacturing Cost

� Maintenance / Operationability

� Damage Tolerance / Reliability

� Weight

Several concepts were developed, using differ-ent technologies with different levels of maturityin order to compare solutions and select the mostpromising ones. Different weights were attributedto each parameter for all the concepts. Figure 3shows the final concepts scores.

The selected concept, shown in Figure 4, is called:”Crocodile Concept”. This concept is based on afinger kinematic system, connected via rollers to themorphing composite skins.

Figure 3: Trade-off plot comparing different mor-phing concepts

Figure 4: Crocodile Concept

4. Target Shape for Morphing

4.1. Input Morphing Shape

Usually, the aerodynamic department defines an op-timum morphing profile shape and the structuraland systems engineers should design a structure andkinematics that could achieve that shape in the bestpossible way. This is the methodology used in theresearch, thus the base aerodynamic shape is con-sidered as an input for the structural assessment.The basis for a morphing profile was provided byAIRBUS Hamburg and it was based on 2D aerody-namic calculations and structural linear analyses, inthe VTP root section. This basis shape was adaptedto get a better profile, but not checked again.

4.2. VTP Section Selection Excel®

An Excel ®program - ”VTP Section Selection” -was created to obtain any desired VTP cross-sectiongeometry and other relevant parameters based onthe reference data available. The VTP 3D extrap-olated geometry, was based on a cross-section drawfrom an AIRBUS A320 Vertical Tail. Figure 5shows the full VTP model obtained.

This program allows the computation of severalrelevant parameters and values listed below:

� Determine any cross-section (Morphed andNon-Morphed) Spanwise;

� Calculate Conserved Skin Length;

� Calculate profile coordinates perpendicularlyto any desired tapered line;

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Figure 5: VTP Geometry Extrapolation

� Ability to calculate X and Z displacements of aspecific skin point, before and after morphing(used as an input in FEM Model);

4.3. Skin Length Conservation

Figure 6 shows the Non-Deformed configuration(with red dots) and the Morphing Input Shape.This shape doesn’t considers the conservation ofthe skin length during morphing movement. Forthis reason the excessive skin length was determined(red line in Figure 6) and subtracted from the orig-inal length.

Figure 6: Morphed and Non-Morphed Input Geom-etry - Excessive Skin Length Computation

The input points of the morphing shape weremoved in order to have a new shape where theskin length is conserved. This geometry was definedbased on the usage of a Spline Ruler, conserving thelength and taking into account some additional rec-ommendations from the aerodynamic department.The new root shape obtained with this process isshown in Figure 7. This shape was then extrapo-lated all across the VTP full model.

This program is able to determine this morph-ing cross-section in planes not parallel to the VTProot. This is important because the skin deforma-tion will probably occur in a sloped plane due toaerodynamic and structural purposes.

Figure 7: Corrected Upper Camber Skin Geometry

4.4. Morphing Starting Line Location

After defining the target aerodynamic profile, thelast point (downstream) connected to the rear VTPspar, where all degrees of freedom are constrained(fixed constrain) should be defined. This point wascalled as Morphing Starting Point (MSP), wherethe bending starts. The group of MSP from theroot until the VTP tip is called Morphing StartingLine (MSL). The geometric final shape obtained inthe 3D Vertical Tail depends a lot in several kine-matic and load-dependent parameters (like VTPbending). The 2D strip model used can’t predictthose effects and considers that the final shape isthe same for all the cross sections. The twisting dueto curvature radius variation spanwise can’t be pre-dicted with this program. It was considered that, toavoid excessive twisting in the real VTP, the Mor-phing Starting Line should be tapered as the wholestructure and the morphing movement should hap-pen in a plane perpendicular to it. This guaranteesthat the aerodynamic shape is constant spanwise,maintaining the geometry proportion, from root totip as shown in Figure 8.

Figure 8: Morphing Perpendicular to MorphingStarting Line

This means that both X and Z distances betweentwo consecutive points in the profile shape vary lin-early spanwise but the global shape remains thesame. This was assumed both in the 2D and in

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the 3D VTP FEM Models. If the rudder is mor-phed in a different direction than the perpendicularone, the proportion of the aerodynamic shape willprobably change spanwise and torsional non-desiredeffects can happen.

The value of 65% of the chord length was chosenfor the Morphing Starting Point/Line. This pointis far enough from the RS (allowing attachment andmaintenance – in the VTP tip the margin is around200 mm – space needed to insert a human hand)and far enough from the region that should move(Trailing edge TIP). This is just a first iteration andother MSL can be analysed in the future. An arbi-trary section, perpendicular to the MSL defined, isshown in Figure 9.

Figure 9: Final Location of the Morphing StartingPoint

5. Morphing Solution

5.1. Optimization Definition

The design of an entire morphing VTP involves alot of different interconnected subjects, thus, someinitial simplifications were done to reduce the num-ber of variables and allow a fastest system initialanalysis. The final objective of a morphing de-sign like this is to find an optimum location of theLoad Introduction Points (LIP) fixing the skin tothe kinematics (see Figure 10) and the thicknessdistribution(span and chordwise), ensuring the fol-lowing design characteristics:

� Correct geometric desired morphed shape;

� Stresses and strains are below the materialstress/strain allowables;

� The skin waviness generated by the air load iswithin an allowed range;

� The structure has stability under air loading;

� Low actuation force needed to move and tomaintain deformed position;

� Light structure;

� The structure has a good damage tolerance.

Due to the high complexity of the problem andthe time constrains, this specific research focusedon the upper camber skin behaviour, the chord-wise distance between Load Introduction Points

Figure 10: Final 3D Optimization Output: LIP Dis-tances Chord and Spanwise

(Fig.10) and the spanwise thickness distribution. Itwas found that the most relevant phenomena con-straining the design would be the strains due tothe skin bending (in the tip) and the airload wavi-ness(in the root section). For this reason it wasdecided to analyse both the lowest and the high-est sections available in the ”VTP Section Selec-tion” Program. In order to simplify this complex3D optimization problem, the chordwise and span-wise effects/deformations were decoupled. Two ini-tial chordwise 2D strip models were build in theextreme sections, as shown in Figure 11.

Figure 11: VTP 2D Strip Models

It was assumed that the strains, stresses and dis-placements in the skin portion bounded by thesetwo strips are also in between the extreme valuesdetermined on those strip models. Thus, a studyon these two sections would be representative and

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conservative in all the sections in between. It wasconsidered that the strains and displacements varylinearly from the root to VTP tip. The consid-ered input variables in the 2D models optimizationstudy, both for Tip and Root Sections were:

� Thickness of the Strip Models

� Chordwise Location of the Load IntroductionPoints

The structural and geometrical output param-eters (strain, shape deviation...) were checkedagainst several criteria and allowables. There aretwo variables and multiple simultaneous goals, thusit is not possible to optimize to a unique designpoint. Instead of that, it was decided to create anoutput matrix of points, for the two variables. Theresultant matrix was evaluated against the definedrequirements.

5.2. Optimization Variables

5.2.1 Number and Location of Load IntroductionPoints

Typically, the minimum number of Load Introduc-tion Points is desired because it usually correspondsto the lightest configuration and the simplest kine-matic system. For this reason, some analyses wereperformed with only one LIP. It was concluded thatthe obtained shapes don’t coincide with the targetones and an additional LIP is needed. In order tosimplify the optimization procedure one LIP wasfixed near the rudder trailing edge, to increase thetip stiffness and to reduce the number of variables.The different locations considered for the secondLIP are shown in Figure 12.

Figure 12: Different Load Introduction Points UsedDuring The Optimization Process

In this concept the number of spanwise LIP lines(represented in Figure 10) is constant all over theVTP. This is important because after defining thenumber of LIP lines and distance between them,thickness is the only parameter that could varyspanwise.

5.2.2 Material and Thickness

The material selected to shape the exterior skin wasa carbon fiber composite layup and the main rea-sons for that were: good fatigue proprieties (the

morphing rudder will be cyclically deflected), thematerial is already certified for several aeronauti-cal applications and the manufacturing techniquesare established. The final thicknesses will only beavailable in discrete steps, depending on the currentmanufacturing techniques and commercialized rawmaterial (plies). To obtain those composite plates,with a constant strain linear behaviour under en-forced bending, all the layups were built accord-ingly to similar design rules, available in the liter-ature. This is important because the analyses andoptimization results will not be influenced by thespecific internal composite stack and a global com-posite behaviour can be considered, depending onlyin thickness. The considered layup thicknesses are:

� A - 2,000 mm

� B - 2,540 mm

� C - 3,048 mm

� D - 4,064 mm

� E - 5,080 mm

� F - 6,096 mm

6. Allowables and Structural Criteria

In order to check the feasibility of the design, thefollowing allowables and criteria were compared tothe analyses output:

� Structural Stability

� Maximum Composite Principle Strain

� Deviation from the target shape

� Maximum Skin Waviness Amplitude andSlope due to Air Loading

6.1. Structural Stability

The morphing skin used in this concept can havean unstable behaviour under air loading as shownin Figure 13.

Instability should be avoided by design. Thus,the following criteria were used to confirm the anal-ysis convergence and skin stability under enforcedbending and air loading:

� The Optistruct® analysis should converge af-ter a finite time;

� The strain points correspondent to the ”Bend-ing+Air Load” case in the Strain vs. Thick-ness plot (see example in Figure 14) should bealigned, with a linear trend.

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Figure 13: Skin Instability under Air Loading

Figure 14: Example of Strain vs. Thickness Plotfor a Specific Load Introduction Point Location

6.2. Maximum Composite Principle Strain

To check if the Strain Allowable is accomplished itis important to verify what is the loading case thatinduces the larger strains in the structure. In thisanalysis, the strains correspondent to the ”bend-ing only shape” were used because they are (onlyfor convergent situations) the largest ones comparedto the ”Bending + Air Loads”. This is interestingbecause, in this case, the air loading reduces thestrains within the structure. The considered strainallowable for this structure is:

� Max. Principle Strain < 5000 µε

Figure 15 shows an example of a Maximum Prin-ciple Strain vs. LIP location plot, for all the avail-able thicknesses. The red line indicates the allow-able. With this information it is possible to findthe optimization points that overcome the maxi-mum composite strain allowable.

6.3. Deviation from the target shape

The deviation between the obtained deformed andtarget shapes was measured. The ”bending only”curve was compared to the target one and the dis-tance between them was determined, perpendicu-

Figure 15: Composite Maximum Principle Strainvs. Load Introduction Point Location for the Avail-able Thicknesses

larly to the second one (Figure 16-A), without ap-plied air loading. The shapes closer to the targetones (the ones with the smaller area integral belowdeviation curve) are preferred. This is not a ”go-no-go” requirement. It simply compares differentshapes and the proximity to the target one.

Figure 16: Deviation from Reference Curves

Figure 17 shows the target shape and all the“bending only” shapes, considering different LIP,in the root strip model. It can be observed that,in general, when the LIP moves downstream, theobtained shape get closer to the target shape.

Figure 17: General view of morphing rudder de-flected for different load introduction points (Root)

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Figure 18 shows the distance between the targetshape and the obtained curves (perpendicularly tothe first one). It can be seen the chord percentagewhere separation is expected, marked with a verti-cal line.

Figure 18: Deviation from target shape perpendic-ular to target shape (Root)

Since no further aerodynamic analysis was per-formed for the different obtained shapes and con-sidering that the target shape is the most efficientone, a simple evaluation plot (Figure 19) was donebased on some assumptions:

� Only the region between the MSP and Sepa-ration point is considered and influences theaerodynamic efficiency;

� The difference between the obtained and targetshapes should be minimum - The total positiveand negative areas between the displacementcurve and the x axis should be minimized (thenegatives areas are added as positive);

Figure 19: Area below displacement curve (Root)

In this example, considering the assumptions ex-plained before, LIP 2, 3 and 4 would be equivalentand the preferred ones because they correspond tothe minimum areas. However, LIP 3 has a smalladvantage when compared to the other ones.

6.4. Maximum Skin Waviness Amplitude and Slopedue to Air Loading

AIRBUS has some criteria to evaluate whichlevel of surface waviness can exist in the finalassembled structure. These allowables can varyper structure and per aircraft type, dependingon the aerodynamic needs. Currently, there areno waviness allowables available for a morphingrudder. The chosen allowables provide a generaltrend, but need to be reassessed in case of a moredetailed morphing rudder study. For this study,the considered limit values are:

� Max. Waviness Amplitude < 2 mm

� Max. Mean Slope < 0,005

It was considered that, in this conceptual phase,only air loads can generate waviness (manufacturingimperfections were neglected). For this reason, thisrequirement evaluated the air loading influence inthe skin geometry. This assessment was done com-paring the ”bending only” shape with the ”bend-ing + air loading” one. The distance between thecurves is measured perpendicularly to the ”bendingonly” shape. This process is represented in Figure16-B.

7. Results

After combining all the output data from the FEManalyses, it is possible to know what are the allowedcombinations of load introduction points locationand thicknesses, for the root and tip considered sec-tions. The following Tables 1 (Root) and 2 (Tip)contain all the analyses output data. It is possibleto observe the regions where the different allowablesare accomplished or not.

Table 1: Root Allowables Evaluation Table

Table 2: Tip Allowables Evaluation Table

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In the root Table 1 it can be seen that LayupsA, B and C aren’t allowed because they are unsta-ble. Only the green points respect the allowables.Considering this and the waviness and slope crite-ria, Points 0,1 and 2 are not allowed for the rootdesign.

In the case of the VTP tip, points 0,1 and 2weren’t analysed because they are not allowed. Thismorphing c concept only works if an OptimizationPoint is accepted both in TIP and ROOT analysis- see Section 5.2.1. This is represented by the darkgrey rectangles in Table 2.

It is assumed that the structure with the thin-ner layup will be the lightest one. With the avail-able information and considering that LIP 4 shapecan achieve successfully the aerodynamic target, thelightest structure accomplishing the objectives willhave the configuration shown in Table 3.

Table 3: Final Optimization LIP-Thickness Combi-nation

The final skin volume was computed for this newconfiguration, thus the conventional and morphingdesigns could be compared. Presentation [3] fromAIRBUS defines a target weight for the morphingpanels in this configuration of 70 Kg. As it is nota real component weight, this value can only beused as a reference for further comparisons in thisproject. The main assumptions consist in not con-sidering the load introduction parts because theywill be needed even within the new morphing con-cept. The weight saving is not linear correlated tothe rudder efficiency. However [3] provides an roughestimation on the total VTP weight saving, if theefficiency is globally increased in 15%. For +/- 5%this is linearly extrapolated, accepting a minor in-accuracy. It is assumed that a horizontal cut in thetop of the current geometry in the VTP Tip can re-duce the total weight, not affecting the global aero-dynamic performance. The extra weight needed dueto the new kinematic system within this morphingconcept wasn’t considered in this preliminary study.Table 4 shows the final weight assesment data.

Table 4: Weight Assessment

8. CONCLUSIONS

It was concluded that it is possible to design amorphing rudder with a short-term entry into ser-vice. To achieve this, existing certified/qualifiedmaterials and technologies were selected: a com-posite stack for the exterior skin and a known“finger” configuration for the internal kinematics.This research concluded that the optimum thick-ness should be different in the different extremeVTP sections (Root and Tip) and a skin thicknessvariation should be considered spanwise to accom-plish the relevant allowables and to minimize theweight. It was determined that, on 2D models con-sidered, the “bending only” shape depend solelyon the Load Introduction Points locations and noton the layup thickness. Furthermore, the analy-sis showed that an optimized aerodynamic shapecan only be achieved with a minimum amount oftwo load introduction points. The chordwise po-sition of those two points in the skin influences alot the obtained shape. The best combination ofskin thicknesses and Load Introduction Points wasselected within the allowed ones confirming that,considering the skin point of view, this concept isfeasible. It was concluded that the morphing rudderskin can globally save 19 kg in the VTP due to thereduction in the size of the structure. This researchproves that bended composite plates can be used asmorphing skin, accomplishing all the requirementsand allowables.

References

[1] L. L. e. R. P. A. Concilio, I. Dimino. MorphingTechnology for Advanced Future CommercialAircrafts. Morphing Wing Technologies, 2018July2014. Elsevier.

[2] Clipground. “Clipground: A320side view”. [Online]. Available:http://clipground.com/images/a320-clipart-7.jpg. [Accessed 14 09 2018].

[3] D. Daandels. Morphing Rudder Weight As-sumption for Master Thesis. AIRBUS, Bremen,26.09.2018.

[4] M. A. C. A. A. e C. Cuerno. Aerodynamic Mod-elling for a Morphing Rudder. July2014. RAeSAerodynamics Conference, Bristol.

[5] C. C.-R. e. M. G.-T. M. A. Castillo-Acero. Mor-phing structure for a rudder. The AeronauticalJournal, 120(1230):1291–1313, August 2016. El-sevier.

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