Continuous morphing trailing-edge wing concept based on multi-stable nanomaterial

HAO, Fengqian; TANG, Tao; GAO, Yuan; LI, Yimeng; YI, Shenghui; LU, Jian

Published in:Chinese Journal of Aeronautics

Online published: 10/06/2020

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Published version (DOI):10.1016/j.cja.2020.03.041

Publication details:HAO, F., TANG, T., GAO, Y., LI, Y., YI, S., & LU, J. (2020). Continuous morphing trailing-edge wing conceptbased on multi-stable nanomaterial. Chinese Journal of Aeronautics. https://doi.org/10.1016/j.cja.2020.03.041

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Download date: 28/07/2021

Chinese Journal of Aeronautics, (2020), xxx(xx): xxx–xxx

Chinese Society of Aeronautics and Astronautics& Beihang University

Chinese Journal of Aeronautics

cja@buaa.edu.cnwww.sciencedirect.com

Continuous morphing trailing-edge wing concept

based on multi-stable nanomaterial

* Corresponding author at: Department of Mechanical Engineering,

City University of Hong Kong, Hong Kong 999077, China.

E-mail address: jianlu@cityu.edu.hk (J. LU).

Peer review under responsibility of Editorial Committee of CJA.

Production and hosting by Elsevier

https://doi.org/10.1016/j.cja.2020.03.0411000-9361 � 2020 Chinese Society of Aeronautics and Astronautics. Production and hosting by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: HAO F et al. Continuous morphing trailing-edge wing concept based on multi-stable nanomaterial, Chin J Aeronaut (2020),doi.org/10.1016/j.cja.2020.03.041

Fengqian HAO a, Tao TANG b, Yuan GAO c, Yimeng LI a, Shenghui YI b,

Jian LU a,b,*

aDepartment of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077, ChinabCentre for Advanced Structural Materials, City University of Hong Kong Shenzhen Research Institute, Greater Bay JointDivision, Shenyang National Laboratory for Materials Science, Shenzhen 518057, ChinacBeijing Thm Technology Co., Ltd., Beijing 100084, China

Received 27 December 2019; revised 12 January 2020; accepted 8 February 2020

KEYWORDS

Aerodynamic performance;

Computational fluid dynam-

ics;

Continuous morphing

trailing-edge wing;

Multi-stable nanomaterial;

Surface mechanical attrition

treatment

Abstract Morphing technology is one of the most effective methods to improve the flight efficiency

of aircraft. Traditional control surfaces based morphing method is mature and widely used on cur-

rent civil and military aircraft, but insufficiently effective for the entire flight envelope. Recent

research on morphing wing still faces the challenge that the skin material for morphing should

be both deformable and stiff. In this study, a continuous morphing trailing-edge wing with a

new multi-stable nano skin material fabricated using surface mechanical attrition treatment tech-

nology was proposed and designed. Computational fluid dynamics simulation was used to study

the aerodynamic performance of the continuous morphing trailing-edge wing. Results show that

the lift coefficient increases with the increase of deflection angle and so does the lift-drag ratio at

a small angle of attack. More importantly, compared with the wing using flaps, the continuous mor-

phing trailing-edge wing can reduce drag during the morphing process and its overall aerodynamic

performance is improved at a large angle of attack range. Flow field analysis reveals that the con-

tinuous morphing method can delay flow separation in some situations.� 2020 Chinese Society of Aeronautics andAstronautics. Production and hosting by Elsevier Ltd. This is an

open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

It is not a secret that human beings imagined flying like birdsfor thousands of years. Inspired by the natural world, early

pioneers invented many flying machines, from kite to the firstpowered, manned aircraft (the Wright Flyer), which rapidlystimulated the development of modern aircraft.1 Nowadays,

designers, as always, are attracted by the excellent aerody-namic performance of bird wings. Observation shows that

https://

2 F. HAO et al.

birds will continually change the shape and size of their wingsduring different flight missions, presumably to exploit the pro-found effect of wing morphology on aerodynamic perfor-

mance.2 However, with the increasing demand for highercruise speeds and payloads, modern aircraft are designed moreand more rigid. The rigid design leaves less and less space for

the morphing design from which aircraft can benefit a lot dur-ing complicated and varied flight conditions. As a result, cur-rent aircraft are always designed with only one typical mission

profile.3 It means that aircraft have to compromise their per-formance with sub-optimal modes in off-designed flight condi-tions. Therefore, breaking this traditional design conflict is oneof the hotspots in the aviation field all the time.4

In fact, researchers have tried different routes for generat-ing morphing structures to improve the flight efficiency. Plentyof moving parts for wing morphing have been designed and

used since World War II, such as flaps, slats, ailerons, spoilers,et al.1 It is well known that, during the flight, flight conditionswill be easily influenced by many factors, such as atmospheric

environment, air traffic control, fuel consumption causedweight decrease, et al. However, existing designed civil andmilitary aircraft are always optimal (maximum lift-drag ratio)

at one or several typical design points of the flight envelopeeven equipping with those moving parts mentioned above.Therefore, once in complex flight conditions, the flight willdeviate from its original optimal design point to a suboptimal

state.5,6 Moreover, when those moving parts are set to work,discontinuities and gaps will be formed on the wing configura-tion, which will inevitably generate extra drag and noise.7 To

say the least, even if only for landing and takeoff, these cum-bersome traditional hinged mechanisms are still not efficientenough.8 For that matter, realizing seamless and continuous

morphing is necessary and promising to improve flight effi-ciency, save energy and reduce noise pollution in the vicinityof airports.

Attracted by such a big profit, the US and the EUR haveconducted many morphing aircraft projects. A large propor-tion is variable camber related research among these projects.From the early conducted Mission Adaptive Wing (MAW)9

and Smart Wing (SW)10 programs to the recently disclosedMission Adaptive Compliant Wing (MACW)11 and VariableCamber Compliant Wing (VCCW)12,13 projects, NASA and

other research institutes and companies developed many waysto realize trailing edge continuous morphing. However, for theMAW and the SW programs, the poor load-bearing ability of

the wing coverings limited the application. Although theMACW was successfully tested on the Gulfstream III aircraftand proved it could save 2% to 11% fuel, this morphingmethod still brought forward a higher request for the skin

material which should be elastic enough for morphing and stiffenough for load-bearing. The VCCW project adopted a single-piece and non-stretchable composite wing skin attracted peo-

ple’s attention as soon as it was announced. According tothe public document, it only can realize two different morph-ing states at present and still needs more in-depth study.

Besides, the EUR also made a series of achievements in thisfield. The ADIF14 project conducted by Deutsches Zentrumfur Luft- und Raumfahrt (DLR) adopted finger and belt rib

concepts to realize wing camber change. However, these twoconcepts were largely dependent on heavy and complex

mechanical structures that are inefficient and uneconomic.Moreover, the EU also invested and led some related wingmorphing projects, such as CHANG’E, NOVEMOR and

Smart Morphing & Sensing (SMS). The CHANG’E15 projectintegrated up to four different morphing mechanisms into asingle wing, however, it still could not bear very high atmo-

spheric pressure. As for the NOVEMOR16 project, it mainlyfocused on the optimization of existing compliant internalstructure and external shape. Recently, the newly conducted

project SMS17,18 has caught people’s attention. It utilized elec-tricity activated SMA to control wing deflection. Perhaps thelow response rate of the SMA would be a key problem influ-enced its practical application.

Observation from the previous morphing studies, smartmaterials or structures play a pivotal role in the morphingmechanism. The principle challenge of flexible skin material

design is to find a balance between the conflict requirements-soft and stiff.3 In addition to the stretchable materials, suchas elastomers, used as wing coverings in the above research

projects, some other most commonly mentioned smart materi-als or structures for wing morphing are auxetic honeycombstructures, variable-stiffness tube structures, corrugated struc-

tures, and multi-stable materials.1,19 The morphing theory ofauxetic honeycomb structures builds on the negative Poisson’sratio character of the material itself.20 It has a potential appli-cation in both variable-span morphing21–23 and variable-

camber morphing24–26 if the connection problem between flex-ible skin and deformation structures is solved. The variable-stiffness tube structures, such as pneumatic tube27,28, shape-

memory materials29,30 and Fluidic Flexible Matrix Composite(F2MC) tube31,32, are mainly used as wing morphing actua-tors. Therefore, this type of wing still needs flexible covering

to keep a smooth configuration. Corrugated structures33–35

can undergo high aerodynamic load and realize large morph-ing, but also face the same problem as auxetic honeycomb

structures do.Multi-stable materials have long been expected to be

applied in the morphing filed due to large deformation abilityand rapid transition rate.19 To date, the most common studied

multi-stable material is the unsymmetric composite laminate36

which has been investigated as morphing skins by manyresearchers. Diaconu et al.37 proposed a bi-stable trailing edge

using prestress stretching-caused morphing material. It canrealize a flap angle of 10�. Gatto et al.38 studied the liftenhancement performance of a bi-stable winglet using experi-

mental method. Mattioni et al.39 also proposed some morph-ing concepts related to variable sweep and camber wingbased on bi-stable or multi-stable composites. Schultz40 pre-sented a bi-stable airfoil-like structure which can be potentially

used for wing twist. Although many studies show that themulti-stable materials have a positive effect on wing morphingperformance, the existing multi-stable materials are difficult to

withstand high aerodynamic load in real flight.19

In this paper, a Continuous Morphing trailing-edge Wing(CMW) concept was proposed. A new multi-stable nanomate-

rial prepared with Surface Mechanical Attrition Treatment(SMAT) technology was used for the wing skin. To analyzethe aerodynamic performance of this morphing wing, Compu-

tational Fluid Dynamics (CFD) simulation was utilized to pro-vide some persuasive results.

Fig. 1 Morphing states with different bi-stable regions (Aluminum, plastic strain ep = 3000 � 10�6).

Continuous morphing trailing-edge wing concept based on multi-stable nanomaterial 3

2. Materials

As multi-stable nanomaterials can realize large continuous

deformation, it has great potential to be applied to the morph-ing wing. In this section, the design scheme and preparationprocess of an aluminum-based multi-stable nanomaterial were

disclosed. A continuous morphing wing concept based on thismulti-stable material was proposed.

2.1. Preparation of bi-stable nanomaterial

In this research, the SMAT technology was used for the prepa-ration of bi-stable nanomaterials. SMAT is an authorized tech-

nical patent which can dramatically improve the mechanicalproperties of metal materials using chaotic ball impact,invented by Lu K and Lu J.41 Recently, Yi et al.42–44 foundmetal sheets, such as steel, aluminum, magnesium, etc., can

obtain local bi-stable property after alternatively processingon both sides using SMAT. The formation mechanism of thebi-stability is due to SMAT induced local residual stress. The

more the residual stress is induced, the larger the force isneeded to trigger the transformation between two stable states.In other words, the larger the trigger force is, the stronger the

load capacity is. Besides, coupled with the increase of deforma-

Fig. 2 Conceptual design of the CMW. Fig. 3 Design of the CMW.

Fig. 4 Prototype and morphing states of the CMW.

Fig. 5 3D and 2D models of each stable state.

Fig. 6 Computational domain, boundary conditions, and 2D

structured mesh.

4 F. HAO et al.

tion ability is the improvement of mechanical property in bi-

stable regions, such as the enhancement in strength and resis-tance to corrosion. Viewed under an electron microscope, Yaet al. found that the surface layer of the bi-stable region exhi-

bits gradient nanostructure which is the decisive factor toimprove mechanical properties.45 In addition to the advantageof mechanical property, compared with the current bi-stable

materials, the bi-stable nanomaterial can be easily used forthe multi-stable morphing by designing multiple bi-stableregions on a metal sheet.

2.2. Design of multi-stable nanomaterial

As mentioned in Section 2.1, the design concept of multi-stablenanomaterials is based on the number and the arrangement of

bi-stable regions.46 As a bi-stable region has two stable states,a sheet with n bi-stable regions should possess 2n morphingstates in theory. Besides, the shape and size of each bi-stable

region and the array format will similarly affect the morphingresults. As shown in Fig. 1, the simulation results show thatdifferent shapes (circle and ellipse), sizes, array formats and

combinations of the bi-stable regions will lead to differentmorphing results, where U represents the absolute node dis-placement of each case. Therefore, these multi-stable nanoma-terials can provide researchers more options for morphing

applications in aviation and other engineering fields.In this study, to simplify problems, the material design fol-

lows a principle that the number, shape and distribution pat-

tern of bi-stable regions are fixed with a single form.Moreover, to make the design close to reality, an aircraft-grade aluminum alloy of 6061 sheet, which is the main material

for wing skin, was chosen as the raw material. 3 mm diameter304 stainless steel balls were used to irregularly impact the alu-minum alloy sheet surface. In any morphing state, the material

does not need any external force to maintain stability. Butwhen the external force exceeds a certain limit, the currentstable state will transform into another stable state with asnap-through phenomenon. The force to trigger the

Fig. 7 Comparison between CFD results and experimental data.

Fig. 8 Comparison between CMW0012 and NACA0012.

Continuous morphing trailing-edge wing concept based on multi-stable nanomaterial 5

snap-through phenomenon can be designed by controlling the

residual stress. The designed multi-stable nanomaterial canrealize upward/downward chordwise deflection and spanwisetwist to simulate the corresponding functions of flaps and

ailerons.

3. Model and methods

A CMW model based on the multi-stable nano aluminumalloy sheet was designed and fabricated. CFD simulationmethod was adopted to study the aerodynamic performance

of this morphing wing.

3.1. Conceptual design

The wing with a continuous and seamless morphing trailing

part was designed based on the NACA0012 airfoil. The designconcept of the CMW is shown in Fig. 2. To increase the mor-phing ability, the morphing part was designed as nearly 50%

chord length (c) of the NACA0012 base airfoil. Traditionalrear-mounted flaps and ailerons were replaced with the morph-ing trailing part which was assembled with two monolithic

multi-stable nano aluminum alloy skins. In Fig. 2, u is thedeflection angle of the traditional 30% chord-length FlapMorphing Wing (FMW). The deflection angle h of CMW is

defined as the angle between the chord lines of the base airfoil(NACA0012) and the morphing trailing part. The anglebetween the chord line of the base airfoil and the flow directionof the free stream is the angle of attack a.

3.2. Prototype fabrication

Considering the follow-up wind tunnel test, the overall dimen-sions of the morphing wing were designed as 390 mm in wing-

span and 515 mm in chord length as shown in Fig. 3(a). Thechordwise length of the morphing section is 265 mm took upnearly 50% of the chord. 24 circular bi-stable regions are uni-

formly distributed on both upper and lower skins in a 4 � 6matrix as shown in Fig. 3(b). To simplify the morphing prob-lem and only consider large deflection states, bi-stable regions

in the same spanwise line were simultaneously controlled forevery actuation. As the number of bi-stable regions along thesame chordwise line is 4, a total of 4 downward deflection

states were mainly studied. The upper and lower multi-stableskins were fixed together at the trailing edge. For the connec-tion methods between the morphing trailing part and the fixedleading part, mechanical fixed connection and groove guided

Fig. 9 Aerodynamic performance of CMW.

Fig. 10 Morphing states of FMW.

6 F. HAO et al.

sliding connection were adopted for the upper and lower junc-tions respectively. The sketch of this sliding connection is

shown in Fig. 3(c). This sliding design plays a crucial role inthe overall design because it can provide a deformation allow-ance for the wing morphing. As the leading part only helps to

keep a complete aerodynamic configuration at this stage, alight-weight 3D printing leading part with a smooth contourwas selected. These two parts could be assembled or disassem-

bled easily. Fig. 4(a) shows the prototype of the CMW. Tofacilitate the expression of each state of the CMW, the initialstate together with 4 downward morphing states, with the

increase of h, are named CMW0012, CMW 1, CMW 2,CMW 3 and CMW 4 shown in Fig. 4(b).

3.3. Numerical analysis and validation

3D scanning was used to capture the details of the multi-stableconfiguration which makes the numerical model close to real-

ity. The Industrial 3D scanner Artec Space Spider with a pre-cision of up to 0.05 mm used here can accurately record theconfiguration parameters of each stable state. The scannedmodels were reasonably optimized with the removal of those

unimportant features introduced during scanning work in thepremise of guarantee calculation precision. The reasonablymodified models can be directly used for 3D simulation or

intercepted to get airfoils for 2D simulation as shown in Fig. 5.In this numerical analysis, flow around morphing airfoils

was simulated by solving the steady turbulent incompressible

2D Navier-Stokes equations. Spalart-Allmaras turbulencemodel which is mainly designed for aerospace applicationswas used to solve this problem. The second-order upwindscheme, second-order central difference scheme, and coupled

algorithm47 were selected for each parameter setting, respec-tively. The computational domain was built large enough toreduce the boundary effect as shown in Fig. 6(a). In Fig. 6

(b), 2D structured mesh with a refined grid in the boundarylayer was adopted to improve computing accuracy. The set-tings of the boundary conditions are depicted in Fig. 6(a).

To verify the feasibility and accuracy of the calculation,simulation results (lift coefficient CL and drag coefficient CD)of the airfoil NACA0012 were compared with the experimental

data published by NASA.48 All the parameters in the simula-tion are consistent with the wind tunnel test. The chord andthe span of this validation model respectively are 60.10 cmand 91.44 cm. The range of the a is set up from �4.25� to

18.25�. The Reynolds number and the Mach number areselected as 2 � 106 and 0.15, respectively. Finally, the feasibil-ity and veracity of the numerical method adopted in this

research were verified by the comparison results.As shown in Fig. 7, the lift coefficients calculated from the

numerical simulation are consistent with the experimental

results except for the stalling angle. As the drag coefficientsare difficult to be precisely predicated using the Spalart-Allmaras model, the simulation results are a little bit largerthan that of experiments. However, the general trend of CD

from the simulation is consistent with that from the experi-ment. As the CFD method is mainly used for the comparisonof two simulation models, in this study it is sufficiently precise

for predicting the aerodynamic performance of the CMW.

4. Results and discussion

This section provides the numerical simulation results for theaerodynamic performance of the CMW. The superiority ofthe CMW is discussed through the comparison with the tradi-

tional FMW.

Fig. 11 Comparison of lift coefficients.

Fig. 12 Comparison of drag coefficients.

Continuous morphing trailing-edge wing concept based on multi-stable nanomaterial 7

Fig. 13 Comparison of lift-to-drag ratio.

8 F. HAO et al.

4.1. Aerodynamic performance analysis of CMW

To study the influence of a and h on the aerodynamic charac-

teristics of the CMW, the 5 airfoils listed in Fig. 5(b) were sim-ulated. The deflection angles of the 5 airfoils are measured as0�, 6.8�, 8.8�, 9.2�, and 11.7�, respectively. The range of the ais set from 0� to 20� with an interval of 2�. The Mach numberand the Reynolds number are 0.17 and 2 � 106, respectively.

4.1.1. Aerodynamic performance verification before morphing

The initial state of the CMW before morphing was pre-designed as NACA0012 airfoil. To verify the fabrication accu-racy of the initial model, CFD simulation results of both

CMW0012 and NACA0012 were compared shown in Fig. 8.And obviously, the results of CL, CD and lift-to-drag ratio(CL/CD) for both models are very close that the relative devi-ation of each parameter is less than 10%. It proves that the

configuration of CMW0012 is almost the same with its baseairfoil NACA0012. As for some specific differences, it’s likelycaused by machining error which can be reduced by correction.

4.1.2. Aerodynamic performance analysis during morphing

Fig. 9 shows the simulation results of the 5 stable states of theCMW. As you can see in Fig. 9(a) and (b), both the CL and the

CD are increased with the deflection of the wing as expected.To study the aerodynamic efficiency of these 5 states, theCL/CD was calculated as shown in Fig. 9(c). It is interesting

to observe that, the CL/CD increases with the increase of theh at a small range of a (0–8�), while it reserves when the a isover 8�. Besides, the CL/CD of the CMW4 is smaller than that

of the other 3 morphing states when the a is 8�. This phe-nomenon indicates that even if the a is small, the CL/CD wouldnot increase infinitely along with the increase of the h.

4.2. Advantage analysis of CMW compared with FMW

To validate the CMW is better than the traditional FMW inaerodynamic performance, 4 corresponding morphing con-

tours of the FMW were modeled according to the relationshipbetween h and u depicted in Fig. 2. Fig. 10 shows the 4 mor-phing airfoils of the FMW. After repeating a series of uniform

simulation procedures as Section 4.1, the comparison resultsare detailed described as follows.

As shown in Fig. 11, the CL relationship between two mod-

els in any morphing state varies greatly. In Fig. 11(a), the CL ofthe FMW is larger than that of the CMW at a small deflectionangle of 6.8�. While, when h increases to 8.8�, the CL values ofboth models are almost the same as shown in Fig. 11(b). In

Fig. 11(c) and (d), as the deflection continues, the unexpectedresults in Fig. 11(a) has been reversed where the CL of theCMW becomes larger. The CL of the CMW4 increases by up

to 46.7% than the FMW4 at a small a of 2�. It can be con-cluded that the lift-enhancement effect caused by continuousmorphing is more apparent at larger deflection angle condi-

tions. As for what causes the results in small morphing states(CMW 1 and CMW 2), the explanation will be explored inthe analysis of the flow field.

To compare the aerodynamic efficiency of the two morph-ing ways, the CD is another important parameter to be studied.It is observed from Fig. 12 that, compared with continuousmorphing, overall, the flap morphing wing creates more drag.

Fig. 14 Comparison of pressure and velocity fields (a= 4�).

Continuous morphing trailing-edge wing concept based on multi-stable nanomaterial 9

The drag reduction effect of the CMW is more remarkableespecially at small h (Fig. 12(a) and (b)). The CMW 1 and

CMW 2 can realize up to about 20% drag reduction in someconditions. Even at large h in Fig. 12(c) and 12(d), it worksfor most of the a. For example, the drag reduction effect of

the CMW 4 is more obvious at a small angle of attack. Thereduction effect of the CMW 4 is amazing which can be upto 50%. A possible reason for the performance of drag reduc-

tion here is speculated that the CMW can influence the flow

separation. This speculation will be further verified in the flowfield analysis in the following Section 4.3.

The CL/CD results of the two morphing ways depicted inFig. 13 are used for evaluating the aerodynamic efficiency.What is clear from Fig. 13(a) is that the advantage of the

CMW is not obvious at small h, and even worse when a issmall. However, with the increase of h, the superiority of thecontinuous morphing has emerged as shown in Fig. 13(b)–

(d). Compared with the FMW 4, the CL/CD of the CMW 4

Fig. 15 Comparison of pressure coefficients (a= 4�).

Fig. 16 Comparison of streamlines.

10 F. HAO et al.

is significantly improved, and the maximum increase can be

200%. Therefore, it’s not hard to see that the aircraft can ben-efit directly from the high lift-drag ratio profit brought by thiscontinuous morphing method.

4.3. Flow field analysis

The detailed results in various forms above could be explained

by the clues hidden in flow fields. As shown in Fig. 14, pressure(color map) and velocity (black lines) fields of the FMW andthe CMW are compared in visualization. The CL relationshipof each morphing state in previous sections can be verified by

the intensity of the color. When the a is small (4� in Fig. 14),

the CL of both wings increases with the h increase as the colorof the pressure field beneath the wing gets darker. Besides, thecolor difference of both wings in the same morphing state also

proves the previous CL results in Fig. 11. The observationshows that the local bulge under the CMW caused by thedeformation theory of the multi-stable skin is probably the

reason for pressure reduction. It makes sense that the CL ofthe CMW gradually exceeds its counterpart when the bulgebecomes small or disappears with the increase of h. To furtherdemonstrate the CL results, the pressure coefficient (Cp) rela-

tionships are depicted in Fig. 15. x is the displacement along

Fig. 17 Comparison of streamlines (a= 16�).

Continuous morphing trailing-edge wing concept based on multi-stable nanomaterial 11

the chordwise direction from the leading edge. The Cp distribu-tions on both upper and lower wing surfaces directly confirm

the aerodynamic characteristics that the pressure fieldsreflected in Fig. 14. It’s not hard to see the initial position ofthe flap morphing can cause pressure concentration which is

easy to induce flow separation.Also, the velocity fields in Fig. 14 show that the CMW has

more advantages on drag reduction than the FMW, especiallyin large h conditions, which is also consistent with the results

shown in Fig. 12. Flow separation is suppressed, and the loca-tion of separation initiation is also delayed due to continuousand moderate morphing. This phenomenon can be further

clearly observed from the streamlines of the FMW 4 and theCMW 4 at middle-higher angles of attack (8� and 12�) shownin Fig. 16. Quite obviously, the drag reduction happens in the

suppression process of separation vortices.Another interesting result in Fig. 12 that the CD of the

CMW is larger than that of the FMW at a high angle of attackaround stalling angle and large deflection angle. The spatial

streamlines in Fig. 17 will help to understand how this happensat the same a of 16�. As you can see in Fig. 17(a) and Fig. 17(b), the extent of flow separation is almost the same at a small

deflection angle (h= 6.8�). However, from Fig. 17(c) toFig. 17(f), when h increases, the flow separation of the CMWis enhanced greatly, and even causes stall. The most likely

cause is that this 50% chord-length morphing design ofCMW will exacerbate the flow separation problem at a highangle of attack due to a wide-range smooth deformation with-

out any resistance just like the sharp edge flap formed duringwork.

In a word, from the above analyzing, there is no doubt thatthe CMW can provide better aerodynamic performance for

the aircraft. The CMW has evident superiority over theFMW in most cases. Those special failed cases are also useful

for determining the range of operation and deserved furtherstudy.

5. Conclusions

A continuous morphing trailing-edge wing based on a multi-

stable nanomaterial was fabricated and numerically analyzedin this study. The steady aerodynamic characteristics of dif-ferent morphing state with different a and h were studied.

Contrastive analysis of aerodynamic efficiency between theCMW and the FMW was conducted. Flow field analysis pro-vided more intuitive evidence to explain the mechanism

behind the results. Some discoveries in this study are asfollows:

(1) With an increase of h, the CL and CD increase accord-

ingly. But the CL/CD increases only at the small a flightcondition. Therefore, the CMW can provide morechoices to adapt to different flight conditions or

missions.(2) The CMW can generate larger CL than the FMW when

the h amounts to a certain degree. In most cases, the

CMW generates less drag than that FMW does. There-fore, at the same flight condition, the overall aerody-namic efficiency of the CMW is superior to the FMW.

(3) At the small-middle angle of attack, the CMW can sup-press the flow separation and reduce separation vortices.This benefit will be more obvious especially in largedeflection states.

12 F. HAO et al.

Acknowledgments

The authors acknowledge the supports of the National KeyR&D Program of China (No. 2017YFA0204403). This workis supported by the Major Program of National Natural

Science Foundation of China (No.: NSFC51590892) and theShenzhen Municipal Science and Technology InnovationCommission of China (No.: JCYJ20160229165310679).

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