American Institute of Aeronautics and Astronautics

1

Fluid-Structure Interaction and Design Optimization of HAR wing for Endurance efficiency of S-HALE aircraft

Sanga Lee, Kyunghyun Park, Junghwa Kim1 College of Engineering, Seoul National University, Seoul 151-744, Republic of Korea

SangOok Jun2

Samsung Electo-Mechanics, Suwon 443-803, Republic of Korea

and

Dongho Lee3 School of Mechanical and Aerospace Engineering/ Institute of Advanced Aerospace Technology,

Seoul National University, Seoul, 151-742, Republic of Korea

In this paper, S-HALE aircraft wing sizing optimization is conducted at 20km altitude by modifying 3 variables including span length, aspect ratio and weight distribution along the spanwise direction of the wing for 24-hour continuous flight. Fluid-Structural Interaction is conducted for static stability of wing and high estimation of aerodynamic performance. 3-dimensional Euler equation and MSC.Nastran are used for aerodynamic and structural analysis. Finally we check the energy flow of S-HALE during a day and confirme that the S-HALE can fly 24-hour continuously.

Nomenclature Wws = weight of wing surface spar Wwer = weight of wing end ribs Wwr = weight of wing surface ribs WwTE = weight of wing trailing edge Wwc = weight of wing covering nult = ultimate load factor Sw = wing surface area bw = wing span

= airfoil thickness to chord ratio

Nwer = Number of wing end ribs Nwr = Number of wing surface ribs Esolar = Energy generated by sun ES-HALE = Energy absorbed by S-HALE Imax = Maximum solar energy per unit area during a day K = weather referential constant

I. Introduction n recent few decades, interest of altenative energy is growing bigger and bigger, because of lack of fossil fuel, environmental pollution, global warming and so on. Among many altenative energies, solar energy has been

applied many fileds because it has parmenency and has no negative effect to environment. By the same token, many people have considering solar energy as a source of aircraft. 1 Graduate student, School of Mechanical and Aerodynamic Engineering 2 Researcher, ellga5@snu.ac.kr 3 Professor, School of Mechanical and Aerodynamic Engineering, donghlee@snu.ac.kr

I

30th AIAA Applied Aerodynamics Conference25 - 28 June 2012, New Orleans, Louisiana

AIAA 2012-2660

Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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In 1974, first solar-powered aircraft Sunrise 1 is developed and about 90 kind of solar-powered aircraft has been developed so far. On july 2010, Zephyr from QinetiQ, UK set a record by flying 14 days and 21 minutes. 1 At 20km altitude, density of air becomes very low so aerodynamic efficiency of wing is decreased. Make up for

this, aircraft flying high altitude has high aspect ratio wing. However, having high aspect ratio weaken the wing structure, so considering nonlinear deformation of wing becomes a necessity.

In the case of solar powered HALE aircraft(S-HALE), it is not a good optimization to sizing wing just for improved L/D or lift coefficient. S-HALE gets its energy from sunlight during a daytime, so if it can store enough energy for night, it can flight permanently. According to this, the more important thing than improving L/D or lift coefficient is reducing required power for flight or increasing energy capacity of aircraft. Sometimes rearranging spanwise weight distribution can help approving endurance efficiency because wing is flexible and how the payloads are set on wing can affects to wing deformation.

From the previous research2, even in the case of S-HALE can’t save enough energy for night, changing cruise altitude during flight can help saving energy and makes S-HALE fly continuously. But over 20km altitude condition, environment like temperature, density changes rapidly, So S-HALE aircraft structure which should be light comparing other aircrafts’ is hard to satisfy these all various flying conditions. Hence, this paper try to make HALE airfraft satisfy continuous flight without changing altitude.

II. Methods

A. FSI method For aerodynamic analysis, steady state three dimensional Euler equations are used. To make up for the drag

coefficient, we used empirical formula of profile drag.3 Spatial discretization is Upwind Method based Finite Volume Method and Roe’s Flux Difference Scheme is applied. For time integration, LU-SGS(Lower Upper Symmetric Gauss Seidel)is used. For structural analysis, MSC.Nastran is used. Nonlinear solver SOL106 for simple beam model is applied and

every step considers remained stress of deformed wing structure for restart run.4 VMT( V:shear force, M:moment, T:torque) is used for transferring aero load to structural load. Only lift direction

force is considered and problem is set for every structural node satisfies each shear force, moment and torque of corresponding aero node.5

B. Design optimization Methods To optimize the wing shape, 3 design variables are selected and 15 design points are identified by Central

Composite Design(CCD) method. In CCD methods, 2 full-factorial experimental design is employed along with 2k star design point(2 design point with each factor) and one center design point(1 design point at center). With these experimental points, meta-model is constructed. Meta-model is Artificial-Neural Network and 9 hidden nodes are used for sufficient reliability6. Optimization tool is NSGA-II7.

III. Optimization process

A. Aerodynamic model and flight conditions Initial variables of S-HALE wing refers to specification of medium size UAV like zyphyr. The wing has very simple shape so has no taper, no twist and no sweepback angle.

Table 1 Initial variables and flight condition

Class Contents

Variable Span length 20 m Aspect ratio 20Weight distribution 0

Fixed value

Altitude 20 km Payload 22 kg GW 45 kg Reynolds number 200000

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Payload is distributed from root to tip and weight distribution is represented as increasing rate of payload. Weight distribution is defined ‘+’ when the root weight is lager then tip’s. DAE31 airfoil which is well used for human-powered aircraft is selected because of its good characteristic in low Reynolds region8. Figure 1 shows shape of DAE31 airfoil and Table 1 describes initial variables and flight condition of S-HALE.

Figure 1 DAE31 airfoil

B. Weight analysis method To calculate reasonable weight corresponding to various span length and aspect ratio, we use weight analysis formula for Daedalus which is very well known human-powered aircraft. Human-powered aircraft has nearly same characteristics with S-HALE, including high aspect ratio, flexible structure and so on. Especially, Daedalus uses Cabor Fiber for structure material as same as what we want to use for S-HALE. Daedalus’ weight fomulars for wing are as below9.

Wing primary structure : W = (0.031 + 0.00756 )(1 + 100 − 24 )

Wing secondary structure : W = 0.662 + 0.00657 W = 0.055 + 0.00191 W = 0.0277 W = 0.0308

In this paper, span length and aspect ratio are firstly determined and total wing weight is calculated from that. To

calculate entire structural weight of S-HALE, we need to know figures of other parts of S-HALE like fuselage, H-tale and V-tale. But this paper deals with only wing so weight of other structural parts is assumed refer to Min’s model.

C. Payload Table 2 shows payload of S-HALE. 10kg of battery is already loaded, so total weight of battery is 10kg adding

spare weight which means total gross weight 45kg minus calculated structural weight.

Table 2 Payload(fixed)

class weightbattery 10.00 kg

solar panel 5.00 kg Motor 1.70 kg

Propeller 1.36 kg Camera 1.10 kg Acionics 1.62 kg MPPT 0.52 kg

etc. 0.70 kg Total 22.00 kg

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D. Structural model Carbon Fiber Reinforced Plastic(CFRP) is considered for conducting wing structure. Assuming main spar supports entire load including lift and weight of wing, torsional deflection is neglected. The shape of main spar is considered as hollowed cylinder and its thickness is fixed by 2mm. The weight determined before by Daedalus weight analysis formulas decide the diameter of main spar.

E. Energy analysis The energy which can be produced by S-HALE is calculated in the condition of north latitude of 36, altitude of 17km, and June. According to Cestino11, solar energy along time during a day is presented by sine formula (1) and maximum energy which can be gained from sun during a day is calculated from (2) E = sin( ( − )) (1)

E = ∙ ∙ (2)

Figure 2 Solar power during a day11

means solar energy per unit area when the sun rises most high. Actual is about 1,250W/ , but this paper set that value 1,100W/ for some uncertainties. is length of daytime, set to 15hr and is time that the sun rises, set to 5:30 in the morning. K is the value which is referential to weather. If it is clean and fine day, it will 1.0 but dark or cloudy, it will be 0.0. At an altitude of 20km, there’s no effect of weather so K is 1.0. Total energy gained from sun by solar cell on the S-HALE is calculated from (3). E = (3)

means area of solar cell, means efficiency of solar cell and means efficiency of MPPT(Maximum Power Point Taker). Solar cell is installed on the wing and produce energy from sunlight during a daytime. The wing using DAE31 airfoil has a curved upper surface so thin film solar cell is more suitable than general product. CIGS(CuInGaSe2) is selected because it is manufactured less than 4μm thickness and have a very high specific

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Class Contents young's modulus 240 GPa poisson ratio 0.35Density 1700 kg/

Table 3 Characteristic of CFRP10

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