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The aerodynamic analysis of the profiles for flying wings.

1. INTRODUCTION

The vector from the robotized aerial system acts the same as piloted airships having the same aerodynamic laws. While projecting a lift surface type flying wing an important aspect would be maneuverability and stability which is directly influences by geometrical characteristics used in the designing phase.

Normally airships type flying wing could have lift surfaces with any aerodynamic profile, everything else resuming to the type of mission and the performance of the bearing surface. To design such an airship at a large operating scale, the wing must present optimal geometrical characteristics so it can maintain induced resistance, the moment coefficient and the lift coefficient at suitable levels. A few aerodynamic profiles used at tailless airships are represented in Figure 1.

The majority of the aerial vectors for the fixed wing have a conventional geometry with the tail, the low cost design determining to create a tailless project that has a blended wing or not, see figure 2. [1]

Mainly there are 3 types of flying wings determined by obtaining longitudinal stabilization: plank (fig. 2a), swept (fig. 2b) and parafoil (fig. 2c).

By choosing one of the longitudinal stabilizations we use a series of aerodynamic profiles corresponding to the momentary coefficient (low values), as examples we choose Phoenix and Clark YH profiles for plank and swept wings and MH 91 for parafoil wings (see figure 3)[2, 3].

2. THEORETICAL LANDMARKS

Aerodynamic analysis for airfoil reveals limits of performances can be obtained: the angle of incidence for zero lift ([[alpha].sub.0]), the angle of the incidence for zero drag [([C.sub.d]).sub.min] maximum smoothness ([C.sub.d]/[C.sub.1]) min (figure 4), the angle of incidence at maximum lift--[([C.sub.1]).sub.max]. [4]. Incidence reference values are shown in Figure 5.

For performances calculus is necessary an explicit dependence of [C.sub.d] = [C.sub.d]([C.sub.1]). Show in figure 4 a linear variation of [C.sub.1] small incidence angles above 7[degrees] but the variation is nonlinear because of the separation of fluid layers. We have: [C.sub.d] = [C.sub.d] (a, M, [R.sub.e], [C.sub.l] = [C.sub.l] (a, M, [R.sub.e]) (1)

removed [alpha], result:

[C.sub.l] = [C.sub.l] ([C.sub.d], M, R) (2)

Variation of the coefficients is show in figure 5 with M and [R.sub.e] known and constant, where: M- nr. Mach, [R.sub.e] - nr. Reynolds

[c.sub.l] = L/[1/2][r.sub.[infinity]][v.sup.2.sub.[infinity]]cb (3)

[c.sub.d] = D/[1/2][r.sub.[infinity]][v.sup.2.sub.[infinity]]cb (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

where: L--lift, D--drag, [M.sub.0]--aerodynamic moment, b--span, c--wing chord.

3. AERODYNAMIC ANALYSIS

3.1. Profili v.2.21software

For the 2D analysis we choose the profiles from figure 6 which are mainly used in building tailless aerial vectors, the analysis being performed by Profili 2.2.1 software [5] using the data from table 1.

The analysis methodology for comparative graphics is described in the diagram in figure 7.

For speed of 10 m/s ([R.sub.e] = 272000) the polars of three airfoils are shown in figure 8 (a, b, c).

The characteristics the three profiles are shown in table 2. [5, 6].

We can see in the graphs (figure 9) pressure coefficient variation ([C.sub.l]) reported at airfoil chord depending angle of incidence ([alpha]) at Re = 272000 for minimum speed of 36 km/h. We observe pressure coefficient ([C.sub.l]) that is influenced by camber.

Analysis of the airfoil, made at Re = 272,000, in the case the curvature changes (flaps out at 5[degrees] and 10[degrees]) to 20% of chord, show increasing lift coefficient ([C.sub.l]) and change the value of the moment coefficient ([C.sub.m]), as graphs in figure 10 (a, b), figure 11 (a, b), figure 12 (a, b).

3.2. Easy CFD v.4.1

It is a calculation instrument used for the fluid dynamic based on the numerical solutions of the fluids and heat transmission in Cartesian coordinates systems. We present the analysis with EASY CFD_G v.4.1 which is realized according to the methodology from figure 13 and the data from table 4 so we can find out the pressure coefficient that is around the three profiles which are subjected to laminar stream.

The speed and pressure prints are remarked in figure 14. The validation of the analysis presented in Easy CFD_G v.4.1 could be start ups for deeper studies with refinements for the initial entry conditions for flow and advection models.

Limits and options of the software

Profili 2.2.1 offers an aerodynamic analysis with a series of instruments and criteria which lead to a high trusted result. The most important analysis instruments are: profile manager (with a data base that contains 2000 profiles), Reynolds calculator (rope functions, speed and altitude), polar analysis profiles, speed and relevant analysis coefficients for profile flow (pressure, friction).

EASY CFDG v.4.1 is simple software, mainly oriented for educational purpose becoming a valorous instrument for a first analysis. It presents a few options and characteristics which recommend him: laminar or turbulent flow, isothermal or non-isothermal flow, steady-state or transient flow, structured and unstructured mesh generation, two turbulence models, conduction in solid and conjugate heat transfer, multi component fluid flow, transport of passive scalars (smoke), geometry import from DXF or point data files [7].

4. CONCLUSIONS

By choosing one of the versions, stabilization for the flying wing is realized with the help of an auto-stabilized profile with negative geometrical torsion of the wings extremities and placing the gravity center under the pressure center of the lift surface. An important aspect is the speed and pressure distribution for small Reynolds numbers which can lead to instability. The best compromise for a UAV flying wings is to adapt curved moderate line of the profile with a maxim curve moved to the leading edge [8].

Recent research with the help of the software led to the design of new profiles with multiple applications without too much experimental effort. The applications in unmanned aerial vehicles request abnormal qualities that are not met at piloted airships: maneuverability at small speeds and high overload.

ACKNOWLEDGMENT

The authors wish to thank the Transilvania University of Brasov and "Henri Coanda" Air Force Academy of Brasov for supporting the research necessary for writing this article.

REFERENCES

[1] UAS Yearbook, Unmanned aircraft systems--The Global Perspective 2011/2012, Blyenburg & Co, june 2011, Paris, ISSN 1967-1709, p. 216

[2] Airfoils database, www. airfoiltools.com, Available at 10.12.2012

[3] Airfoils, ww.aerodesign. de/english/profile/profile_s.htm, Available at 10.12.2012

[4] Deliu Ghe., Mecanica aeronavelor, Editura Albastra, 2001, Cluj-Napoca, ISBN 973-650-029-2, p. 375

[5] Duranti S. Profili 2.21 software, 2012, Feltre-Italia, www.profili2.com, Available at 12.12.2012

[6] Airfoil tools, www. worldofkrauss.com, Available at 14.12.2012

[7] Gameiro Lopes A.M., Easy CFD_G user manual, 2012, p. 98

[8] Airfoils for tailless airplanes, http://www.mh-aerotools.de/airfoils/ nf_1.htm, Available at 18.01.2013

Vasile PRISACARIU

Transylvania University, Brasov, Romania

Table 1. The conditions of the analysis

Features                          Value

CMA(mm)                            400
Flight height (m)                  100
Angle of incidence (0)            -5/15
Speed (m/s)              10       20       30
Reynolds number Re       272000   543000   815000

Table 2. Airfoil features

Features          Phoenix   Clark YH   MH 91

Thickness:         8,2%      11,9%      15%
Camber:            2,8%        6%      1,7%
Max CL:            1,17       1,11     1,11
Max CL angle:       11         15       15
Max L/D:           92,61     32,83     20,09
Max L/D angle:       7        4,5        3
Zero lift angle    -0,69       -2        0

Table 3. Analysis data

Meshing               unstructured

Mesh spacing method     uniform
Mesh elements (max)       3000
Regime                steady-state
Thermal effects        isothermal
Iteration                 100
Airfoil incidence      0[degrees]
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Author:Prisacariu, Vasile
Publication:Journal of Defense Resources Management
Article Type:Abstract
Geographic Code:4EXRO
Date:Apr 1, 2013
Words:1328
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