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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 9, Issue 2, February 2018, pp. 36–48, Article ID: IJMET_09_02_004

Available online at http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=2

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

AN APPROACH TO THE DESIGN OF

COMPOSITE RADOME FOR AIRBORNE

SURVEILLANCE APPLICATION

S. Ilavarasu

Research Scholar, Department of Aerospace Engineering, Jain University,

Bangalore, India & Scientist, Center for Airborne Systems, DRDO, Bangalore, India

A. C. Niranjanappa

Scientist, Center for Airborne Systems, DRDO, Bangalore, India

Mandeep Singh

Scientist, Center for Airborne Systems, DRDO, Bangalore, India

P.A. Aswatha Narayana

Visiting Professor, IIAEM, Jain University, Bangalore, India

ABSTRACT

Design of radome is a multidisciplinary task involving structural and electrical

studies. In contrary, design of radome is always a delicate compromise between

electrical and structural requirements. A- Sandwich and C- Sandwich radome are

widely used in airborne application. C - Sandwich offer wide working bandwidth and

withstand high mechanical strength. This paper discusses the approach for the design

of radome, especially for airborne surveillance application. This paper presents a

review of radome design, which includes material selection, various load cases, static

structural analysis, estimation of aerodynamic pressure load, bird impact study,

fabrication aspects, material characterization, and electromagnetic study of radome

structure. Mechanical characterization and bird impact analysis of C – Sandwich

structure are discussed in detail.

Key words: Radome, Surveillance, Sandwich, Bird Impact.

Cite this Article: S. Ilavarasu, A.C. Niranjanappa, Mandeep Singh, P.A. Aswatha

Narayana, An Approach to the Design of Composite Radome for Airborne

Surveillance Application, International Journal of Mechanical Engineering and

Technology 9(2), 2018, pp. 36–48.

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=2

1. INTRODUCTION

Radome is a structural weather proof enclosure or cover that protects radar system from

environment. The radar system transmits and receives the signals and hence the radome

An Approach to the Design of Composite Radome for Airborne Surveillance Application


should be designed in such way to not only minimally attenuate electromagnetic signals

transmitted but also must have high mechanical strength [1]. E glass, S-glass D-glass,

Terylene, Quartz, Kevlar are the most common reinforcement fibres for the radome. S-glass,

possesses good structural strength while D-glass provides better electrical properties and

structural stability. Quartz fibre is a good reinforcement since it possesses good thermo

mechanical properties and has a dielectric constant close in value to many of the recipient

resin materials [2]. Epoxy and polyester are common matrix materials used for radome

applications.

Figure 1 Radome Wall Structure. (a) Thin wall, (b) Half wavelength (c) A - Sandwich, (d) B -

Sandwich, (e) C - Sandwich, (f) Multilayer

Radome material is divided in to those forming a) wall b) core c) finish and coating.

Radome styles are defined according to the dielectric wall construction. There are 5 basic

styles, thin walls, half wavelength wall, A- Sandwich, B - Sandwich, C-Sandwich and

multilayer. Figure. 1 shows the radome wall structure. The core material of A-Sandwich type

radomes typically has the form of either honeycomb structures or foams. For airborne

applications, foams have the advantage that a closed-cell structure can be obtained and so

penetration by moisture can be avoided. Foam offers the potential of a high strength to weight

ratio, a low dielectric constant and low loss. A number of polyurethane rigid foams are used

in industry having advantages and disadvantages. This paper focuses on approach followed

for the design radome for airborne surveillance applications.

2. REVIEW OF LITERATURE

Glass Fibre Reinforced Plastic (GFRP) when used as face sheets for A-Sandwich radomes,

offer low weight and relatively low production costs, but these materials cannot withstand

high temperatures and for higher-velocity missile applications. The use of inorganic materials,

such as glass ceramics are widely used for missile application. Honeycomb aramid paper is

the most common core materials used for aircraft. Honeycomb core cells for aerospace

applications are usually hexagonal. Apart from hexagonal cell shape honeycombs are

available in over expanded core and flex core.

Due to the closed cell formation of polymethacrylimide (PMI) moisture absorption is

minimal and also it saturates at a very low value compared to aramid based honeycombs[3].

Despite many benefits over traditional honeycomb cores, foams lack in few mechanical

strength aspects, specially in compressive strength. Also thermoforming of the thick foam for

complex shapes is still in a progressive stage and lacks in maturity, whereas flex core based

honeycombs can be used with ease to perform these tasks.

S. Ilavarasu, A.C. Niranjanappa, Mandeep Singh, P.A. Aswatha Narayana


Kyung Won Lee [4] proposed a simple design equation for A-sandwich radome design for

aircrafts, missiles as well as fixed ground installations. The input parameters required are

operating frequency and transmittance. The electrical performance of A-sandwich radome

depends on the distance between the thin walls and skin material. When the material constant

(Ɛr1 & Ɛr2) and thickness (d) of the skin are selected, the optimum operable frequency (fop) of

the radome is estimated as in equation (1). The thickness of core can be obtained from

equation (2) by assuming half wavelength at fop.

√

√

The manufacturing tolerance for fabrication of airborne radome is important. A detailed

study on the effect of variation of core thickness from the baseline thickness up to 5% was

studied by A Kedar and U.K Revankar [5] and detailed analysis show the value of Radome

Transmission Efficiency(RTE) is above 97% for different core thickness wherein electrical

performance can be maintained within tolerable limits ± 2 mm. Effect of variation in skin

thickness (range ± 5% of skin thickness) does not have much effect on RTE and Insertion

Phase Delay (IPD) for angle of incidence 0 deg. It was absorbed to have some effect for

incidence angle for 60 deg. To protect from environmental effects paint layer is provided and

the study reveals a variation of ± 1% of paint thickness will have no effect on performance of

radome.

C - sandwich consists of two equal thickness outer skins, a center skin whose thickness is

twice that of outer skins and two equal thickness core. This configuration is called symmetric

C sandwich and may be of two identical A sandwich back to back. A technique for optimizing

the symmetric C - sandwich by means of cancelling reflection co efficient does not seem to be

available in open literature. V. Dicaudo [6] proposed a procedure using simple Smiths chart

and concluded that the procedures for determining the A - sandwich and C - sandwich

optimum core thickness at parallel polarization are identical to those outlined for

perpendicular polarization. It has been found that not much research activities have been

carried out on characterization of C - Sandwich structure.

A. G. Hanssen, Y. Girard et al [7] studied an experimental bird strike test carried out on a

sandwich specimen and the same problem was simulated in a nonlinear explicit finite element

program LS Dyna. To model the bird, Lagrangian Eulerian formulation is used. In the

numerical simulation of impact test, formulation of the bird is very important. The simulation

results will match the experimental results only when the formulation of the bird had done

properly. Based on the studies done on different species of bird that involve in impact, bird

shapes are finalized to flat or hemispherical ended cylinder. The bird will have a length to

diameter ratio of two [8].

Hongxu Wang [9], experimentally investigated the medium velocity impact response of

sandwich panels with five different cores and were compared in terms of contact force, energy

absorption, depth of indentation, overall bending deflection. They observed that the core

material is found to be very important in the target deformation, energy absorption, damage

mechanisms, and penetration resistance of the sandwich panel.

Anderson and Madenci [10], have examined the low-velocity impact characteristics for

sandwich composites with a Rohacell foam core. They have concluded that the damage



resistance of a sandwich structure can be improved by increasing the thickness of the face

sheets and increasing the density of the core.

3. DESIGN APPROACH FOR RADOME

Design of radome is a multidisciplinary task involving structural and electrical studies. The

electrical performance and mechanical properties are often mutually exclusive and

compromise to be taken while selecting the material. The following block diagram is

proposed for the design of airborne radome structure. The important phase of design is

requirement capture. Radomes for airborne surveillance applications may also be required to

operate in either a dual, broad or multiband role. The electrical requirements like frequency of

operation, band width are important parameters.

The required electrical property of radome material selected to be as low or optimum

dielectric constant & loss tangent with material density, temperature, frequency, radiation

effects etc. In the same time the required material property like strength, stiffness and impact

to be good enough for withstanding aerodynamic and static load conditions. Aramid fiber

radome structures have better electromagnetic transmission and mechanical characteristics

than those of E-glass/ epoxy radomes [5]. Quartz fibre possesses good thermo mechanical

properties with low dielectric constant.

A – Sandwich radome are widely used in airborne application as offer low weight and

relatively low production costs, but these materials cannot withstand high temperatures. The

thickness of core and operational frequency can be selected as discussed in equation (1) & (2)

shown above. C – Sandwich structure offer wide working bandwidth and withstand high

strength.

Figure 2 Block diagram for radome design



Determination of layer orientation defines the strength of face sheet. Finite element

analysis gives an idea about the strength and stiffness of the material. Having finalized the

design of core, fiber and number of layers, flat panels of 1 m by 1m is required to be

fabricated to assess electromagnetic performance & mechanical characteristics of radome.

Electromagnetic analysis using HFSS simulation software can be used for EM evaluation.

A sample of radome material is analysed by keeping periodic boundary conditions in the

simulation to form an infinite sheet

Coupons are fabricated as per ASTM standard for estimation of material allowable. Wet

layup technique or prepreg can be used for the fabrication of coupons followed by post

curing. Mechanical tests on sandwich coupons are performed as per ASTM standards for the

estimation of strength and stiffness. Four-point bending test allows to estimate the flexural

strength of sandwich specimen.

Impact test or analysis using LS Dyna software assesses the impact strength capability of

radome panel. The material allowable estimated are thus used for finite element analysis.

Smooth Particle Hydrodynamics (SPH) method is used to model the bird for impact analysis.

SPH is a meshless lagrangian numerical technique used to model the fluid equations of

motion. In SPH a fluid is represented with a set of moving particles evolving at the flow

velocity.

Based on the results obtained, the structural and electrical parameters are analysed and

reviewed. If the required parameters are not achieved, revisit of material specification is done

carefully.

4. AERODYNAMIC PRESSURE ANALYSIS

Radome is an external part of aircraft is subjected to external aerodynamic loads, hence must

be structurally strong enough to sustain aerodynamic pressure loads. Aerodynamic pressure

distribution can be estimated by Computational Fluid Dynamics (CFD) methods or wind

tunnel test. Aerodynamic forces can be estimated by force method or pressure distribution

methods. Pressure co efficient for different aerodynamic angles are to be estimated by wind

tunnel tests. The coefficient obtained is to be extrapolated using Prandtl Glauert

compressibility correction for higher Mach number.

CFD methods are popular nowadays and many commercial codes are available. Fluid and

material properties, flow physics model, and boundary conditions are applied with the help of

solver. The radome structure has to be designed for critical design case as specified by

original equipment manufacturer (OEM). In the absence of critical design cases as a

conservative measure extreme pressure (maximum and minimum) values generated for a

range of aerodynamic angles and velocity are considered for finite element structural analysis

[11,12]

5. FABRICATION OF TEST PANEL

The test panels are manufactured as per ASTM standard and cured in autoclave with specified

cure cycle. The testing specimens are cut from solid laminated panels, sandwich panel in

accordance with the testing standards. Laminated panels were inspected using non-

destructive method ultrasonic pulse echo method. The test coupons are to be tested in Room

Temperature Dry (RTD) and Elevated Temperature Wet (ETW). Specimen dimensions should

be taken before moisture conditioning. Test environments were defined as follows:



RTD = 23ºC ± 3ºC, 50 ± 10% Relative Humidity (RH)

ETW = 82ºC ± 3ºC.

In the present case C- sandwich coupons were tested in RTD condition. The test panels

and coupons were tested for some electromagnetic performance and mechanical

characterization and the results are discussed as below. Figure 3 show the photograph of C –

Sandwich test panel.

Figure 3 C – Sandwich test panel

6. MECHANICAL PROPERTIES OF SANDWICH PANEL

Ilbeom Choi [13] constructed low-observable radome with a sandwich construction composed

of aramid/epoxy composites faces, foam core which had the abilities of transmitting EM

waves selectively in the X-band range. The mechanical properties of radome made of

aramid/epoxy composite were measured by the 3-point bending test and compared with

conventional low-observable radome made of E-glass/epoxy composite. The dielectric

constant and loss tangent of the aramid/epoxy composite measured by the free space

measurement method. Unlike metallic material, composite sandwich materials lack of reliable

data, hence it is essential that an accurate experimental characterization of the materials is

essential. The data generated will help the designer in choosing the more appropriate material

and to assess the stress field in the sandwich panel. Flatwise compression, tension and flexure

tests were carried out to estimate the mechanical properties of sandwich radome. The specific

mechanical tests were carried out as per ASTM standards to estimate the stiffness and

strength properties.

6.1. Flatwise Compression Test

Figure 4 Load-Displacement curve of flatwise compression test



Figure 4 represents the load vs. displacement plot of flatwise compression test for C-

sandwich structure with Nomex as core material. Flatwise tensile test provides data for

design, related to flatwise tensile strength & modulus of the cores alone and also the

combined strength of core & core-to-facing bond under tensile loading.

6.2. Flatwise Tensile Test

Figure 5 shows the load vs displacement plot for flatwise tensile test. The flatwise tensile

strength for C-sandwich structure with Nomex honeycomb core has maximum loads carrying

capacity of 6890 N, with a related displacement of 3.59 mm. The average equivalent tensile

strength of the honeycomb core is about 2.756 MPa. The equivalent tensile strength of the

core is about 1.56 times of the equivalent compressive collapse strength.

Figure 5 Load-Displacement plot for Flatwise Tensile test

6.3. Flexural Test (ASTM C393)

Flexural test provides data for design, pertaining to flatwise flexural strength and stiffness of

sandwich panel. Four-point and three-point loading fixtures are employed to load the long

beam specimens and to determine the flexural parameters like facing modulus and panel

bending stiffness. Short span three-point test is done to determine the core shear strength and

panel shear rigidity. Figure 6 show the arrangement of test for short beam, 3- point and 4-

point flexure.

Figure 6 C-Sandwich in Short Beam flexure test fixture mounted with AE Sensor (a) short Beam

Flexure (b) 3 Point Flexure (c) 4 Point Flexure

Figure 7 represents the load-displacement plot for flatwise flexure test where the linear-

elastic behaviour was observed initially. The failure was localised failure under concentrated

loading point



Figure 7 Load-displacement plot for flatwise flexure test

The short beam flexural testing had core shear strength of 0.58 Mpa respectively. The

specimens failed by transverse shear in the core. Initial failure occurred by wrinkling of top

face sheet at the loading point, further core crushing occurs due to localised indentation. The

failure occurs when the maximum shear stress reaches the critical value of the core material.

Figure 8 shows the comparison of core shear strength.

Figure 8 Comparison of Core shear strength for C- Sandwich

The flexural strength is expressed by the following equation

The shear stress can be computed as below

where, τ = core shear stress in MPa, P = load on specimen in N, L = length of specimen in

mm, b = width of specimen in mm. The shear modulus can be calculated as follows:



where G = core shear modulus in MPa, S = ΔP/Δu, slope of initial portion of load-

deflection curve in N/mm, u = displacement of loading plates, t = thickness of core in mm

7. STRUCTURAL MODELLING AND ANALYSIS

It is mandatory to estimate the stress levels of radome when subjected to aerodynamic loading

conditions, emergency landing conditions and bird impact loads as per FAR 25 specifications.

Emergency landing conditions are defined below

9g forward

6g down

3g side

1.5g rear

Popular tools such as CATIA V5 R20, Altair Hypermesh, finite element codes such as

ANSYS and bird impact analysis tool LS-Dyna are used for generating the solid model, FE

mesh generation, static analysis and impact analysis respectively. The material properties

obtained from the coupon test is used for FE analysis. Following methodology is followed to

perform structural analysis of the composite radome structure.

Face sheet is idealized using 2D Shell elements and Core is idealized using 3D Solid elements.

All the bolts are simulated using beam elements, beam element ends are connected to the bolt

holes in the structural components using RBE3 elements.

Perfect bonding is assumed between core and face sheet for static analysis.

Maximum strain criteria are used for Composite Radome analysis.

A detailed FE analysis will provide radome displacement and the stress distribution. This

intern will give the margin of safety.

8. BIRD IMPACT DAMAGE

The Radome must be capable of successfully completing a flight during which likely

structural damage occurs as a result of impact with a 4-pound bird when the velocity of the

airplane relative to the bird along the airplane‟s flight path is equal to Vc at sea level or 0.85 x

Vc at 8000 feet, whichever is more critical. Bird impact test or analysis can be used for

substantiating the impact strength of radome material.

Bird impact analysis is modeled using a mesh less lagrangian technique called smooth

particle hydrodynamics. In high velocity impact simulation and crash simulation mesh

distortions are more. SPH is the best proven method for this kind of problems involving large

mesh distortions. The fluid is represented by a group of moving particles changing their

position with respect to fluid velocity. The particles of SPH are denoted as an interpolation

points on which all fluid properties are known. Particle distribution of bird is similar to mesh

of the target in order to maintain a proper interaction between nodes to surface contact.



Figure 9 Bird Impact on C-sandwich panel at different intervals of time

The shape of the bird is chosen to be a cylindrical to represent an experimental bird

model. The bird is modeled such that their length is twice that of diameter and possesses a

weight of 4 pound with a density of 938.5 Kg/m3. The bird is modeled with a diameter of

0.112m and a length of 0.224m. The different stage of the bird impact on 1m X 1m, C-

Sandwich panel is given in figure 9. The following are the observations for the impact of 4-

pound bird at a speed of 150m/s.

No penetration observed.

No fragments of secondary projectiles formed.

Hence C-Sandwich panel is capable of withstanding the impact of 4-pound bird at a speed

of 150 m/s.

9. ELECTROMAGNETIC SIMULATION

C - Sandwich panel was tested in planar near field measurement facility for EM transparency

in S band for 2.5 GHz and 4.5 GHz. The test set up for measurement is shown Fig. 10. The

measured antenna patterns with and without radome material is as shown in Fig. 11 and 12.



Figure 10 Electro Magnetic measurement set up

Figure 11 Pattern for Antenna and radome (frequency = 2.5 GHz)

Figure 12 Pattern for Antenna and radome (frequency = 4.5 GHz)



There is no significant change in the antenna patterns in main lobe as well as side lobe

level, beam width and the beam pointing angle. The radome hence concluded as

electromagnetically transparent with very minimal effect on antenna radiation properties in S-

Band.

10. ACCEPTANCE CRITERIA

The radome structure must be designed so that material ultimate strength will not be exceeded

at ultimate loads. Ultimate load is limit load multiplied by 1.5. The allowable stress must

include the effects of material strength reduction due to action of moisture and elevated

temperature. The material allowable as estimated in para 6 to be used. The criteria for

deflection of radome in general can be defined as the radome should not interfere with any

other component or the deflection of radome should not be more than 10 to 15 mm to the

nominal value [12]. The acceptance criteria for impact load are as follows. The bird should

not penetrate or debris due to damage should not come out. Any crack on the test specimen as

a result of impact is acceptable.

11. CONCLUSIONS

The design of radome involves multi-disciplinary activities that involve deign of electrical

and structural aspects. Structural design involves aerodynamic load estimation, bird impact

analysis, mechanical testing in Room temperature dry and elevated temperature wet

conditions, FEM analysis, certification etc. The design approach for an airborne surveillance

system mounted on top of aircraft fuselage has been discussed. The experimental test such as

flatwise compression, flatwise tension, flexural short beam, long beam 3-point test and 4 point

tests were performed accordingly as per ASTM standards. Some of the results for impact

strength, pattern for antenna and radome for flat panel sandwich radome estimated are also

discussed.

ACKNOWLEDGEMENTS

The authors would like to thank MS Easwaran, Director, CABS and Suma Varughese,

Programe Director, AEW&C & USHAS for their technical support. The authors express their

sincere thanks to Dr. V. Arumugam, MIT, Chennai for the conduct of mechanical tests at

departmental facility.

REFERENCES

[1] Military specifications radomes, General Specifications for, MIL-R-7705B.

[2] G. A. E Crone, A. W. Rudge, Sen. Mem and G.N. Taylor, “Design and performance of

airborne radomes: A review”, IEE Proc, Vol 128, Dec 1981

[3] O. Russo, A. Colasante, G. Bellaveglia, F. Maggio, L. Rolo, “State of the Art Materials

for KU and KA band Mobile Satellite Antenna Radomes”.

[4] Kyung Won Lee, Yeong Chul Chung, Ic-Pyo Hong and Jong Yook, “An Effective Design

Procedure for A-Sandwich Radome”. IEEE, 2010

[5] A. Kedar, U. K. Revankar, “Parametric study of flat sandwich multilayer radome,”

Progress in Electromagnetics Research, PIER 66, pp 253-265, 2006.

[6] V. Dicaudo, Determining optimum C sandwich radome thickness by means of smith chart.

IEEE Transactions on Antenna and propagation. Year 1967, pp 821-822



[7] A. G. Hanssen, Y. Girard, L Olovsson, T Berstad, M Langseth, „A numerical model for

bird strike of aluminium foam based sandwich panel‟. International journal of impact

engineering 32 (2006), 1127-1144.

[8] M. A. Lavoie, A. Gakwaya, M Nejad Ensan, D. G. Zimcik and D. Nandlall. „Birds

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[9] Hongxu Wang, Karthik Ram Ramakrishnan, Krishna Shankar, Experimental study of the

medium velocity impact response of sandwich panels with different cores. Materials and

Design 99 (2016) 68–82

[10] Anderson T, Madenci E. Experimental investigation of low-velocity impact characteristics

of sandwich composites. Composite Structure 2000; 50 (3):239–47

[11] S. Ilavarasu, Aerodynamic Studies on Airborne Surveillance Aircraft, DRDO Science

Spectrum, March 2009, pp 1-3.

[12] G. Pulvirenti, P. D. Tromboni, M. Marchetti, A. Delogu, “Surveillance System Airborne

Composite Radome Design”, Dip. Ing. Aerospaziale Astronautica, Universita di Roma

“La Sapienza”- Italy

[13] Ilbeom Choi a, Jin Gyu Kim a, Dai Gil Lee, Sung Seo, “Aramid/epoxy composites

sandwich structures for low-observable radomes” Composites Science and Technology,

17(2011), pp 1632 - 1638

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