The document discusses using a honeycomb structure with microwave absorbing materials to create radar absorbing structures (RAS) for aircraft. It aims to develop a broadband microwave absorbing composite with a wide absorption bandwidth. The research fabricates a honeycomb structure from glass/epoxy composite sheets containing carbon nanotubes. Simulation and testing show the fabricated structure achieves over 10dB return loss from 3-16GHz, demonstrating excellent broadband absorption performance.

1. STEALTH
TECHNOLOGY FOR
AIRCRAFTS BASED ON
MICROWAVE
ABSORBER USING
HONEYCOMB
STRUCTURESubmitted by
NIRMAL S
S8MB
GUIDED BY : ASST PROF.
SREEJITH N K

2. OBJECTIVES
o Introduction to Stealth Technology
o What is Radar Stealth technology?
o What is Radar absorbing structure (RAS)?
o Latest research work – microwave absorbing honeycomb
structure
2

3. ?Stealth
“Cautious and surreptitious
action or movement.”
(noun)
OXFORD DICTIONARY
2

4. Harry Potter –
The invisibility cloak
Avengers 1 –
Helicarrier scene
4

5. SIGNATURE
“Any unique indicator of its
presence.”
Signature can be concluded as any
activity or radiation or the
characteristic of the body that helps to
revile its presence at a particular point.
“Signature indicates observability of
an object.”
Red hot Iron Ball
5

6. SIGNATURES OF AN
AIRCRAFT
Active Passive
Aircraft Signatures
Radar Visual Acoustics Infrared Others
6

7. RADAR STEALTH
RADAR is the acronym for Radio Detection and Ranging.
Radar is an object-detection system that uses radio waves to
determine the distance, direction, height and speed of the
objects. It helps in early detection of surface or airborne objects.
RADAR basically works on two
major principles.
1) Echo
2) Doppler shift
7
Figure credit:
purbeckradar.org.uk

8. Radar cross-section (RCS)
“It is the measure of a target's ability to reflect radar
signals in the direction of the radar receiver.”
The RCS of a target can be viewed as a
comparison of the strength of the
reflected signal from a target to the
reflected signal from a perfectly
smooth sphere of cross sectional area
of 1 m^2. The size of targets image on
radar is measured by the radar cross
section (RCS) measured in Square
meters. 8
Figure credit: Bipin Kumar Jha, Mayur Somnath Aswale,
“Mechanical Aspects in Stealth Technology: Review”,
International Journal of Engineering and Technical Research
RCS = Projected cross section x Reflectivity x Directivity

9. 9
A conventional
aircraft’s shape
agrees with the
laws of
aerodynamics
and the
principles of
engineering. But
it is entirely
random in terms
of the way it
scatters radar
energy.

10. Major aspects of RCS (signature) minimization
techniques.
1) Effort to shape the airframe, the geometric design
considerations
2) Radar absorbent materials and Radar-absorbent
structures
Shaping and RAM are the most practical and tend to provide
good results. These two axes are of course not taken in isolation
during the design; trade-offs often have to be made between
them.
10

11. Radar-absorbing materials
(RAMS)
Materials that are capable of attenuating the reflection
of microwaves are known as radar-absorbing
materials (RAMs).
They can do this in two main ways:
1) By being absorbed into a material that converts the
microwave energy into another form of energy such as
heat; and
2) By destructive interference.
11

12. Radar absorbing structures
(RAS)
Aircraft structures made out of Radar absorbingmaterials
is called Radar absorbing structures (RAS).
RAS have both the functions of load bearing and electromagnetic
energy absorbing capability.
Most important aspect of a radar wave absorption or an
electromagnetic wave absorption is to soak up the incident
electromagnetic energy, thereby reducing the net energy available for
reflection.
12

13. 13
Composite = Reinforcement + Matrix
Various polymers are used as matrix in polymeric composites.
Glass / epoxy systems are cost efficient and have high strength, so
they are widely used in aerospace applications.

14. 14
Figure credit: W.-H. Choi and C.-G. Kim, “Broadband microwave-absorbing honeycomb structure with novel,” Composites Part
B: Engineering, pp. 14-20, 2015.

15. REASON FOR THE RESEARCH
Microwave absorbing composite structures have been intensively
studied. However, most of the reported microwave absorbing
composite structures has narrow operating frequency bands.
Recently, researches on counter stealth, are actively progressed
in the military area. Since stealth technology had mostly been
focused on defeating monostatic radar systems of narrow
frequency bands, broadband or low frequency radar have become
significant threats for conventional stealth aircraft.
15

16. 16
Although the bandwidth can be enlarged by using magnetic
materials, such methods usually rely on a very high wt% of the
magnetic materials for sufficient absorption performance.
However, such methods are also accompanied by weight
increases and degrade the mechanical properties due to its high
wt% and particle size.
To obtain a wide absorption bandwidth in microwave absorbing
structures using non-magnetic materials, large thicknesses are
required. However, thick microwave absorbing structures are not
feasible for aircraft due to the weight increase.
Generally, to implement microwave absorbing composite
structures, non-magnetic materials are used. Since mechanical
properties are greatly influenced by fillers, nonmagnetic
materials such as carbon black, MWCNT, or graphene are used
primarily.

17. 17
To overcome the weight increase caused by such thick thickness,
the use of an alternative structure, such as a honeycomb sandwich
structure, is recommended.
Honeycomb sandwich composite plates have been widely applied
to aeronautical structures as well as building, automobile and train
structures, because they possess many advantages, such as
o Lighter weight,
o Higher stiffness, high stiffness-to-mass ratio
o Heat insulation and preservation,
o Anti-radiation properties
Honeycomb structures have hexagonal passages, which does not
cause considerable weight increases despite the significant
thickness.

18. 18
Commercial
software package
CST Microwave
Studio was used
for simulating the
return loss.
Figure credit: W-H Choi and C-G Kim, “Broadband microwave-absorbing honeycomb structure with novel,” Composites Part B:
Engineering, pp. 14-20, 2015.

19. 19
The basic design concept of
radar absorbing honeycomb
structures is to use the
transverse to the ribbon
direction (y-axis) instead of
the thickness direction (z-
axis). Since the transverse
direction can utilize the
whole volume of the
honeycomb structure, the
trapped incident waves are
greatly reduced due to its
large effective depth of the
direction becomes greater
than that in the thickness
Figure credit: W-H Choi and C-G Kim, “Broadband microwave-
absorbing honeycomb structure with novel,” Composites Part B:
Engineering, pp. 14-20, 2015.

20. FABRICATION
Fabricated using an autoclave process.
a) 12 plies of glass/epoxy-MWCNT 1.8 wt% prepregs produced in
advance were layered onto a flatplate mold. A flat-plate
glass/epoxy-MWCNT composite was fabricated.
20
Figure credit: W-H Choi and C-G Kim, “Broadband microwave-absorbing honeycomb structure with
novel,” Composites Part B: Engineering, pp. 14-20, 2015.

21. 21
b)Tool-tags were attached onto the honeycomb shape molds in order to
easily release the cured glass/epoxy- MWNCT composite sheets from
the honeycomb molds.
c) A corrugated piece of the honeycomb part was fabricated using two
folded honeycomb molds with a glass/epoxy-MWCNT prepreg.
Figure credit: W-H Choi and C-G Kim, “Broadband microwave-absorbing honeycomb structure with novel,” Composites Part B:
Engineering, pp. 14-20, 2015.

22. 22
d) The bagging layers, the breather, and non-perforated release film
were placed onto the stacked honeycomb molds. Finally, the
layered honeycomb specimens with the bagging layers were
vacuum-bagged and cured in an autoclave for 120 min at 130°C.
The cured corrugated composite plates were bonded to make a
honeycomb structure.
Figure credit: W-H Choi and C-G Kim, “Broadband microwave-absorbing honeycomb structure with novel,” Composites Part B:
Engineering, pp. 14-20, 2015.

23. 23
Figure credit: W-H Choi and C-G Kim, “Broadband microwave-
absorbing honeycomb structure with novel,” Composites Part B:
Engineering, pp. 14-20, 2015.
By overlapping two corrugated half
cells, the thickness of those faces was
set to 2twall.

24. 24
Return loss measurement
was done by free-space
measurement system.
Figure credit: Y. He, R. Gong, H. Cao, XianWang and Y. Zheng,
“Preparation and microwave absorption properties of metal magnetic
micropowder-coated honeycomb sandwich structures,” SMART
MATERIALS AND STRUCTURES, p. 1501–1505, 2007.

25. 25
The fabricated absorbing
honeycomb structure satisfied
the 10 dB return loss from 3
GHz to 16 GHz. The fabricated
absorbing structure showed
very excellent absorbing
performance in terms of the
absorption bandwidth.
Figure credit: W-H Choi and C-G Kim, “Broadband microwave-
absorbing honeycomb structure with novel,” Composites Part B:
Engineering, pp. 14-20, 2015.

26. CONCLUSION
oStealth Technology- the key to achieving air superiority
oStealth is achieved mainly by the reduction of signature. The major signatures to
be reduced is the RCS for an airborne system.
oOne of the most fundamental aspects of radar stealth technology remain the
radar-absorbing materials (RAMs) and radar absorbing structures (RAS). RAS
have both the functions of load bearing and electromagnetic energy absorbing
capability. Composites come into greater focus when considering radar-
absorbing structures (RAS). Glass / epoxy systems are cost efficient, high
strength and modulus structures and are widely used in industry, from aerospace
applications to protective equipment.
o However, thick microwave absorbing structures are not feasible for aircraft due
to the weight increase. To overcome the weight increase caused by such thick
thickness, the use of an alternative structure, such as a honeycomb sandwich
structure, is recommended. 26

27. REFERENCES
[1] D. Howe, “Introduction to the Basic Technology of Stealth Aircraft: Part 1—
Basic Considerations and Aircraft Self-Emitted Signals (Passive Considerations),”
ASME. J. Eng. Gas Turbines Power, pp. 75-79, 1991.
[2] B. K. Jha and M. S. Aswale, “Mechanical Aspects in Stealth Technology:
Review,” International Journal of Engineering and Technical Research (IJETR), vol.
4, no. 4, 2016.
[3] N. Kumar and S. R. Vadera, “Stealth Materials and Technology for Airborne
Systems,” in Aerospace Materials and Material Technologies. Indian Institute of
Metals Series, Springer, Singapore, 2017, pp. 519-537.
[4] J. Paterson, “Overview of Low Observable Technology and Its Effects on
Combat Aircraft Survivability,” JOURNAL OF AIRCRAFT, vol. 36, no. 2, 1999.
[5] M. A. Alves, R. J. Port and M. C. Rezende, “Simulations of the radar cross
section of a stealth aircraft,” in International Microwave & Optoelectronics
Conference, 2007.
27

28. REFERENCES
[6] B. Jang, M. Kim, J. Park and S. Lee, “Design Optimization of Composite
Radar Absorbing Structures to Improve Stealth Performance,” International
Journal of Aeronautical & Space Science, p. 20–28, 2016.
[7] S. Kangal, “Structures, development of radar-absorbing composite,” 2013.
[8] W.-H. Choi and C.-G. Kim, “Broadband microwave-absorbing honeycomb
structure with novel design concept,” Composites Part B: Engineering, pp. 14-
20, 2015.
[9] Y. He, R. Gong, H. Cao, XianWang and Y. Zheng, “Preparation and
microwave absorption properties of metal magnetic micropowder-coated
honeycomb sandwich structures,” Smart materials and structures, p. 1501–
1505, 2007.
28

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