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1.0
How to get Stealthy
1.1 Ingredients of Stealth
technology
To make a stealthy aircraft, designers had to consider
five key ingredients:
- reducing the imprint on radar screens / stifling
radio transmissions
- turning down the heat of its infrared picture
- Improve aerodynamics
- making the plane less visible.
- muffling noise
To understand more about each ingredient, here is some
theory first.
1.2 Radar Cross section
(RCS)
The first goal is to cut down the size of the aircraft's
radar image, called its "radar cross section,"
or RCS. This normally involves using radical design features
and some nonmetallic materials.
A conventional fighter aircraft has an Radar Cross
Section (RCS) in the region of 6 square metres. The
much larger B-2B bomber, using the latest stealth technology,
displays an RCS of only 0.75 square metres. By comparison,
a bird in flight displays an RCS of 0.01 square metres.
Stealth plane designers have to take in account that
the used materials (for instance composites) may not
be transparant to radar, but they are also not completely
reflective. In other words, the parts behind the skin
of the plane may be invisible for the eye, but they
are not for radar waves, thus causing echos.
2.0 Getting invisible
This section explains more about what radar echos look
like and how they can be prevented to reach the radar
receiver again after hitting the plane.
2.1 Echo scattering
Curving surfaces on conventional aerodynamic bodies
act as scatterers, reflecting radar waves from any angle
and giving the radar operator a clear signal. The right-angled
surfaces at the wing and tail roots also reflect radar
signals straight back to their source.
Scintillation is a measure of how rapidly the size of
the return varies with the angle. The greater this variation,
the more difficult a target is to track. The lower the
number of lobes and the narrower the lobes, the lower
the probability of detecting any return.
Panels on planes are angled so that radar is scattered
and no signal goes back to base.
The F-117 airframe for instance has a large number
of faceted surfaces, not unlike a crystal.
The facets are presumed to |
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reflect radar energy away from the aircraft in any other
direction than that of the radar emitter.
A flat plate at right angles to an impinging radar wave
has a very large radar signal, and a cavity, similarly
located, also has a large return. Thus the inlet and exhaust
systems of a jet aircraft would be expected to be dominant
contributors to radar cross section in the nose-on and
tall-on viewing directions, and the vertical tail dominates
the side-on signature.
2.2 Radar absorbtion
A second way of stopping radar reflections is by coating
the plane with material that soaks up radar energy.
These typically consist
of carbon, carbon fibre componsites, or magnetic
ferrite-based substance.
The result is that for instance the B-2 is reported
to have the same RCS as a child's tricycle!
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Flight-control surface can be made from honeycombed
materials which reflect incoming radar waves internally
rather than back to the radar. Radar-absorbing coatings
can be applied to the surface of the body which effectively
drain the energy of the radar signal.
2.3 Echo cancellation
The key dimension of a quarter wavelength can vary in
practice from millimeter to one meter. Although the
coating designer will frequently try to use materials
whose dielectric constant varies in a way that maintains
a constant wavelength independent of frequency, the
reality is that a number of different coatings and absorbers
are needed to cover the required bandwidth.
Imagine a low frequency absorber that might be made
of glass fiber hex-cell material. Its resistance is
graded from front to back so that the edge is initially
electro-magnetically soft and gradually becomes more
attenuating as the wave passes through. This approach
is particularly taken when, for practical reasons, the
layer cannot be as deep as a quarter wavelength. The
inner absorber is covered by a high-frequency ferromagnetic
coating, which completes the frequency coverage.
Metal components such as the engine, which produce
significant radar reflections, can be shielded using
a metal and plastic sandwich whose layers are spaced
in such a way as to create a standing wave, cancelling
out any radar reflections.
3.0 Heat radiation reduction
Infrared radiation (heat) should be minimized by a combination
of temperature reduction and masking, although there is
no point in doing these past the point where the hot parts
are no longer the dominant terms in the radiation equation.
The main body of the airplane has its own radiation, heavily
dependent on speed and altitude, and the jet plume can
be a most significant factor, particularly in afterburning
operation.
The jet-wake radiation follows the same laws as the engine
hot parts. Various ways have been developed and tested
to cool down the engine exhaust gasses. The ilustration
above shows how the hot exhaust gasses can be surrounded
by cooler air, significantly reducing the IR signature
of the plane.
Air has a very low emissivity, carbon particles have a
high broadband emissivity, and water vapor emits in very
specific bands. Infrared seekers have mixed feelings about
water-vapor wavelengths, because, while they help in locating
jet plumes, they hinder in terms of the general attenuation
due to moisture content in the atmosphere. There is no
reason, however, why smart seekers shouldn't be able to
make an instant decision about whether conditions were
favorable for using water-vapor bands for detection.
4.0 Turbulence reduction
By optimizing the aerodynamics of the stealth plane,
the for the eye invisible turbulence trail in the air,
can be kept to a minimum. This way it becomes harder
for the very special laser equipment to detect the trail
and trace it back all the way to the plane which created
it.
5.0 Visual detection reduction
5.1 Hiding smoke contrails
(jet wake)
Reducing smoke in the exhaust is accomplished by improving
the efficiency of the combustion chambers. Getting rid
of contrails - that distinct white line in the sky caused
by high flying jets - is a harder task.
Tests have been done using exotic chemicals to be inserted
into the engine outlet gases to modify infrared signature
as well as to force water molecules in the exhaust plume
to break up into much finer particles, thus reduce or
even eliminate contrails. One of the chemical used for
this was chloro-fluoro-sulphonic acid. Several other
acids were tested too, but the result was that the chemicals
were too corrosive and the system was waved.
5.2 Low visibility
An aircraft at low to medium altitudes tends to be a black
dot against the background of the sky. To avoid this,
the plane a given a special medium gray color.
The gray, when combined with light scattering at low to
medium altitudes ensures about as low observability as
can be possible, or a reduction to 30% in visibility.
5.3 Low level flight
Another technique used by aircraft to avoid radar is
to fly at very low levels where there is a great deal
of 'ground clutter' ... radar reflections given off
by buildings and other objects. Low-level aircraft can
go undetected by most radar systems.
The latest ground-defence systems however are designed
to discriminate between ground-clutter and hostile planes.
In addition, ground-clutter is partly avoided by using
'look down' radar systems, which track aircraft from other
aircraft flying above.
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