Is it easier to make a reduction in the frontal aspect rcs or top aspect rcs?
With the current aircraft technology, designs and shapings it is not possible to reduce top/bottom RCS. Basic radar principles tells us that surface area have a direct effect on aspect RCS value. However, there is one technique that can difuse reflected signals...
With known behaviors of radar waves upon a surface, tapered conductivity would have the traveling wave be interupted and portions of the wave be discharged or to put it simplistically 'bounced' off the segmented surface. The B-2, F-22 or F-35 'may' have this technique applied to their designs...May have it.
In frontal aspect you have engine inlets that needs to be reduced,radome and sharp points like wings. Reducing the cross section of them is difficult but their overall rcs is lower than the top aspect stealth. If you face directly to the enemy radar it will be late to pick you up but if a missile is coming to you and you maneuver you will expose non rcs reduced parts to enemy radar and rely on your jammers.
On the frontal aspect, the engines are the most problematic, far more than the radome, cockpit and leading edges of intake inlets and flight control surfaces combined. Moving surfaces, especially rapidly moving like jet engine blades or helo rotors, create a phenomenon called 'Doppler scintillation', much like spinning a propeller in front of a light source, then the wave superposition principle come into play where the reflection from each engine blade sum itself with the next blade and the next blade and the next blade and so on...All these signals then bounces around the inlet tunnel ending in an electronic flare that no receiver could miss. Reducing engine related radar reflectors should take precedent over other miscellanies. Unfortunately, this would require so much of a radical relocation of the engines that one might as well design another airframe.
In top aspect reduction there is a large area on top of the plane to cover with ram that increase the expenses but the engine inlets of most planes are underside the plane(Mig 29, J 10, JF 17) and your job is reduced to handling the airframe geometry, giving angle to tail fins and ram.
The current technology of Radar Absorbent Material are highly narrow in frequency bands.
There are four main types of passive absorbers:
1- Jaumann layering
2- Dallenbach layering
3- Salisbury screen
4- Analog circuit sheet
Basically, the first three are ferrite particles embedded in a liquid solution then painted or somehow applied to a surface. The distribution of those particles are not very finely controlled, hence their limited frequency range.
Here is an example of the drawbacks of those three...
Salisbury screen - Wikipedia, the free encyclopedia
There are a few disadvantages inherent to this model (some of which have been solved). One would be the fact that salisbury screens work well only for a very narrow portion of the radar spectrum thus making it very vulnerable to multiple radar protected areas. Another problem is the thickness of the screen itself, the radar wavelengths are between 10 cm and 1 mm, thus for a longer wavelength, the thickness gets up to 2.5 centimeters which is quite difficult to cope with (e.g., in aerospace applications). Thus, research is being conducted for ultrathin Salisbury screens involving the Sievenpiper HIGP (high impedance ground plane) (source: Wiley Periodicals, Inc., Microwave Opt. Technol. Lett.), which shows remarkable improvements to the thickness of the screen.
Item four has potential to make an aircraft a truly active RCS manipulation vehicle...
A smart radar absorber
This paper proposes a configuration for a smart radar absorber which is capable of both self-tuning and absorb while scan operation. The discussion is complemented by modelled and measured performance data.
Basically...The ferrite particles are somehow deposited into a material in a uniform formation. They can be of various sizes to affect different frequencies. They can even be in layers and be electrically charged, like a transistor, to either completely negate (or absorb) a radar signal, or to amplify (emit) a reflection so the aircraft can present itself to be something else, like an unmanned drone only a few meters long electronically masquerading as a B-52. This is not science fiction. Photolithography technology in the semiconductor industry, which I currently hold employment, is very applicable to active RCS manipulation. This is one step up from jamming, part of which is about brute force overpowering a reflected signal off a body. The US is currently actively engaging in active RCS manipulation technology, not only for the current generation of 'stealth' aircrafts, but also for the next evolution. Keep in mind that this would require appropriate computational power as the enemy would undoubtedly employ frequency agility tactics and the response would have to be in picoseconds to deny him any coherent radar returns and that an aircraft has limited internal space for avionics. So the more miniaturized the core computers, the more capable the avionics and the 'stealthier' the aircraft.
The F-117 relied upon angled facetings to have some measure of control on radar signals. That was a 1970s technology design where engineers were still adept with the sliderule. The B-2, F-22 and F-35 were designed with supercomputers that calculate nuclear explosions. Curved surfaces required far more computational power and knowledge of radar wave behaviors to produce a consistently low RCS design. Radar waves behave differently on a curved surface than on a planar surface. On a planar surface, most of the signal reflects. But on a curved surface, the wave does what is called a 'creeping wave' effect...
Creeping wave - Wikipedia, the free encyclopedia
According to the principle of diffraction, when a wave front passes an obstruction, it spreads out into the shadowed space. A creeping wave, in electromagnetism or acoustics is the wave that is diffracted around the shadowed surface of a smooth body such as a sphere.
Creeping waves greatly extend the ground wave propagation of long wavelength (low frequency) radio. They also cause both of a person's ears to hear a sound, rather than only the ear on the side of the head facing the origin of the sound. In radar ranging, the creeping wave return appears to come from behind the target.
Vladimir **** made important contributions to the understanding and calculation of creeping waves. They are described by Airy functions.
In other words, if you cannot or refuse to master angled facetings, the most basic of RCS reduction techniques, odds are good that whatever designs you produce will be just as visible to the F-22 and its brethens as if you had produce a 'non-stealth' design. This is why those who have relevant experience and knowledge about radar and avionics chuckled at how the Russians and the Chinese rushed to the media proclaiming their versions of 'stealth' aircrafts.
(personal experience)...
