I’m just pointing out that given the careful shaping and faceting of the inner intake walls
I think the point both that image and APA are making is that the angle of the facet shown below (I also think APA got the shape of that facet wrong) results in an RCS spike at about 10 degrees below horizontal.
The facet…
The cant angle of that facet…
I think the corner reflectors between the elevators and the engines (remember, those big, stealthy cylinder things with an “RCS” of -50 “dBm” in lab tests ;P ) are much more significant though. pofacets didn’t display the full story here because it doesn’t factor double reflections. The RCS problem at around 5 degrees would actually be worse in real life than in a pofacets simulation.
I’d bet the third of the “shape”s refers to panel joints though 😉
My comment on surface joints was actually about the intersections between differently angled surfaces which can cause scattering of creeping waves if not smoothly and gradually blended.
Panel seams and edges along a large flat’ish surface are not such a big thing as long as surface waves are managed effectively and the edges themselves are consistent with the plan-form shaping of the aircraft.
Tapered R-Cards and magnetic absorbers are adhered to the surface and are extremely efficient at breaking up and softening the creeping waves as they propagate across the surface. They scatter the waves gradually and in predefined directions rather than having one big scatter at the surface trailing edge.
This pic of a raptor which hasn’t had its outer coat of paint applied in a while shows a number of the small sections of ram (ignoring the sawtooth vents) which have their edges aligned with the aircraft’s surface leading and trailing edges. You can faintly see that particular care has been taken in front of the canopy frame.
One of the images you provided seems to show a similar solution being used on the T-50, albeit executed with a little less precision.
http://forum.keypublishing.com/attachment.php?attachmentid=206796
I understand that with the F-35, they’ve adopted a laser system for applying the cards at precisely the correct angle for maximum effect. There are portable versions of the unit that are just placed on the ground around an aircraft. The devices automatically calculate their location in relation to the aircraft and accurately draw the ram boundaries on the surface with laser. Its a good way to allow crews in the field to reapply the RAM accurately and easily.
http://www.youtube.com/watch?v=JwvZLqtAZWY
Still, the return scatter from surface waves is nowhere near as significant as that from specular reflection, certainly not after the various RAM’s and methods are applied.
Couldn’t help but be amused by those trying (maturely :rolleyes:) to discredit the previously posted plots as “completely” incorrect based on pyhsical optics methods being used, especially the guy taking a 2yo, unused account out of mothballs to post on it for the first time.
PO provides decent estimations of the specular RCS of objects, period. It’s more than acceptable for comparing the exposed surfaces of to-scale shapes and good enough for numerous military and civilian universities to use in their engineering courses.
PO does not take into account corner reflectors, edge diffraction, creeping waves, and it ignores shadow regions, but as mentioned earlier these things would actually accentuate RCS spikes of cluttered shapes with little surface blending and alignment, not negate them.
Cross sections are used in these samples rather than entire aircraft to help keep models simple and avoid too much inaccuracy.
The same material selection was used in sampling both shapes (PEC) so RAM is irrelevant. Applying RAM to both models would not change the angle of RCS spikes, nor would it benefit one shape and not the other.
Cylinders are not a stealthy object, they provide a decent, uniform return all the way around. Applying RAM to one makes it a slightly less detectable, non-stealthy object (hint, cylindrical poles are not used on RCS test stands). No cylinders were used in these cross section samples in any case.
I’ll get to those for my own curiosity later, along with comparing the LO impact of all moving rudders, ventral fins and those enormous flap actuators on the J-20, wing leading edge length, sweep angle, sharpness and edge design, and the lumpy-guts F-35 (though I have a feeling it might have been cleverly done).
As Obligatory said, I think if they were going for an all out stealth fighter, they would have. It seems that the basic design of the PAK FA all centres around the engine configuration. They just want to keep those engine spread wide and in that V arrangement. This dictates the centre tunnel being a part of the design which in turn dictates how the IR missiles will be mounted and what cant angle can be used on the nacelle inner surface.
Wait a second, are you seriously saying that those two RCS charts are an “apples to apples” comparison between the “lower fuselage cross sections of the F-22 (left) and T-50”!
I can assure you this was an apples to apples, physical optics calculation method simulation using the same width cross section of both aircraft in the area just behind their intakes. Also because PO method was used rather than MOM or FDM, double reflection of waves are not taken into account which would act even more in the F-22’s favor. T-50 has a couple corner reflectors, but in areas that would only matter while its banking.
The result is hardly surprising. The PAK FA has a lot going on underneath the fuselage compared to the F-22 and J-20 due to the underslung nacelles.
The inner surface of the engine nacelles are almost perpendicular to source radar at around 10 degrees below horizontal causing that upper spike seen on the polar diagram (indicated by the arrow).
There is a lot about certain relevant aspects of the T-50 that may well change before it enters production – are the current airframes the equivalent of the X-35, F-35AA-1 or F-35AF-1 in terms of development progress? We have no way to be sure, so any quantitative assessment at this time is pretty much void from the word go.
To fix the issues with the RCS spikes at -10 degrees a redesign of the nacelles is required. Altering this too much effectively would mean a different aircraft.
Trident, the X-35’s had a slightly stealthier shape than the current F-35’s, some compromises were required for aerodynamics and avionics.
People love eyeballing man.
That and sticking to preconceived notions of what is stealth, and what isn’t.
🙂 and some have done the hard yards and have access to the tools to actually test and validate this stuff 😉
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