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One of the keys to the stealth capability was its faceted design. The concept is that curved surfaces have a larger radar cross-section. The idea of making all surfaces flat to reflect radar away from the receiver seems a pretty sound concept. It must have been pretty effective, since they had to treat the windows because the pilot's helmet had a major effect on cross-section.

But all of the low-observable aircraft made since, such as the B-2 Spirit and even Lockheed's own F-22 and F-35 have a significant amount of curved surface area.

Why has the fully-faceted concept not carried over into newer low-observable designs? Does it just have too much negative affect on handling characteristics?

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The F-117 was designed at a critical point in aeronautical history where we didn't have enough computing power to analyze the radar cross section of more complicated geometry, but did have the computing power to provide artificial stability to unstable designs.

The faceted design is not ideal for stealth or aerodynamics. Obviously the corners are less aerodynamic than a smooth surface. But it also means that if the facets align with a radar source they will reflect a strong return. The compromise is that it's easy to calculate the reflecting angles of radar energy off of flat surfaces based on various locations of radar sources relative to the aircraft.

A curved surface means that only a small section of the surface is really reflecting directly back to a radar source, while the rest is scattered. Together with radar absorbing materials, this can provide effective stealth. It just takes more analysis to determine how different curves and features will reflect radar energy.

You can see the evolution of stealth designs from the Have Blue (prototype for the F-117) in the mid 70's, to the Tacit Blue prototype in the late 70's, to the B-2 in the early 80's.

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  • $\begingroup$ This is an interesting concept regarding radar reflection, but did they really need powerful computers to tell them that a round object scatters radar better at all angles, vs a faceted design only being effective at certain angles? $\endgroup$
    – YAHsaves
    Jul 30, 2018 at 18:31
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The faceted design makes the aircraft aerodynamically unstable. This is stated in the Wikipedia Article. The designers had to use a complicated fly-by-wire system to compensate for it and is one of the first instances of fly-by-wire aircraft. The F117A could not fly faster than the speed of sound because of this. The F117A was designed to fly in places where there were surface to air missiles so they wouldn't be identified on radar and shot down. It was not designed to participate in a dog fight. Current stealth technology relies on radar absorbing materials so that the aircraft can be aerodynamically efficient.

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    $\begingroup$ Not to say that aerodynamics are not the major concern, but modern tactical aircraft are pretty much all aerodynamically unstable, to the point of being unflyable without FBW, including all of the low-observables. Despite its F- designation, the F117A is not a fighter. It was no more designed for dogfighting than the B-2, which is also not capable of supersonic flight. $\endgroup$
    – TomMcW
    Apr 27, 2018 at 19:30
  • $\begingroup$ "and is one of the first instances of fly-by-wire aircraft." The LLRV (1964) was one of the first FBW aircraft. Also the F8 (1972) used FBW. I kind of disagree that an aircraft introduced almost 20 years after the LLRV would be considered "one if the first instances". But I stand to be corrected. $\endgroup$
    – idkfa
    Apr 27, 2018 at 23:27

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