How does "continuous curvature" give stealth?

Question

Wikipedia says

RCS reduction as a result of shape had already been observed on the Royal Air Force's Avro Vulcan strategic bomber,[117] and the USAF's F-117 Nighthawk. The F-117 used flat surfaces (faceting technique) for controlling radar returns as during its development (see Lockheed Have Blue) in the early 1970s, technology only allowed for the simulation of radar reflections on simple, flat surfaces; computing advances in the 1980s made it possible to simulate radar returns on more complex curved surfaces.[118] The B-2 is composed of many curved and rounded surfaces across its exposed airframe to deflect radar beams. This technique, known as continuous curvature, was made possible by advances in computational fluid dynamics, and first tested on the Northrop Tacit Blue.[119][114]

That makes no sense. The whole point of stealth shaping is to reduce the number of directions radar waves go. Faceting is the only way to achieve this.

Answer 1

Electromagnetic radiation is reflected differently depending on wavelength and angle. While light will always be reflected back or scattered, with centimeter waves this only happens if the incidence is almost vertical. Only then will the radar pulse be thrown back like a tennis ball.

In the case of grazing incidence, the radio wave continues to travel through the material and is scattered at the edges. That is why edge alignment is so important for stealth. And this also determines the inclination of all surfaces: You will not find any vertical surface on a well-designed stealth airplane.

For the early modern designs (Have Blue, F-117, MBB Lampyridae) the computational capabilities (both computer power and the available codes!) only allowed to simulate plane surfaces. The result was the multi-faceted design with edge alignment even for the serrated edges of maintenance panels and landing gear doors. Especially the early Lockheed designs sported many edges and, consequently, their stealthiness was rather poor and only remedied by the generous application of RAM (radar absorbing material). With better computational methods, also curved surfaces could be simulated and it became possible to reduce edge count. But ideally the surface should be horizontal near all edges, which explains why fuselages and wings look like they had ironing edges. Note the pillow shape of the lateral edges of flaps on modern stealth airplanes: The same applies here, too.

Faceting is the only way to achieve this.

Not really. This only applied to the early designs of the Seventies and Eighties, and even then RAM helped a lot already. Generally, the number of edges should be reduced as much as practical and vertical surfaces avoided completely. While inclination against the horizontal is possible in the center of continuous surfaces, the surface orientation should be as horizontal as possible at all edges and the transition from the horizontal edge to the half-vertical center should be gradual.

Answer 2

Stealth capabilities of military aircraft (or any vehicles or apparatus for that matter) are not reliant on the form of the object only. Other major contributing factors are materials and joinery. Different materials can be used to absorb radar frequency radiation and joinery of parts is designed such that no favorable condition to for radiation to be reflected back towards its source is created (as in prisms for example.

The main goal of the form of a stealth object is to create a controlled RCS pattern that is directionally optimized such that for certain strategic directions the radar signature of the object is minimal. In other directions radar absorbing materials can be used to minimize the return pulse of the radar signal.

The following study simulates the radar signature of the F-22 Raptor fighter. Please note that this study does not represent the actual radar signature of the F-22 but it can be considered a close enough approximation of that https://www.hindawi.com/journals/ijap/2018/5435837/

From the diagram you can see that the radar signature has substantial directional variation, so the F-22 is by no means invisible to radar at certain directions. However from directions that are considered necessary for successful missions completion the F22 is very, very difficult to see with radar.

The same applies for faceted designs such as the F-117. The difference between faceted and constant curve designs is that faceted designs have relatively large flat surfaces at certain directions and this provides strong radar return, whereas the curve designs give a return across larger sweep angle but the return is smaller.

For aircraft delivering ordinance it must be taken into account that releasing bombs, missiles and such will most certainly give out the position for the aircraft. Therefore if it for example turns away right after release, the radar signature during that maneuver is not that important, approach and departure signatures are. For loitering surveillance aircraft the design will be different.

Since modern materials and designs can produce low enough radar signatures for necessary directions using curved designs, they are preferred since they are by far superior aerodynamically. The preference for flight performance was actually the case already in the 60's, when the CIA overhead recommaisance program was commissioning a supersonic spy plane.Locheed A-12 was chosen over Convair Kingfish despite having inferior radar properties, because its flight performance was projected to be better. What A-12 and its successor SR-71 lacked in stealth, they made up in pure grunt. They simply were too fast for the missiles of the day.

Answer 3

Well, since you know "the only way to achieve" stealth is through faceting, then obviously the B-2, F-22, F-35, B-21, and quite a few drones must not be stealthy.

The Wikipedia article is weak. They usually are for deeply technical articles. Don't go there to learn real engineering or real depth. Go there to learn a layperson's perspective on topics.

One of the problems with cutting edge military technologies is that people who know about it -- aren't allowed to talk about it.

Given that constraint, it seems unreasonable for Wikipedia to be a strong source of information.