Чем выгоднее двухкилевое оперение?

Вуду>- Ценю ваш юмор, но если вы объясняете развал килей на F-18, а так же на F-22 исключительно необходимостью уменьшения таким способом ЭПР, то объясните тогда положение килей на SR-71, когда фактор малой РЛ-заметности ещё, мягко говоря, не был доминирующим при проектировании? И почему на F-15 кили совершенно параллельны, если положительное V вертикального оперения так здорово помогает?

Ну во первых давайте отделим ср-71, ф-117 и ф-22 от ф-15 и 18. Если присоздании последних и задумывались об эпр, то эти размышления оказались малопродуктивными. С первыми же машинами ситуация совершенно другая. Нижеизложенное является в основном компиляцией нескольких книг на аглицком по истории авиации:

Все они являются продукцией фирмы локхид (а именно ее отделения известного под названием skunk works) и в некоторм смысле преемственны. А началось все с у-2. По каким-то (лично мне не до конца понятным причинам) амы были убеждены, что его обнаружение будет проблемой. Скорее всего просто рассчитывали что он не попадет в диаграмму антенны рлс по высоте. Однако уже в первых реальных полетах над ссср делались многочисленные попытки целенаправленно навести на него истребители, что говорило о том, что он замечен и сопровождается. Тогда начались попытки снизить его эпр. Ни к чему хорошему эти попытки не привели.

President Eisenhower was extremely disturbed by the ease with which
the Soviets were detecting the U-2 overflights.He directed that work be
undertaken to reduce the U-2's "radar cross section."Called Project Rain-
bow,it had the highest priority and the attention of all the Skunk Works
engineers.Eisenhower threatened to end the overflight program should
Rainbow fail.Johnson asked advice from two radar experts,Dr.Frank
Rogers and Ed Purcell.
They suggested stringing wires of varying lengths from the nose and tail
to the wings.The idea was to scatter the radar signals away from the re-
ceiver,which would weaken the radar echo.The modification worked,but
with major shortcomings —the U-2's range was cut and its maximum alti-
tude was reduced by seven thousand feet.The wires also whistled and
sometimes broke,flapping against the cockpit and fuselage.
The other attempt was more elaborate.The U-2's underside was covered
with a metallic grid,called a Salisbury Screen,and then overlayed with a
black foam rubber called Echosorb.The grid would deflect the radar signal
into the absorber.The modifications proved to have only limited usefulness.
At some radar frequencies,they did reduce the U-2's radar cross section.At
others,however,the plane's radar echo was made worse.The coating also
prevented the engine's heat from dissipating out the skin.

Урок однако был усвоен и при разработке 71 его модель проверялась на эпр на довольно ранних стадиях разработки и были внесены изменения в конструкцию (в частности фюзеляж стал некруглым).

By the spring of 1959,Johnson and his Skunk Works engineers had
worked their way up to the A-10,but success seemed elusive.President
Eisenhower was intent on a plane with a zero radar cross section.He did not
want the Soviets to even know it as there.Kelly Johnson told the CIA that
there was no way to accomplish this.
Work continued on reducing the radar cross section.One idea involved
adding wedge-shaped chines made of radar absorbing material to the A-10's
cylindrical fuselage.Tests of a small model were successful,and by May
1959 the chines had been incorporated into the A-ll design.This showed a
reduction of a full 90 percent in radar cross section.Although not invisible,
success was now within reach.In July,a final revised design of the A-ll
was prepared.It made full use of the chines,as well as elements from the
previous designs,and was the sum of fifteen months of work.

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Тем не менее тот факт, что конструкцию, изначально разработанную исключительно из требований аэродинамики, доработать на последующих стадиях проектирования до по настоящему малозаметной (т.е. снизить эпр хотя бы в 100, не говоря уж о 1000 раз) нельзя, еще не был до конца осознан, как впрочем и то, что такого снижения эпр вообще можно добиться. А вот при работе над 117 все перевернулось (имхо у нас этот не только технический, но и психологический барьер преодолен так и не был), самолет был спроектирован под малую заметность не аэродинамиками, и не из аэродинамических соображений, а по результатам компьютерного моделирования его эпр (а смоделировать тогда с с требуемой точностью с помощью метода Кирхгофа (физической оптики) с поправками Уфимцева удалось только формы составленные из плоских пластин, и, кстати, результатам этих расчетов сперва не очень-то верили до изготовления моделей), а к аэродинамикам попал уже потом, с требованием заставить это летать.

The keys to Lockheed's efforts were Lockheed mathematician Bill Schroe-
der and Skunk Works software engineer Denys Overholser.
The Maxwell,Sommerfeld,and Ufimtsev equations still could not pre-
dict the RCS for a complex shape like that of an airplane.Schroeder's con-
ceptual breakthrough was to realize that the shape of an airplane could be
reduced to a finite set of two-dimensional surfaces.This reduced the num-
ber of individual radar reflections that would have to be calculated to a.
manageable number.Rather than a surface made of smoothly curving sur-
faces,the whole airplane would be a collection of flat plates,which re-
flected the echo away from the radar.This system of flat,triangular panels
became known as "faceting,"because it resembled the shape of a diamond.
Schroeder asked Overholser to develop a computer program that could
predict the RCS of a faceted aircraft shape.It took only five weeks for the
Echo I program to be completed.Now,with the faceting concept and the Echo I
program,it would be possible to predict the RCS of an aircraft.Possible
designs could be tested and refined in the computer.The way was clear to
build a truly invisible aircraft."
The initial design was dubbed the "Hopeless Diamond."
The Hopeless Diamond met with a frosty reception by Kelly Johnson,
who was still working as a consultant to the Skunk Works.Having built
some of the most graceful planes ever to take to the skies,he was not im-
pressed with this alien design.Johnson's opinion was shared by many of the
Skunk Work's senior engineers and aerodynamicists.
Johnson thought the radar return from the Hopeless Diamond would be
larger than that of the D-21.A ten-foot mock-up of the Hopeless Diamond
was built.On September 14,1975,it was tested against the original mock-
up of the D-21.The Hopeless Diamond had a radar return one thousandth
that of the D-21.This was exactly that predicted by te Echo I program.

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Аэродинамиками впоследствии были внесены некоторые изменения, в частности уменьшена стреловидность и удлинены наружные части крыла.

The Have Blue's wing sweep was an extreme 72.5-degree angle.This
resulted in a poor lift-drag ratio,which cut into payload and range perfor-
mance.Highly swept,low-aspect ratio wings also lose airspeed rapidly in a
sustained high-g turn.The sweep angle was reduced to 67.5 degrees,and
the wings were extended as far back as possible to improve performance.

Вот тут Вуду об устойчивости говорил (что для традиционных машин наверняка так и есть), но у 117 ни с сильно загнутыми внутрь килями (первый прототип ), ни с растопыренными впоследствии наружу устойчивость все равно не обеспечивалась и была поставлена эдсу, с которой оба варианта успешно летали

Have Blue was the first airplane whose shape was determined by electri-
cal engineering,rather than aerodynamics.The design was inherently unstable
in all three axes —pitch (longitudinal stability),roll (lateral stability),and
yaw (directional stability).Every aircraft ever built had curved wing sur-
faces.On the Have Blue,the wings were made of long,wedged-shaped flat
plates,meeting at a sharp edge.
Because of the design's instability,it used a fly-by-wire control system,
built for the F-16A,that was modified to make the Have Blue stable in all
three axes.(The F-16 was unstable only in the pitch axis.)

Зато утверждается, что первая конфигурация килей обладая низкой эпр не способствовала экранированию ик излучения выхлопа, ну их вот так запросто и поменяли на наклоненные в совсем другую сторону (впрочем управляемости это тоже малость помогло, однако думаю в основном за счет большего выноса назад, просто плечо возросло)

The most obvious change to emerge during the redesign was the tail.The
Have Blue's twin fins were canted inward to shield the platypus exhausts
from infrared detectors above the aircraft.In practice,however,the fins
reflected the heat toward the ground,making the plane more visible from
below.The twin fins were also mounted on a pair of booms,which proved
structurally inefficient.In the stealth fighter,the fins were moved farther aft
and canted outward,in a V shape (similar to the V-tail of the Beech Bo-
nanza light plane).This also improved control effectiveness.

Что до б-2 то его разработчики стояли перед еще более сложной задачей, поскольку самолет физически очень большой, имхо маленькие стабилизаторы им не помогли бы, а большие ставить все равно нельзя из-за эпр, вот они от них и отказались совсем. Зато у них было больше возможностей по расчету эпр, поэтому они справились с кривыми поверхностями. Хотя сама задумка – создать аэроплан способный автономно разыскивать и отстреливать тополя вольно порхая над нашей территорией была бредовой.

Ну и наконец ф-22 носит следы уже довольно сбалансированного подхода, и даже похлже снва с некоторым креном в сторону аэродинамики (но это дважды имхо ибо мои познания в сей науке более чем скромны, и мне было бы очень интересно узнать какие вещи в конструкции ф-22 никак не могут быть оправданы аэродинамически, может таких и нет?).

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At the end of the 1990s,as this is written,stealth is no longer the first and onlydesign consideration.Rather,it must be balanced with the more traditional re-
quirements of speed,range,payload,weight,maneuverability,and cost.
An example of the changing view of stealth is the F-22 fighter.In today's air
combat,the preferred method is a first-shot kill with long-range air-to-air missiles.
Historically,most pilots lost in air combat were shot down by planes they never
saw.The F-22's stealth features are an extension of this,giving it the ability to de-
stroy an enemy plane,without warning,with a missile fired from miles away.If
the ability to make a long-range kill was the only consideration,then a pure stealth
design would serve.
In a close-in dogfight,however,a pure stealth-centered design would be at a
disadvantage,as it could be slower,underpowered,and much less maneuverable
than its conventional opponents.In the case of the F-22,the designers were un-
willing to totally compromise its performance in the name of stealth.Several fea-
tures of the F-22 increase its radar cross section,but that was the price for better
maneuverability.

Ну и так, к слову

Although the basic shape of the aircraft may be the primary driver of stealth,
the design considerations now include existing airframes.An aircraft's RCS can
come from relatively small features,such as a gap in an access door.It therefore
makes sense that a "clean-up"program,involving discovery and fixing of such re-
flection sources,as well as addition of Radar Absorbing Material to areas such as
intakes,could result in a militarily significant reduction in a plane's RCS.The prime
example is the B-IB.This was a redesign of the original B-1A to incorporate a re-
duced RCS.Subsequently the "Have Glass"program was undertaken in the late
1990s to add stealth features to F-16 fighters.Although this cannot give an F-16
the tiny RCS of a true stealth aircraft such as an F-l 17A or a B-2,Have Glass has
made F-16s much more difficult to spot in air-to-air engagements.
The effect of both the more balanced design philosophy and the addition of
stealth to existing airframes is to blur the boundaries.In the 1970s and 1980s,there
were stealth airplanes,and there were conventional airplanes.The boundaries be-
tween them were fixed and rigid ones of secrecy and technology.Today the lines
are not hard and fast.Is the F-22 a stealth aircraft?Yes,without question,but not
in the same ways as the F-l 17A or B-2.Are the B-lBs and Have Glass F-16s stealth
aircraft? No,but they are stealthier now than when the prototypes made their first
flights.