delta
wings
Sometimes, a technology persists despite
its problems and eventually is rescued by other technologies. The delta
wing story provides an excellent example.
A delta wing is a wing whose shape when
viewed from above looks like a triangle, often with its tip cut off. It
sweeps sharply back from the fuselage with the angle between the leading
edge (the front) of the wing often as high as 60 degrees and the angle
between the fuselage and the trailing edge of the wing at around 90
degrees. Often delta-wing airplanes lack horizontal stabilizers. Despite
the fact that paper airplanes have delta wings and appear to fly quite
well when launched from a height, delta wings actually perform poorly at
low speeds and often are unstable (i.e., they do not stay in level flight
on their own). Their primary advantage is efficiency in high-speed flight.
The first patent for a delta-wing
aircraft design was granted to Englishmen J.W. Butler and E. Edwards in
1867. Their aircraft design would have used a jet-propulsion system, with
thrust provided by rockets, compressed air jets, steam, or gunpowder, had
it ever flown. Like many advanced concepts, not until the combatants in
World War II conducted actual wind tunnel tests did large numbers of
aircraft designers start to take the delta wing seriously. Professor
Alexander M. Lippisch of Germany, best known for developing the
Messerschmitt Me 163 Komet rocket fighter, began thinking about supersonic
airplanes during the 1940s. He chose the delta shape and constructed a
wooden glider to be launched from high altitude by a transport plane. The
Allies captured the unflown glider at the end of the war and sent it to
the United States for study. Lippisch also came to the United States where
he worked on supersonic flight for the U.S. Army Air Forces, the
predecessor to the U.S. Air Force.
Soon after the end of the war, Convair,
a U.S. manufacturer of bombers, began work on a supersonic interceptor
aircraft with a delta wing. The company's engineers began testing models
in wind tunnels before building a full-size aircraft. The XF-92A first
flew from Muroc Air Base (later Edwards Air Force Base) in 1948. Its
designers gave it an extremely large vertical tail, thought necessary
because of fears that the large delta wing might block airflow to the tail
and make the plane impossible to control. Flight tests with the XF-92
proved that a large tail was unnecessary.
By 1953, Convair's engineers had
developed the YF-102 Delta Dagger, a radical design that lacked a
horizontal tail and featured a large, sharply swept delta wing. Wind
tunnel tests of small-scale models indicated that the aircraft could
accelerate through Mach 1 (the speed of sound) with relative ease, rather
than "punching" through it like earlier experimental planes that had to
burn a lot of fuel to go faster than Mach 1. However, the first prototype
unexpectedly encountered immense drag as it approached Mach 1. This
so-called "transonic" region presented a major problem for the aircraft.
Near the same time, Richard T. Whitcomb,
an aeronautical scientist at the National Advisory Committee for
Aeronautics (NACA), was studying transonic drag. Whitcomb developed what
he called the "supersonic area rule." This theory stated that aircraft
that would fly at supersonic speed should increase in cross-sectional area
from a pointed nose. Anything that protruded into the air stream, such as
the canopy over the cockpit, wings, or tail, should be accompanied by a
reduction in cross-section elsewhere. In 1954, Whitcomb, who was then only
33-years old, was awarded the prestigious Collier Trophy for this
contribution to aeronautics.
Convair's designers quickly applied the
supersonic area rule to a new aircraft, the YF-102A, pinching the fuselage
near its mid-point to give it a slightly hourglass (or Coke-bottle)
appearance. This was a compromise for an existing aircraft; later
airplanes included the area rule in their designs in much less obvious
ways. When the first YF-102A with this new design took flight, it easily
accelerated through Mach 1.
To
overcome high drag loads at transonic speeds, Convair engineers redesigned
the fuselage and wing.
The fuselage design used the "area-rule" which resulted in the
characteristic "coke bottle" shape.
During the 1950s the delta wing was used
on several aircraft that had a need for speed, including the B-58 Hustler
and the cancelled XB-70 Valkyrie bomber. The Soviet Union used a delta wing
for its failed Tu-144 supersonic passenger jet, and for its famed MiG-21,
one of the most widely-used fighter jets of the Cold War. The French also
adopted the delta for its successful Dassault Mirage III.
The two most famous current aircraft to
use the delta wing are the Concorde and the Space Shuttle. The Concorde's
delta wing made the plane's sustained cruising speed of Mach 2 possible.
The Space Shuttle's wing, known as a "cranked delta" because the leading
edge of the wing has a slight bend near its midpoint, is used for a
different purpose. The Space Shuttle originally had what was known as a
"high cross-range" requirement, which was the ability to glide for
thousands of miles to either side of its flight path when landing.
Conventional straight wings did not provide enough lift at high speeds and
altitudes to achieve this type of range, and so the large delta wing was
necessary.
While delta wings are critical to
achieving high lift for supersonic flight, they also have a number of
disadvantages for less high-performing aircraft. They require high landing
and takeoff speeds and long takeoff and landing runs, are unstable at high
angles of attack, and produce tremendous drag when "trimmed" to keep the
plane level. Of these disadvantages, pilots and designers usually consider
the high landing and takeoff speeds the most important because they make
flying the plane dangerous. Indeed, when the Concorde had its first ever
crash in 2000, after two decades of safe operations, the high-speed
takeoff was a factor in this terrible accident, for the plane's high
ground speed before becoming airborne placed major stress upon the
aircraft's tires, which exploded upon striking an object on the runway.
By the 1980s, except for the Concorde
and Space Shuttle, the delta wing appeared headed for obsolescence. Its
drawbacks made it unattractive and changes in fighter warfare reduced the
requirement for sustained supersonic speed. Few aircraft spend much time
travelling at high supersonic speeds because it burns so much fuel,
rendering the delta wing, which is primarily useful for supersonic flight,
less attractive. But the computer and an additional flight control device
called the canard have rescued the delta wing from obsolescence.
Computer-controlled "fly-by-wire" flight
control systems have allowed designers to compensate for some of the delta
wing's poor control qualities. Canards are small horizontal fins (or small
wings) mounted on the fuselage in front of an aircraft's main wings to
provide greater control, particularly during high angles of attack. When
they are part of a delta-wing aircraft, they improve its stability and
manoeuvrability.
The
delta wings can easily be seen on this photograph of the Shuttle orbiter
Columbia.
Several aircraft appeared in the 1980s
and 1990s that incorporated both delta wings and canards. The latest
delta-wing aircraft are the Swedish JAS 39 Grippen, the Dassault Rafale
naval fighter (designed to be launched from the French aircraft carrier
Charles de Gaulle), the Indian Light Combat Aircraft, or LCA, and the
Eurofighter Typhoon. The Typhoon, which had its first flight in the
mid-1990s, is a joint European effort (Britain, Germany, Italy and Spain,
with France withdrawing early) to develop an advanced fighter to replace a
number of different aging aircraft in their air forces. Thus, the delta
wing, which seemed destined for obsolescence, has gained a new lease on
life.
Dassault Rafale M |