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
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.
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.
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.
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 traveling 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
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
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.
Certainly Concorde flies better than a lot of deltas, but one fact of life is that deltas come in on the back side of the drag curve, and you've got to remember it... B. Trubshaw, Director of Concorde Flight Test, 1969
Delta wings (Deltas) are symmetrical triangular wings
designed to fly at subsonic or supersonic speeds. At
supersonic speeds the leading-edge can be subsonic, sonic or
supersonic, depending on the relation between sweep angle
and speed (see below).
Leading edges are generally linear, although there are
cases of more complex geometries, such as the ogive delta
(Concorde SST), the gothic delta, the cranked delta
(Lockheed CL-823), the double delta (SAAB Viggen), delta +
canards (North American XB-70 and others).
Almost all delta wings fall into the category of low
aspect-ratio wings. Their aspect-ratio is defined by AR =
4/tan(D), where D is the leading edge sweep angle (this lead
to AR less than 3 in most cases; about 1.8 in the case of
Concorde). Wing thickness is generally small.
The problem is to find the aerodynamic properties of the
wing (CL, Cd, Cm, Cp distribution, etc.), along with the
lateral and longitudinal stability characteristics of the
wings at different operation points.
The technical literature on deltas is huge, and it is
safe to say that all speeds and sweep angles have been
investigated (experimental, theoretical and computational
Delta Wing in Subsonic Flow
Flows past delta wings are severely compounded by the
leading edge separation, by the roll-up structure of the
concentrated vortices, and by the lateral and longitudinal
instability that is consequent to large sweeps, high-angle
of attack, and sharp maneuvers.
Although the aerodynamics of the delta wing is
non-linear, most of the research has relied for a long time
on liberalized small perturbation theory (shortly reviewed
Computational methods (ex. vortex
lattice methods, panel codes) have proved tremendously
effective at low speeds and unsteady flows (Katz, 1984,
Linearized theory for the slender body with small angle
of attack (Munk, 1924; RT Jones, 1945) leads to a very
simple conclusion: lift is produced in the conical flow
created by the stream-wise variation of span b(x). There is
one singularity at the apex of the delta, where the
theoretical pressure would be infinite.
The expression for the lift coefficient
that is correct only for very low aspect-ratios (AR=1). The
corresponding induced drag coefficient is
, that is just half the value that is expected at angle of
The center of pressure is found at 2/3 chord from the
pointed leading-edge, where the pressure is also singular.
The effect of a fuselage can also be estimated by a more
general formulation (Ashley-Landhal, 1965) that gives a lift
coefficient in the wing-body configuration is lower that the
Lifting surface theory (for ex., vortex
lattice method) is a better approach to the prediction of
the basic coefficients. There are also methods for arrowhead
wings (Mangler, 1955) and wings in yawed flow (Carafoli,
Delta Wings in Supersonic Flow
Delta wings are appropriate plan forms to fly at
supersonic and hypersonic speeds, therefore there has been a
long time interest in investigating the effects of high Mach
The principle of independence (Buseman,
1935) allows to investigate separately wings with subsonic
and supersonic leading edges (e.g. for which the normal Mach
number is below or above the speed of sound).
In general, wings with subsonic leading edges are
characterized by leading edge separation; wings with
supersonic leading edge are characterized a Prandtl-Meyer
The main parameters of the wing
problem are sweep, free stream Mach number, angle of attack
and wing thickness. The effects of all these parameters can
be collapsed in one single plane alfan-Machn, where the
occurrence of subsonic or supersonic flow can be diagnosed
as function of the parameters (Stanbrook- Squire, 1964).
Delta Wing with Subsonic Leading-Edge
The wing is inside the Mach cone if the sweep angle is
greater than the Mach angle, thus yielding leading-edges
that are fully subsonic, Fig. 1. Linearized theory leads
again to simplified expressions for the main aerodynamic
characteristics, which are quite powerful to describe the
operation of the wing.
The lift coefficient depends on the aspect-ratio,
according to en expression that is fairly approximate for
incidences less than 5 degrees (Ashley-Landhal, 1965).
According to the theory, the strong leading edge suction
gives rise to a leading edge thrust that decreases the
amount of drag (in practice only a small amount of this
suction can be realized.)
Delta Wing with Supersonic Leading-Edge
Figure 1: Delta wing with subsonic leading-edge
Figure 2: Delta wing with supersonic leading-edge
The leading-edge is inside the Mach cone, by virtue of
the comparatively larger sweep angle (Fig. 2). In such a
case there is no interaction of flows between upper and
The pressure jump at any given point on the wind surface
has a definite expression, which is constant along lines
through the wing vertex (conical flow). By integration of
the pressure jump one finds that the lift coefficient is
independent from the sweep angle, and the lift-curve slope
is also independent from the angle of attack for as long as
the leading edge is supersonic.
Carafoli (1969) report analytical
studies of a wide array of delta wings, polygonal wings, and
T-wings, also in yawed flow.
Flow Separation on Highly Swept Wings
Real cases of flow past slender delta wings (wings of
small aspect-ratios) are almost certainly separated, and to
a great extent. Separation starts from the leading-edge and
produces a series of vertical regions that have a conical
shape growing stream wise. The angle of attack at which
these vortices appear depends on the slenderness.
Separation is at the leading-edge when the leading edge
is sharp, and leads to performances largely independent from
the Reynolds number. The presence of the leading-edge
vortices is the cause of a number of phenomena:
The lift coefficient is larger than that predicted
with linearized theory (see below). This is due to the
suction effect of the separation vortices. The
difference between the linear value of the lift and its
actual value is called vortex lift.
The leading-edge vortices induce a field of low
pressure on the suction side of the wing. The increased
suction is a reason for increased lift (point above).
Stall occurs at a large angle of attack, because of
the vortex instability, leading to vortex burst. When
the vortex core bursts the suction effect disappears. A
a vortex burst far behind the trailing edge, the burst
has little or no effect; vortex burst on the wing itself
will reduce the vortex lift.
The vortex pattern behind the delta wing depends on
the slenderness, because slenderness, together with
angle of attack, is what decides the vortex burst.
Vortex asymmetry appears on very slender wings at
lower and lower angles of attack, because the vortex
finds less physical limits for development, therefore
becoming soon unstable.
Flow separation characteristics depend on speed (Mach number), wing sweep, angle of attack and wing thickness.
Wings with subsonic leading edge are dominated by leading
edge separation. Secondary separation appears at moderate to
high angles of attack, Fig. 3.
Wings with supersonic leading edges are characterized by
a Prandtl-Meyer expansion behind the bow shock and by an
attached leading edge flow Fig. 4.
Figure 3: Flow separation on delta wing with subsonic leading edge.
A = attachment; S = separation; V = vortex.
Figure 4: Flow separation on delta wing with supersonic leading edge.
SW = shock wave
Dr. Alexander M. Lippisch, Aviation Pioneer, 1894-1976
Dr. Alexander Martin Lippisch (November 2, 1894 - February
11, 1976) was a German pioneer of aerodynamics who made
important contributions to the understanding of flying wings and
ground effect craft. His most famous design was the
Messerschmitt Me 163 rocket-powered interceptor.
Lippisch was born in Munich, Germany. He later recalled that his
interest in aviation was first kindled by watching a
demonstration by Orville Wright in September 1909 in Berlin. He
was, however, planning to follow in his father’s footsteps and
enter art school when World War I intervened. During his
service with the German Army from 1915 – 1918, Lippisch had the
chance to fly as an aerial photographer and mapper.
Following the war, Lippisch worked for a while with the Zeppelin
Company, and it was at this time that he first became interested
in tail-less aircraft. In 1921 the first such design of his
would reach fruition in the form of the Lippisch-Espenlaub E-2
glider, built by Gottlob Espenlaub. This was the beginning of a
research program that would result in some fifty designs
throughout the 1920s and 30s. Lippisch’s growing reputation saw
him appointed the director of Rhon-Rossitten Gesellschaft (RRG),
a glider research group.
Lippisch’s work led to a series
of tail-less designs numbered Storch I – Storch IX between 1927
and 1933. These were greeted with almost complete indifference
by both government and private industry. During this time, one
of Lippisch’s designs, the Ente (Duck), would enter history as
the first aircraft to fly under rocket power. It was a sign of
things to come.
Experience with the Storch series led
Lippisch to concentrate increasingly on delta-winged designs.
These would find expression in five aircraft (simply numbered
Delta I – Delta V) built between 1931 and 1939. In 1933, RGG had
been reorganized into the Deutsche Forschungsanstalt für
Segelflug (DFS - German Institute for Sailplane Flight) and the
Delta IV and Delta V were designated as the
DFS 39 and
DFS 40 respectively.
In early 1939,
the Reichsluftfahrtsministerium (RLM) – (Reich Aviation
Ministry) transferred Lippisch and his team to work at the
Messerschmitt factory to design a high-speed fighter aircraft
around the rocket engines then under development by Hellmuth
Walter. They quickly adapted their then-current design, the
DFS 194 to
rocket power, successfully flying in early 1940. This was the
direct ancestor of the Messerschmitt
Me 163 Komet.
Although technically brilliant, the Komet did not prove to
be a successful weapon, and friction between Lippisch and
Messerschmitt was frequent. In 1943, Lippisch transferred to
Vienna’s Luftfahrtforschungsanstalt Wien (LFW), to concentrate
on the problems of high-speed flight. That same year, he was
awarded a doctoral degree in engineering by the University of
Wind tunnel research in 1939 had suggested
that the delta wing was a good choice for supersonic flight and
Lippisch set to work designing a supersonic, ramjet-powered
fighter, the Lippisch P-13. By the time the war
ended, however, the project had only advanced as far as a
development glider, the DM-1.
Like many German
scientists, Lippisch was taken to the United states after the
war under Project Paper Clip.
[Originally called Operation Overcast, Operation Paperclip
was the code name for the operation by the government of the USA
to extract rockets (e.g. V-1, V-2), chemical weapons (e.g.
Zyklon-B) and medical scientists from Germany, after the
collapse of the Nazi government during World War II.
Scientists were deployed at
White Sands Proving Ground, New Mexico and Fort Bliss, Texas to
work on guided missile and ballistic missile technology, and led
to the foundation of NASA and the US ICBM program.
700 members of the Nazi scientific community were brought to the
US as a direct result of Operation Paperclip, many of whom were
still ardent Nazi supporters
Although President Harry S.
Truman gave explicit orders not to allow any scientists who were
thought to have strong Nazi leanings to enter the US under
Operation Paperclip, many dossiers were re-written to "clean-up"
the histories of many of the scientists involved, to avoid their
knowledge falling into the hands of another power.
of the information surrounding Operation Paperclip is still
Advances in jet engines were making his original interceptor
designs more practical, and Convair became interested in a
hybrid jet/rocket design that they proposed as the F-92. In
order to gain experience with the delta wing, they first built a
jet powered test aircraft, the 7003, which became the first
powered delta-wing aircraft to fly. Although the USAF lost
interest in the F-92, Convair's experience with the delta-wing
design led them to proposing it for most of their projects
through the 1950s and into the 1960s, including the F-102 Delta
Dagger, F-106 Delta Dart and B-58 Hustler.
From 1950 -
1964 Lippisch worked for the Collins Radio Company in Iowa,
which had an aeronautical division. It was during this time that
his interest shifted toward ground effect craft. The results
were an unconventional VTOL
[Vertical Take-Off and Landing] describes airplanes that can lift off
vertically. This classification includes only a very few
aircraft; helicopters are not considered VTOL.
Nikola Tesla received patents for an apparatus for aerial
transportation. It is one of the earliest example of VTOL
aircraft. In the late 1950's and early 1960's almost all fighter
aircraft designed included some VTOL features. This was a
response to the worrying possibility that a first-strike against
airfields by nuclear armed bombers would leave a country open to
attack by following bombers. The "solution" was to use VTOL
fighters that could be moved to open fields around the
countryside, making them immune to widespread destruction.
In reality the costs of VTOL performance were huge, and
while it turned out to be fairly easy to move the plane, moving
the support equipment and fuel was not so easy. By the mid-1960s
interest in VTOL had faded, perhaps due much to the widespread
introduction of ICBMs as the main nuclear delivery system.
Currently there are believed to be two types of practical
VTOL aircraft in operation:
- Bell Boeing V-22 Osprey "tilt-rotor" and the
- British Aerospace Hawker Harrier "Jump jet" ]
aircraft (an aerodyne) and an aerofoil boat. Lippisch
resigned from Collins because of ill health caused by cancer.
When he recovered in 1996, he formed his own research
company, Lippisch Research Corporation, and attracted the
interest of the West German government. Prototypes for both the
aerodyne and the ground-effect craft were built, but no further
development was undertaken. The Kiekhaefer Mercury company was
also interested in his ground-effect craft and successfully
tested one of his designs as the Aeroskimmer, but also
eventually lost interest.
Lippisch died at Cedar Rapids,