A New Shape For A New
Century
With the advent of
the rectangle,
and then the tube with wings, twin characteristics of airliners and
commercial
flight since the early Twenties, airline manufacture has adhered to a
well-established
pattern. Whether a Ford Trimotor, Douglas DC-3 or -4, the long
line
of Boeings up through the 777, and the entire Airbus family, passengers
have entered a more or less long cylindrical tube, which given enough
push
(jet engines) or pull (propellers) has then sped them away to their
various
destinations.
In 1966, when Boeing's
747
was launched, it was believed that the tube with wings configuration
had
reached the apogee of the form's usefulness. That very large
aircraft
was powered by four engines, yet in 1990 Boeing offered still another
tube
with wings airliner, almost as large, but with only two engines, the
777.
In its latest -300 models, already in production, that design will have
a fuselage length which exceeds the 747's by more than ten feet.
Boeing has also been contemplating an even larger four-engined 747, the
747 XL (Xtra Large), a sort of scaled up flying watermelon that could
seat
up to 650 passengers. But that what many engineers consider to be
a foreshortened freak will not be translated to metal, at least not by
Boeing.
Boeing's 777
variant
and those spun off from it will not greatly alter current airport
facilities
or operations, but a new giant from Airbus will. Known as the
A3XX,
and currently under final design, this behemoth, which may debut as
early
as 2005, will have a wingspan of 253 feet and a length of 250
feet.
The fin will tower 75 feet above the tarmac, but no ordinary tarmac
will
be able to support its massive one million, eighty four thousand pound
takeoff weight. Such a huge shape will also cause problems with
wake
vortex, passenger circulation and comfort, plus the daunting attendant
psychology in flying in something that big. At a greatly higher
price
than the newest 747, it will only hold 75 more passengers without any
significant
increase in speed or range.
The reason these super jumbos
are
going to become reality has already been discussed. In the
future,
the world's airlines will have to move many more people than they do
now,
and each international takeoff will have to be maximized in terms of
passengers
transported. This prediction brings us to the subject of this
article.
Long before Airbus decided to go ahead with their super-jumbo Boeing
evaluated
a potential successor for its own 747, since the extended range 777 was
only an intermediate solution to moving more people.
At a symposium held in
January
1998 at Reno, Nevada, Boeing came up with two ambitious, but practical
alternatives to answer the real need for a very large transport
airplane
of the future. They are presented here. Each one would cost
at least seven billion dollars to bring to fruition, but in the
following
discussion, the reader will see that either design is far superior to
the
standard tube with wings already chosen by Airbus, and one is a truly
breathtaking
solution.
The C-Wing
Klingon Battlecruiser
To paraphrase John
McMasters
of the Boeing Company, "innovation for it own sake can be a great waste
of time, but individuals with a sufficient depth of knowledge in more
than
one technical discipline can, working in teams, exploit the unorthodox
to create a very workable design.
The ideal cruising
aircraft
is a simple, elegant flying wing, and everything that does not
contribute
directly to generating lift should be integrated in or on that wing, if
it is to retain an aerodynamic purity. In every large aircraft of
this type, one that might accommodate up to 800 passengers, the
possible
laminarization of the wing could not be taken advantage of until it was
think enough and large enough to carry that many people. So, if
your
goal is 600 passengers or more, you might want to
choose
the very thick subsonic Griffith airfoil,
invented
over a half century ago in England. With slots top and bottom and
a number of additions, this basic football-shaped section, when viewed
in profile, would provide the necessary lift, but its span would be on
the order or 300 feet and passengers in the center section of this
flying
wing would sit 50 abreast.
A more promising alternative
would
be to take the basic wing structure, as described, graft
a central tubular fuselage extending ahead and behind its center
section
on it, remove the hybrid laminar flow control outer wing panels and
replace them with inward and rear-facing smaller horizontal winglets
located
at the tips of standard vertical winglets. In addition to
reducing
the span and eliminating the horizontal tail of a conventional
alternative,
sweeping the wing and the horizontal winglets by 35 degrees, allows the
latter act as a horizontal stabilizer relative to the rest of the
wing.
This configuration,
which
was patented by Boeing in 1995, not only lessens induced drag, keeping
it within acceptable limits, but also down sizes the airplane all
around,
resulting in a fin and rudder 20 feet lower than what would be
necessary
on a scaled up conventional shape.
As conceived by John
McMasters,
I.M. Kroo and Richard J. Pavek, the C-Wing shape would be thick enough
for spanwise distribution of payload, thus reducing high lift
requirements,
and would be commodious enough to seat 36 abreast. A canard, or
foreplane,
would act as a control surface during cruise, becoming part of the
necessary
high lift system when flaps were extended, as would the stabilizing
surfaces
of the aft swept horizontal winglets. Two engines forward and two
aft would supply adequate power and also reduce noise. In effect,
the C-Wing maximizes the positives found in the basic Griffith Wing
layout.
An alternative layout with
only
three engines showed even more promise, with approach speeds of 135
mph,
compared to 155 mph for a conventional shaped aircraft accommodating
the
same 126,00 pound payload. Range would be identical, 7,400 miles,
although the C-Wing design in all its variants would be heavier by some
125,000 pounds and would require an additional 700 feet of runway
to get off, but it could land within 5,400 feet, nearly 1,000 feet
shorter
than its large conventional rivals. It would also require and
burn
more fuel per passenger mile. ( http://www.lmasc.com/ama/gallery.htm)
The Blended Wing Body
Additional weight and more
fuel necessary
to transport the same payload just as far will probably doom the C-Wing
alternative, but when Boeing absorbed McDonnell Douglas, they also
acquired
the thinking of three more innovative design engineers, R.H. Liebeck,
M.A.
Page and B.K. Rawdon, who were working on the Blended Wing Body (BWB)
transport
of the future.
(http://www.boeing.com/news/releases/mdc/97-158.html)
If ever a design represented
innovation
matched with utility, this one is the embodiment of that concept.
According to intensive, well-reasoned calculations, the aircraft
they propose would carry 800 passengers over a 7,100 nautical mile
range
and be ready to enter service in the year 2010. Quite an
accomplishment
considering that its fuel burn will be 27% lower than its conventional
Airbus A3XX rival, with a take off weight 15% lower. Empty weight
will be 12% less. It will only require three instead of four
engines,
and will match or exceed conventional performance, despite having
27% less thrust. Those factors combined with 20% better lift/drag
capability translates to the phenomenal savings in fuel already
mentioned.
With a double-decked interior
cabin
located in the central portion of the blended wing, the extension
serves
to stiffen, buttress and extend structural integrity and aerodynamic
overlap
to the entire wing structure. The blended wing
layout
also serves as a very resilient bending structure, dramatically
reducing
the cantilever span of the thin wing section, distributing weight along
the span more efficiently. This reduces the peak bending moment and
shear
to half that of a conventional configuration. Its shape also
reduces
total wetted area, or those portions of the aircraft which come in
contact
with the air. In this imaginative layout there is no need for a
conventional
tail. Unlike standard configurations, the blended wing's outboard
leading edge slats are the only high lift devices required and, because
the three buried engines aft of the central wing structure ingest the
wing's
boundary layer airflow, effective ram drag is also reduced.
A cylindrical pressure vessel
was
the starting point for what became the BWB fuselage. In order to
seat passengers in reasonable comfort, it originally had a volume of
55,000
sq. ft. The minimum wetted area for this given volume, enclosing
a passenger cabin for 800, plus galleys, lavatories and baggage, is
best
realized as a sphere, but a sphere is not conducive to streamlining, unless
it can be flattened into a disk. In the case of the BWB, a
streamlined
disk integrated with the wing initially resulted in reducing total
wetted
area by 7,000 feet. Further revisions and modifications dealing
with
engine and control surface integration led to a total surface area of
29,700
feet, a reduction of an additional 33%.
( http://oea.larc.nasa.gov/PAIS/BWB.html
)
In this deign the fuselage is
not
only a wing, but a mounting for the engines that power it, along with
their
inlets, as well as a pitch control surface. By continuing to blend and
smooth the streamlined disk, with a bullet nose added for enhanced
visibility
from the flight deck, the designers have come up with an aircraft that
will fly at Mach .85, with an optimized wing loading fully 33% lower
than
that of conventional large size, long-range aircraft with less
passenger
carrying capacity. Since the wing blending hides most of the
trapezoidal
wing within the centerbody of the aircraft, the cost of wing area on
drag
is greatly lessened. In short, because the BWB planform has such
a large chord, it requires a much lower sectional lift coefficient to
preserve
an elliptic span load, thus allowing the centerbody's thickness to
maximize
payload volume without a high compressibility drag penalty.
In layman's terms, the low
effective
wing loading of the BWB meant that exotic high lift systems are not
needed.
A leading edge slat is necessary on the outboard wing, but all trailing
edge devices are simple hinged flaps, which also serve as
elevons.
Low wing loading reduces control power demands. The small
winglets
provide primary directional stability and control, and split drag
rudders,
similar to those found on the B-2 bomber, are used for low-speed,
engine-out
conditions.
On a 5% scale model
tested
in Langley's wind tunnel (http://lisar.larc.nasa.gov/ABSTRACTS/EL-1998-00245.html),
the BWB showed relatively small center of gravity variations, good
stall
characteristics and excellent control power through the stall, the BWB
handling extremely well in the normal flight envelope. Further
tests
at Stanford
University explored extreme flight envelope characteristics and
revealed
so significant problems that could not be readily addressed and
solved.
( http://aero.stanford.edu/BWBProject.html
)
Like all next generation
aircraft,
the BWB will be constructed with composites. Bending and pressure
loads on the structure can be carried by a 5-inch thick sandwich and
deep
hat stringer shell, or a deep skin/stringer alternate, both of which
are
already in wide application. Passengers will be
accommodated
in five longitudinal bays, each the width of a DC-8 cabin. Coach
class will have six seats with an aisle between. Business class
will
have two/two seating with an aisle. Each separated section will
be
the length of a DC-9 fuselage.
Galleries and lavatories will
be
located aft. In addition to a forward view through windows
mounted
along the curve of the wing, flanking the flight deck's bullet nose, an
additional promenade aisle will allow passengers to walk along the
curve
of the leading edge. On the ground, entrance and exit will be
accomplished
by means of main cabin doors in the wing's leading edge and through
doors
aft of the rear spar. Cargo will be carried outboard of the
passenger
bays, with fuel in cells further out on the wing, thus allowing a great
deal of space between the tanks and the passenger compartment.
In addition to performance,
comfort
and capacity, the BWB concept has an inherently low acoustic
signature.
Exhaust noise will not be reflected off the wing's undersurface.
There is little additional airframe noise caused by complex mechanism,
such as slotted flaps. The aft location and staggered positions
of
the engines lessens the possibility of shards and debris from a failed
powerplant penetrating the pressurized cabin or fuel tanks, destroying
flight controls or causing the remaining engines to fail.
Compared
to conventional cylindrical tube fuselages, the center body pressure
vessel
of the BWB is much stronger, thus improving chances of survival in a
crash.
Will such an
aircraft
ever be built? That's the decision the manufacturer will have to
make. But if a large subsonic aircraft to take the place of the
747
is really needed, it appears that the BWB concept offers the most for
the
necessary investment. It's lighter, more commodious, more fuel
efficient,
requires far less power, and is certainly more aesthetic in
appearance.
True, looks aren't everything, but that old aviation adage still holds
true, "If it looks good, it will fly good," and the BWB aircraft, in
addition
to much improved economy, simplicity and handling, certainly has any
potential
flying watermelon beaten hands down.
Eigthy-five years ago, when
Boeing
first began making a name for itself, in addition to farseeing design
and
exceptional engineering excellence, a willingness to invest in the
cutting
edge of future flight inevitably characterized it planes.
Innovation
has always been a prominent feature of Boeing's corporate
initials.
Taking a page out of it enviable history, it should, one again, invest
in that future. The BWB airliner is the right plane at the right
time and will, once again, keep Boeing in the forefront of economically
viable aerospace technology. (Click
here to read an article from the Seattle Times on the blended wing.)
(The remainder of the
article covered
the history of commercial airliners from other countries, and it
included
a large number of very good pictures of these aircraft.)
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