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. (Clickhere
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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