Re: Blended-body snag?

Date:         13 Feb 97 01:37:36 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
References:   1 2 3
Followups:    1 2 3
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   Various people have written:

> >>How do the new blended-body airliners claim to handle pressurisation
> >>loads?

> >
> >Not a technical answer... but do wonder if a design could be
> >similar to a submarine.  The guts of the passenger/flight crew
> >pressure shell could still be cylinderical... although blended
> >with the whole 'wing'.
>

> What about the engines?

   Published drawings show the engines on short pylons on the upper
rear surface of the wing.
   I can't believe that the designers at McDonnell Douglas, Airbus, and
Lockheed (which has been looking at flying wings they call "Spanloaders"
for a long time) have all neglected pressurization. For that matter,
weren't the B-35 and B-49 flying wing bombers pressurized? The B-35 met
the same range and payload specs as the B-36 but with four R4350 engines
instead of six. Hopefully one of the engineers working on these things
will comment.
   Lockheed's drawing of a cargo spanloader showed standard 8x8 cargo
containers loaded into a hollow wing spar, so the compartments were
spanwise instead of fore and aft. Current drawings for blended-body
airliners show passengers behind the leading edge of the center section,
also with spanwise compartments. It seems to me that the main wing spars
would be well positioned to take the tension loads from floor to ceiling.
One of the advantages that has long been claimed for blended-body aircraft
is that by combining fuselage with wing the structure that distributes the
aerodynamic loads also contributes to the strength of the passenger
compartment, whereas in a conventional aircraft, the cabin area is pretty
much a dead load, and the designers natural inclination is to make it as
light as possible. Every pound of weight added to the fuselage to improve
crashworthiness means extra weight in the wing structure to carry it.
In a blended design, heavier skinning on the passenger area will still
increase the weight to be lifted, but it will also contribute to the
maximum load factor the aircraft will sustain and to the fatigue life.
   As you point out, structurally efficient pressure vessels do not have
to be circular in cross section. A super-imposition of circular arcs
with the vertices linked by tension members will also work. This is
probably most familiar in the case of the "double bubble" fuselage of the
Boing Stratocruiser and KC-97. For a large blended-wing airliner, if we
take several B747 fuselages (about twenty feet in diameter) and place them
with their centerlines ten feet apart and remove the sides where they
intersect, the missing sides can be replaced by vertical bulkheads about
seventeen feet high. At a maximum cabin presurization of 8 psi these
bulkheads will have a tensile load of 12,000 pounds per linear foot.
Assuming a safe working load for aluminum of 10,000 psi, it would need to
be just a tenth of an inch thick, for a total weight of 24 pounds per
foot of length. This comes to 2.4 pounds per square foot of cabin floor.
Assuming ten square feet of cabin area per passenger, this is 20,000
pounds of structure for an 800 passenger aircraft. The interior height
would range from seventeen to twenty feet, and the upper and lower
surfaces of the cabin would need to be no heavier than on the present
B747. Actually, this height would probably allow for two passenger decks
and need only 10,000 pounds of wall structure for 800 people.
These bulkheads would also be contributing to the longitudinal bending
strength of the aircraft.
  If the interior height were made 17 feet everywhere, the upper and lower
1.5 feet of the cabin would incorporate the ribs and stringers of the
aircraft skin, and the pressure cabin would be adding buckling resistance
to the wing surface. The bulkheads could be trusswork for much
of their height, giving the cabin interior more of the look of an open
office than of a railroad car.
   These are of course very rough figures based on my limited knowledge
of aeronautics and structures, but I think they serve my purpose, since if
I use these same assumptions to figure the weight of a circular fuselage
to hold 800 passengers, I get a higher number. The fact that the cabin of
a blended-wing aircraft is wider (and therefore shorter) than that of a
conventional plane will in itself reduce the bending loads and hence the
structural weight. Also note that a relatively flat cabin like this has
more passengers per unit of frontal area than a circular one, giving less
drag for the same capacity. All in all, I see no reason to doubt the
claimed advantages for this type of aircraft, certainly not before they
try building one.


                        Peter Wezeman, anti-social Darwinist

                             "Carpe Cyprinidae"