Date: 13 Feb 97 01:37:36 From: "P. Wezeman" <email@example.com> 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"