Date: 01 Mar 97 02:45:02 From: "P. Wezeman" <firstname.lastname@example.org> Organization: The University of Iowa References: 1 2
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On 19 Feb 1997, Ostreger wrote: > I'm a FW nut and want BBAs to work; I just can't see how they beat the > pressurisation problem. Estimates of 10,000 pounds weight penalty don't > approach mine - the catch is distortion, and consequent fatigue. > The B-35 and B-49 had tiny cabins where this didn't apply. > The pressure cabins of aircraft are structurally efficient when they every part is in pure tension, and no part is subjected to bending stress. Clearly, a sphere and a cylinder are good shapes for this. After all, the surface of a common balloon has no bending strength at all, and so it naturally assumes a spherical or cylindrical shape (depending on what kind of balloon) when inflated. Suppose that you took an inflated spherical balloon and attached a string to its inner surface at two points on opposite sides of the balloon. If you then gradually shortened the string, you would pull the sides of the balloon together, making the sides pucker in. It would no longer be spherical, but it would still be loaded in pure tension. The string that you added and that is holding the sides together is no more capable of resisting a bending load than the balloon's rubber surface is. If you added more strings parallel to the first and pulled them tight, the balloon would gradually assume the appearance of a quilted chair cushion. It would still be a structurally efficient shape. You could also take a cylindrical balloon and put in a closely set row of strings down the middle. If you then shortened all the strings, the balloon would be pulled into a figure 8 cross section. The strings would form an internal partition loaded in pure tension. This is also an efficient shape. The Boing Stratocruiser airliner had a fuselage of this configuration, commonly called "double bubble", with the tension link between the two lobes forming the floor of the upper passenger deck. You could also build a triple bubble pressure cabin with two internal partitions carrying the tension forces from one side to the other. If you had about a dozen partitions you would have a basically flat shape, looking much like a common air mattress, that would fit well into a blended-body airliner. The Goodyear company once built an inflatable airplane (called the Inflatoplane, what else) that could be rolled up and packed in a big duffel bag, and then inflated with a pump running off the engine. The wings were rubberized fabric with closely spaced internal ties holding them in a flat shape. The whole plane was a pressure vessel built entirely from flexible materials. This really worked; you can find photographs of it flying. Peter Wezeman, anti-social Darwinist "Carpe Cyprinidae"