Re: Blended-body snag?

Date:         01 Mar 97 02:45:02 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
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"