RWM <RWM@RWMann.com> wrote:
>I suspect the reference was to "double bubble" fuselage section,
>i.e. not a simple "hoop", but two joined, partial hoop sections
>of different diameter, designed to achieve more section height
>than width.
>
>A circular hoop section is in (essentially) pure tension when
>pressurized. A double-bubble is in tension, and also in bending
>at the join.
>
>- Bob Mann
Bob,
A double bubble fuselage consists of two circular sections, which
are each under pure tension as far as pressurization loads go (there
is of course shear or bending for other loads). The cabin floor has
to be at the joint line, and the floor beams are under tension. As
such, in a double-bubble fuselage, the floor beams are an essential
part of the pressure hull (in a circular fuselage, the floor beams
do not carry any pressurization loads).
A double-bubble fuselage always has a sharp crease line where the
two sections meet. This may be faired over for aerodynamic reasons,
but the actual pressure hull should have a 'figure 8' cross section.
In the front fuselage, it is usually impossible to maintain purely
circular sections all the way, and then the frames will carry some bendin=
g.
The most famous is of course the front fuselage of the
Boeing 747 and the non-circular sections cause considerable fatigue
problems. Ask any owner of old 747s.
Kees de Lezenne Coulander
-- =
C.M. de Lezenne Coulander
Aircraft Development and Systems Engineering B.V.
Hoofddorp, The Netherlands
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