On Wed, 5 Aug 2009 05:19:21 -0700 (PDT), mhorowit
wrote:


I’m trying to understand Figures 4.45, and have a question about a
technique shown in 4.44.

First, 4.44 – here they show fabricating an inner tube by cutting,
reforming and welding outer tube material. Is there any reason this
can’t be done with round tubing if one can’t locate the correct size
inner tubing for an internal splice?

my answer. yes, the reason is that the round tubing isnt used here is
that it isnt stiff enough and would provide only line contact. you may
as well just use a piece of flat for all that achieves. the use of a
streamline tube inside a streamline tube provides full support for the
damaged wall and it also provides the necessary wall contact for the
rosette welds.
streamline tube might be about $35 a foot but I'm sure aircraft spruce
can sell you a foot or so.

if you look carefully you will notice that the repair is made in the
same size streamline tube as what you are trying to repair. you cut it
at 80% of the chord, heat it and persuade it into a shape that will
slide neatly into the other tube, then weld the back edge up again.


Second, 4.45 – are they showing cutting slots in the outer tubing?
In the lower drawing what is that streamline shape illustrating?

the top view is from the wide side, the lower view is from the narrow
side and all it really shows is the appearance of the end of the
oblique slice through the tube. the left most illustration at the top
is an end on view of the tube showing how the plates protrude through
each side.
that method of repair uses two plates slotted into the streamline
tube. each plate is made slightly wider (not thicker, wider) than the
slot it goes into.

mike what you are trying to achieve with each of these welds is a
repair that will resist the buckling under compression that an
undercarriage strut will see.
round tube would simply not provide the necessary buckling resistance.

I should imagine that there are a few failed undercarriage struts
behind those recommendations.
Stealth Pilot