Welded tube frames
I
have been building for the last few years almost all the
scale reproductions airplanes with a fuselage structure
made with welded tubes.
The first airplane I built using such technique was
the Fieseler Storch.
Not only I like them very much, but these structures are
incredibly light and strong, (see the article about
the Piper Cub). Also they are very
quick to build.
The materials
The
material I normally use is a stainless steel tube in
various diameter sizes, ranging from 3 to 9 mm (0,118” to
0,354”), having a thickness of 0,25mm (0,01”) or 0,5mm
(0,02”) for the most stressed out parts.
I use 30%
silver alloy bars for welding. It’s sufficient to use a
GPL or Butan gas welding pipe, you can find also some
pipes that work with the cigarette lighter type of gas,
in this case they don’t last long.
Even an oxygen type of pipe is ok, however I recommend to
keep the flame very low and at a certain distance because
it’s very powerful and , if the stainless steel overheats
and turns black can not be welded.
Anyway a slight blackening of the tube is normal.
The
welding technique.
The
gas pipe, which has a wide flame, is good to weld the
tubes together, the oxygen pipe is better for the welding
of massive parts and the heat can be concentrated in
precise spots
I suggest the beginners to buy only the gas pipe.
The tubes
must be prepared so to fit very closely with each other,
using a round file to make a groove in the tubes that
will be welded end to end, small cracks can be filled
with the welding alloy.
After positioning the two tubes, the tip of the welding
bar has to be slightly preheated and it’s layed at the
tubes crossing, and heated until it melts.
When the welding alloy starts to melt make sure that
flows where you want to, keep in mind that the welding
alloy flows towards the hotter parts, therefore it’s
sufficient to heat ahead of it so as to guide it to the
required point.
It sounds easy but it needs some practice.
I suggest to practice with some tube scraps before making
the final weldings.
As a general rule keep in mind that if you have to weld
two tubes or plates of different thickness the flame has
to be on the side of the thicker object so to heat this
first. You can use the welding bar to protect from the
flame the part that don’t want to be heated .
Don’t be in a hurry but don’t dwell too much on the same
spot, oxidation is waiting to happen.
If the tube becomes incandescent then oxidizes, so when
you see the tube that starts turning red lower the flame
and move it away from it.
To go over a welding, add some welding alloy drops in the
cracks or to fill holes, wait until the welding cools
down , then soften the bar tip using the flame and lay
the bar on the point to be rewelded moving the flame away
often and dwelling at the right time, otherwise you may
melt the first welding.
Fuselage
construction
I have found out that an iron made working table is very
suitable for the job. My table is a 2mm galvanized iron
sheet (picture below) measuring 2x1mt with the long sides
bent at 90° for 4 cm, resting on a wooden table.
I start building a fuselage outlining with a marker the
structure on the metal sheet, then I line up the tubes
blocking them in position with strong magnets that I
retrieved from old computers hard disks.
At this stage I begin welding, starting from the plane
nose towards the tail. It’s important to proceed in
subsequent steps because the tube tends to expand and
must be left free to stretch out towards the tail. The
second side must be built aligned with the first one so
as to have two sides exactly the same. Having done that
the two sides must be aligned with each other using some
reference points on the table , magnets and right angle
squares. The first cross braces and diagonals will then
be welded paying lot of attention to the alignment, as
the work goes on it will be easier and easier.
Usually it takes me a day to frame up a fuselage.
Appropriate stainless steel plates will then be attached
to the structure for the motor mount, the landing gear,
the wing, the wire cabana struts (on the biplane), etc.,
these details vary depending on the model.
Other
steel components
Many
planes had also steel tailplane, some had the ribs made
with small tubes (see Bucker) , others with bent
plates (see RO41). The landing gear is
another component that most of the times is made with
stainless steel tubes, I always try to copy the full
scale landing gear, shock absorbers included, using
chromemolibdenium tubes (1).
Residues
removal and painting
Once finished, the structures will have to be carefully
cleaned from the antioxidant, which usually tends to
leave very resistant glass residues.
They can be removed with sand paper, small files and some
patience or, as I do, using some residues remover in an
acid gel form., you can find it in workshops specialized
in welding.
You can also leave the structures unpainted, being in
stainless steel it will not be a problem. I often have
them painted with epoxidic paint, but it’s only matter of
taste.
Covering
Usually a framed style fuselage is covered with fabric in
the tail area and aluminum in the front. You will find
these techniques explained in my articles especially in
the Piper Cub instruction manual, which is covered with
Solartex, I suggest also to use the Dacron (1), it’s more
difficult but it’s worth the effort.
1) The same used on the real airplanes.