Tube Cluster Weld Question

[Dick]

Other than a weight consideration, what concern is there with replacing a
typical fuselage tube cluster (all legs in the same geometric plane) with a
half circle plate where the slotted end legs are slide onto and then welded?

Kind of like a bridge with it's clip angles on I-beams. Then the diagonals
and other bridge members are riveted/bolted onto the clip angles.. Or
similiar to those wooden semi-circular gussets used to sandwich a wooden
fuselage member joint.

It appears to me that the linear inches of weld could be greater than those
fishmouthed connections so tedious to make.

Thanks, Dick

[Steve Beaver]

The Sukhoi 26/29/31 aerobatic a/c use a system similar to the one you
describe. A plate is welded on the outside and the inside of the cluster but
there is a slot, about 1/16 in wide and 4 or 5 inches long cut in the plate
exactly in line with the center of each tube. It is through this slot that
the welds are made between the plate and the tubes.

The tubing on the Sukhoi is VN02 stainless, not 4130 but it is an
interesting piece of design.

"Dick" > wrote in message
. com...
> Other than a weight consideration, what concern is there with replacing a
> typical fuselage tube cluster (all legs in the same geometric plane) with
a
> half circle plate where the slotted end legs are slide onto and then
welded?
>
> Kind of like a bridge with it's clip angles on I-beams. Then the diagonals
> and other bridge members are riveted/bolted onto the clip angles.. Or
> similiar to those wooden semi-circular gussets used to sandwich a wooden
> fuselage member joint.
>
> It appears to me that the linear inches of weld could be greater than
those
> fishmouthed connections so tedious to make.
>
> Thanks, Dick
>
>

[Veeduber]

>
>Other than a weight consideration, what concern is there with replacing a
>typical fuselage tube cluster (all legs in the same geometric plane) with a
>half circle plate where the slotted end legs are slide onto and then welded?

---------------------------------------------------------

Dear Dick,

In simple terms, the reason for NOT using the method you've suggested has to do
with stress (or load) concentration.

Maximum stress is said to appear in the 'outer-most fiber of the material,'
meaning on the surface of the tubing, not its center. The accepted method of
making a joint provides a continuous load-path between the outer-most fibers of
the members making up the cluster.

-------------------------------

With regard to the tedious task of fitting tubes to be welded, the tedium is
usually in inverse proportion to your tooling & experience. First off, the
tubes do not have to be a zero-zero fit. Indeed, there MUST be some gap,
otherwise you'll end up with a distorted cluster. Secondly, when a smaller
tube intersects with one (or more) of larger diameter, the fishmouth is little
more than a minor amount of scalloping. Finally, step-drills and a few
simply-made jigs will allow you to fabricate most of your fitted-ends on the
drill press, or on the lathe using milling cutters. The benefit of the latter
methods are more significant than they may appear because most structures of
welded steel tubing are symmetrical; such set-ups make it quite easy to produce
two identical pieces.

As with all of the manual arts, producing a nice cluster is largely the product
of practice. The use of mild steel tubing, such ERW, or even EMT with the
galvanizing removed, allows you to practice without going bankrupt buying
4130.

-R.S.Hoover

[Dick]

Steve, Are the tube ends cluster welded together also or is the only
connection via that slot? Thanks, Dick
"Steve Beaver" > wrote in message
...
> The Sukhoi 26/29/31 aerobatic a/c use a system similar to the one you
> describe. A plate is welded on the outside and the inside of the cluster
but
> there is a slot, about 1/16 in wide and 4 or 5 inches long cut in the
plate
> exactly in line with the center of each tube. It is through this slot that
> the welds are made between the plate and the tubes.
>
> The tubing on the Sukhoi is VN02 stainless, not 4130 but it is an
> interesting piece of design.
>
> "Dick" > wrote in message
> . com...
> > Other than a weight consideration, what concern is there with replacing
a
> > typical fuselage tube cluster (all legs in the same geometric plane)
with
> a
> > half circle plate where the slotted end legs are slide onto and then
> welded?
> >
> > Kind of like a bridge with it's clip angles on I-beams. Then the
diagonals
> > and other bridge members are riveted/bolted onto the clip angles.. Or
> > similiar to those wooden semi-circular gussets used to sandwich a wooden
> > fuselage member joint.
> >
> > It appears to me that the linear inches of weld could be greater than
> those
> > fishmouthed connections so tedious to make.
> >
> > Thanks, Dick
> >
> >
>
>

[Steve Beaver]

The tube ends are fish-mouthed and welded but the weld bead is tiny. perhaps
Only 1/8" wide so that the joint is not "buried" in weld, as a 4130 cluster
would be. I suspect this is the way this type of stainless needs to be
welded and that it requires the reinforcement of the special gussetts.


"Dick" > wrote in message
m...
> Steve, Are the tube ends cluster welded together also or is the only
> connection via that slot? Thanks, Dick
> "Steve Beaver" > wrote in message
> ...
> > The Sukhoi 26/29/31 aerobatic a/c use a system similar to the one you
> > describe. A plate is welded on the outside and the inside of the cluster
> but
> > there is a slot, about 1/16 in wide and 4 or 5 inches long cut in the
> plate
> > exactly in line with the center of each tube. It is through this slot
that
> > the welds are made between the plate and the tubes.
> >
> > The tubing on the Sukhoi is VN02 stainless, not 4130 but it is an
> > interesting piece of design.
> >
> > "Dick" > wrote in message
> > . com...
> > > Other than a weight consideration, what concern is there with
replacing
> a
> > > typical fuselage tube cluster (all legs in the same geometric plane)
> with
> > a
> > > half circle plate where the slotted end legs are slide onto and then
> > welded?
> > >
> > > Kind of like a bridge with it's clip angles on I-beams. Then the
> diagonals
> > > and other bridge members are riveted/bolted onto the clip angles.. Or
> > > similiar to those wooden semi-circular gussets used to sandwich a
wooden
> > > fuselage member joint.
> > >
> > > It appears to me that the linear inches of weld could be greater than
> > those
> > > fishmouthed connections so tedious to make.
> > >
> > > Thanks, Dick
> > >
> > >
> >
> >
>
>

[D.Smith]

Veeduber,

I'm uncomfortable with the loading you describe for tube clusters. Bending loads
cause the maximum stress at the 'outer-most fiber of the material'. However tube
clusters are usually from a truss design using tension/compression loads only.
There are probably local bending loads around the welded cluster ends but these are
developed mainly due to the stiffness of the connection itself. Ideally the
connection would be a "pin joint" but that's not practical.

I don't see a reason that an alternate attachment of the tubes can't be used. I
would advise caution to ensure that the connection design does handle the loads
presented.

Doug


Veeduber wrote:

> >
> >Other than a weight consideration, what concern is there with replacing a
> >typical fuselage tube cluster (all legs in the same geometric plane) with a
> >half circle plate where the slotted end legs are slide onto and then welded?
>
> ---------------------------------------------------------
>
> Dear Dick,
>
> In simple terms, the reason for NOT using the method you've suggested has to do
> with stress (or load) concentration.
>
> Maximum stress is said to appear in the 'outer-most fiber of the material,'
> meaning on the surface of the tubing, not its center. The accepted method of
> making a joint provides a continuous load-path between the outer-most fibers of
> the members making up the cluster.

> .......

> -R.S.Hoover

[Veeduber]

>I don't see a reason that an alternate attachment of the tubes can't be used.
>I
>would advise caution to ensure that the connection design does handle the
>loads
>presented.

-------------------------------------------------------

Dear Doug,

No problem. Other methods can (and have) been used. (Numerous examples of
gusset & pin fuselages using square tube and even angle-stock, both extruded &
self-bent).

A point often overlooked is that Mr's Warren, Pratt & Howe didn't do the twist
:-) They just sat there, waiting for the next train to come along. The tricky
bit with a fuselage is what happens when you have to deal with a complex load
(ie, tension, compression & torque). The usual solution was a mare's nest of
wire diagonals and the need to 'tune' the fuselage every time the weather
changed. Attempting to emulate this method using using pinned joints
consisting of struts & gussets throws you back into the task of transferring
the load through the cluster, which is now a morningstar collection of gussets,
at least as difficult to weld (or otherwise fabricate) as a conventional tubing
cluster.

I think the real point here is that if you're going to use round steel tubing
you'll find there are more reasons to stick with accepted welding practices
than to depart from them. But it's really up to you. It's your plane; you can
build it any way you want.

Best example of gussetted steel structures is probably towers of various types
that are subjected to wind loads (ie, complex loading). Interesting to note
that when minimum weight is a consideration they abandon gusseted angles in
favor of cluster-welded tube. (Rohn, etc.)

-R.S.Hoover