Monday, 2 May 2011

The fittings are in there somewhere

I had the various thickness 4130 steel sheets, and I had a pretty good idea what the fittings should look like.
But turning the sheets into this:
is not so easy, and takes an amazing amount of time.

The main reference for the fittings was photographs and the actual wing structure. I knew kind of what they should look like, but turning photos into practical fittings is an altogether different exercise.
There were no dimensioned drawings of the various fittings.

The above fitting is the RH I strut attach point, landing wire, and drag wire attach fitting. This assembly is comprised of 5 separate steel fittings.
there is an opposite assembly on the LH lower wing.

The first issue with the various fittings is deteriming the actual dimensions. I started by making them all in cardboard, then dry fitting the ribs where fittings would be attached, and then by using dummy wires, and measuring angles, I could determine whether the specific fitting would work as drawn.
The next step is to determine the fitting material thickness, some of which were shown in various documentation, some not, but there were enough shown in various laird drawings to allow the rest to be accurately determined.
The original fittings were 1025 mild steel, but 4130 carbon steel is used instead as it has superior strength  for the same thickness.
If the original thickness was no longer available, the next larger size was used, ie; if the original was .059, .063 would be used.
All fittings requiring welding are TIG welded.
For some assemblies, fixtures were required, as below, the lower wing root attach fitting weld fixture.

Each of these assemblies is made up of three parts, a bent U section made of .063, and two plates made of .100 4130 sheet.
They are match drilled and then edge welded on the assembly fixture. The front spar assembly is about 15% larger than the rear spar assembly.

 and this is where the two assemblies will end up, at the spar roots.

Some of the other wing fittings include the rear spar I strut attach point
and the various drag / anti drag wire attach fittings. Each lower wing has 19 separate metal fittings

this set does not include the aileron fittings, which are a separate assembly all together.
Once all the lower wing fittings had been cut, drilled, bent, filed and welded, they were placed in the relative positions on the spars, and dry fit. Once it was determined they were in the correct locations, and did what they were supposed to, the spars were marked and drilled.
Before the spars could be drilled the plywood had to be cut and dry fit to the spars, both to determine the exact thickness of the fittings, and also to assure the holes would all line up.
*also shown are the fittings I welded up to attach the spars securely to a fixed root point for future assembly.
Once all spar holes were final drilled, and it was determined all fittings would work, they were taken off, cleaned up, and powder coated a gloss black.
(It appears you could have wing fittings in any colour in the 1930`s as long as it was black)

With all wing fittings in place, and the ribs in position, the various drag / anti drag wire lengths were measured and the wires ordered.
The wires are being produced with each a LH and RH rolled thread. The inner and mid bays will have 1/4" diameter wires, and the outer bays are 3/16th wires. All wires are 4130 steel, with a hot dipped finish to replicate the original finish 1025 of mild steel wires.
The wires are being produced in New Zealand, and should be delivered by mid May 2011.

The LH and RH lower wing fittings took a total of 5 months to build from initial cardboard mockups to finish power coat, and all were ready for final assembly by the beginning of April 2011.
Unfortunately there would have been no time savings by drawing the fittings in CAD and having them laser cut, as each fitting went through many versions before the final dimensions were determined.
During the fitting build process, the wings were dry assembled and taken apart at least 8 times.


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  2. Thanks for sharing your working process!
    Lots of interesting pictures!