Home       Aeronca    Stinson                                                                                                               ... by John Baker

Stinson 108 Fuselage Tubing Repair
Page created 2/5/99
Updated 2/13/99

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Trouble Spots

Though my Stinson appeared to be in pretty good shape before I began the restoration in 1994, there were a number of problems hidden under the fabric, some that made me very glad I disassembled the airframe.  My first tip that there were problems that needed attention occurred in 1993, when I noticed the tail spring flexing laterally when pulling the plane out of the hangar.  The tail spring mount, rear ends of the longerons, and lower tailpost were severely rusted. In 1993 it was necessary to cut the fabric away from the tailpost and weld in a new "rear end" (#1 in the top photo) including the last four inches of the longerons,  spar attach and tail wheel attach fittings, and about eight inches of the tailpost.  In the photo below, the areas repaired in 1993 are painted in dark green epoxy.  Note the spliced tailpost.  Less noticeable in this photo are the splices in the lower longerons.

After removing the fabric and sandblasting the entire fuselage structure, I found other problems.  The lower left longeron had a number of pinholes along it's entire length aft of the cabin (from #1 to #2 in the top photo).  There were also pinholes in one of the lower horizontal diagonal tubes and two of the diagonal tubes on the right side of the fuselage. The most startling corrosion damage, however, was on the bottom of the upper right longeron, just forward of the crossmember to which the forward vertical stabilizer spar attaches (#3 in the top photo).  A section about two and one half inches long, and one quarter of the circumference of the longeron, was rusted away completely - just a big black hole.  This last trouble area is in a "low spot" just before the longeron kicks up again as it extends aft.  See below:

I have heard from other restorers that there are two other areas that should be checked carefully.  These are:

  • Lower longerons in the area of the landing gear attach fittings
  • Aft door frame where the square tubing meets the lower longeron and the square tubing the forms the lower door frame.
Look for cracks in these two areas (#4 in the top photo)

Fuselage Jig

Making plans to replace tubing, I knew I needed to keep the fuselage in alignment.  Since I would be replacing once piece of tubing at a time, I was advised by my AI that it would not be necessary to jig the fuselage.  However, as an extra precaution, I chose to jig the fuselage so that the relationship between the forward fuselage and the tail would stay fixed.  My method was to use the motor mount bolt holes to mount the forward fuselage solidly to the back wall of my garage.  I used a two-by-four attached to the wall with lag bolts and a piece of heavy angle iron borrowed from an old bed frame.

If you look carefully at the photo above you can see the brown angle iron.  It rests on and is bolted
to the the two-by-four and is bolted solidly to the fuselage via the upper motor mount bolt holes.

At the rear of the fuselage, I used more angle iron liberated from an old bed frame to construct a tripod to support the tail.  It is bolted to the fitting at the lower tailpost that normally used to attach the stabilizer vertical spar.  The tripod is bolted with lag bolts to the concrete floor in three spots. In this photo  you can see the jig and you can see that the lower left longeron has been removed (from point #1 to #2 in the top photo). You can see also that I share my shop space with some younger family members.

In this photo you can see details of how the jig attaches to the rear fuselage.

Replacing the Old Tubing

I'm sure there is more than one way to cut out the corroded tubing.  I found that it was easy to cut out major lengths of tubing using a plumber's tubing cutter and then to use small cutoff wheels on my Dremel tool to do the final cutting of tubing around the clusters.  In some cases I used a rotary file on my die grinder, but the Dremel did the best work.  You MUST wear eye protection when using these cut off wheels - they shatter often.  I followed the repair methods of EA-AC 4313.13-1A &2A, Acceptable Methods, Techniques, and Practices, Aircraft Inspection and Repair/Aircraft Alterations.  The lower left longeron, as I mentioned, was replaced all the way from the rear of the cabin (where the tubing steps down in diameter) to the earlier repair at the tailpost.  I used an inner sleeve at the tail, drilling out the rosette weld from the earlier repair.  Up front at the rear of the cabin, I cut the larger tubing just 1/4" forward of the weld point, diagonally as originally welded, and was able to slide the old smaller tubing out of the larger tubing.  The new longeron, nearly 12 feet long, slides into the forward tubing just as originally built.

I replaced the section of tubing of the upper right longeron that had the "black hole", cold bending the tubing to match the original at the "kickup".  The section was spliced in with sleeves and rossette welds.  All tubes were cut at a 30 degree angle as specified in EA-AC 4313 to increase the weld area.

The following photo sequence shows the replacement of a diagonal on the right side of the fuselage.  I found pinholes in the lower part of this diagonal late in the game - after I had sandblasted and primed the fuselage.

First the old tube is removed.

A new  tube is carefully cut to fit.

The new tube is welded in place, along with the bracket for the rudder cable pulley and stringer.  The welding is done with traditional oxy-acetylene equipment.

The welded sections are sandblasted in preparation for painting with epoxy primer.

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