The wing tip bows were a real challenge. I had very little of the originals and a sketch of the tip bow profile. First step was to layout the shape on the jig table to correspond with the wing drawing. Then make a jig to match the wing and use masonite board to create a template.
In the top picture, you can see how far the piece of masonite goes inboard on the leading edge. It has to go all the way into the last ‘A’ rib. So, the piece that needs to be made is pretty long. There was a pattern for the bow, included with the set of drawings that came with the project.
You can see a 1” by 1” template pattern marked off on the jig table. This corresponds to a similar patter marked off on the wing assembly print. This was used to transfer the tip shape to the jig. I made “L” shaped stand-offs which were screwed to the jig table along the shape of the tip. These were used to bend the tip laminations to the proper shape and provided a clamping spot every 3-4 inches.
The ends of the spars were trimmed to match the shape of the tip. The “L” shaped stand-offs were placed closer together where the tip has the maximum bend. Each stand-off actually has a piece of 3/4” dowel, that had been ripped length wise, attached on the outward facing surface. This allowed the bow te exactly line up with the template.
The tip bow is made of 3 laminations of 1/4” thick spruce for a total thickness of 3/4”. In order to bend the spruce around the shape of the tip, a steamer was constructed.
The steaming box is on top of the roller stands and is wrapped in insulation. It is a wood box, about 4” by 4” inside dimensions and long enough to hold the laminations. The inside walls were double lined with heavy duty aluminum foil before assembly. The sides and end cap were sealed with a bead of high temp RTV before being screwed together. One end was removable for insertion of pieces.
A 5-gallon jerry can filled with pre-heated water, sitting on top of a kerosene heater provided the steam to the box via some left over heater hose from a car project.
The removable end cap had some copper tubing connected to a valve, which allowed me to control the pressure in the system to maximize the steam production. Once the whole thing got cooking, it worked great. I found, through trial and error, that 1.5 hours in the steamer would be enough to allow the 3/4” thick laminations to wrap around the jig without distortion.
Working quickly, with gloves of course, the lamination was removed from the steamer and immediately clamped in the jig and allowed to dry. The first lamination is steamed and clamped in the jig. Again, from this set of pictures, you can get a feel for the length of this piece. The total length is about 10’.
Next step was to steam and clamp the second lamination. Then glue the two together. Let dry overnight. Then steam and clamp the third lamination. Let dry. Then glue and let dry. By the time the third lamination was glued and dried, there was essentially no “spring back” in the piece.
The mirror image template and jig was then done and the wing tip bow for the other wing was made using the same process.
The tip bow was later attached to the wing while the wing was clamped in the vertical assembly jig. Many hours with the plane and a belt sander and finally the long board sander where required to get the desired
final shape.
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