Wednesday, September 17, 2008

TWO-SPAR WING TRUSSES

The wire-braced truss is probably the original method used to fabricate a ladder-like truss. The only jig required is a pair of saw-horses, the only tools a carpenter's framing square and a level. The method is also extremely versatile in that the compression members, shown here as a strengthened rib, may be completely independent of the ribs. The Volksplane uses a heavy dowel-rod, for example, while the Fly Baby uses a steel tube. The turn-buckles may be located on either end of the tensioning cable.

Following World War II new turn-buckles, cable eyes and Nicopress sleeves were available as surplus, often priced at their metal value. This allowed homebuilders to utilize this type of wing structure which would otherwise have been too expensive.

By comparison, the Pitts-type of truss makes use of rods as the tension elements with re-enforced ribs as the compression members. The rods, typically of .156" to about .185" in diameter were threaded. The fixed end was fitted with a T-nut or other fitting that prevented the wire from turning. The other end of the wire was fitted with a coupling nut, secured with an elastic stop-nut. Tightening the coupling nut provided the required tension. The assembly was then locked in place by the elastic stop-nut. The
key factor in this method was the use of filler blocks that presented a perpendicular face to the tensioning wire. For lightly loaded wings, mild steel rods and cut threads (vs rolled) provided more than enough strength.


I've called the third type of truss the Ison Truss because I first saw it on a set of drawings from Wayne. In this truss a wooden spar serves as both the tension and compression member. The key to success with this truss is to ensure adequate gluing area between the spars, the ribs and the diagonal tension/compression member. This is accomplished through the use of plywood gussets having a generous surface area.

This truss is specifically designed for use with a C-type built-up spar, in which the plywood gussets are glued directly to the spar caps. To increase the load-carrying capacity you need only increase the area of the glued surface between the spar and the diagonal strut. Indeed, the advantages of this method are almost too numerous to mention but first among them would be low cost, followed by ease of fabrication and light weight.

This method is popular among ultralights and may be found on Leonard Milholland's 'Eagle' series of VW-powered airplanes. When fitted with a D-cell leading edge of adequate depth, the wing proves remarkably rigid making it suitable for use in a Primary Glider.
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