Page 33 - Volume 14 Number 4

Laird selected the RAF No. 15 airfoil because it possessed good stall characteristics that contributed to a landing speed of about 38 mph. The four wing panels, all of which were interchangeable, had a chord of 58 inches and the two upper panels attached to a center section that was 30-inches wide. Only 1 inch of dihedral was incorporated to improve lateral stability. The main spars were built up from two laminations that formed a box-type configuration. Ribs were made of spruce battens screwed to basswood webs, and No. 8 hardwire was used to brace the wing’s internal structure. Cotton fabric was doped and sewn to the ribs, followed by two coats of pigmented dope.
The fuselage and empennage were of wood construction with wire bracing, but the elevators panels and rudder were made of welded steel tubing with brazed joints. The front cockpit was carpeted, featured a thick, well-upholstered seat and a footrest to improve overall comfort of the passengers. Flight controls used a conventional stick and rudder arrangement with steel cables to deflect the ailerons, elevators and rudder. Streamline steel tubing was used for the main landing gear.
Fortunately for Laird, there was a good supply of skilled wood and metal workers in Wichita. Early in 1920 he hired a few men to begin fabricating the first airframe, and that number soon grew to 11 by February. Progress was swift, and in March all of the major airframe assemblies had been completed. Although working conditions in the makeshift factory were less than ideal, an air of anticipation began to sweep through the workforce as the airplane emerged in its final form.
Matty was not a trained engineer, but his years of designing and building aircraft provided him with
a basic understanding of testing airframe structures to determine their ultimate strength. In 1920 the science of stress analysis was still in its infancy and Laird had little or no knowledge of the complex equations that were necessary to compute loads imposed on a structure, such as wings.
Instead, he employed an empirical standard test long used by the Army Air Service and the United States Navy. A fuselage with wings attached was inverted and suspended in a special jig only by the wing attach points. Cloth bags filled with sand at a specific weight were laid on the structure at intervals, beginning at the wing root and progressing outward to the wingtips. During the lengthy process, which often
required days to complete, mea- surements were carefully taken and recorded to document how far the wings deflected from their original, static position. In addition, inspec- tions of the spars, ribs and other components were made before in- creasing weight to the next level. According to Laird’s notes, 5,400 pounds of sand were applied ini- tially, followed by more weight until the wings supported 10,028 pounds of load. That amount was equivalent to six times the airplane’s maximum gross weight of 1,750 pounds. The empennage surfaces were tested in a similar manner.
Early in April the first Laird Tractor was completed, disass- embled and transported by truck to the flying field where the ship
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