PRESENT-DAY efforts to produce wood aircraft in large quantities have uncovered many new problems, for wood has certain peculiarities that must be taken into consideration by the engineer, if he is to design structures that make full use of the benefits to be derived from wood.
Attempts to take advantage of the high tensile strength of wood will lead to failures in shear, because loads theoretically in tension practically always have shear components that are great enough to overcome the low shear strength of wood. Moisture content also has a great effect on the strength of wood; and the moisture equilibrium of a piece of wood will vary with the relative humidity and temperature to which it is exposed.
Mr. Peterson discusses some of the problems confronting the wood aircraft manufacturer under three headings: fabrication, static testing, and detail design.
The fabrication of wood structures revolves around the production of strong glue joints. Sufficient glue spread on surfaces that have been carefully smoothed, and a correct balance between temperature, pressure, and glue consistency at the time pressure is applied are the basic requirements.
The problem of “reducing” static-test data for wood structures is very difficult and cannot be associated with the more-or-less standard correction procedures that have been established for metal structures. Certain factors that affect the strength of wood tend to affect one another, and corrections to static-test data for complete wood structures are unnecessary, provided some control is maintained over the specific gravity of the material in the test article.
Lack of knowledge of plywood strength and elastic properties, and poor detail design practices are responsible for the somewhat poor reputation that wood aircraft structures have acquired recently.
Both of these items will be overcome as a background of design information and experience is obtained similar to that already available for metal structures.