2(;t JOURNAL OF FORESTRY 



ally we became had practically exhausted the stock of seasoned spruce 

 when our own aircraft program was undertaken. It requires from one 

 to two years to air-dry green spruce 3 inches thick. The Forest Service 

 supplied specifications for kiln-drying spruce comparatively early in 

 the war, under which satisfactory material in the thicknesses mentioned 

 can be secured in from 20 to 40 days. It should be remembered that 

 before this time aircraft designers maintained that only air-seasoned 

 material could be accepted. Intensive investigations are now showing 

 that material kiln-dried under proper specifications is equally or even 

 more satisfactory than that secured naturally. This comparative im- 

 provement becomes the more important when it is recognized that only 

 a slight advantage in lightness and strength over the best substitutes 

 makes spruce the preferred airplane wood and warranted the organi- 

 zation of a Spruce Production Division. The problem of drying does 

 not end with spruce. Xo one could predict on what additional species 

 it might be necessary to draw, and work was well advanced on a num- 

 ber of the best substitutes, so that had the war continued at no time 

 would the United States have been embarrassed through lack of knowl- 

 edge as to satisfactory methods to dry any species which might have 

 been required. 



The highly technical character of airplane design required the best 

 of fundamental data on the strength of woods. Before the end of the 

 w'ar- service strength tests of many years past had risen to a total of 

 more than 300,000, covering about 130 species. This data adapted for 

 use in airplane design in a table showing strength values at 15 per cent 

 moisture were adopted by both the Army and the Navy as a basis for 

 the design of all wooden aircraft parts. It served further in the selec- 

 tion of the species most suitable for airplanes and aided in the rejection 

 of unsuitable species. Even with the best of strength data available, 

 the preparation of satisfactory specifications is a difficult task which 

 demands consideration and advice and constant check by specialists on 

 strength properties of wood. Our experts were therefore able to render 

 assistance the value of which cannot be definitely measured by prepar- 

 ing or reviewing practically all of the airplane specifications for lumber 

 and for airplane parts which were adopted by either the Army or the 

 Navy. The data available enabled our laboratory men by recommend- 

 ing density requirements on the one hand to insure the selection of the 

 strongest stock; on the other hand, data on the effect of defects made 

 it possible to admit specified defects in lightly stressed parts and thus 

 practically double the quantity of acceptable stock without sacrificing 



