January 10, 1921 



Hardwood Record — ^Veneer & Panel Section 



31 



Wood and Plywood in the Commercial Airplane 



*By Glenn L. Martin 



It seems quite reasonable that out of the great forests 

 of the world any amount of timber of any given quality 

 could be easily obtained. Such, however, is not the case. 

 To secure the quality of timber that will comply with the 

 demands of designers and builders of commercial air- 

 craft, it is conservative to say that hundreds of kinds of 

 wood have been scientifically analyzed and found unfit 

 for airplane construction. Statistics compiled during The 

 Great War in the extensive search for the ideal type of 

 lumber suitable for airplane construction revealed the fact 

 that only timber having exceptional qualities of dynamic 

 resilience, lightness, strength and uniformity of grain can 

 be used successfully in airplane construction. 



Of the great mass of timber available throughout the 

 world, it has been proven that Alaskan or Sitka (Picea 

 Sitchensis) silver spruce is the wood par excellence for 

 the manufacture of aircraft. Eighty per cent of the tim- 

 ber used during the war in the construction of airplanes, 

 both for the Allies and the U. S. Government, was sup- 

 plied by the great forests of the Pacific Northwest. Over 

 180,000,000 feet of aircraft lumber was procured by the 

 Government Spruce Division. The Allies received 120,- 

 000,000 feet and the U. S. Army and Navy 60,000,000 

 feet. 



Sitka or silver spruce is light and has greater strength 

 than any other woods of similar weight. It is moreover, 

 stiff, with long fibers, and has an unusual degree of im- 

 pact resistance. It can be worked with ease to relatively 

 close tolerances. These combined properties have made 

 spruce the most satisfactory wood for use in the con- 

 struction of struts, w^ing beams and ribs. 



In the same manner in which spruce has proven itself 

 to be the best timber for the construction of the main 



'President of The Oleiin L. Martin Company, Cleveland, O. 



wing members, ash has become considered as of equal 

 importance in the construction of longerons, engine stand- 

 ards, and wing tips, while walnut, white oak and cherry 

 are unrivalled as propeller material. 



During the war, numerous difficulties were encountered 

 in procuring hardwood timbers for the airplane industry. 

 Spruce could only be obtained in a green condition, and 

 seasoned mahogany and walnut could not be obtained in 

 adequate quantities. This deplorable situation led to the 

 erection of innumerable dry kilns throughout the aircraft 

 industry for the speedy seasoning of wood on a scale 

 which would comply with the requisites of airplane 

 production. 



On the signing of the Armistice, over 20,000,000 feet 

 of aircraft lumber had been dried under the supervision 

 of Government inspection in special dry kilns built to 

 comply with the great demand for seasoned timber. To- 

 day most manufacturers of aircraft maintain dry kiln 

 operations to insure the proper as well as timely seasoning 

 of the wood used in the construction of theJr planes. 



The pressure of the war resulted in the accumulation of 

 considerable experience and a high standard of efficiency 

 in kiln drying. Toward the latter part of the war the 

 British Government developed a method of examining 

 aircraft timber and wood workmanship by means of the 

 X-Ray. Although this method is too much involved to 

 be used for inspection, it gives evidence of being a very 

 valuable aid to the existing methods of timber research. 



With the advance in development of airplane design, 

 plywood is coming into greater use not only in fuselage 

 construction but in wing construction as well. The greater 

 bulk of plywood is produced from birch and yellow pop- 

 lar. Birch is used almost exclusively for the faces of the 

 plies, and poplar for the core. Ordinarily three, five or 

 (Continued on page 38) 



Martin Bi-plane, Showing Plywood Construction of Fuselage 



