FOREST SERVICE. 195 



interplane struts between the wings. Sitka spruce from the west 

 coast and red and white spruce from the east are all satisfactory air- 

 plane woods when properly selected and dried. Not only must 

 checks and knots be avoided, but the direction of the grain must be 

 carefully examined. Only a slight deviation from straight grain can 

 be alloAved. 



At the beginning of the war it was customary to air-dry all wood 

 used in airplane construction, on account of the danger of injuring 

 the strength by methods employed in commercial kiln-drying. As it 

 takes about two years to air-dry spruce in airplane sizes, and quan- 

 tities of material were needed at onc€, kiln-drying was absolutely 

 necessary. Investigations in kiln-drying had been under way at the 

 Madison laboratory for several years, and methods had been worked 

 out for a number of woods. An extension of these investigations to 

 include spruce showed that it could be kiln-dried without loss of 

 strength in less than a month. This information was used as a basis 

 for preparing specifications for diying airplane stock. Numerous 

 dry-kilns designed on the principles of experimental kilns at Madi- 

 son, so as to allow the regulation of temperature, humidity, and 

 amount of air passing over the Avood to be dried, have been built by 

 companies with airplane contracts. A large battery of kilns designed 

 by the Forest Service has been erected by the Government at the 

 cut-up plant of the Bureau of Aircraft Production at Vancouver, 

 Wash. 



The necessity for high-grade material in airplanes in order to keep 

 down the size and weight of parts makes only a small proportion 

 (from 10 to 20 per cent) of the lumber cut at the mill available. The 

 high quality of material needed, together with the difficulties attend- 

 ant on getting large spruce production and the increasing demands 

 of the Allies, have necessitated the use of other species. The results 

 of the extensive earlier investigations at Madison, supplemented by 

 special tests, have made this possible. Port Orford cedar, Douglas 

 fir, and other woods are now allowed for airplane construction, and 

 a serious situation in the shortage of material is being relieved. 



The test data which have been secured cover the relative suita- 

 bility of many possible substitutes for spruce. These will make pos- 

 sible' not only the selection of the best species to meet the maximum 

 requirements" which may be developed by the war but also (what 

 is equally important) avoidance of the use of unsuitable species. 



The work of past years has shown that practically no two woods 

 should be kiln-dried under the same conditions of temperature, hu- 

 midity, and circulation. It has been necessary, therefore, to develop 

 safe methods of artificial kiln-drying for the best spruce substitutes. 



A problem has also been encountered in finding how to select and 

 treat wood for airplane propellers so that the finished propeller will 

 give satisfactory'- results. Propellers are built up of several lamina- 

 tions glued together and then cut to shape. Much trouble has been 

 due to the joints failing or the blades warping on account of unequal 

 shrinkage or swelling in the adjacent laminations. Data covering 

 density and moisture in laminations are being obtained with a view 

 to reducing the percentage of failures. 



Heretofore most propellers have been made of black walnut or 

 mahogany. The heavy demand on these woods has made a search for 

 substitutes necessary. Studies to find satisfactory substitutes are 



