48 The Forest Products Laboratory 



a search for a new and satisfactory coating! Thus was hrought about 

 the invention and perfection of the ahiminum leaf coating, which is 

 practically impervious to moisture so long as it remains intact. 



The change in the moisture content of projDellers due to atmos- 

 pheric conditions, with the consequent warping, twisting, and unbal- 

 ancing of the blades was only one of many troubles experienced in the 

 manufacture and use of aircraft propellers. The results were, how- 

 ever, much the same. There are many variables entering into the 

 manufacture of propellers, such as differences in weight, shrinkage 

 and moisture among the various laminations, and to separate all of 

 these variables and their effects upon the finished propeller required 

 a very elaborate and costly series of tests on full sized propellers of 

 various species. The species entering into this study were: Central 

 American. African and Philippine mahoganies, white oak, red oak, 

 hard maple, yellow birch, yellow poplar and red gum. A complete 

 propeller manufacturing shop and a number of storage rooms were 

 erected, all provided with automatic temperature and humidity con- 

 trol. Lumber of the different species was kiln dried at the laboratory, 

 and the task of making up several hundred propellers, each in accord- 

 ance with a definite plan, was got under way. These propellers were 

 subjected to various atmospheric conditions ranging from very moist 

 to very dry, and the changes in them carefully measured at stated in- 

 tervals. This project was not finished during the war, but is now 

 nearing completion. The results are already quite evident, and 

 shortly the desired specifications for manufacturing limits and con- 

 trol will be drawn up in correct and final form. 



Many small but important problems in connection with the air- 

 craft program were solved, and much miscellaneous help given, prin- 

 cipally through numerous field trips by the technical staff. Permanent 

 record of this service, and of the results of the major studies as well, 

 is found largely in the specifications of the army and navy aircraft 

 bureaus. 



Kiln Drying of Heavy Woods 



The sudden increase in the demand for vehicle stock brought 

 about by the need for tremendous quantities of army escort wagons, 

 automobile trucks, and artillery wheels almost immediatelv exhausted 



