Shrinkage.
Another point, and one which is intimately concerned with the proper seasoning of timber, is the amount of moisture contained in a specimen, and this latter point is of some considerable importance, as not only is a large amount of moisture detrimental to the strength values of the timber, but it also renders useless any attempt at precision of workmanship. It is this very point of shrinkage, which constitutes the greatest bar to the achievement of a measure of component standardization, and it is also one of the most serious disabilities of wood as a material for aircraft construction. It is now necessary in the production of finished parts to make some allowance for resultant shrinkage, which is a matter of guesswork, and only practicable where some time will elapse between the finishing of the part and its erection in the complete machine. Under present conditions, more often than not the parts are assembled almost immediately they are made, which means that no allowance over the actual size is possible, this being due to the various fittings which in the majority of machines are of set dimensions and clip or surround the material.
As a natural sequence shrinkage occurs subsequent to the attachment of the fitting, followed by looseness and loss of alignment in the structure. Until the proper period for seasoning can elapse, between the cutting of the tree and its conversion into aeroplane parts, it is difficult to see how this disability can be obviated, although latterly some considerable advances have been made with artificial methods of seasoning. The prejudice against kiln drying is founded on the belief that the strength of the timber is reduced, and that extraneous defects are induced. A method which is a distinct improvement on those systems, using superheated steam and hot air, is now being used with apparently good results. In this system, steam under very low compression is constantly circulated through the timber, drying being effected by a gradual reduction in the humidity of the atmosphere.