Previous investigations on the effect of moisture on the elastic constants of spruce have been confined to its effect on Young's modulus in the direction of the grain. The most important work in this connection is probably that of H. D. Tiemann, who deduced curves for pine spruce and chestnut showing that Young's modulus along the grain decreased as the moisture increased up to the fibre saturation point, and that further addition of moisture had little or no effect on the value of the modulus. Tiemann thus found that the modulus decreased to a minimum at the fibre saturation point and then remained nearly constant and independent of any further addition of moisture. The reason for the independence of the modulus on the moisture beyond the fibre saturation point becomes evident if it is considered that when the fibres are saturated, any further moisture added is retained in the cells and in all probability cannot further affect the stiffness and strength of the wood.

Airscrews are generally made of wood and owing to the relatively low Young's modulus of timber the blades of an airscrew have a large measure of flexibility. This flexibility is reflected in the comparatively low frequency of vibration of the blades with consequent undesirable effects. Firstly, there is a tendency for the blades to heat resulting in the laminations becoming unstuck, and secondly, there is considerable danger from resonance between the frequency of the blades and the firing impulses of the engine driving the airscrew. The result of this is heating and burning of the boss and ultimately failure of the airscrew. It may even result in failure of the airscrew shaft by shearing.