Strut Materials.
Referring again to the material generally employed for struts, i.e. silver spruce, it is perhaps necessary to explain further the reasons for its predominance over ash, as on a strength-for-weight ratio the latter wood is slightly the better material. The points already detailed, indicate that an interplane strut is stressed essentially in compression, and therefore the chief characteristic of ash, great tensile strength, is of but secondary importance. There is also the fact that, for the same weight, spruce would be thicker, and correspondingly more able to resist collapse. However, in machines of the flying-boat class, where the engine is invariably mounted between the four central plane struts, and consequently subjected to an amount of vibration varying with the type of engine used, ash forms the material.