Torsion.—This is a twisting stress compounded of compression, tension, and shear stresses. Example: the propeller shaft.
Nature of Wood under Stress.—Wood, for its weight, takes the stress of compression far better than any other stress. For instance: a walking-stick of less than 1 lb. in weight will, if kept perfectly straight, probably stand up to a compression stress of a ton or more before crushing; whereas, if the same stick is put under a bending stress, it will probably collapse to a stress of not more than about 50 lb. That is a very great difference, and, since weight is of the greatest importance, the design of an aeroplane is always such as to, as far as possible, keep the various wooden parts of its construction in direct compression. Weight being of such vital importance, and designers all trying to outdo each other in saving weight, it follows that the factor of safety is rather low in an aeroplane. The parts in direct compression will, however, take the stresses safely provided the following conditions are carefully observed.
Conditions to be Observed:
1. All the spars and struts must be perfectly straight.
The above sketch illustrates a section through an interplane strut. If the strut is to be kept straight, i.e., prevented from bending, then the stress of compression must be equally disposed about the centre of strength. If it is not straight, then there will be more compression on one side of the centre of strength than on the other side. That is a step towards getting compression on one side and tension on the other side, in which case it may be forced to take a bending stress for which it is not designed. Even if it does not collapse it will, in effect, become shorter, and thus throw out of adjustment the gap and all the wires attached to the top and bottom of the strut, with the result that the flight efficiency of the aeroplane will be spoiled.