SHEAR STRESS IS such that, when material collapses under it,
one part slides over the other. Example: all the locking pins.
Some of the bolts are also in shear or ``sideways'' stress,
owing to lugs under their heads and from which wires are
taken. Such a wire, exerting a sideways pull upon a bolt,
tries to break it in such a way as to make one piece of the bolt
slide over the other piece.
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.
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