The decision as to what strength shall be given to the structure of an aeroplane is made as a result of previous experience. It is well known that it is quite impossible so to design the structure as to render it unbreakable under any condition of flight without increasing the weight to such an extent that the aeroplane is useless as a flying proposition.

The result has been that load factors have been specified for different types of aeroplanes such as experience has shown will render the structure safe for any reasonable treatment in the air.

The principal factor to be fixed is that for the case when the centre of pressure is in its most forward position. Once this is determined the remainder of the structure can be designed consistently with it.