The advent of the use of aircraft for purposes other than warfare will involve the appearance of several additional aspects into the already numerous considerations which have to be applied during the evolution of the design of an aircraft.

During the recent war, designers were called upon to produce designs in which performance and manœuvrability were the foremost considerations, and sacrifices of other factors, including safety and stability, had to be made in order to attain supremacy in these directions. It is true that during the latter stages of the war matters concerning ease of production, the use of alternative materials and accessories, and facilities for maintenance did receive a certain amount of attention; but in view of the speed at which designs and production had necessarily to mature, the amount of thought which could be given towards such considerations was very limited.

So far as is known, the only papers containing reference to the principles of rigid airship design or construction are those delivered by Herr Dornier to the German “ Schiffbantechnischen Gessellschaft ” in 1915 and by Mr. C. I. R. Campbell before the Institute of Naval Architects in 1919. The former paper dealt particularly with the Zeppelin, but while it contains valuable information yet it does not deal with the fundamental principles underlying rigid airship construction.

Further, the figures and estimates that are deduced are based on performances of the earlier Zeppelins and have been substantially improved upon during the war. The present paper is intended to state as far as possible the various principles underlying the design of a rigid airship, in the hope that such information may be of value in the progress of this important branch of aeronautics.

The inclusion of incidence wires in strength calculations, by the method of strain energy, makes very little difference in normal flight conditions, but shows, large changes in wing loads in nose diving and in centre section loads with a broken flying wire. There is a serious increase of load in the bottom plane in nose diving.

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.

High tensile steel wire has many uses in connection with aircraft. A single wire has the advantage of a high breaking stress, greater than that of the bulk material from which it is made, because, as Goodman has pointed out, the drawing produces an elongated stricture of the steel, and the wire strength approximates to the true breaking stress of a larger specimen, which is the tension the latter resists at the moment of fracture divided by the reduced area. The British Engineering Standards Association's specification, 2wi., requires a minimum breaking stress of 80 tons per sq. in. up to 0.160in. diameter, 90 to 0.116, and 100 to 0.064. These stresses can safely be exceeded when required particularly with wire of small diameter.