THE LONGITUDINAL STRENGTH OF RIGID AIRSHIPS. 
By ProFessor WILLIAM HovGAArRD, MEMBER. 
[Read at the thirtieth general meeting of the Society of Naval Architects and Marine Engineers, held in 
New York, November 8 and 9, 1922.] 
INTRODUCTION. 
The general strength of rigid airships is a problem so much akin to the corresponding 
one for ordinary ships that it is believed to be of interest to the members of this Society, 
the more so as some of them are actively engaged in the design and construction of this new 
craft. Yet it must be admitted that the structure of an airship is in several respects radically 
different from that of an ordinary vessel and in fact from any construction hitherto 
attempted by man. The extreme requirements to a saving in weight make it necessary to 
adopt an entirely novel combination of girders with steel wires of high tensile strength. The 
girders are constructed of the lightest metal we possess. At the same time the factor of 
safety is cut down to a figure which would not for a moment be considered either in ship- 
building or in civil engineering. These features necessitate a nicety of design, an accuracy 
in calculations, and a perfection in construction unparalleled in structural work in the two 
branches of engineering just referred to. They call for an extensive theoretical and experi- 
mental study of a most difficult nature, involving, as they do, new and intricate problems in 
structural design and in aerodynamics. An airship in its complete form is one of the highest 
and boldest achievements in engineering. 
The present paper results from a study of this subject carried out by the author when 
retained by the Navy Department in a consulting capacity in connection with the design of 
the rigid airship ZR-1. The fact that the paper introduces for the first time a new and im- 
portant subject to the members of this Society will, I hope, serve as an excuse for its great 
length. 
I. GENERAL DISCUSSION OF THE STRUCTURE OF A MODERN RIGID AIRSHIP. 
For the sake of those who are not familiar with the general construction of airships a 
brief description is given of the principal features of a ship of the Zeppelin type and I choose 
as an example the German L-49, which was completed in 1917. In the fall of the same year 
she was forced down intact at Bourbonne-les-Bains and was captured by the French, who 
made a careful survey of her. She has served as a basis for the design of the ZR-1, now 
under construction at Philadelphia and Lakehurst. 
Referring to Plate 53, the form of the sections of L-49 is that of a fairly regular poly- 
gon, approaching very closely to a circle. The longitudinal main girders, with the excep- 
tion of the two bottom girders, are equidistant, 6 meters apart, and midway between them 
run lighter, so-called secondary longitudinals, which cannot, however, be considered as inte- 
gral parts of the structure, since they are not in efficient shearing connection with the main 
girders or with one another. The two bottom or M-girders are more closely spaced and 
