THE LONGITUDINAL STRENGTH OF RIGID AIRSHIPS. 171 
DISCUSSION. 
THE PRESIDENT:—You have heard Paper No. 9, “The Longitudinal Strength of Rigid 
Airships,” by Prof. William Hovgaard, a Member of the Society. This is one of those highly 
valuable scientific papers which it is one of the aims of the Society to encourage and to include 
in our Transactions. 
I do not know whether there is anybody here who is prepared to discuss this paper or 
not. If there is anyone here who would care to discuss it, we would be glad to have him 
do so. 
CoMMANDER Emory S. Lanp, (C. C.), U.S. Navy, Member:—I am not quite prepared 
to discuss this paper on my own behalf exactly, but with a little assistance from the Bureau of 
Aeronautics of the Navy Department, I have here a discussion which was contributed jointly 
by C. P. Burgess, Commander J. C. Hunsaker, and myself, which is as follows: 
Professor Hovgaard has presented a very clear and interesting exposition of the diffi- 
culties involved in the theoretical investigation into the distribution of the loads among the 
longitudinal and diagonal members of the hull of a rigid airship, resulting from the primary 
bending and shearing forces. The study of the stresses and deflections in rectangular panels 
due to combined end and transverse forces is believed to be unique in its exhaustive analysis 
of this particular problem, and it serves as a most useful introduction to the study of the com- 
plex structure of a rigid airship. Care must be taken, however, to bear in mind that the case 
of rectangular panels, free to deflect, each by itself, under the influence of Professor 
Hovgaard’s P and Q forces, is very different from the actual case of a rigid airship, in which 
the problem is vastly more complicated owing to the panels being bound together and re- 
strained in their deflections, so that they interact with each other in a way which makes the 
determination of the P and Q forces for each panel the dominant part of the problem, instead 
of something given in the hypothesis. 
The two rival methods of calculation mentioned by Professor Hovgaard, and designated 
by the method of bending moments and the method of transverse shears, seem to have oc- 
curred independently to several investigators; and, indeed, they follow directly from well- 
known principles of analysis of complex structures. Both these methods are based on the 
assumption, known to be only approximately true, that transverse sections plane before bend- 
ing remain plane after bending, that the longitudinals alone take the primary bending forces, 
and the shear wires alone take the primary shearing forces. 
Professor Hovgaard declares himself in favor of the bending theory, and he has con- 
tributed to it the addition of fictitious longitudinal bars to be used as equivalents for the diago- 
nal wires in calculating to what degree these members assist the longitudinal girders in re- 
sisting the primary bending moments in the hull. These fictitious bars increase the effective 
moment of inertia of the cross sections of the hull of a modern rigid airship by about 5 to 
6 per cent. 
Engineers have been accustomed to regard methods of analysis by moments and by 
shears as alternatives which should give identical results; but it is found that in the problem 
of the rigid airship the results are the same only when the longitudinal girders are of equal 
areas and situated at the apices of regular polygons, and the diagonal wires are of equal size 
throughout each main frame space, and the shear and counter-shear wires are both effective. 
