THE LONGITUDINAL STRENGTH OF RIGID AIRSHIPS. 173 
that Professor Hovgaard has done much of the wheel horse work of going into the design 
and the calculation of these airships; the results of some of these investigations have been 
. presented to the Society this morning. The Navy Department in general and the Bureau of 
Aeronautics in particular owe this committee a hearty vote of thanks, and I assure you we 
are very grateful. 
I should like to clarify, if possible, one point on page 143. After the sentence reading 
“The girders are constructed of the lightest metal we possess,” I think should be added “due 
consideration should be given to the strength requirements.” The metal referred to 
is duralumin. 
In connection with the matter on page 146, it might be of minor interest to refer to the 
catastrophe to R-38 (ZR-2). Many of the eye-witnesses insisted that the ship first opened 
at the bottom, and the statements made with reference to the accident indicated that the 
strength of the keel girders was the weakest part of the ship. Such is not the case, and the 
final testimony is to the effect that the ship first opened on the side. The idea of some of 
the eye-witnesses was prebably obtained from seeing one of our own officers fall out through 
the bottom (Lieut. Commander E. W. Coil, U. S. N.). There was evidently a good deal 
of misstatement from so-called eye-witnesses. However, Professor Hovgaard’s statement 
relative to keel structure definitely holds so far as R-38 (ZR-2) is concerned. 
On the same page, last sentence of the second paragraph, where the author says: “The as- 
sumption being that the dynamic forces would be less serious.’”’ I take slight exception to 
that, because I think it was not a question of their assumption being incorrect, as that their 
knowledge was inconclusive and very incomplete. They felt that if the static forces were 
taken care of, the dynamic forces would be taken care of by the factor of safety. The known 
aero-dynamic forces and the data available relative to them were very meager, and unless 
the Germans have more than we think they have, they still are very meager. The assump- 
tion that the aero-dynamic forces will be taken care of by the factor of safety has been proven 
to be incorrect. 
The last paragraph on page 159 puts before you gentlemen the extreme difficulty of 
attempting to solve the problem before you, where you have shear, tension, compression and 
torsion forces and do not know where or when you are getting one or more of these stresses 
acting on a structure as complicated as the rigid airships we have under consideration. I 
am particularly glad that Professor Hovgaard brought out the analogy to the submarine, be- 
cause there is altogether too little attention paid in this country to the question of strength 
of rigids on the part of those interested in the design of submarines; they should get a special 
benefit from these comparisons made by the professor, and they should help us out. The 
analogy is very close, the problem is here, and it seems to me that we are up against a stiff 
problem; there should be more interest displayed in attacking it, in an endeavor to solve it. 
The work on this difficult problem has only started, and while Professor Hovgaard has 
given us a way out, it will require a good deal more work and a good deal more experi- 
mental work, both in model design in the laboratory and actual tests of the ships in the air, 
before we will come to any definite and final results with the aero-dynamic forces. An effort 
is to be made to go through some actual tests in air with all of the equipment in our airships. 
Actual air tests were very strongly recommended by the committee which sat on the investi- 
gation of the R-38, but lack of funds prevented it being carried out. It will be necessary to 
check our experimental data with trial data in the air. We have only just begun the attacks 
on the strength problems. 
