to Mr. Parkinson for his most interesting and informative contribution to the hydro- 

 dynamics of high speed water-based aircraft. 



C. L. Fenn 



Mr. Parkinson's complete coverage of the high speed hydrodynamic situation 

 has left me with practically nothing to say. I might, however, make one or two points. 

 They are very small. 



It might be possible in future development to arrange a hydroski or hydrofoil, 

 but for the moment let us consider a hydroski, of such a nature that it would lift the 

 aircraft out of the water at a speed at which the aerodynamic, but not the hydro- 

 dynamic, loads would design the fuselage. This would put the seaplane on an even 

 footing with the land plane. It is something to think about. 



The next things that occurs to me is where does the hydrofoil sit in relation 

 to the hydroski, development-wise? It seems to be somewhat further back. At the 

 moment, we have no correlation between the model tests and full scale operation. 



It also appears — and I will not say the dangerous thing — the dangerous thing 

 to say is that there has been no successful hydrofoil aircraft. When you say that, 

 someone says "Oh, they had one in Hammondsport in 1908." But, let's put it this way 

 — there has been no hydrofoil aircraft so good that it spawned a family of hydrofoil 

 airplanes. 



Such is not true with the hydroski. There have been complete families of 

 hydroski airplanes. I know of one, four in number, which started at 8,000 pounds 

 and which has grown to 55,000 pounds, and they have lent themselves to high-speed 

 operation. 



R. M. Hopkins 



Mr. Parkinson is to be congratulated on a very interesting and comprehensive 

 paper. While listening to it, however, several thoughts arose which might be of some 

 interest to you. On looking back — particularly over the past year — I can imagine that 

 Jack may well feel that we never meet without complaints on my part over things he 

 hasn't done. It may therefore be appropriate at this point to mention a few of the 

 things he has done. 



First, Jack and his group at Langley did the pioneering work on hydro-skis, 

 which in conjunction with the development of the turbojet engine was the initial step 

 toward high speed equality between land-based and water-based aircraft. More recently 

 Jack has combined forces with the aerodynamics people at N.A.C.A. to show that 

 there are no fundamental reasons why the famed "area rule" concept may not be 

 applied equally as well to water-based as to land-based aircraft. This type of leader- 

 ship has caused many to re-evaluate the future role the seaplane may play. 



I should now like to bring to your attention one or two high speed hydrodynamic 

 problems in somewhat more detail. In the slides just presented, you will recall the 

 one showing the very large facility designed to examine the problems involved in a 

 land-plane landing gear when it makes contact with a nice smooth runway. It is 

 suggested that the high speed towing tank auxiliary to that facility which Jack discussed 

 might be used to examine the much more severe problems of high speed seaplane 

 landing gears in contact with a runway that is far from smooth. 



These problems are closely allied to the self-excited or hydroelastic vibrations 

 which Jack discussed, and for which some insight may be gained through the brief 

 theoretical treatment of the flow involved contained in Professor Milne-Thomson's 

 book on "Theoretical Hydrodynamics." It is believed, however, that the crux of the 

 matter is best stated as that case for which the unsteady input forces (whether self- 

 excited in character or due to wave impacts) excite the natural or resonant frequency 

 (or frequencies) of the aircraft structure. The serious nature of this problem may be 

 appreciated by a reference to an early incident in Sea Dart development. This seaplane 



209 



