D. Savitsky 



Mr. Parkinson has presented a fine summary of the "status of the art" of high 

 speed seaplane hydrodynamics — both as regards the special hydrodynamic problems 

 which have been accentuated by the high speed water based aircraft and as regards 

 some of the new hydrodynamic developments which are available and necessary to the 

 designer of high speed seaplanes. 



I would like to amplify further Mr. Parkinson's reference to the beneficial poten- 

 tials which may develop with the application of hydrofoil alighting gear to the aerody- 

 namically refined high length beam ratio hulls. In particular, it appears that a hydrofoil 

 system shows a good deal of promise in improving and extending both the high and low 

 speed rough water capabilities of the seaplane. As pointed out by Mr. Parkinson, high 

 speed hulls, while having a satisfactory lift drag ratio, are subjected to heavy impact 

 loads and motions in a seaway. Small, highly loaded, penetrating hydro-skis alleviate 

 the severe rough water loads and motions but have excessive water resistance during ski 

 emergence. The high aspect ratio hydrofoil, on the other hand, has a lift drag ratio 

 comparable to the hull and can be designed to alleviate the impact loads and motions 

 to a degree comparable to the hydroski. A significant design feature which is inherent 

 in a hydrofoil system is that the hydrodynamic support area of the main foil will prob- 

 ably be distributed vertically rather than longitudinally as in the case of either a hull 

 or hydroski. This would keep the center of pressure essentially in a fixed longitudinal 

 position and hence eliminate generation of excessive pitching moments. Further, the 

 vertical arrangement of support area can be designed to contact the water surface with 

 small areas during the impact process and hence lead to alleviation of the impact forces. 



Another advantage of the hydrofoil system which has not been generally dis- 

 cussed, is the effective damping in pitch that a tandem foil system provides at prehump 

 speeds. In these speed ranges, aerodynamic damping is negligible and the high density, 

 low frontal area hulls are subjected to severe pitching and excessive bow spray genera- 

 tion when operating in a seaway at low speeds. In some cases the development of 

 severe wetting of engines and structure may preclude take-off. The tandem hydrofoil 

 system has the capability of providing significant pitch damping which in turn would 

 reduce the pitch motions and the corresponding bow spray development. 



To achieve these hydrofoil potentials we must continue our research on the basic 

 hydrodynamic problems attendant with hydrofoils, e.g. cavitation, ventilation, longitudi- 

 nal and lateral stability of foil systems suitable for seaplane application. 



The high landing speeds associated with projected high speed water based air- 

 craft has put a strain on the existing facilities of the Stevens Experimental Towing 

 Tank. It appears that the top speed of our No. 3 Tank will have to be increased to 

 accommodate the increasing landing speed of seaplanes and to handle some of the 

 cavitation problems which exist with the hydrofoil systems. Also, an irregular wave 

 maker has been installed in our No. 3 Tank to provide for more realistic evaluation of 

 the rough water characteristics of seaplane forms. Our present No. 2 rectangular tank 

 facility which is being altered to examine ship behavior in oblique seas, will also be able 

 to accommodate tests of seaplanes at oblique headings to a sea. Combining these new 

 test facilities with the so-called ETT "free-to-surge" towing apparatus, which permits a 

 model to be towed with complete longitudinal freedom, will make for a more realistic 

 evaluation of the new seaplane designs. 



M. C. Eames 



The writer was most gratified to learn of the progress being made in the study 

 of the mechanics of ventilated flow on submerged lifting surfaces, but was disappointed 

 to discover the relative states of development of the hydro-ski and the hydrofoil for 

 application to water based aircraft. 



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