It is to be hoped that recent work by Tulin and Wu on the theory of hydrofoils 

 operating in a fully cavitated state will provide the additional incentive required to 

 initiate an active research program in this field. 



Sufficient evidence must now exist to convince the aircraft designer of the hydro- 

 dynamic superiority of the hydrofoil. One's thoughts, therefore, naturally turn to the 

 structural problems involved, and it seems inevitable that these will be very much more 

 severe in the case of the hydrofoil. One thought in this direction appears appropriate 

 to a symposium on hydrodynamics. The writer is treading on dangerous ground, but it 

 is believed that several of the problems met in high speed flight have a counterpart in 

 the design of hydrofoils. (One elementary example that comes to mind is the simple 

 relation that exists between the critical Mach number and the critical cavitation number 

 of a foil section.) It might not, therefore, prove to be beyond feasibility to design a 

 hydrofoil system which, rather than being an awkward encumberance in flight, could 

 perform a useful aerodynamic function — thus obviating the need for retraction. If this 

 were possible the weight normally associated with retraction gear would presumably be 

 the hydrofoil structure, and it is even feasible that an increased payload might result. 



/. B. Parkinson 



The discussers of this paper have enhanced its value by supporting and ampli- 

 fying two major points which were made but not developed to any extent. The first is 

 that water loads and their effects on airplane structures are of increasing concern be- 

 cause of current design trends and operating requirements. The second is that there 

 remain valid reasons for further research and development in the area of hydrofoil- 

 supported aircraft. The two are interrelated since the hydrofoil provides an additional 

 promising approach to alleviation of prohibitive loads and motions due to wave impacts. 

 With this clarification of objective, it now should be possible to arrive at practical 

 hydrofoil applications for water-based aircraft that are competitive with or perhaps 

 superior to presently available solutions. 



An extended discussion of water loads is beyond the scope of the paper. It may 

 be mentioned that authorities on this subject are reluctant to prescribe upper limits on 

 loads, pressures, and rates of build-up resulting from rough-water operation. Transient 

 pressures up to 50 psi have been measured on a seaplane and up to 100 psi on a large 

 hull model in the Langley impact basin. The effects of local resonances and flexibili- 

 ties, finite expanse of the waves, etc., are not too well understood, but in general are 

 believed to be small compared to the total loads, statistical variations of the dominating 

 parameters, and the orders of alleviation required for significant improvements in rough- 

 water capabilities. 



The author wishes to express his thanks and those of his colleagues in the NACA 

 for the favorable comments on the paper and the stimulating thoughts contributed in 

 its discussion. 



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