Propeller-Induced Appendage Forces 



This form manifested itself in nearly all the data obtained in an extensive series 

 of tests, but it is possible to expect a power- law decay in an asymptotic sense 

 for large spacing distances. In that case, at large distances, the magnitudes of 

 the forces would be extremely small and it is not expected that any useful infor- 

 mation could be obtained if such a power law was adhered to in the evaluation 

 and interpretation of the results. The other comment by Dr. Milgram is con- 

 cerned with the effects of boundary layers on the walls of the water tunnel and 

 how they influence our experimental results. All of our data are the values of 

 the amplitudes of oscillatory dynamic forces induced on the foil appendages by 

 the action of a propeller. Boundary layer effects, as mentioned by Dr. Milgram, 

 will influence only the steady lift and drag forces, which are of no concern in this 

 study. It is not expected that the influence of the boundary layers on our water 

 tunnel will affect the dynamic forces of concern in this program. 



Dr. Breslin's comment concerning the magnitude of the theoretical forces, 

 as compared to the experimental measured forces, must be examined in greater 

 detail before assuming that the two-dimensional theory has no applicability to 

 force prediction in the present case. The axial forces are predicted in a much 

 closer fashion than the transverse forces, as illustrated in our figures. It is 

 optimistic to expect the very simple blade element treatment for the propeller 

 thrust, assuming an equivalent single vortex, lack of interference between blades, 

 equivalent contributions of angle of attack and camber, etc., to provide a useful 

 model of the disturbance strength provided by the propeller. Since the magni- 

 tude of the relative contribution of various components to the total force depends 

 upon the strength of a single vortex derived from the simplified thrust represen- 

 tation, it can be seen that it is difficult to expect good agreement under those 

 circumstances. A more sophisticated two-dimensional treatment of the propel- 

 ler representation would be expected to lead to better agreement. 



The use of our data as a means of comparison with the extensive theoretical 

 work that has been carried out at the Davidson Laboratory and other organiza- 

 tions is an important aspect of our work. We certainly look forward to the re- 

 sults of a comparison with the theoretical work at the Davidson Laboratory so 

 that a rational explanation of the variation of the forces with separation distance, 

 especially for the rudder problem, will be available. 



233 



