Morgan and Caster 



Fig. 23 - Pitching moment of the Doak ducted propeller at 

 angles of incidence (19) 



comparisons were based on predicting the total system thrust. It can be con- 

 cluded that a more complete mathematical treatment is required for the propel- 

 ler, stator vanes, and guide vanes to be able to predict all the forces satisfac- 

 torily. Also, more complete experimental data are required where the forces 

 on the various components are measured separately, especially for an angle of 

 incidence. 



Tip Clearance 



Several investigations have been made of the effect of tip clearance on the 

 performance of ducted propellers, e.g., Refs. (28 - 32). Results of a theoretical- 

 experimental comparison made by English (31) are shown in Fig. 24. The basic 

 conclusions of this work is that, for increasing tip clearance, the duct thrust de- 

 creases and the propeller thrust increases, while the net effect is a small re- 

 duction in efficiency. The theoretical predictions of English slightly under - 

 predict the change in efficiency with tip clearance. The comparisions made by 

 Turbal (30) show similar results. In summary, the comparisons show that the 

 theory indicates the trends of the experimental data. 



CONCLUSIONS 



The following conclusions as to the adequacy of the theory are based on a 

 limited number of geometric shapes. Consequently, general applicability of the 

 conclusions may be limited. 



1338 



