COMPARISON OF THEORY AND 

 EXPERIMENT ON DUCTED PROPELLERS 



Wm. B. Morgan and E. B. Caster 

 Naval Ship Research and Development Center ' ' . . 



Washington, D. C. 



ABSTRACT •■. ■ : ..i;.; 



The adequacy of the various theories in predicting the performance of 

 annular airfoils and ducted propellers is discussed. Force data and 

 pressure distributions are presented and conclusions are drawn as to 

 the limitations of the theories. In general, the available theories can 

 give an adequate prediction of the forces and the pressure distribution 

 if no separation occurs on the annular airfoil and, in addition for ducted 

 propellers, if a sufficient mathematical model of the propeller is used. 



INTRODUCTION 



Many studies have been made during the past few years of annular airfoils 

 and of the use of annular airfoils as shroud rings around propellers. These 

 studies have been both theoretical and experimental and have been directed to- 

 ward application in both air and water. An extensive summary of this work was 

 made by Burnell and Sacks (1) in 1960. 



Ducted propellers of two types have been investigated: (1) where the duct 

 accelerates the flow at the propeller, and (2) where the duct decelerates the flow 

 at the propeller. The first type is used for thrust augmentation; the second has 

 been suggested for increasing the limiting Mach number of the propeller in air 

 and for increasing the cavitation inception speed of the propeller in water. For 

 these applications, the annular foil, or duct, is an integral part of the propulsor, 

 and a theoretical treatment must consider the interaction between the propeller 

 and duct. Because of the complexity of the problem, this interaction is consid- 

 ered by an iterative procedure. 



The results of the various experimental studies have direct application to 

 either air or water, providing the Mach number is not too high for the studies in 

 air and cavitation does not occur for the studies in water. The theoretical stud- 

 ies, of course, have application to any fluid, i.e., providing the mathematical 

 model gives a reasonable approximation to the real flow. It is the purpose of 

 this paper to discuss the adequacy of the various theories in predicting the ac- 

 tual performance of annular airfoils and ducted propellers. Are the various 

 theories adequate to predict the pressure distribution on the duct or to predict 

 the duct and propeller forces? What are the shortcomings of either the theories 

 or experiments and where does additional work need to be done? These ques- 

 tions will be considered in the subsequent discussion. It is not the intent of this 

 paper to present new data, but to synthesize comparisons already made. 



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