Theory of the Ducted Propellers— A Review 



In the fore part of the duct, the difference is about the same for all ducts. 

 In the rear part, however, where the influence of the propeller wake is consid- 

 erable, very large differences are obtained. Duct No. D4 is neutral, which 

 means that it coincides with the mean slipstream of a conventional propeller. 

 The radius of the slipstream must in this case decrease continuously from 1.0 

 at the propeller to 0.86 in the ultimate wake, where the propeller-induced ve- 

 locities are about twice that at the propeller. If the continuity law is used to 

 determine the shape of the duct the radius of duct D4 will be 0.92 at the trailing 

 edge, while corresponding value obtained with the boundary condition method is 

 0.80 — a quite unrealistic result. 



Figure D-1 also illustrates a fact enforced by the authors —that it is not 

 advisable to ignore the induced axial velocity in comparison to the advance ve- 

 locity, when satisfying the boundary condition. As seen quite different camber 

 and thickness distributions are obtained with the complete and the simplified 

 boundary condition equations. 



The authors state in their final remarks that "it might be desirable to drop 

 the assumption of slenderness for the propeller blades." As described in SSPA 

 publication no. 62/18/, our design method includes also lifting surface calcula- 

 tions. Figure D-2 shows an example of the results obtained. Starting from the 

 effective camber distribution, the camber correction due to the propeller is cal- 

 culated according to the method by Pien. An additional correction caused by the 

 mean vorticity of the duct is then determined. In the present case the camber 

 correction due to the duct generally counteracts the correction caused by the 

 propeller. 



0.03 r- 



0.02 - 



0.01 - 



Geometrical camber 



Without duct camber corrections 



Fig. D-2 - Radial distribution of maximum cam- 

 ber of blade profiles. Ducted propeller P1315 

 D6 (SSPA publication No. 63) 



1263 



