Theory of the Ducted Propeller— A Review 



of the Laplace equation or the Navier-Stokes equations. A first study of a ring 

 airfoil in a nonuniform flow has been presented by Maass (122). 



Another problem, perhaps more important in practice, is the boundary 

 layer on ring airfoils and ducted propellers. At angle of attack, the ring airfoil 

 yields a comparatively simple model for investigations in three-dimensional 

 boundary-layer theory which is interesting in itself (109, 113). 



Lastly, one should not forget, that— from the point of view of pure mathe- 

 matics—almost nothing has been proven rigorously in the three-dimensional 

 theory of airfoil and propellers, not even in the linear theory. There is a wide 

 field of open problems. 



From a practical point of view, the dominating task is to check the existing 

 theory by carefully designed experiments in order to find out if and where im- 

 provements are desirable. But this is not the concern of this review. 



NOTATION 



c Duct (chord) length 



Cl = l/C/jV^ttRc) Lift coefficient 



