Schmiechen 



in developing the conceptual framework for the definition of the performance 

 criteria looked for. 



Propulsion devices considered as subsystems of complex systems, i.e., ve- 

 hicles, may be described by their representative points in multidimensional 

 spaces of performance criteria, the definition of the appropriate criteria de- 

 pending on the contexts of the particular types of vehicles. In cases where all 

 relevant definitions are established and a best-practice envelope may be deter- 

 mined accordingly, it is easy to judge on the merits of any propulsion device, 

 especially those newly proposed as compared with the state-of-the-art. 



In this paper only few aspects of propeller performance will be considered. 

 In view of the multidimensionality of the parameter space, it should be kept in 

 mind that optimizing with respect to the aspects considered will in general not 

 result in an optimum overall system. It is this point that is often rightly 

 stressed by inventors claiming simplified power plants, etc., for their devices 

 as compared with conventional propulsion. 



2. PRINCIPLES OF PROPULSION 



Although the basic principles have been stated over and over again, a short 

 review is presented here for ready reference. 



Any propulsion device of the type to be considered may be defined as a me- 

 chanical system, parts of which consist of the surrounding fluid; see Fig. 1. The 

 latter will be considered as incompressible here, having the mass density p . 

 The construction of the boundary of the system is in general undoubtedly one of 

 the basic problems, to which many other problems may be reduced; see Sec. 5.2. 



.BOUNDARY OF THE SYSTEM 

 (INVARIANT) 



Fig. 1 - Propeller: a 

 mechanical system 



In any case, there exists in the system a dividing surface S between the 

 propeller proper and the surrounding fluid. While the relative velocities i>t of 

 the elements dSj of this surface with respect to the invariant boundaries of the 

 system characterize the kinematics of the propeller, the stresses S^m at these 



1086 



