DURAND— MECHANICAL ASPECTS. 187 



spending loss in efficiency. Again if too large or if the relation 

 between speed of advance, slip and torque are unsuitable, the pro- 

 peller will perhaps hold down the motor to a rotative speed entirely 

 too low and thus render impossible the development of the desired 

 power. These and other relations are of course well known and 

 are only mentioned here in order to emphasize the importance of the 

 most careful inter-adaptation between the aeroplane, the motor and 

 the propeller. 



In this field there is still important work to be done in a more 

 complete study of the characteristics of the aeroplane and propeller 

 separately and when combined in their normal relation, all with a 

 view of insuring a more perfect adaptation of the one to the other 

 and of the prime mover to both. 



The air propeller has thus far been normally made of wood and 

 of the two-bladed form. Outstanding problems which are awaiting 

 investigation relate to the best modes mechanically of making three 

 and four blade propellers with the consequent saving of diameter 

 for the same thrust, revolutions and slip ; also to the practicability 

 of propellers of light metal alloys instead of wood. Some work has 

 been done along these lines and some hopeful indications have 

 appeared. 



A further problem, structually, relates to the thickness neces- 

 sary for strength under the complex stress due to centrifugal force 

 and air pressure, and also the distortion of the blade under these 

 loads and the extent to which such distortion may modify the geo- 

 metrical characteristics of the propeller itself. 



Concluding we may in resume sum up for the aeroplane as a 

 whole, the insistent demands on the realization of which future 

 progress must depend. These are : 



I. Minimum weight of structure in relation to area of svipport- 

 ing surfaces and of power plant per unit of power developed. This 

 will secure increased carrying capacity for fuel and supplies and for 

 useful weight such as passengers, mail, etc., and this will serve as a 

 factor in either long life in the air or heavy carrying capacity for 

 short distance. On the other hand such extra weight carrying 

 capacity may be put into additional power plant, engine and fuel, 

 for correspondingly increased speed over shorter distances. 



