144 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



tainecl at the sacrifice of efficiency. In the aeroplane, however, light- 

 ness per output is a prime consideration, and certainty and reliability 

 of action is demanded, since if by chance the motor stops the ma- 

 chine must immediately glide to the earth. A technical discussion 

 of motors would of itself require an extended paper, and may well 

 form the subject of a special communication. 



Propellers. — The fundamental principles of propellers are the 

 same for air as for water. In both elements the thrust is directly 

 proportional to the mass of fluid set in motion per second. A great 

 variety of types of propellers have been devised, but thus far only 

 the screw propeller has proved to be of practical value in air. The 

 theory of the screw propeller in air is substantially the same as for 

 the deeply submerged screw propeller in water, and therefore does 

 not seem to call for treatment here. There is much need at present 

 for accurate aerodynamic data on the behavior of screw propellers 

 in air, and it is hoped that engineers will soon secure such data and 

 present it in practical form for the use of those interested in airship 

 design. 



Limitations. — Euclid's familiar " square-cube " theorem connect- 

 ing the volumes and surfaces of similar figures, as is well known, 

 operates in favor of increased size of dirigibles and limits the pos- 

 sible size of heavier-than-air machines in single units and with 

 concentrated loads. 



It appears, however, that both fundamental forms of aerial craft 

 will likely be developed, and that the lighter-than-air type will be 

 the burden-bearing machine of the future, whereas the heavier-than- 

 air type will be limited to comparatively low tonnage, operating at 

 relatively high velocity. The helicopter type of machine may be 

 considered as the limit of the aeroplane when, by constantly increas- 

 ing the speed, the area of the supporting surfaces is continuously 

 reduced until it practically disappears. We may then picture a 

 racing aeroplane jiropelled by great power, supported largely by 

 the pressure against its body, and with its wings reduced to mere 

 fins which serve to guide and steady its motion. In other words, 

 starting with the aeroplane type, we have the dirigible balloon on 

 the one hand as the tonnage increases, and the helicopter type on 

 the other extreme as the speed increases. Apparently, therefore, 

 no one of these forms will be exclusively used, but each will have its 

 place for the particular work required. * * * 



