84 EXPERIMENTS IN AERODYNAMICS. 



VI. The investigation of the power that must be expended to furnish this end- 

 thrust, and the determination of the best form and size of propeller for the pur- 

 pose, combines the use of the two instruments. 



In the center of the Recorder is provided a place (see plate VII) for the 

 electric motor already referred to, whose power is transmitted by a belt to the 

 pulley of the Dynamometer-Chronograph, which is mounted on the end of the rigid 

 arms. It may be observed that, in this manner of establishing the motor, the 

 tension of the pulley, however great, in no way interferes with the freedom of 

 motion of the arms of the Recorder — a very essential mechanical condition, and 

 one not otherwise easily attainable. With the various pieces of apparatus thus 

 disposed, and with the propeller to be tested fastened to the shaft of the Dyna- 

 mometer, the whirling table is rotated at any desired speed. The propeller is then 

 driven by the motor with increasing amounts of power until the forward motion 

 of the Recorder arm about its vertical axis indicates that the propeller is driving 

 the Dynamometer ahead at a velocity just exceeding the velocity of the whirling- 

 table. This is the moment at which all the records admit of interpretation. The 

 work that is being done by the propeller is that of overcoming the resistance of 

 the air to the bulk of the Dynamometer, and in place of this we may substitute, 

 in thought, the resistance that would be caused bv an aerodrome of such a size 



CD ' */ 



as to produce the same effect. The power put out and the resistance to advance 

 are both registered on the cylinder of the Dynamometer. The result realized is 

 found by multiplying the static pressure indicated by the pencil which registers 

 the end-thrust by the velocity of the turn-table at the moment when the pro- 

 peller's independently acquired velocity is just about to exceed it. The static 

 pressure represents the resistance overcome, and the velocity of advance gives 

 the distance through which it is overcome per unit of time. The product there- 

 fore represents the effective work done per unit of time. If the adopted velocity 

 of the whirling-table be the soaring velocity of an aerodrome which would have 

 the actually observed resistance, the experiment will virtually be made under all 

 the conditions of actual horizontal flight. In practice, the experiments were 

 made at a series of velocities, and the results obtained — power expended and 

 useful work done — can be interpolated for any desired speed. 



Preliminary experiments were made with wooden propellers having four, 

 six, and eight blades set at different angles with the axis. Lastly, two aluminum 

 propellers were used having only two blades each, extending 24 and 30 inches, 

 respects ely, from tip to tip. 



In order that the reader may follow the method of experiment in detail, the 

 following description of experiments made November 4, 1890, is here given, 

 together with abstracts from the original record of observations for that date: 



