86 EXPERIMENTS IN AERODYNAMICS. 



(5) 5.52 is the number of inches of the Di/namometer-Chronograph barrel 



revolved in a minute, as determined by measuring the time trace. An entire 



revolution corresponds to the entire cii'cumference of the ban'el, 10.7 inches, and 



(with the gearing used in this exj^eriment) to 2,000 revolutions of the Dynamometer 



pulley shaft. 



Hence 



5.52 X 2000 



10.7 



= 1,032 



is the number of revolutions of the Dynamometer pulley per minute at the time 

 of this experiment. The effective diameter of the pulley being 4 inches, this 

 gives for the velocity of the crrd 1,063 feet per minute. 



(c) The extension of the power i^encil P" = 0.65 inches. From the calibra- 

 tion tables we iind that this corresponds to a tension of 0.67 pounds on the pulley 

 cord. The product of this tension by the pulley speed gives the power put out, 

 viz., 712 foot-pounds per minute. 



{d) The extension of the end-thrust trace, 0.20 inch, corresponds to a 

 pressure of 20 pound. 



{e) The horizontal spring has no appreciable extension, except as caused by 

 puifs of wind. This indicates that the propeller is not di'iving quite fast enough 

 to equal or exceed the velocity of the turn-table; but the deficiency of velocity is 

 so small that we shall not discard the experiment, but compute the record as if 

 the requisite velocity were just attained. 



(/) The speed of turn-table multiplied by the end-thrust gives the work 

 done per minute by propeller, viz., 373 foot-pounds per minute. 



We have, then, as a result of the experiment, that the ratio of work done 

 by the propeller to the power put out is 52 per cent., the form of the propeller 

 blades not being a very good one. 



The whole series of experiments is not given here in detail, but their prin- 

 cipal results will be communicated in general terms. The first result is that the 

 maximum efficiency of a j^ropeller in air, as well as in water, is obtained with a 

 small number of blades. A propeller with two blades gave nearly or quite as 

 good results as one with a greater number. This is strikingly different from the 

 form of the most efficient wind-mill, and it may be well to call attention to the 

 essential difference in the character of the two instruments, and to the fact that 

 the wind-mill and the movable propeller are not reversible engines, as they might 

 at first sight seem to be. It is the stationary propeller — i. e., the fan-blower — 

 which is in reality the reversed wind-mill ; and of these two, the most efficient 

 form for one is essentially the most efficient form for the other. The efficiency 

 of a fan-blower of given radius is expressed in terms of the quantity of air 

 delivered in a unit of time for one unit of power put out ; that of the wind-mill 



