8G EXPERIMENTS IN AERODYNAMICS 



(b) 5.52 is the number of inches of the Dynamometer- Chronograph barrel 

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

 revolution corresponds to the entire circumference of the barrel, 10.7 inches, and 

 (with the gearing used in this experiment I to 2.000 revolutions of the Dynamomi tt r 



pulley shaft. 



I [.'lice 



_' x 2000 _ 1 ,,..,, 

 10.7 



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 ccrd 1,063 feet per minute. 



(c) The extension of the power pencil P" = 0.6"> inches. From the calibra- 

 tion tables we find 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. 



i Tin' 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 

 puffs of wind. This indicates that tin- propeller is not driving quite fast enough 

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

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

 the requisite velocity were just attained. 



i f) The sp 1 of turn-table multiplied by the end-thrust gives the work 



done per minute by propeller, viz.. .'!7"> 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 lie communicated in general terms. The first result is that the 

 maximum efficiency of a propeller 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 

 essentia] 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 mosl efficient 

 form for one i- 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 



