80 



EXPERIMENTS IN AERODYNAMICS. 



The end-thrust spring was calibrated by suspension of weights in a similar 

 way. The following calibration was obtained from experiments made March 8, 

 1888: 



Calibration of End-Thrml Spring. 



The method of computing the horse-power expended, and the return in end- 

 thrust obtained, may now be illustrated in the reduction of the following observa- 

 tions taken without change from the original notes : 



October 30, 1888. 



Six-bladed propeller, with l_)lades set at 45° with axis. Dynamometer driven by belt from a 

 small dynamo. Belt driving 2.1 inch pulley. Dynamometer geared so as to give one revolution 

 of cylinder for 2,000 revolutions of pulley. Time of one revolution of cylinder, 295 seconds. 

 Departure of pencil of clock-spring (set-screw in " " hole), 1.43 inches. 



Driving pulley makes — ^ t revolutions per minute. Circumference of 



2'J5 



pulley equals 



feet. Velocity of belt equals 



feet 



per minute. From calibration of March 8, 1888, an extension or departure of 

 1.43 inches of the pencil of the clock-spring, with the set-screw in "0" hole, 

 is equivalent to a weight of 1.35 pounds on a 3.9-inch pulley. The tension on 



the present 2.1-inch driving pulley is thei'efore 1.35 x |^ pounds. Multiplying 



tension of belt by velocity of belt and dividing by 33,000, we have the work 

 expended per minute expressed in horse-power, viz : 



60 X 20 00 „.._ 135x3.9 ^„,^ 1.35 ^^,^ 

 295 X 12 ^ ^-1^16 ^ -^3000- = 3-713 X 395 =0.017. 



It will be noticed that in this expression the factor 2.1 lias dropped out, and the 

 only variables are the time of one revolution of cylinder and the tension on the 

 spiral spring taken from the calibration curve. If the former be represented by 

 a and the latter by b, and the gearing remain unchanged, the horse-power in any 



experiment will be given bv the formula 3.713 x -• 

 ° ■ a 



