PROPELLER PERFORMANCE CURVES 



105 



v. 



Then fill in the V and P R columns by copying from the table 

 of propeller performance in standard density air, and fill in the 

 last two columns by using the formulae 



V = V and P R ' = <rP R . 



The last two columns contain the data for plotting the con- 

 stant revolutions propeller performance curve at the altitude. 

 The revolutions to which it corresponds are the same as the full 

 revolutions for standard density air. 



The above completes the determination of the propeller per- 

 formance curves for the general case where the propeller has not 

 yet been designed. In the particular case, however, where the 

 propeller design has already been carried out, the required data 

 will be supplied by the propeller designer, and the above method 

 need not of course be used. 



Racing Propellers. Reference has already been made to 

 the case where it is desired to have a propeller to give the maxi- 

 mum possible top speed to the machine in standard density air : 

 in this case the best value of V is definitely fixed and can be 

 found as follows : 



Having decided the question of 2-bladed or 4-bladed pro- 

 peller, draw up a table in the following form : 



J. 



Fill up the first column with three or four values of V say 

 differing successively by 10 miles per hour so as to form a 

 range of speed ample to cover any top speed that the machine 

 may develop. Then fill in the second column by using which- 

 ever of Watts' formulae applies. Then fill up the third column 

 by using the definition of J. Then fill up the fourth column by 



