io6 AEROPLANE PERFORMANCE CALCULATIONS 



referring to the curves on page 100, making use only of the 

 curve marked I -o. Finally, fill in the last column by using the 

 formula P T = H?7 T . 



The first and last columns suffice to plot P T on a velocity 

 base : if this curve is plotted (at a later stage) on the machine 

 performance curve and the speed at which the curves cut is noted, 

 this speed is the best value of V for the racing machine, but this 

 curve of P T is not the propeller performance curve. 



If it is desired to get the greatest possible top speed at an 

 altitude instead of in standard density air, the above procedure 

 is not applicable, and it is necessary to resort to trial and error, 

 as will appear later. 



Multi-Propeller Machines. When each propeller is direct 

 driven by a single separate engine, first treat each power unit 

 separately and then add up their separate contributions to P T , P R , 

 P T ', and P R '. 



When a propeller is driven by more than one engine or an 

 engine drives more than one propeller, no difficulty will be found 

 in modifying the above processes to meet the case. Note, how- 

 ever, that the mechanical losses in the gearing will modify the 

 values of H,/>, and q: this applies also to a geared propeller in 

 the case where part or the whole of the gearing is specially 

 designed for the machine, and has not therefore been included in 

 bench tests at the engine works : in considering this case refer- 

 ence should be made to Chapter III., page 16. 



Tandem Propellers. The case of tandem propellers is too 

 complex to admit of treatment on the lines of the rest of this 

 book. If such a case occurs, reference should be made to "The 

 Design of Screw Propellers for Aircraft," by H. C Watts.* The 

 subject is treated fully in Chapter XII. of that work. 



Examples. For numerical examples illustrating the methods 

 of this chapter, see Chapter XVII., page 153, and Chapter XXII. , 

 pages 1 88, 195, and 196. 



Theory. For theoretical explanations see Chapter III., 

 page 1 6. 



* Published by Messrs. Longmans, Green & Co. 



