i6 



CHAPTER III. 



PROPELLER PERFORMANCE CURVES. 



General. The work of Harold Bolas* has placed in the hands 

 of the machine designer a ready means of estimating the per- 

 formance of a propeller designed to meet given conditions. 

 The curves of pages 100 and 101 are obtained from his formulae. 

 They are used as indicated in Chapter X., page 102, and give 

 the propeller performance curves, i.e. the plottings of P T ,'the 

 output horse-power at full torque, and P R , the output power at 

 full revolutions, on a base of V, the speed of the machine, for 

 standard density air with very little trouble. 



Before using Bolas' curves, however, it is necessary to deter- 

 mine a suitable diameter for the propeller. This is readily done 

 with the aid of formulae due to H. C. Watts,f which will be found 

 in Chapter X., page 99. 



It remains for us to see how a propeller, already determined 

 for conditions of standard density air, will behave at an altitude. 



Engine Power at an Altitude. Definitions .- 



I is the indicated horse-power. 



H is the brake horse-power. 



F is the frictional horse-power lost in the engine. 



P T is the effective horse-power at full torque. 



P R is the effective horse-power at full revolutions. 



Q is the indicated torque. 



N is the revolutions per minute of the engine (whereas n is 

 used for the revolutions per minute of the propeller which are 

 proportional to those of the engine but not necessarily equal to 

 them). 



T is the propeller thrust in pounds. 



V is the machine speed in miles per hour. 



All the above symbols refer to standard density air. Cor- 

 responding symbols with dashes refer to an altitude where the 

 relative air density is a. Corresponding symbols in italic 



* See C.I.M. No. 704 issued by the Air Board. 



f See " The Design of Screw Propellers for Aircraft," published by Longmans, 

 Green & Co. 



