107 



CHAPTER XL 



MACHINE PERFORMANCE CURVE. 



General. The machine performance curve is the plotting of P 

 on a base of V for standard density air, or of P' on a base of V 

 when flight at an altitude is under consideration. 



The Machine Performance Curve in Standard Density Air. 

 One of the four following methods should be used. 



The First Method is for approximate work only. 



The Second Method should not generally be used. 



The Third Method is for accurate work, except on flying boats. 



The Fourth Method is for accurate work on flying boats. 



There is also a very useful Variant of the Third Method 

 which is recommended for use in the majority of cases. 



First Method.- 



W is the total weight of the machine in pounds. 



S is the total area of the wings in square feet 



R is the total body resistance in pounds at 100 miles per 

 hour, and has been found by the method of Chapter VIII., page 

 77. If, however, the work of Chapter VIII. has been done in 

 the form of finding R : and R 2 instead, then R = R x + R 2 . 



k^max an d L/D are corrected values for values of X from 'I to 

 I'O and have already been found by the method of Chapter IX., 

 page 85, where they will be found in the last column of the first 

 table. 



V is the speed of the machine in miles per hour in standard 

 density air. 



P is the effective horse-power required to maintain horizontal 

 flight at V miles per hour in standard density air. 



Now write down the numerical values of W, S, k imax , and R. 



Then work out and write down the numerical values of 



a - and ft - 



10* 



Then construct a table in the following form : 



