MACHINE PERFORMANCE CURVE 31 



Method The extension of this method to the case of multi- 

 propeller machines presents no special difficulty. 



Comparison of the Above Four Methods. Examples (i) to 

 (4) inclusive in Chapter XV1IL, pages 155 to 161 inclusive, show 

 the different numerical results obtained on the same machine by 

 following the four different methods of calculation. 



A comparison of the four curves on page 157 shows that there 

 is little difference between the First Method and the Second 

 Method : this being so it is advisable not to use the Second 

 Method in ordinary work, as the slightly quicker First Method 

 gives practically identical results. It must be remembered, how- 

 ever, that if the centre of gravity of the machine is unusually 

 placed, the difference between the methods is greater : this applies 

 particularly to flying boats having a negative tail in the slip 

 stream and the e.g. far forward as a means of preventing a sudden 

 stall on engine failure. 



Comparing now the Third Method with the Fourth Method, 

 we again find little to choose, so that it is usually preferable to 

 use the quicker Third Method : this, however, again may not be 

 advisable in the case of flying boats, since the unusually high 

 centre of propeller thrust which is inseparable from this type 

 causes the difference between the two methods to be greater than 

 in the examples under consideration. 



For ordinary work, therefore, the choice lies between the First 

 Method and the Third Method. Here we see a wide departure, so 

 that the selection of the approximate First Method instead of the 

 practically accurate Third Method, in the case under consideration, 

 though it would not cause much error on the climb, would result 

 in the top speed being over-estimated by about 2 miles per hour 

 and in the landing speed under power being over-estimated by 

 about 4 miles per hour the landing speed with the engine shut off 

 (which is usually taken as the specified landing speed) must, of 

 course, be taken from curve (i or 2) in any case. 



This being so it is advisable to use the First Method always 

 when doing rough or first approximation work, the Second Method 

 practically never, the Third Method always for accurate work 

 except on flying boats, and the Fourth Method always for accurate 

 work on flying boats. 



Inclination of the Propeller Shaft. The assumption 

 hitherto made that the propeller shaft is horizontal at all speeds 

 of flight is not of course true. Consequently it is not really 

 correct to assume, as has been done, that there is no vertical 



