35 



CHAPTER V. 



AIR PERFORMANCE. 



General. By the air performance of a machine is meant the 

 numerical evaluation of everything that the machine can do when 

 air borne. The subject can be conveniently divided into three 

 parts according to the proportion of the available engine power 

 which is being used. We shall therefore consider the subject of 

 air performance under three main heads, Gliding, Full Power, 

 and Throttled. 



I. GLIDING FLIGHT. 



General. Referring to the four methods of machine per- 

 formance calculation of Chapter IV., we observe that the Third 

 Method and the Fourth Method are not applicable, seeing that 

 we are supposing that the engine has broken down or else has 

 been intentionally switched off, so that there is no propeller slip 

 stream. 



Of the two remaining methods, the Second Method is the 

 accurate one to use, but the First is generally close enough to 

 the mark except in the case of a flying boat. In what follows 

 therefore under the present heading of Gliding Flight, when the 

 machine performance curve is referred to it is to be understood 

 that it is to have been found either by the First Method or the 

 Second Method. 



Landing Speed on Glide. When making a landing without 

 his engine the pilot, of course, pulls his control back at the last 

 moment so that when it actually lands the machine is flying 

 horizontally. In this case, therefore, the Second Method may be 

 taken as accurate (or the First Method as a close approximation) 

 without further investigation. 



For the purpose of finding the landing speed, however, we do 

 not need the whole performance curve but only the one point 

 on it corresponding to X = I -o. 



First Method. Referring to page 22 we find that under the 

 assumptions of the First Method of Chapter IV. 



