BODY RESISTANCE n 



and R 2 the part corresponding to bodies which are clear of it* 

 Let V be the velocity of the machine in miles per hour and let 

 bV be the velocity in miles per hour added to the air by the 

 propeller, 'so that the relative air velocity in the slip stream is 

 (l + )V miles per hour. 



Then when the machine is flying under power in air of 

 standard density 



/ V \ 

 total body resistance = [R x (i + frf + R 2 ](^ - ) 



2 

 while, if the machine is gliding, so that b = o, we get 



total body resistance = [R x + 



Total Body Resistance at an Altitude. At an altitude the 

 density of the air is less than that at ground level. The ratio of 

 the density of the air at an altitude to the standard air density 

 (which differs slightly from that at ground level) is denoted by 

 a and the variation of a with altitude is shown in the curve 

 plotted on page 104. 



This reduction of density has an effect on the air resistance 

 of any body which is such that the resistance is reduced directly 

 in the ratio of the densities. 



We therefore have for a machine flying under power at an 

 altitude 



/ V 

 total body resistance 



and for a machine gliding at an altitude 

 total body resistance = o"R(- 



v y. 



IOO/ 



Line of Action of Body Resistance The height of the line 

 of action of the body resistance is required in order to enable 

 certain refinements to be -included in the machine performance 

 calculation if necessary. This line of action is the line of action 

 of the resultant of the numerous parallel forces which together 

 make up the body resistance. The ordinary method of taking 

 moments which is used for finding the resultant of a set of 

 parallel forces is therefore applicable. 



* Bodies in front of a propeller are approximately clear of the slip stream. 



