77 



CHAPTER VIII. 



BODY RESISTANCE. 



General. The object of the work is to find the numerical values 

 of R! and R 2 , that is to say, the aggregate resistance in pounds 

 at 100 miles per hour in air of standard density of parts of the 

 machine subject and not subject to the action of the slip stream 

 respectively.* If great accuracy is not sought for, the slip stream 

 effect will be disregarded and the total figure will then be R. 



The procedure is simple and not particularly laborious : it 

 consists in making two lists, one of parts in the slip stream, the 

 other of parts outside it, and noting the necessary areas and 

 other dimensions the need for which will appear in the following 

 pages. Then with the aid of the following pages a figure r, the 

 resistance of the item in pounds at 100 miles per hour, is entered 

 opposite each item of the list and finally these figures are added 

 up and give R x and R 2 . 



It is most advisable to deal with the machine as a whole and 

 not to attempt to work to half the machine and double the result. 



Of course at the present stage the diameter of the propeller 

 is not known, and therefore the propeller circle is not accurately 

 known. Therefore we must either make a reasonable guess at 

 the propeller diameter and leave it at that ; or we can disregard 

 slip stream, find R and then go far enough with the work of the 

 next two chapters to determine a propeller diameter, and finally 

 return and repeat the work of this chapter ; alternatively we can 

 make a reasonable guess at the propeller diameter, find R L and 

 R 2 , proceed with the work of the next two chapters till we find 

 the propeller diameter, and then come back and correct. See 

 also the method used on page 182. 



* Bodies in front of a propeller are to be taken as not subject to slip stream 

 action : further, it is sufficiently accurate to assume that the parts affected by slip 

 stream action are those within the propeller circle in the front-view drawing of the 

 machine; the slip stream of a propeller contracts, but this is offset by the forcible 

 separation due to the presence of the body : also the slip stream is deflected down- 

 wards by the action of the main planes, but in this respect " what is lost on the- 

 swings is gained on the roundabouts ". 



