CHAPTER I. 



BODY RESISTANCE. 



General. The total body resistance of a machine can be briefly 

 defined as the force resisting the passage through the air of the 

 complete machine with the exception of the main planes and the 

 propeller. 



This total body resistance is made up of terms due to various 

 parts of the machine, all of which will be investigated in detail 

 presently. Each of these terms is affected in value by the 

 relative speed of travel through the air (generally referred to 

 briefly as the " speed ") and by the direction in which the relative 

 air stream meets that particular part of the machine to which the 

 term in question refers. 



It may be stated at once as a general approximation that the 

 resistance of any part of the machine, other things being equal, 

 is proportional to the area in front view multiplied by the square 

 of the speed. This is known as "the V 2 Law". There are, 

 however, a number of corrections of various degrees of import- 

 ance to be applied to this statement and we will now pass to a 

 consideration of them. 



Correction for Dimension Effect. Extensive and prolonged 

 researches carried out at the National Physical Laboratory have 

 satisfactorily established that as long as we are considering 

 objects of exactly the same shape (though not necessarily of the 

 same size), and as long as the air stream meets these bodies in 

 identical relative directions, the resistance, i.e. the component of 

 the air reaction in the direction of the air stream, can be ex- 

 pressed accurately by the equation 



resistance = K^V 2 , 



where / is a linear dimension of the body, V is the speed and K 

 is a function of the product IV.* 



* Variations in the viscosity of the air are neglected in this statement of the 

 law, as they are unimportant for our purpose. 



