BODY RESISTANCE 9 



Large Bodies. In order to avoid the tedious process of 

 finding the full scale resistance at 100 miles per hour from the 

 nearest available model test in each case, formulae based on 

 average cases are given on page 78, and will be found to give 

 sufficiently close approximation in practice. In working out 

 the numerical values the speed has been taken as 100 miles per 

 hour and a reasonable average size has been chosen. 



It will be noted that cockpits and wind screens are not 

 included in these formulae. It is best to consider the resistances 

 of these independently of that of the body proper. In finding 

 the resistance of a fuselage or other body therefore, the shape to 

 take is the shape the body would have if no wind screens had been 

 fitted and no cockpits cut in it. 



Cockpits. Owing presumably to the swirling of air round 

 the edges, a cockpit has a very high resistance. For this reason 

 it is deemed best to estimate its resistance separately from the 

 fuselage in which it is cut. A formula for cockpit resistance is 

 given on page 82. In the case of tandem cockpits it is very 

 doubtful if any effective shielding takes place, and it is therefore 

 advisable to take all cockpits at full value for resistance. When 

 a cockpit is fitted with a wind screen, the resistance of the latter 

 is, of course, an addition. Cockpits which are entirely enclosed 

 do not, of course, come under this heading, but have to be treated 

 as modifications of the fuselage shape. 



Tail Surfaces. The rudder and fin present no difficulty 

 as they can simply be estimated as proportional to their area. 

 A numerical figure is given on page 78. The tail plane, 

 however, is often so set that even at top speed the elevators are 

 at an angle to the fixed portion of the tail plane : moreover, the 

 relative air stream meeting the tail is seldom inclined downwards 

 at just the angle which will give zero angle of incidence to the 

 tail plane. For these reasons the numerical figure given on page 

 78 is higher than that given for the rudder and fin. Again the 

 resistance is proportional to the area. 



Struts, Stream-line Wires, and Cables. In each case the 

 resistance is proportional to the frontal area, which is most easily 

 computed by measurements of length on a front view drawing of 

 the machine coupled with reference to a schedule for the dia- 

 meters. Numerical values are given on pages 80 and 82. A 

 number of strut sections are given in order that a numerical 

 value may be chosen which corresponds to a strut section closely 

 approximating to the one used on the machine. King-levers and 

 other such small parts built to stream-line strut section should 



