BODY RESISTANCE 7 



would otherwise assume an unmanageable complexity, it is 

 customary to neglect the angle of attack effect in the case of 

 these bodies. 



Horizontal and Inclined Stream-lined Bodies such as Faired 

 Axles and Stream-line Wires. The same can be said of these as 

 of those considered above : no correction for angle of attack is 

 attempted. 



Incidence Wires. The obliquity in this case is allowed for 

 simply by measuring their length on a front view drawing of the 

 machine instead of taking their true length from a side view 

 drawing. 



Control Cables which are not Approximately Transverse to the 

 Wind. Cases of this occur in the control cables from the fuselage 

 to the king-levers on the tail unit, in the cables from the front 

 spar to the aileron king-levers, and in the cables from all these 

 king-levers back on to the control surfaces themselves. Again, 

 the lengths should be scaled from the front view drawing of the 

 machine. 



To sum up the influence of angle of attack or obliquity, it is 

 not customary to bother about it at all except that certain 

 wire lengths are scaled from the front view drawing. It must 

 be admitted that this attitude is adopted not only because 

 such obliquity has no great bearing on the performance of the 

 machine but also because the performance calculations would 

 become impossibly long if all these minor points were punctiliously 

 dealt with. 



Correction for Shielding. When two similar objects are 

 placed exactly one behind the other, the front one obviously 

 disturbs the wind flow in such a way that the back one is sub- 

 jected to a wind of less effective velocity. Therefore we might 

 expect to find that the total resistance of the combination of two 

 objects so placed is less than double the resistance of either con- 

 sidered separately. This expectation is confirmed by experi- 

 ments made at the National Physical Laboratory, from which the 

 corrections plotted in the lower curve, on page 81, have been 

 deduced. These curves give the corrections to be applied in the 

 case of two cables or two stream-line wires placed one behind 

 the other, according to the fore and aft distance between centres 

 in terms of the fore and aft dimension of either wire. These 

 corrections are applicable to the case of double flying wires but 

 not to the case of front and rear gap struts, since these are too 

 widely separated for shielding to be effective. 



