8 AEROPLANE PERFORMANCE CALCULATIONS 



It should be noted in this connection, however, that if one of 

 the wires is vibrating the shielding effect will obviously disappear, 

 or may even be replaced to some extent by something of the 

 nature of interference. Again, the effect of angle of attack in the 

 case of double flying wires will be to bring the back wire partially 

 into view and so reduce shielding. For this reason the majority 

 of designers reject this correction entirely and reckon the resist- 

 ance of each of the two wires at full value, and on the whole this 

 practice is to be recommended. 



Correction for Interference. It may be that two stream- 

 line struts are placed side by side at a small interval. This case 

 is most likely to occur at the junction of a folding wing with the 

 stationary centre section. It is usual to design these two 

 struts in such a way that they form a complete stream-line shape, 

 either by virtue of their own shape or by the shape of their 

 fairings. When this is done the resistance of the combination 

 can of course be obtained at once by treating it as a single stream- 

 line strut. 



When, however, the designer has adopted two separate stream- 

 line struts side by side, further consideration is necessary. It 

 appears likely on the face of it that when the struts are so close 

 that the clear interval between them is not large in comparison 

 to the width of either strut, the air would have less freedom of 

 passage than when the struts are far apart, and consequently we 

 might expect the resistance of the combination to exceed the 

 sum of the resistances of the two struts considered separately. 

 This expectation is confirmed by experiments made at the 

 National Physical Laboratory, from which the upper curve of 

 p. 8 1 has been prepared. This curve gives the correction to be 

 applied for various values of the distance between the centres of 

 the struts, in relation to the width of either. If the struts are of 

 unequal widths, the mean of the two widths should be taken. 



Numerical Values. As has been explained above, the square 

 law can be applied direct from the model test in the case of the 

 blunter shapes, but it should be applied from the full scale 

 resistance at TOO miles per hour in the case of those shapes which 

 have really fine lines. 



The best plan, therefore, is to find the full scale resistances at 

 100 miles per hour for all the objects that contribute to the total 

 body resistance. Then by adding up we obtain a single term 

 for the total full scale body resistance at this speed : from this 

 we can work conveniently on the square law for all other speeds 

 of the machine with which we are concerned. 



