AEROPLANE PERFORMANCE CALCULATIONS 



In this case it is therefore usual to work on the square law 

 direct from the model test, particularly as the error is on the safe 

 side and only affects a moderate proportion of the total body 

 resistance of the machine ; moreover, the example analysed above 

 is rather an extreme case. 



To sum up the influence of Dimension Effect on body re- 

 sistance, we may say that the square law can be used direct from 

 model tests except for bodies coming within Case (2), namely, 

 Fuselages, Flying Boat Hulls, Floats (if they have long fine tails, 

 not blunt sterns), and Engine Nacelles (if they are designed on 

 fine lines) : in the case of these bodies, however, the square law 

 is to be used from the actual full scale resistance at 100 miles 

 per hour, which must first be found. 



Correction for Angle of Attack. Any part of the machine 

 which is subject to air resistance is, of course, designed to have 

 as small a resistance as practicable when it is head on to the 

 wind. Many parts of a machine, however, are not always so 

 placed, but are either placed permanently at an angle to the wind 

 direction or else take up an angle to the wind when the attitude 

 of the machine changes in flight. We will now consider these. 



Fuselages, Hulls, Floats, and Similar Bodies. The designer 

 usually aims as far as possible at getting these placed so that 

 they are head on to the wind when the machine is at top speed 

 (except in the case of commercial machines, when he will of 

 course work to the cruising speed instead), thus he makes their 

 resistance a minimum under the conditions which are most im- 

 portant for practical reasons. When, however, the machine is 

 flying more slowly and is consequently in a " tail down " attitude 

 the resistance of such bodies as these is undoubtedly increased. 

 This increase in resistance is accompanied by the introduction of 

 a lifting force from the air, which tends to compensate for the 

 loss of power due to increased resistance, at least in its influence 

 on the rate of climb attainable. Partly because of this tendency 

 to compensation and partly because the performance calculations 



