io AEROPLANE PERFORMANCE CALCULATIONS 



have their frontal areas measured and should be treated as 

 struts. 



Wheels. Figures for the resistance of a wheel with different 

 types of fairing are given on page 82, and in this case it is con- 

 venient to take the resistance as proportional to the product of 

 the diameter and the tyre diameter. 



Radiators. When the machine is flying at top speed the 

 radiator shutters are usually nearly closed if not quite shut. For 

 this reason the figure given on page 83 for a radiator mounted 

 outside the fuselage is the flat plate figure, while a compromise 

 figure is given for the case of a nose radiator. When a retract- 

 able radiator is used, it will only have about half its area exposed 

 at top speed, hence a lower figure (if reckoned on the total 

 radiator area) is given for this case on page 83 



Flat Plates. A number of minor parts of a machine ap- 

 proximate to being flat plates normal to the wind : for instance, 

 wind screens, tail skids and parts of fittings. The frontal area 

 of these should be added up throughout the machine and multi- 

 plied by the coefficient given on page 83. . 



Circular Cylinders. Under this heading are included pro- 

 jecting heads of engine cylinders, any unfaired wheel axles, etc. 

 The frontal area should be added up for the whole machine and 

 the coefficient given on page 83 used. 



Miscellaneous. Any parts of the machine subject to air re- 

 sistance which remain to be dealt with must be carefully con- 

 sidered and estimated on the basis of their frontal area. Most 

 designers prefer to estimate these at the "flat plate" figure, 

 perhaps taking a somewhat reduced area in dealing with such 

 items as look to be not very bad from the resistance point of view. 



A word may be put in here about wire attachments. These 

 are so numerous that it is best not to deal with them under this 

 head, and for that reason the fork ends of stream-line wires and 

 the splices and wire strainers used on cables are allowed for by 

 adding a constant to the length of each wire, as is explained on 

 p. 82. 



Total Body Resistance. When all the items of the body 

 resistance have been found as described above, they must be 

 added, and the sum is R, the total body resistance of the 

 machine at 100 miles per hour. It is convenient, however, to 

 note the parts of R corresponding to bodies in and not in the 

 propeller slip stream. 



Let Rj be the part corresponding to bodies in the slip stream 



