142 AEROPLANE PERFORMANCE CALCULATIONS 



Then fill up the first two columns with the data in hand, 

 work out the other columns, and plot P^,, the total water horse- 

 power on V. Plot P T on V on the same paper. 



If the curves do not cut, the machine will get off in a calm. 

 If they do cut, she will not. The condition for getting off in a 

 wind is easier than that for getting off in a calm, so it need not 

 be considered. 



The point V ls P l is the last point on the P w curve, of course. 



When a Special Model Test is not Available. In this case 

 the float system should have been designed geometrically similar 

 to a known system, tests of which are available, and should be 

 fixed on the machine at the same angle. 



With the same notation as before, let capital letters refer to 

 the new design, italicised capitals to the full scale machine being 

 copied from, and small letters to the model tests of the latter. 



Then the similarity is only correct if, in addition to geometri- 

 cally similar lines and identity of angular setting we have 



Pi 



w 



and = 



If all these conditions are not fulfilled, the designer is work- 

 ing in the dark, and can only rely on his judgment to determine 

 if the machine will come off in a calm. There are cases on 

 record where designers have so relied on their judgment and 

 have been wrong. 



If, however, all the above conditions are satisfied, and we 

 have a table of /and v, we can convert it to a table of F and V 

 by the formulae 



'/ 



and V = A/7" 



while if, instead, we have a table of F and V, we can convert it 

 to a table of F and V by the formulae 



and 



