72 AEROPLANE PERFORMANCE CALCULATIONS 



This is Froude's law of similarity, on which all model testing 

 of hulls and floats for flying boats and seaplanes depends. 



When a model is tested specially for a design of seaplane, 

 the model is made to a convenient scale and the relation of W w 

 to V is worked out from the consideration that W w plus W a 

 (the air-borne weight) equals W (the total weight) while 



where V l is the minimum flying speed of the machine at the 

 angle of the wings corresponding to the angle at which the float 

 is to be run. 



The model is then run at a series of speeds from quite slow 

 ones up to z//, loaded at each speed to the value of w w which 

 satisfies the above, and then /is measured. 



We have, then, a series of values of /for a series of values 

 of z;. 



L 2 V 2 

 Then F = -^- ^/ from (i) 



L' 2 L 

 = p // from (2) 



. (F) 



and V = v (V) 



which is all that we require from the model test to enable us to 

 calculate whether the machine will get off in still air. 



If the model is made specially for the machine in question, 

 the designer will probably be given F plotted on V instead of 

 f plotted on v, and therefore he will not have to do any con- 

 verting. 



If, on the other hand, he works as is more usual by copy- 

 ing the underwater lines of some float system of which tests are 

 available, then he must convert the data available to his own 

 case by means of equations (F) and (V), being careful to see 

 that the conditions on page 71 are all satisfied. This method 

 can be used equally to convert from model figures to the machine 

 in question or from any available set of full scale figures to the 



