PROPELLER PERFORMANCE CURVES 99 



Propeller Diameter. If the propeller has already been 

 designed the diameter is of course known : otherwise the best 

 diameter for an ordinary case is given by the following formulae 

 due to H. C. Watts : 



For 2-bladed propellers, 



4/ H~~ 



d = 1 0,000 J 



and for 4-bladed propellers, 



4 / H~~ 



d = 10,000 J : . 



\ui* *V fl 



The designer is here confronted with the necessity of deciding 

 whether to use a 2-bladed or a 4-bladed propeller: this is a 

 problem of aeroplane design, not of aeroplane performance cal- 

 culation, and, therefore, we will merely here point out the 

 obvious advantages in reduced height of chassis, increased 

 efficiency and steadier running enjoyed by the 4-bladed type as 

 an offset to its greater cost and its inconvenience for transport 

 the weights being approximately equal in the two cases. 



The designer must also now decide what value of V he will 

 use. This again is a design problem, but it is interlocked with 

 the performance of the machine, and hence in a sense with the 

 performance calculation. It may, therefore, be opportune to 

 make some remarks on the choice of V . 



For a racing machine the end to be aimed at is to make V 

 close to the top speed at ground level as yet not known. 

 On the other hand, for a weight carrying machine in which top 

 speed is not important, the aim would be to make V approxi- 

 mate more to a speed of good cruising economy. Again, for a 

 machine designed to meet a definite circumscribed specification, 

 if the conditions laid down tend to produce a machine defective 

 in climb, but on the safe side for top speed, a low value of V is 

 suitable, and vice versa. 



Probably the best thing to do will always be to make a shot 

 at the best value of V , and then, at a later stage, go back and 

 try a larger or smaller value to see if that will improve the 

 machine for its intended purpose: it will be found, by the way, 

 that an alteration of V by 10 miles per hour does not make a 

 lot of difference to the performance. 



In brief, the designer is advised to find the best value of V 

 by a system of intelligent trial and error. 



The above decisions having been made, d can be found from 

 the appropriate formula, and noted. 



