294 SOME APPLICATIONS OF THE PRINCIPLES OF 
present form and of the same pitch ratio could hardly be expected to give a 
much better efficiency. 
Inspection of equation (6) indicates that for a given thrust and ereed 
the larger the diameter the better for efficiency, and this would be true but 
for friction and similar losses. If the propeller is made small the slip loss is 
large and if made large the friction loss is excessive. The best diameter is a 
compromise between the two extremes. 
Vv 
In the paper referred to above the writer proposed the formula a-f 
where T is the thrust in pounds and V the speed in knots as giving about 
the best diameter for a marine propeller. If allowance is made for the differ- 
ence in the densities of air and water and V is changed to miles per hour the 
corresponding formula for the air propeller is 
33 VT 
jew Se 
V 
For a thrust of 100 pounds and at a speed of 40 miles per hour this gives, 
q=23 V 100 
40 
= 84 feet. 
This result agrees remarkably well with what the Wrights use for this thrust 
and speed. They use an 83-foot propeller under these conditions. The pitch 
of their propeller is about 1.3 d, and in order to use this pitch with a fast 
running motor it is interesting to note that they make use of a reduction gear, 
the principle being similar to that of reduction gears which are now beginning 
to be used for marine propulsion. 
For the best type of marine propellers the actual efficiency obtained is 
about 80 per cent of that indicated as the limit by theory. If equation (6) 
is multiplied by 0.80 we have 
32 
e=—__ 2 
34+ ts (7) 
This indicates what should be expected from a good air propeller provided 
the experience with marine propeller can be extended to the latter. —The curve 
corresponding to (7) is shown in Fig. 4, Plate 142. 
A full size two-bladed air propeller tested at Vickers’ on a whirling table 
gave the following results :— 
