212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
Flying machines of this latter type should have their auxilary sur- 
faces located as far as possible from the main bearing planes, because 
the greater the distance the greater is the leverage and consequently 
the smaller the amount of surface required. The auxiliary surfaces 
are usually placed either in front or in the rear of the main supporting 
surfaces, since they act with greater efficiency in these positions than 
when placed above or below. 
With a view to high efficiency, no part of either the main surfaces 
or the auxiliary surfaces should be exposed on their upper sides in a 
way to create downward pressures. One pound of air pressure exerted 
downward costs as much in propelling power as 2 pounds of downward 
pressure produced by actual weight carried. This is due to the fact 
that the total pressure on an aeroplane is not vertical, but approxi- 
mately normal to the plane of the surface. This pressure may be re- 
solved into two forces, one acting in a line parallel with the direction 
of travel, and the other at right angles to the line of travel. One is 
termed “lift”? and the other “drift.” With a given aeroplane surface, 
the drift and lift for any given angle of incidence always bear a definite 
ratio to one another. This ratio varies from 1 to 12, to 1 to 1, accord- 
ing to the angle of incidence and the shape of the surface. On an 
average it is about 1 to 6, so that the thrust. required of the propeller 
in the ordinary flying machine is approximately one-sixth of the 
weight carried. When traveling on a horizontal course the lift is 
sential and is exactly equal to the total weight of the machine and 
load. This load may be real weight, or it may be partly real weight 
and partly downward pressures exerted on parts of the surfaces. For 
every pound of weight carried, a thrust of approximately one-sixth 
pound is required. If, however, instead of real weight a downward 
air pressure is exerted on some part of the machine, this downward 
pressure must be overcome by an equal upward pressure on some 
other part of the machine to prevent the machine from descending. 
In this case the horizontal component of the one pound downward 
pressure will be about one-sixth pound, and the horizontal component 
of the compensating upward pressure also will be about one-sixth 
pound, making a total of one-third pound required in thrust from the 
propellers, as compared with one-sixth pound thrust required by one 
pound actual weight carried. It is, therefore, evident that the use of 
downward air pressures in maintaining equilibrium is exceedingly 
wasteful, and, as far as possible, should be avoided. In other words, 
when the equilibrium of an aeroplane has been disturbed, instead of 
using a downward air pressure to depress the elevated side an upward 
pressure should be utilized to elevate the low side. ‘The cost in power 
is twice as great in one case as in the other. 
The dynamically less efficient system of downward air pressures 
is used to some extent, however, on account of its adaptability in 
