NAVAL ARCHITECTURE TO AERONAUTICS. 291 
and the surface becomes unstable. The forces acting then tend further to 
reduce the angle of incidence until the supporting forces become nil. If, 
however, the angle of incidence is increased above the critical point, the action 
and stability for the curved surfaces is similar to that for a plane one. It 
therefore seems that the fore and aft stability of the ordinary aeroplane is 
less than it would be if its supporting surfaces were plane. Itis probable that 
in some of the cases where aviators making vol-planes at sharp angles of 
descent, have lost control of their machines and been unable to right them, 
are explainable by the fact that the angle of incidence has gotten so far 
below the critical angle that the horizontal rudder was insufficient to restore 
the equilibrium. | ; 
Transverse stability for the curved surface does not differ from that for 
the plane. 
At the present transverse stability of aeroplanes depends almost entirely 
on the skill of the aviator in flexing the wings or ailerons which are in effect 
horizontal side rudders. 
If automatic means can be provided whereby the flexing of the wings 
can be made proportional to and simultaneous with the transverse inclination 
of the aeroplane, the resulting metacentric height can be made relatively 
very large and satisfactory stability will be insured. 
It may be of interest to note here that in the Wright aeroplane, some 
use is made of the principle of distributing weights transversely so as to 
increase the natural period of rolling as the motor is some distance on one 
side of center line and the operator and passenger a corresponding distance 
on the other side. 
PROPULSION. 
Given a reliable motor not excessive in weight the question of propulsion 
reduces practically to the selection of a propeller of proper form and dimen- 
sions for the necessary thrust at the estimated speed. Examination of the 
literature on the subject published either in books or in aeronautical journals 
discloses wide diversity of opinions and theories as to the action of air 
propellers and the different elements affecting their efficiencies. There is 
also a great dearth of reliable data taken from actual trials of model or full 
size propellers in motion. The diversity of theories can readily be understood 
by those who have struggled with similar questions as regards the water pro- 
peller, and Marine Engineers will scarcely be surprised to learn that the 
efficiencies obtained by the propellers in use on some of the best known types 
of aeroplanes, vary from somewhere below 50 per cent. to the vicinity of 
70 per cent. ‘ 
