290 SOME APPLICATIONS OF THE PRINCIPLES OF 
In a biplane of the ordinary type with distance of 6 feet between planes, 
the center of gravity is sometimes as much as 12 inches below the center of 
the buoyancy taken as midway between the center of pressure of the upper 
and the lower planes. This of course gives a transverse metacentric height of 
about 12 inches for such machines. It is dangerous, however, to place the 
center of gravity very low because of the pendulum action tending to capsize 
the aeroplane when its motion is accelerated or retarded, and for that reason 
it is not possible to get satisfactory transverse stability simply by placing 
weights low. 
From the foregoing discussion of the stability of plane surface in motion, 
it appears clear that the stability in the direction of motion is much greater 
than that at the right angles to that direction. 
On account of the greater proportional supporting effect, aeroplanes, 
as you know, are built with curved surfaces. It remains to determine what 
effect this curvature has on the fore and aft stability and what modifications 
in the conclusions as regards the stability of plane surfaces are necessary. 
The variation of the position of the center of pressure with changes in 
the angle of incidence for the curved surface differs markedly from that for 
the plane surface for similar changes in the angle of incidence. The exact 
law for curved surfaces is not definitely known but its general character can 
be indicated. 
In Fig. 1 are shown curves giving the distance of the center of pressure 
from the leading edge of a plane surface and of a curved surface at different 
angles of incidence. 
For the plane it will be seen that the distance increases from about 0.2 
at zero inclination to 0.5 at 90 degrees, while for the curved surface the 
center of pressure is somewhere near the middle at zero inclination, moves for- 
ward as the inclination increases to about 5 degrees, and then backward until 
it isin the same position as fora plane at an inclination of 90 degrees. The 
reason for this difference will be seen by referring to Figs. 2 and 3, Plate 141, 
which represent the side views of curved and plane surfaces respectively. The 
small arrows indicate in general the direction of the forces acting on the two. 
The large arrows indicate the resultant forces in the two cases. At the angle 
of incidence shown the forces act downward near the leading edge of the 
covered surface and upward on the plane. This causes the position of the 
resultant, or the center of pressure of the first to be considerably farther from 
the leading edge than for the second. 
The conclusion to be drawn from the difference shown in Fig. 1, is that 
for a curved surface after the angle of incidence is decreased below a certain 
point, the center of pressure moves backwards instead of moving forward 
a 
