If the plane can be so formed as to make a large
and effective area it will add greatly to the value
of the sustaining surface.

Unfortunately, the long fiat plane does not lend
any aid in this particular, as the stream line flows
down along the top, as shown in Fig. 23, without
being of any service.

_Fig. 23. Air lines on the upper side of a Plane._

THE CONCAVED PLANE.--These considerations
led to the adoption of the concaved plane formation,
and for purposes of comparison the diagram,
Fig. 24, shows the plane B of the same length and
angle as the straight planes.

In examining the successive stream lines it will
be found that while the 1st, 2d and 3d lines have
a little less angle of impact than the corresponding
lines in the straight plane, the last lines, 5, 6
and 7, have much greater angles, so that only line
4 strikes the plane at the same angle.

Such a plane structure would, therefore, have
its center of pressure somewhere between the
lines 3 and 4, and the lift being thus, practically,
uniform over the surface, would be more effective.

THE CENTER OF PRESSURE.--This is a term used