In Fig. 11a three positions of a monoplane are
shown, each in horizontal flight. Let us say that
the first figure A is going at 40 miles per hour,
the second, B, at 50, and the third, C, at 60 miles.
The body in A is nearly horizontal, the angle of
the plane D being such that, with the tail E also
horizontal, an even flight is maintained.

When the speed increases to 50 miles an hour,
the angle of incidence in the plane D must be
decreased, so that the rear end of the frame must
be raised, which is done by giving the tail an angle
of incidence, otherwise, as the upper side of the
tail should meet the air it would drive the rear
end of the frame down, and thus defeat the attempt
to elevate that part.

_Fig. 12. Angles of Flight._

As the speed increases ten miles more, the tail
is swung down still further and the rear end of
the frame is now actually above the plane of flight.
In order, now, to change the angle of flight, without
altering the speed of the machine, the tail is
used to effect the control.

Examine the first diagram in Fig. 12. This
shows the tail E still further depressed, and the
air striking its lower side, causes an upward movement
of the frame at that end, which so much decreases