[Illustration: TRAJECTORY DURING DESCENT]
What happens when the motor stops? The velocity of the machine
gradually decreases: the resistance to forward movement stops its
forward movement and the excess of weight over upward pressure due to
velocity causes it to descend. It behaves like a projectile, but the
details of behavior are seriously complicated by the variation in head
resistance and sustaining force due to changes in the angle of the
planes. The "angle of inclination" is now not the angle made by the
planes with the horizontal, but the angle which they make with the path
of flight. Theory indicates that this should be about two-thirds the
angle which the path itself makes with the horizontal: that is, the
planes themselves are inclined downward toward the front. The forces
which determine the descent are fixed by the velocity and the angle
between the planes and the path of flight. Manipulation of the rudders
and main planes or even the motor may be practised to ensure lancing
to best advantage; but in spite of these (or perhaps on account of
these) scarcely any part of aviation offers more dangers, demands more
genius on the part of the operator, and has been less satisfactorily
analyzed than the question of "getting down." It is easy to stay up
and not very hard to "get up," weather conditions being favorable; but
it is an "all-sufficient job" to _come down_. Under the new rules of
the International Aeronautic Federation, a test flight for a pilot's
license must terminate with a descent (motor stopped) in which the
aviator is to land within fifty yards of the observers and come to
a full stop inside of fifty yards therefrom. The elevation at the
beginning of descent must be at least 150 feet.
[Illustration: DESCENDING]