The Wright machines (at least in their original form) have usually
been started by the impetus of a falling weight, which propels them
along skids until the velocity suffices to produce ascent. The
preferred designs among French machines have contemplated self-starting
equipment. This involves mounting the machine on pneumatic-tired
bicycle wheels so that it can run along the ground. If a fairly long
stretch of good, wide, straight road is available, it is usually
possible to ascend. The effect of altitude and atmospheric density on
sustaining power is forcibly illustrated by the fact that at Salt Lake
City one of the aviators was unable to rise from the ground.
[Illustration: WRIGHT BIPLANE ON STARTING RAIL, SHOWING PYLON AND
WEIGHT]
To accelerate a machine from rest to a given velocity in a given time
or distance involves the use of propulsive force additional to that
necessary to maintain the velocity attained. Apparently, therefore, any
self-starting machine must have not only the extra weight of framework
and wheels but also extra motor power.
[Illustration: LAUNCHING SYSTEM FOR WRIGHT AEROPLANE
(From Brewer's _Art of Aviation_)]
Upon closer examination of the matter, we may find a particularly
fortunate condition of things in the aeroplane. Both sustaining power
and resistance vary with the inclination of the planes, as indicated
by the chart on page 24. It is entirely possible to start with no such
inclination, so that the direct wind resistance is eliminated. The
motor must then overcome only air friction, in addition to providing
an accelerating force. The machine runs along the ground, its velocity
rapidly increasing. As soon as the necessary speed (or one somewhat
greater) is attained, the planes are tilted and the aeroplane rises
from the ground.
[Illustration: THE NIEUPORT MONOPLANE
Self-Starting with an 18 hp. motor (From _The Air Scout_)]
The velocity necessary to just sustain the load at a given angle of
inclination is called the _critical_ or _soaring_ velocity. For a given
machine, there is an angle of inclination (about half a right angle)
at which the minimum speed is necessary. This speed is called the
"least soaring velocity." If the velocity is now increased, the angle
of inclination may be reduced and the planes will soar through the
air almost edgewise, apparently with diminished resistance and power
consumption. This decrease in power as the speed increases is called
_Langley's Paradox_, from its discoverer, who, however, pointed out
that the rule does not hold in practice when frictional resistances
are included. We cannot expect to actually save power by moving more
rapidly than at present; but we should have to provide much more power
if we tried to move much more slowly.
[Illustration: A BIPLANE
(From _Aircraft_)]
[Illustration: ELY AT LOS ANGELES
(Photo by American Press Association)]
Economical and practicable starting of an aeroplane thus requires a
free launching space, along which the machine may accelerate with
nearly flat planes: a downward slope would be an aid. When the planes
are tilted for ascent, after attaining full speed, quick control is
necessary to avoid the possibility of a back-somersault. A fairly
wide launching platform of 200 feet length would ordinarily suffice.
The flight made by Ely in January of this year, from San Francisco
to the deck of the cruiser _Pennsylvania_ and back, demonstrated the
possibility of starting from a limited area. The wooden platform built
over the after deck of the warship was 130 feet long, and sloped. On
the return trip, the aeroplane ran down this slope, dropped somewhat,
and then ascended successfully.
If the effort is made to ascend at low velocities, then the motor
power must be sufficient to propel the machine at an extreme angle of
inclination--perhaps the third of a right angle, approximating to the
angle of least velocity for a given load. According to Chatley, this
method of starting by Farman at Issy-les-Moulineaux involved the use of
a motor of fifty horse-power: while Roe's machine at Brooklands rose,
it is said, with only a six horse-power motor.