The air pressure, direct and frictional resistances, and power depend
upon the _relative_ velocity of flying machine and air. It is this
relative velocity, not the velocity of the balloon as compared with a
point on the earth's surface, that marks the limit of progression.
Hence the speed of the wind is an overwhelming factor to be reckoned
with in developing an aerial time table. If we wish to travel east at
an effective speed of thirty miles per hour, while the wind is blowing
due west at a speed of ten miles, our machine must have an independent
speed of forty miles. On the other hand, if we wish to travel west, an
independent speed of twenty miles per hour will answer.
[Illustration: THE SANTOS-DUMONT DIRIGIBLE NO. 2 (1909)]
Again, if the wind is blowing north at thirty miles per hour, and the
minimum (relative) velocity at which an aeroplane will sustain its load
is forty miles per hour, we cannot progress northward any more slowly
than at seventy miles' speed. And we have this peculiar condition of
things: suppose the wind to be blowing north at fifty miles per hour.
The aeroplane designed for a forty mile speed may then face this wind
and sustain itself while actually moving backward at an absolute speed
(as seen from the earth) of ten miles per hour.
We are at the mercy of the wind, and wind velocities may reach a
hundred miles an hour. The inherent disadvantage of aerial flight is
in what engineers call its "low load factor." That is, the ratio of
normal performance required to possible abnormal performance necessary
under adverse conditions is extremely low. To make a balloon truly
dirigible throughout the year involves, at Paris, for example, as
we have seen, a speed exceeding fifty-four miles per hour: and even
then, during one-tenth the year, the _effective_ speed would not
exceed twenty miles per hour. A time table which required a schedule
speed reduction of 60% on one day out of ten would be obviously
unsatisfactory.
[Illustration: IN THE BAY OF MONACO SANTOS-DUMONT'S NO. 6
The flights terminated with a fall into the sea,
happily without injury to the operator]
Further, if we aim at excessively high independent speeds for our
dirigible balloons, in order to become independent of wind conditions,
we soon reach velocities at which the gas bag is unnecessary: that
is, a simple wing surface would at those speeds give ample support.
The increased difficulty of maintaining rigidity of the envelope, and
of steering, at the great pressures which would accompany these high
velocities would also operate against the dirigible type.
With the aeroplane, higher speed means less sail area for a given
weight and a stronger machine. Much higher speeds are probable. We have
already a safe margin as to weight per horse-power of motor, and many
aeroplane motors are for stanchness purposely made heavier than they
absolutely need to be.