One of the first difficulties which the novice will encounter is the
uncertainty of the wind currents. With a low velocity the wind, some
distance away from the ground, is ordinarily steady. As the velocity
increases, however, the wind generally becomes gusty and fitful in its
action. This, it should be remembered, does not refer to the velocity of
the machine, but to that of the air itself.
In this connection Mr. Arthur T. Atherholt, president of the Aero
Club of Pennsylvania, in addressing the Boston Society of Scientific
Research, said:
"Probably the whirlpools of Niagara contain no more erratic currents
than the strata of air which is now immediately above us, a fact hard to
realize on account of its invisibility."
Changes In Wind Currents.
While Mr. Atherholt's experience has been mainly with balloons it is all
the more valuable on this account, as the balloons were at the mercy of
the wind and their varying directions afforded an indisputable guide as
to the changing course of the air currents. In speaking of this he said:
"In the many trips taken, varying in distance traversed from twenty-five
to 900 miles, it was never possible except in one instance to maintain
a straight course. These uncertain currents were most noticeable in
the Gordon-Bennett race from St. Louis in 1907. Of the nine aerostats
competing in that event, eight covered a more or less direct course
due east and southeast, whereas the writer, with Major Henry B. Hersey,
first started northwest, then north, northeast, east, east by south,
and when over the center of Lake Erie were again blown northwest
notwithstanding that more favorable winds were sought for at altitudes
varying from 100 to 3,000 meters, necessitating a finish in Canada
nearly northeast of the starting point.
"These nine balloons, making landings extending from Lake Ontario,
Canada, to Virginia, all started from one point within the same hour.
"The single exception to these roving currents occurred on October 21st,
of last year (1909) when, starting from Philadelphia, the wind shifted
more than eight degrees, the greatest variation being at the lowest
altitudes, yet at no time was a height of over a mile reached.
"Throughout the entire day the sky was overcast, with a thermometer
varying from fifty-seven degrees at 300 feet to forty-four degrees,
Fahrenheit at 5,000 feet, at which altitude the wind had a velocity of
43 miles an hour, in clouds of a cirro-cumulus nature, a landing finally
being made near Tannersville, New York, in the Catskill mountains, after
a voyage of five and one-half hours.
"I have no knowledge of a recorded trip of this distance and duration,
maintained in practically a straight line from start to finish."
This wind disturbance is more noticeable and more difficult to contend
with in a balloon than in a flying machine, owing to the bulk and
unwieldy character of the former. At the same time it is not conducive
to pleasant, safe or satisfactory sky-sailing in an aeroplane. This is
not stated with the purpose of discouraging aviation, but merely that
the operator may know what to expect and be prepared to meet it.
Not only does the wind change its horizontal course abruptly and without
notice, but it also shifts in a vertical direction, one second blowing
up, and another down. No man has as yet fathomed the why and wherefore
of this erratic action; it is only known that it exists.
The most stable currents will be found from 50 to 100 feet from the
earth, provided the wind is not diverted by such objects as trees,
rocks, etc. That there are equally stable currents higher up is true,
but they are generally to be found at excessive altitudes.
How a Bird Meets Currents.
Observe a bird in action on a windy day and you will find it continually
changing the position of its wings. This is done to meet the varying
gusts and eddies of the air so that sustentation may be maintained and
headway made. One second the bird is bending its wings, altering the
angle of incidence; the next it is lifting or depressing one wing at a
time. Still again it will extend one wing tip in advance of the other,
or be spreading or folding, lowering or raising its tail.
All these motions have a meaning, a purpose. They assist the bird in
preserving its equilibrium. Without them the bird would be just as
helpless in the air as a human being and could not remain afloat.
When the wind is still, or comparatively so, a bird, having secured the
desired altitude by flight at an angle, may sail or soar with no wing
action beyond an occasional stroke when it desires to advance. But, in a
gusty, uncertain wind it must use its wings or alight somewhere.
Trying to Imitate the Bird.
Writing in _Fly_, Mr. William E. White says:
"The bird's flight suggests a number of ways in which the equilibrium
of a mechanical bird may be controlled. Each of these methods of control
may be effected by several different forms of mechanism.
"Placing the two wings of an aeroplane at an angle of three to five
degrees to each other is perhaps the oldest way of securing lateral
balance. This way readily occurs to anyone who watches a sea gull
soaring. The theory of the dihedral angle is that when one wing is
lifted by a gust of wind, the air is spilled from under it; while
the other wing, being correspondingly depressed, presents a greater
resistance to the gust and is lifted restoring the balance. A fixed
angle of three to five degrees, however, will only be sufficient for
very light puffs of wind and to mount the wings so that the whole
wing may be moved to change the dihedral angle presents mechanical
difficulties which would be better avoided.
"The objection of mechanical impracticability applies to any plan
to preserve the balance by shifting weight or ballast. The center of
gravity should be lower than the center of the supporting surfaces,
but cannot be made much lower. It is a common mistake to assume that
complete stability will be secured by hanging the center of gravity very
low on the principle of the parachute. An aeroplane depends upon rapid
horizontal motion for its support, and if the center of gravity be far
below the center of support, every change of speed or wind pressure will
cause the machine to turn about its center of gravity, pitching forward
and backward dangerously.
Preserving Longitudinal Balance.
"The birds maintain longitudinal, or fore and aft balance, by elevating
or depressing their tails. Whether this action is secured in an
aeroplane by means of a horizontal rudder placed in the rear, or by
deflecting planes placed in front of the main planes, the principle is
evidently the same. A horizontal rudder placed well to the rear as in
the Antoinette, Bleriot or Santos-Dumont monoplanes, will be very much
safer and steadier than the deflecting planes in front, as in the Wright
or Curtiss biplanes, but not so sensitive or prompt in action.
"The natural fore and aft stability is very much strengthened by placing
the load well forward. The center of gravity near the front and a
tail or rudder streaming to the rear secures stability as an arrow is
balanced by the head and feathering. The adoption of this principle
makes it almost impossible for the aeroplane to turn over.
The Matter of Lateral Balance.
"All successful aeroplanes thus far have maintained lateral balance by
the principle of changing the angle of incidence of the wings.
"Other ways of maintaining the lateral balance, suggested by observation
of the flight of birds are--extending the wing tips and spilling the air
through the pinions; or, what is the same thing, varying the area of the
wings at their extremities.
"Extending the wing tips seems to be a simple and effective solution of
the problem. The tips may be made to swing outward upon a vertical axis
placed at the front edge of the main planes; or they may be hinged to
the ends of the main plane so as to be elevated or depressed through
suitable connections by the aviator; or they may be supported from a
horizontal axis parallel with the ends of the main planes so that they
may swing outward, the aviator controlling both tips through one lever
so that as one tip is extended the other is retracted.
"The elastic wing pinions of a bird bend easily before the wind,
permitting the gusts to glance off, but presenting always an even and
efficient curvature to the steady currents of the air."
High Winds Threaten Stability.
To ensure perfect stability, without control, either human or automatic,
it is asserted that the aeroplane must move faster than the wind is
blowing. So long as the wind is blowing at the rate of 30 miles an hour,
and the machine is traveling 40 or more, there will be little trouble as
regards equilibrium so far as wind disturbance goes, provided the wind
blows evenly and does not come in gusts or eddying currents. But when
conditions are reversed--when the machine travels only 30 miles an hour
and the wind blows at the rate of 50, look out for loss of equilibrium.
One of the main reasons for this is that high winds are rarely steady;
they seldom blow for any length of time at the same speed. They are
usually "gusty," the gusts being a momentary movement at a higher speed.
Tornadic gusts are also formed by the meeting of two opposing currents,
causing a whirling motion, which makes stability uncertain. Besides, it
is not unusual for wind of high speed to suddenly change its direction
without warning.
Trouble With Vertical Columns.
Vertical currents--columns of ascending air--are frequently encountered
in unexpected places and have more or less tendency, according to their
strength, to make it difficult to keep the machine within a reasonable
distance from the ground.
These vertical currents are most generally noticeable in the vicinity
of steep cliffs, or deep ravines. In such instances they are usually of
considerable strength, being caused by the deflection of strong winds
blowing against the face of the cliffs. This deflection exerts a back
pressure which is felt quite a distance away from the point of origin,
so that the vertical current exerts an influence in forcing the machine
upward long before the cliff is reached.