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ABOUT WIND CURRENTS






From: Flying Machines Construction Operation

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.





Next: THE ELEMENT OF DANGER.

Previous: PECULIARITIES OF AIRSHIP POWER.



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