ABOUT WIND CURRENTS





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.





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