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SOARING FLIGHT.






From: Flying Machines Construction Operation

By Octave Chanute.

[5]There is a wonderful performance daily exhibited in
southern climes and occasionally seen in northerly
latitudes in summer, which has never been thoroughly
explained. It is the soaring or sailing flight of certain
varieties of large birds who transport themselves on rigid,
unflapping wings in any desired direction; who in winds
of 6 to 20 miles per hour, circle, rise, advance, return and
remain aloft for hours without a beat of wing, save for
getting under way or convenience in various maneuvers.
They appear to obtain from the wind alone all the necessary
energy, even to advancing dead against that wind.
This feat is so much opposed to our general ideas of
physics that those who have not seen it sometimes deny
its actuality, and those who have only occasionally
witnessed it subsequently doubt the evidence of their own
eyes. Others, who have seen the exceptional performances,
speculate on various explanations, but the majority
give it up as a sort of "negative gravity."

[5] Aeronautics.

Soaring Power of Birds.

The writer of this paper published in the "Aeronautical
Annual" for 1896 and 1897 an article upon the sailing
flight of birds, in which he gave a list of the authors who
had described such flight or had advanced theories for
its explanation, and he passed these in review. He also
described his own observations and submitted some computations
to account for the observed facts. These computations
were correct as far as they went, but they were
scanty. It was, for instance, shown convincingly by
analysis that a gull weighing 2.188 pounds, with a total
supporting surface of 2.015 square feet, a maximum body
cross-section of 0.126 square feet and a maximum cross-
section of wing edges of 0.098 square feet, patrolling on
rigid wings (soaring) on the weather side of a steamer
and maintaining an upward angle or attitude of 5 degrees
to 7 degrees above the horizon, in a wind blowing 12.78
miles an hour, which was deflected upward 10 degrees
to 20 degrees by the side of the steamer (these all being
carefully observed facts), was perfectly sustained at its
own "relative speed" of 17.88 miles per hour and extracted
from the upward trend of the wind sufficient energy
to overcome all the resistances, this energy
amounting to 6.44 foot-pounds per second.

Great Power of Gulls.

It was shown that the same bird in flapping flight in
calm air, with an attitude or incidence of 3 degrees to 5
degrees above the horizon and a speed of 20.4 miles an
hour was well sustained and expended 5.88 foot-pounds
per second, this being at the rate of 204 pounds sustained
per horsepower. It was stated also that a gull in its observed
maneuvers, rising up from a pile head on unflapping
wings, then plunging forward against the wind and
subsequently rising higher than his starting point, must
either time his ascents and descents exactly with the
variations in wind velocities, or must meet a wind billow
rotating on a horizontal axis and come to a poise on its
crest, thus availing of an ascending trend.

But the observations failed to demonstrate that the
variations of the wind gusts and the movements of the
bird were absolutely synchronous, and it was conjectured
that the peculiar shape of the soaring wing of certain
birds, as differentiated from the flapping wing, might,
when experimented upon, hereafter account for the performance.

Mystery to be Explained.

These computations, however satisfactory they were
for the speed of winds observed, failed to account for the
observed spiral soaring of buzzards in very light winds
and the writer was compelled to confess: "Now, this
spiral soaring in steady breezes of 5 to 10 miles per hour
which are apparently horizontal, and through which the
bird maintains an average speed of about 20 miles an
hour, is the mystery to be explained. It is not accounted
for, quantitatively, by any of the theories which have
been advanced, and it is the one performance which has
led some observers to claim that it was done through
'aspiration.' i, e., that a bird acted upon by a current,
actually drew forward into that current against its exact
direction of motion."

Buzzards Soar in Dead Calm.

A still greater mystery was propounded by the few
observers who asserted that they had seen buzzards soaring
in a dead calm, maintaining their elevation and their
speed. Among these observers was Mr. E. C. Huffaker,
at one time assistant experimenter for Professor Langley.
The writer believed and said then that he must in some
way have been mistaken, yet, to satisfy himself, he paid
several visits to Mr. Huffaker, in Eastern Tennessee and
took along his anemometer. He saw quite a number of
buzzards sailing at a height of 75 to 100 feet in breezes
measuring 5 or 6 miles an hour at the surface of the
ground, and once he saw one buzzard soaring apparently
in a dead calm.

The writer was fairly baffled. The bird was not simply
gliding, utilizing gravity or acquired momentum, he was
actually circling horizontally in defiance of physics and
mathematics. It took two years and a whole series of
further observations to bring those two sciences into
accord with the facts.

Results of Close Observations.

Curiously enough the key to the performance of circling
in a light wind or a dead calm was not found
through the usual way of gathering human knowledge,
i. e., through observations and experiment. These had
failed because I did not know what to look for. The
mystery was, in fact, solved by an eclectic process of
conjecture and computation, but once these computations
indicated what observations should be made, the results
gave at once the reasons for the circling of the birds, for
their then observed attitude, and for the necessity of an
independent initial sustaining speed before soaring began.
Both Mr. Huffaker and myself verified the data
many times and I made the computations.

These observations disclosed several facts:

1st.--That winds blowing five to seventeen miles per
hour frequently had rising trends of 10 degrees to 15
degrees, and that upon occasions when there seemed to be
absolutely no wind, there was often nevertheless a local
rising of the air estimated at a rate of four to eight miles
or more per hour. This was ascertained by watching
thistledown, and rising fogs alongside of trees or hills of
known height. Everyone will readily realize that when
walking at the rate of four to eight miles an hour in a
dead calm the "relative wind" is quite inappreciable to
the senses and that such a rising air would not be noticed.

2nd.--That the buzzard, sailing in an apparently dead
horizontal calm, progressed at speeds of fifteen to eighteen
miles per hour, as measured by his shadow on the
ground. It was thought that the air was then possibly
rising 8.8 feet per second, or six miles per hour.

3rd.--That when soaring in very light winds the angle
of incidence of the buzzards was negative to the horizon
--i. e., that when seen coming toward the eye, the afternoon
light shone on the back instead of on the breast,
as would have been the case had the angle been inclined
above the horizon.

4th.--That the sailing performance only occurred after
the bird had acquired an initial velocity of at least fifteen
or eighteen miles per hour, either by industrious flapping
or by descending from a perch.

An Interesting Experiment.

5th.--That the whole resistance of a stuffed buzzard,
at a negative angle of 3 degrees in a current of air of
15.52 miles per hour, was 0.27 pounds. This test was
kindly made for the writer by Professor A. F. Zahm in
the "wind tunnel" of the Catholic University at Washington,
D. C., who, moreover, stated that the resistance
of a live bird might be less, as the dried plumage could
not be made to lie smooth.

This particular buzzard weighed in life 4.25 pounds,
the area of his wings and body was 4.57 square feet, the
maximum cross-section of his body was 0.110 square feet,
and that of his wing edges when fully extended was
0.244 square feet.

With these data, it became surprisingly easy to compute
the performance with the coefficients of Lilienthal
for various angles of incidence and to demonstrate how
this buzzard could soar horizontally in a dead horizontal
calm, provided that it was not a vertical calm, and that
the air was rising at the rate of four or six miles per
hour, the lowest observed, and quite inappreciable without
actual measuring.

Some Data on Bird Power.

The most difficult case is purposely selected. For if
we assume that the bird has previously acquired an initial
minimum speed of seventeen miles an hour (24.93
feet per second, nearly the lowest measured), and that
the air was rising vertically six miles an hour (8.80 feet
per second), then we have as the trend of the "relative
wind" encountered:

6
-- = 0.353, or the tangent of 19 degrees 26'.
17

which brings the case into the category of rising wind
effects. But the bird was observed to have a negative
angle to the horizon of about 3 degrees, as near as could be
guessed, so that his angle of incidence to the "relative
wind" was reduced to 16 degrees 26'.

The relative speed of his soaring was therefore:

Velocity = square root of (17 squared + 6 squared) = 18.03 miles
per hour.

At this speed, using the Langley co-efficient recently
practically confirmed by the accurate experiments of Mr.
Eiffel, the air pressure would be:

18.03 squared X 0.00327 = 1.063 pounds per square foot.

If we apply Lilienthal's co-efficients for an angle of
6 degrees 26', we have for the force in action:

Normal: 4.57 X 1.063 X 0.912 = 4.42 pounds.

Tangential: 4.57 X 1.063 X 0.074 = - 0.359 pounds,
which latter, being negative, is a propelling force.

Results Astonish Scientists.

Thus we have a bird weighing 4.25 pounds not only
thoroughly supported, but impelled forward by a force
of 0.359 pounds, at seventeen miles per hour, while the
experiments of Professor A. F. Zahm showed that the
resistance at 15.52 miles per hour was only 0.27 pounds,
17 squared
or 0.27 X ------- = 0.324 pounds, at seventeen miles an
15.52 squared
hour.

These are astonishing results from the data obtained,
and they lead to the inquiry whether the energy of the
rising air is sufficient to make up the losses which occur
by reason of the resistance and friction of the bird's body
and wings, which, being rounded, do not encounter air
pressures in proportion to their maximum cross-section.

We have no accurate data upon the co-efficients to apply
and estimates made by myself proved to be much
smaller than the 0.27 pounds resistance measured by
Professor Zahm, so that we will figure with the latter
as modified. As the speed is seventeen miles per hour, or
24.93 feet per second, we have for the work:

Work done, 0.324 X 24.93 = 8.07 foot pounds per second.

Endorsed by Prof. Marvin.

Corresponding energy of rising air is not sufficient at
four miles per hour. This amounts to but 2.10 foot pounds
per second, but if we assume that the air was rising at
the rate of seven miles per hour (10.26 feet per second),
at which the pressure with the Langley coefficient would
be 0.16 pounds per square foot, we have on 4.57 square
feet for energy of rising air: 4.57 X 0.16 X 10.26 = 7.50
foot pounds per second, which is seen to be still a little
too small, but well within the limits of error, in view of
the hollow shape of the bird's wings, which receive
greater pressure than the flat planes experimented upon
by Langley.

These computations were chiefly made in January,
1899, and were communicated to a few friends, who found
no fallacy in them, but thought that few aviators would
understand them if published. They were then submitted
to Professor C. F. Marvin of the Weather Bureau, who
is well known as a skillful physicist and mathematician.
He wrote that they were, theoretically, entirely sound
and quantitatively, probably, as accurate as the present
state of the measurements of wind pressures permitted.
The writer determined, however, to withhold publication
until the feat of soaring flight had been performed by
man, partly because he believed that, to ensure safety, it
would be necessary that the machine should be equipped
with a motor in order to supplement any deficiency in
wind force.

Conditions Unfavorable for Wrights.

The feat would have been attempted in 1902 by Wright
brothers if the local circumstances had been more favorable.
They were experimenting on "Kill Devil Hill,"
near Kitty Hawk, N. C. This sand hill, about 100 feet
high, is bordered by a smooth beach on the side whence
come the sea breezes, but has marshy ground at the back.
Wright brothers were apprehensive that if they rose on
the ascending current of air at the front and began to
circle like the birds, they might be carried by the
descending current past the back of the hill and land in
the marsh. Their gliding machine offered no greater
head resistance in proportion than the buzzard, and their gliding
angles of descent are practically as favorable, but
the birds performed higher up in the air than they.

Langley's Idea of Aviation.

Professor Langley said in concluding his paper upon
"The Internal Work of the Wind":

"The final application of these principles to the art of
aerodromics seems, then, to be, that while it is not likely
that the perfected aerodrome will ever be able to dispense
altogether with the ability to rely at intervals on
some internal source of power, it will not be indispensable
that this aerodrome of the future shall, in order to
go any distance--even to circumnavigate the globe without
alighting--need to carry a weight of fuel which
would enable it to perform this journey under conditions
analogous to those of a steamship, but that the fuel and
weight need only be such as to enable it to take care of
itself in exceptional moments of calm."

Now that dynamic flying machines have been evolved
and are being brought under control, it seems to be
worth while to make these computations and the succeeding
explanations known, so that some bold man will
attempt the feat of soaring like a bird. The theory
underlying the performance in a rising wind is not new,
it has been suggested by Penaud and others, but it has
attracted little attention because the exact data and the
maneuvers required were not known and the feat had
not yet been performed by a man. The puzzle has always
been to account for the observed act in very light
winds, and it is hoped that by the present selection of
the most difficult case to explain--i. e., the soaring in a
dead horizontal calm--somebody will attempt the exploit.

Requisites for Soaring Flights.

The following are deemed to be the requisites and
maneuvers to master the secrets of soaring flight:

1st--Develop a dynamic flying machine weighing
about one pound per square foot of area, with stable
equilibrium and under perfect control, capable of gliding
by gravity at angles of one in ten (5 3/4 degrees) in still air.

2nd.--Select locations where soaring birds abound and
occasions where rising trends of gentle winds are frequent
and to be relied on.

3rd.--Obtain an initial velocity of at least 25 feet per
second before attempting to soar.

4th.--So locate the center of gravity that the apparatus
shall assume a negative angle, fore and aft, of about 3 degrees.

Calculations show, however, that sufficient propelling
force may still exist at 0 degrees, but disappears entirely at
+4 degrees.

5th.--Circle like the bird. Simultaneously with the
steering, incline the apparatus to the side toward which
it is desired to turn, so that the centrifugal force shall
be balanced by the centripetal force. The amount of the
required inclination depends upon the speed and on the
radius of the circle swept over.

6th.--Rise spirally like the bird. Steer with the
horizontal rudder, so as to descend slightly when going
with the wind and to ascend when going against the
wind. The bird circles over one spot because the rising
trends of wind are generally confined to small areas or
local chimneys, as pointed out by Sir H. Maxim and
others.

7th.--Once altitude is gained, progress may be made
in any direction by gliding downward by gravity.

The bird's flying apparatus and skill are as yet infinitely
superior to those of man, but there are indications that
within a few years the latter may evolve more accurately
proportioned apparatus and obtain absolute control over
it.

It is hoped, therefore, that if there be found no radical
error in the above computations, they will carry the conviction
that soaring flight is not inaccessible to man, as
it promises great economies of motive power in favorable
localities of rising winds.

The writer will be grateful to experts who may point
out any mistake committed in data or calculations, and
will furnish additional information to any aviator who
may wish to attempt the feat of soaring.





Next: FLYING MACHINES VS. BALLOONS.

Previous: LAW OF THE AIRSHIP.



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