SOARING FLIGHT.

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, ad ance, 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.