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第28章

flying machines-第28章

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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 pe

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