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