flying machines-第20章
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
〃In the case of giving warranties on aeroplanes； we
have yet to see just what a court is going to say。 It is
easy enough for a manufacturer to guarantee to build a
machine of certain dimensions and according to certain
specifications； but when he inserts a clause in the contract to
the effect that the machine will raise itself from
the surface of the earth； defy the laws of gravity； and
soar in the heavens at the will of the aviator； he is to
say the least contracting to perform a miracle。
〃Until aeroplanes have been made and accepted as
practical； no court will force a manufacturer to turn out
a machine guaranteed to fly。 So purchasers can well
remember that if their machines refuse to fly they have
no redress against the maker； for he can always say；
'The industry is still in its experimental stage。' In
contracting for an engine no builder will guarantee that
the particular engine will successfully operate the aeroplane。
In fact he could never be forced to live up to
such an agreement； should he agree to a stipulation of
that sort。 The best any engine maker will guarantee
is to build an engine according to specifications。〃
CHAPTER XX。
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； 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 Lil
