Construction and Operation
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[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.
CHAPTER XXI.
FLYING MACHINES VS. BALLOONS.
While wonderful success has attended the development
of the dirigible (steerable) balloon the most ardent
advocates of this form of aerial navigation admit that it
has serious drawbacks. Some of these may be described
as follows:
Expense and Other Items.
Great Initial Expense.--The modern dirigible balloon
costs a fortune. The Zeppelin, for instance, costs more
than $100,000 (these are official figures).
Expense of Inflation.--Gas evaporates rapidly, and a
balloon must be re-inflated, or partially re-inflated, every
time it is used. The Zeppelin holds 460,000 cubic feet
of gas which, even at $1 per thousand, would cost $460.
Difficulty of Obtaining Gas.--If a balloon suddenly
becomes deflated, by accident or atmospheric conditions,
far from a source of gas supply, it is practically worthless.
Gas must be piped to it, or the balloon carted to
the gas house--an expensive proceeding in either event.
Lack of Speed and Control.
Lack of Speed.--Under the most favorable conditions
the maximum speed of a balloon is 30 miles an hour.
Its great bulk makes the high speed attained by flying
machines impossible.
Difficulty of Control.--While the modern dirigible balloon is
readily handled in calm or light winds, its bulk
makes it difficult to control in heavy winds.
The Element of Danger.--Numerous balloons have
been destroyed by lightning and similar causes. One of
the largest of the Zeppelins was thus lost at Stuttgart
in 1908.
Some Balloon Performances.
It is only a matter of fairness to state that, under
favorable conditions, some very creditable records have
been made with modern balloons, viz:
November 23d, 1907, the French dirigible Patrie, travelled
187 miles in 6 hours and 45 minutes against a
light wind. This was a little over 28 miles an hour.
The Clement-Bayard, another French machine, sold
to the Russian government, made a trip of 125 miles at
a rate of 27 miles an hour.
Zeppelin No. 3, carrying eight passengers, and having
a total lifting capacity of 5,500 pounds of ballast in
addition to passengers, weight of equipment, etc., was
tested in October, 1906, and made 67 miles in 2 hours
and 17 minutes, about 30 miles an hour.
These are the best balloon trips on record, and show
forcefully the limitations of speed, the greatest being not
over 30 miles an hour.
Speed of Flying Machines.
Opposed to the balloon performances we have flying
machine trips (of authentic records) as follows:
Bleriot--monoplane--in 1908--52 miles an hour.
Delagrange--June 22, 1908--10 1/2 miles in 16 minutes,
approximately 42 miles an hour.
Wrights--October, 1905--the machine was then in its
infancy--24 miles in 38 minutes, approximately 44 miles
an hour. On December 31, 1908, the Wrights made 77
miles in 2 hours and 20 minutes.
Lambert, a pupil of the Wrights, and using a Wright
biplane, on October 18, 1909, covered 29.82 miles in 49
minutes and 39 seconds, being at the rate of 36 miles
an hour. This flight was made at a height of 1,312 feet.
Latham--October 21, 1909--made a short flight, about
11 minutes, in the teeth of a 40 mile gale, at Blackpool,
Eng. He used an Antoniette monoplane, and the official
report says: "This exhibition of nerve, daring and ability
is unparalled in the history of aviation."
Farman--October 20, 1909--was in the air for 1 hour,
32 min., 16 seconds, travelling 47 miles, 1,184 yards, a
duration record for England.
Paulhan--January 18, 1901--47 1/2 miles at the rate of
45 miles an hour, maintaining an altitude of from 1,000
to 2,000 feet.
Expense of Producing Gas.
Gas is indispensable in the operation of dirigible balloons,
and gas is expensive. Besides this it is not always
possible to obtain it in sufficient quantities even in large
cities, as the supply on hand is generally needed for
regular customers. Such as can be had is either water
or coal gas, neither of which is as efficient in lifting
power as hydrogen.
Hydrogen is the lightest and consequently the most
buoyant of all known gases. It is secured commercially
by treating zinc or iron with dilute sulphuric or
hydrochloric acid. The average cost may be safely placed
at $10 per 1,000 feet so that, to inflate a balloon of the
size of the Zeppelin, holding 460,000 cubic feet, would
cost $4,600.
Proportions of Materials Required.
In making hydrogen gas it is customary to allow 20
per cent for loss between the generation and the introduction
of the gas into the balloon. Thus, while the
formula calls for iron 28 times heavier than the weight
of the hydrogen required, and acid 49 times heavier, the
real quantities are 20 per cent greater. Hydrogen weighs
about 0.09 ounce to the cubic foot. Consequently if we
need say 450,000 cubic feet of gas we must have 2,531.25
pounds in weight. To produce this, allowing for the 20
percent loss, we must have 35 times its weight in iron,
or over 44 tons. Of acid it would take 60 times the
weight of the gas, or nearly 76 tons.
In Time of Emergency.
These figures are appalling, and under ordinary conditions
would be prohibitive, but there are times when
the balloon operator, unable to obtain water or coal gas,
must foot the bills. In military maneuvers, where the
field of operation is fixed, it is possible to furnish supplies
of hydrogen gas in portable cylinders, but on long
trips where sudden leakage or other cause makes descent
in an unexpected spot unavoidable, it becomes a question
of making your own hydrogen gas or deserting the balloon.
And when this occurs the balloonist is up against
another serious proposition--can he find the necessary
zinc or iron? Can he get the acid?
Balloons for Commercial Use.
Despite all this the balloon has its uses. If there is to
be such a thing as aerial navigation in a commercial
way--the carrying of freight and passengers--it will
come through the employment of such monster balloons
as Count Zeppelin is building. But even then the carrying
capacity must of necessity be limited. The latest
Zeppelin creation, a monster in size, is 450 feet long,
and 42 1/2 feet in diameter. The dimensions are such as
to make all other balloons look like pigmies; even many
ocean-going steamers are much smaller, and yet its passenger
capacity is very small. On its 36-hour flight in
May, 1909, the Zeppelin, carried only eight passengers.
The speed, however, was quite respectable, 850 miles
being covered in the 36 hours, a trifle over 23 miles an
hour. The reserve buoyancy, that is the total lifting
capacity aside from the weight of the airship and its
equipment, is estimated at three tons.
CHAPTER XXII.
PROBLEMS OF AERIAL FLIGHT.
In a lecture before the Royal Society of Arts, reported
in Engineering, F. W. Lanchester took the position that
practical flight was not the abstract question which some
apparently considered it to be, but a problem in locomotive
engineering. The flying machine was a locomotive
appliance, designed not merely to lift a weight,
but to transport it elsewhere, a fact which should be
sufficiently obvious. Nevertheless one of the leading scientific
men of the day advocated a type in which this, the
main function of the flying machine, was overlooked.
When the machine was considered as a method of transport,
the vertical screw type, or helicopter, became at
once ridiculous. It had, nevertheless, many advocates
who had some vague and ill-defined notion of subsequent
motion through the air after the weight was raised.
Helicopter Type Useless.
When efficiency of transport was demanded, the helicopter
type was entirely out of court. Almost all of
its advocates neglected the effect of the motion of the
machine through the air on the efficiency of the vertical
screws. They either assumed that the motion was
so slow as not to matter, or that a patch of still air
accompanied the machine in its flight. Only one form of this
type had any possibility of success. In this there were
two screws running on inclined axles--one on each side
of the weight to be lifted. The action of such inclined
screw was curious, and in a previous lecture he had
pointed out that it was almost exactly the same as that
of a bird's wing. In high-speed racing craft such inclined
screws were of necessity often used, but it was
at a sacrifice of their efficiency. In any case the efficiency
of the inclined-screw helicopter could not compare
with that of an aeroplane, and that type might be
dismissed from consideration so soon as efficiency became
the ruling factor of the design.
Must Compete With Locomotive.
To justify itself the aeroplane must compete, in some
regard or other, with other locomotive appliances, performing
one or more of the purposes of locomotion more
efficiently than existing systems. It would be no use
unless able to stem air currents, so that its velocity must
he greater than that of the worst winds liable to be encountered.
To illustrate the limitations imposed on the
motion of an aeroplane by wind velocity, Mr. Lanchester
gave the diagrams shown in Figs. 1 to 4. The circle
in each case was, he said, described with a radius equal
to the speed of the aeroplane in still air, from a center
placed "down-wind" from the aeroplane by an amount
equal to the velocity of the wind.
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