few hundred feet were up and down as before, but by the time three
hundred feet had been covered, the machine was under much better
control. The course for the next four or five hundred feet had but
little undulation. However, when out about eight hundred feet the
machine began pitching again, and, in one of its starts downward, struck
the ground. The distance over the ground was measured and found to be
852 feet; the time of the flight 59 seconds. The frame supporting the
front rudder was badly broken, but the main part of the machine was not
injured at all. We estimated that the machine could be put in condition
for flight again in a day or two.
While we were standing about discussing this last flight, a sudden
strong gust of wind struck the machine and began to turn it over.
Everybody made a rush for it. Wilbur, who was at one end, seized it in
front, Mr. Daniels and I, who were behind, tried to stop it by holding
to the rear uprights. All our efforts were vain. The machine rolled over
and over. Daniels, who had retained his grip, was carried along with it,
and was thrown about head over heels inside of the machine. Fortunately
he was not seriously injured, though badly bruised in falling about
against the motor, chain guides, etc. The ribs in the surfaces of the
machine were broken, the motor injured and the chain guides badly bent,
so that all possibility of further flights with it for that year were at
an end.
[Illustration]
Some Aeronautical Experiments
_By Wilbur Wright_
The difficulties which obstruct the pathway to success in flying machine
construction are of three general classes: (1) Those which relate to the
construction of the sustaining wings. (2) Those which relate to the
generation and application of the power required to drive the machine
through the air. (3) Those relating to the balancing and steering of the
machine after it is actually in flight. Of these difficulties two are
already to a certain extent solved. Men already know how to construct
wings or aeroplanes which, when driven through air at sufficient speed,
will not only sustain the weight of the wings themselves, but also that
of the engine, and of the engineer as well. Men also know how to build
engines and screws of sufficient lightness and power to drive these
planes at sustaining speed. As long ago as 1893 a machine weighing 8,000
lbs. demonstrated its power both to lift itself from the ground and to
maintain a speed of from 30 to 40 miles per hour; but it came to grief
in an accidental free flight, owing to the inability of the operators to
balance and steer it properly. This inability to balance and steer still
confronts students of the flying problem, although nearly ten years have
passed. When this one feature has been worked out the age of flying
machines will have arrived, for all other difficulties are of minor
importance.
The person who merely watches the flight of a bird gathers the
impression that the bird has nothing to think of but the flapping
of its wings. As a matter of fact, this is a very small part of its
mental labour. Even to mention all the things the bird must constantly
keep in mind in order to fly securely through the air would take a
very considerable treatise. If I take a piece of paper, and after
placing it parallel with the ground, quickly let it fall, it will not
settle steadily down as a staid, sensible piece of paper ought to do,
but it insists on contravening every recognized rule of decorum,
turning over and darting hither and thither in the most erratic
manner, much after the style of an untrained horse. Yet this is the
style of steed that men must learn to manage before flying can become
an everyday sport. The bird has learned this art of equilibrium, and
learned it so thoroughly that its skill is not apparent to our sight.
We only learn to appreciate it when we try to imitate it. Now, there
are two ways of learning how to ride a fractious horse: one is to get
on him and learn by actual practice how each motion and trick may be
best met; the other is to sit on a fence and watch the beast awhile,
and then retire to the house and at leisure figure out the best way of
overcoming his jumps and kicks. The latter system is the safest; but
the former, on the whole, turns out the larger proportion of good
riders. It is very much the same in learning to ride a flying machine;
if you are looking for perfect safety you will do well to sit on a
fence and watch the birds; but if you really wish to learn you must
mount a machine and become acquainted with its tricks by actual trial.
* * * * *
My own active interest in aeronautical problems dates back to the death
of Lilienthal in 1896. The brief notice of his death which appeared in
the telegraphic news at that time aroused a passive interest which had
existed from my childhood, and led me to take down from the shelves of
our home library a book on "Animal Mechanism," by Prof. Marey, which I
had already read several times. From this I was led to read more modern
works, and as my brother soon became equally interested with myself, we
soon passed from the reading to the thinking, and finally to the working
stage. It seemed to us that the main reason why the problem had remained
so long unsolved was that no one had been able to obtain any adequate
practice. We figured that Lilienthal in five years of time had spent
only about five hours in actual gliding through the air. The wonder was
not that he had done so little, but that he had accomplished so much. It
would not be considered at all safe for a bicycle rider to attempt to
ride through a crowded city street after only five hours' practice,
spread out in bits of ten seconds each over a period of five years; yet
Lilienthal with this brief practice was remarkably successful in meeting
the fluctuations and eddies of wind gusts. We thought that if some
method could be found by which it would be possible to practice by the
hour instead of by the second there would be hope of advancing the
solution of a very difficult problem. It seemed feasible to do this by
building a machine which would be sustained at a speed of 18 miles per
hour, and then finding a locality where winds of this velocity were
common. With these conditions a rope attached to the machine to keep it
from floating backward would answer very nearly the same purpose as a
propeller driven by a motor, and it would be possible to practice by the
hour, and without any serious danger, as it would not be necessary to
rise far from the ground, and the machine would not have any forward
motion at all. We found, according to the accepted tables of air
pressures on curved surfaces, that a machine spreading 200 square feet
of wing surface would be sufficient for our purpose, and that places
could easily be found along the Atlantic coast where winds of 16 to 25
miles were not at all uncommon. When the winds were low it was our plan
to glide from the tops of sand hills, and when they were sufficiently
strong to use a rope for our motor and fly over one spot. Our next work
was to draw up the plan for a suitable machine. After much study we
finally concluded that tails were a source of trouble rather than of
assistance, and therefore we decided to dispense with them altogether.
It seemed reasonable that if the body of the operator could be placed in
a horizontal position instead of the upright, as in the machines of
Lilienthal, Pilcher and Chanute, the wind resistance could be very
materially reduced, since only one square foot instead of five would be
exposed. As a full half-horse-power could be saved by this change, we
arranged to try at least the horizontal position. Then the method of
control used by Lilienthal, which consisted in shifting the body, did
not seem quite as quick or effective as the case required; so, after
long study, we contrived a system consisting of two large surfaces on
the Chanute double-deck plan, and a smaller surface placed a short
distance in front of the main surfaces in such a position that the
action of the wind upon it would counterbalance the effect of the travel
of the center of pressure on the main surfaces. Thus changes in the
direction and velocity of the wind would have little disturbing effect,
and the operator would be required to attend only to the steering of the
machine, which was to be effected by curving the forward surface up or
down. The lateral equilibrium and the steering to right or left was to
be attained by a peculiar torsion of the main surfaces, which was
equivalent to presenting one end of the wings at a greater angle than
the other. In the main frame a few changes were also made in the details
of construction and trussing employed by Mr. Chanute. The most important
of these were: (1) The moving of the forward main cross-piece of the
frame to the extreme front edge; (2) the encasing in the cloth of all
cross-pieces and ribs of the surfaces; (3) a rearrangement of the wires
used in trussing the two surfaces together, which rendered it possible
to tighten all the wires by simply shortening two of them.
[Illustration]
With these plans we proceeded in the summer of 1900 to Kitty Hawk,
North Carolina, a little settlement located on the strip of land
that separates Albemarle Sound from the Atlantic Ocean. Owing to the
impossibility of obtaining suitable material for a 200-square-foot
machine, we were compelled to make it only 165 square feet in area,
which, according to the Lilienthal tables, would be supported at an
angle of three degrees in a wind of about 21 miles per hour. On the very
day that the machine was completed the wind blew from 25 to 30 miles per
hour, and we took it out for a trial as a kite. We found that while it
was supported with a man on it in a wind of about 25 miles, its angle
was much nearer 20 degrees than three degrees. Even in gusts of 30 miles
the angle of incidence did not get as low as three degrees, although the
wind at this speed has more than twice the lifting power of a 21-mile
wind. As winds of 30 miles per hour are not plentiful on clear days, it
was at once evident that our plan of practicing by the hour, day after
day, would have to be postponed. Our system of twisting the surfaces
to regulate the lateral balance was tried and found to be much more
effective than shifting the operator's body. On subsequent days, when
the wind was too light to support the machine with a man on it, we
tested it as a kite, working the rudders by cords reaching to the
ground. The results were very satisfactory, yet we were well aware that
this method of testing is never wholly convincing until the results are
confirmed by actual gliding experience.
We then turned our attention to making a series of actual measurements
of the lift and drift of the machine under various loads. So far as we
were aware, this had never previously been done with any full-size
machine. The results obtained were most astonishing, for it appeared
that the total horizontal pull of the machine, while sustaining a weight
of 52 lbs., was only 8.5 lbs., which was less than had previously been
estimated for head resistance of the framing alone. Making allowance for
the weight carried, it appeared that the head resistance of the framing
was but little more than 50 per cent. of the amount which Mr. Chanute
had estimated as the head resistance of the framing of his machine. On
the other hand, it appeared sadly deficient in lifting power as compared
with the calculated lift of curved surfaces of its size. This deficiency
we supposed might be due to one or more of the following causes:--(1)
That the depth of the curvature of our surfaces was insufficient, being
only about one in 22, instead of one in 12. (2) That the cloth used in
our wings was not sufficiently air-tight. (3) That the Lilienthal tables
might themselves be somewhat in error. We decided to arrange our machine
for the following year so that the depth of the curvature of its
surfaces could be varied at will and its covering air-proofed.
Our attention was next turned to gliding, but no hill suitable for the
purpose could be found near our camp at Kitty Hawk. This compelled us to
take the machine to a point four miles south, where the Kill Devil sand
hill rises from the flat sand to a height of more than 100 feet. Its
main slope is toward the northeast, and has an inclination of 10
degrees. On the day of our arrival the wind blew about 25 miles an hour,
and as we had had no experience at all in gliding, we deemed it unsafe
to attempt to leave the ground. But on the day following, the wind
having subsided to 14 miles per hour, we made about a dozen glides. It
had been the original intention that the operator should run with the
machine to obtain initial velocity, and assume the horizontal position
only after the machine was in free flight. When it came time to land he
was to resume the upright position and alight on his feet, after the
style of previous gliding experiments. But in actual trial we found it
much better to employ the help of two assistants in starting, which the
peculiar form of our machine enabled us readily to do; and in landing we
found that it was entirely practicable to land while still reclining in
a horizontal position upon the machine. Although the landings were made
while moving at speeds of more than 20 miles an hour, neither machine
nor operator suffered any injury. The slope of the hill was 9.5 deg., or
a drop of one foot in six. We found that after attaining a speed of
about 25 to 30 miles with reference to the wind, or 10 to 15 miles over
the ground, the machine not only glided parallel to the slope of the
hill, but greatly increased its speed, thus indicating its ability to
glide on a somewhat less angle than 9.5 deg., when we should feel it
safe to rise higher from the surface. The control of the machine proved
even better than we had dared to expect, responding quickly to the
slightest motion of the rudder. With these glides our experiments for
the year 1900 closed. Although the hours and hours of practice we had
hoped to obtain finally dwindled down to about two minutes, we were very
much pleased with the general results of the trip, for, setting out as
we did with almost revolutionary theories on many points and an entirely
untried form of machine, we considered it quite a point to be able to
return without having our pet theories completely knocked on the head
by the hard logic of experience, and our own brains dashed out in the
bargain. Everything seemed to us to confirm the correctness of our
original opinions--(1) that practice is the key to the secret of
flying; (2) that it is practicable to assume the horizontal position;
(3) that a smaller surface set at a negative angle in front of the
main bearing surfaces, or wings, will largely counteract the effect
of the fore-and-aft travel of the center of pressure; (4) that
steering up and down can be attained with a rudder without moving
the position of the operator's body; (5) that twisting the wings so
as to present their ends to the wind at different angles is a more
prompt and efficient way of maintaining lateral equilibrium than that
employed in shifting the body of the operator of the machine.
When the time came to design our new machine for 1901 we decided to make
it exactly like the previous machine in theory and method of operation.
But as the former machine was not able to support the weight of the
operator when flown as a kite, except in very high winds and at very
large angles of incidence, we decided to increase its lifting power.
Accordingly, the curvature of the surfaces was increased to one in 12,
to conform to the shape on which Lilienthal's table was based, and to be
on the safe side we decided also to increase the area of the machine
from 165 square feet to 308 square feet, although so large a machine had
never before been deemed controllable. The Lilienthal machine had an
area of 151 square feet; that of Pilcher, 165 square feet; and the
Chanute double-decker, 134 square feet. As our system of control
consisted in a manipulation of the surfaces themselves instead of
shifting the operator's body, we hoped that the new machine would be
controllable, notwithstanding its great size. According to calculations,
it would obtain support in a wind of 17 miles per hour with an angle of
incidence of only three degrees.
[Illustration]
Our experience of the previous year having shown the necessity of a
suitable building for housing the machine, we erected a cheap frame
building, 16 feet wide, 25 feet long, and 7 feet high at the eaves. As
our machine was 22 feet wide, 14 feet long (including the rudder), and
about 6 feet high, it was not necessary to take the machine apart in any
way in order to house it. Both ends of the building, except the gable
parts, were made into doors which hinged above, so that when opened they
formed an awning at each end and left an entrance the full width of the
building. We went into camp about the middle of July, and were soon
joined by Mr. E. C. Huffaker, of Tennessee, an experienced aeronautical
investigator in the employ of Mr. Chanute, by whom his services were
kindly loaned, and by Dr. A. G. Spratt, of Pennsylvania, a young man who
has made some valuable investigations of the properties of variously
curved surfaces and the travel of the center of pressure thereon. Early
in August Mr. Chanute came down from Chicago to witness our experiments,
and spent a week in camp with us. These gentlemen, with my brother and
myself, formed our camping party, but in addition we had in many of our
experiments the valuable assistance of Mr. W. J. Tate and Mr. Dan Tate,
of Kitty Hawk.
* * * * *
It had been our intention when building the machine to do most of the
experimenting in the following manner:--When the wind blew 17 miles an
hour, or more, we would attach a rope to the machine and let it rise as
a kite with the operator upon it. When it should reach a proper height
the operator would cast off the rope and glide down to the ground just
as from the top of a hill. In this way we would be saved the trouble of
carrying the machine uphill after each glide, and could make at least 10
glides in the time required for one in the other way. But when we came
to try it we found that a wind of 17 miles, as measured by Richards'
anemometer, instead of sustaining the machine with its operator, a total
weight of 240 lbs., at an angle of incidence of three degrees, in
reality would not sustain the machine alone--100 lbs.--at this angle.
Its lifting capacity seemed scarcely one-third of the calculated amount.
In order to make sure that this was not due to the porosity of the
cloth, we constructed two small experimental surfaces of equal size, one
of which was air-proofed and the other left in its natural state; but we
could detect no difference in their lifting powers. For a time we were
led to suspect that the lift of curved surfaces little exceeded that of
planes of the same size, but further investigation and experiment led to
the opinion that (1) the anemometer used by us over-recorded the true
velocity of the wind by nearly 15 per cent.; (2) that the well-known
Smeaton coefficient of .005 V^2 for the wind pressure at 90 degrees is
probably too great by at least 20 per cent.; (3) that Lilienthal's
estimate that the pressure on a curved surface having an angle of
incidence of three degrees equals .545 of the pressure at 90 degrees is
too large, being nearly 50 per cent. greater than very recent
experiments of our own with a special pressure testing machine indicate;
(4) that the superposition of the surfaces somewhat reduced the lift per
square foot, as compared with a single surface of equal area.
[Illustration]