importance than the ratio of lift to drift, as this alone decides the
angle of gliding descent. In a plane the pressure is always
perpendicular to the surface, and the ratio of lift to drift is
therefore the same as that of the cosine to the sine of the angle of
incidence. But in curved surfaces a very remarkable situation is found.
The pressure, instead of being uniformly normal to the chord of the arc,
is usually inclined considerably in front of the perpendicular. The
result is that the lift is greater and the drift less than if the
pressure were normal. While our measurements differ considerably from
those of Lilienthal, Lilienthal was the first to discover this
exceedingly important fact, which is fully set forth in his book, "Bird
Flight the Basis of the Flying Art," but owing to some errors in the
methods he used in making measurements, question was raised by other
investigators not only as to the accuracy of his figures, but even as to
the existence of any tangential force at all. Our experiments confirm
the existence of this force. At Kitty Hawk we spent much time in
measuring the horizontal pressure on our unloaded machine at various
angles of incidence. We found that at 13 degrees the horizontal pressure
was about 23 lbs. This included not only the drift proper, or horizontal
component of the pressure on the side of the surface, but also the head
resistance of the framing as well. The weight of the machine at the time
of this test was about 108 lbs. Now, if the pressure had been normal to
the chord of the surface, the drift proper would have been to the lift
(108 lbs.) as the sine of 13 degrees is to the cosine of 13 degrees, or
(.22 x 108) / .97 = 24+ lbs.; but this slightly exceeds the total pull
of 23 lbs. on our scales. Therefore, it is evident that the average
pressure on the surface, instead of being normal to the chord, was so
far inclined toward the front that all the head resistance of framing
and wires used in the construction was more than overcome. In a wind of
14 miles per hour resistance is by no means a negligible factor, so that
tangential is evidently a force of considerable value. In a higher wind,
which sustained the machine at an angle of 10 degrees, the pull on the
scales was 18 lbs. With the pressure normal to the chord the drift
proper would have been (.17 x 98) / .98 = 17 lbs., so that, although the
higher wind velocity must have caused an increase in the head
resistance, the tangential force still came within one pound of
overcoming it. After our return from Kitty Hawk we began a series of
experiments to accurately determine the amount and direction of the
pressure produced on curved surfaces when acted upon by winds at the
various angles from zero to 90 degrees. These experiments are not yet
concluded, but in general they support Lilienthal in the claim that the
curves give pressures more favorable in amount and direction than
planes; but we find marked differences in the exact values, especially
at angles below 10 degrees. We were unable to obtain direct measurements
of the horizontal pressures of the machine with the operator on board,
but by comparing the distance traveled in gliding with the vertical
fall, it was easily calculated that at a speed of 24 miles per hour the
total horizontal resistance of our machine when bearing the operator,
amounted to 40 lbs., which is equivalent to about 2-1/3 horse-power. It
must not be supposed, however, that a motor developing this power would
be sufficient to drive a man-bearing machine. The extra weight of the
motor would require either a larger machine, higher speed, or a greater
angle of incidence in order to support it, and therefore more power. It
is probable, however, that an engine of six horse-power, weighing 100
lbs., would answer the purpose. Such an engine is entirely practicable.
Indeed, working motors of one-half this weight per horse-power (9 lbs.
per horse-power) have been constructed by several different builders.
Increasing the speed of our machine from 24 to 33 miles per hour
reduced the total horizontal pressure from 40 to about 35 lbs. This was
quite an advantage in gliding, as it made it possible to sail about 15
per cent. further with a given drop. However, it would be of little or
no advantage in reducing the size of the motor in a power-driven
machine, because the lessened thrust would be counterbalanced by the
increased speed per minute. Some years ago Professor Langley called
attention to the great economy of thrust which might be obtained by
using very high speeds, and from this many were led to suppose that high
speed was essential to success in a motor-driven machine. But the
economy to which Professor Langley called attention was in foot-pounds
per mile of travel, not in foot-pounds per minute. It is the foot-pounds
per minute that fixes the size of the motor. The probability is that the
first flying machines will have a relatively low speed, perhaps not much
exceeding 20 miles per hour, but the problem of increasing the speed
will be much simpler in some respects than that of increasing the speed
of a steamboat; for, whereas in the latter case the size of the engine
must increase as the cube of the speed, in the flying machine, until
extremely high speeds are reached, the capacity of the motor increases
in less than simple ratio; and there is even a decrease in the fuel
consumption per mile of travel. In other words, to double the speed of a
steamship (and the same is true of the balloon type of airship) eight
times the engine and boiler capacity would be required, and four times
the fuel consumption per mile of travel; while a flying machine would
require engines of less than double the size, and there would be an
actual decrease in the fuel consumption per mile of travel. But looking
at the matter conversely, the great disadvantage of the flying machine
is apparent; for in the latter no flight at all is possible unless the
proportion of horse-power to flying capacity is very high; but on the
other hand a steamship is a mechanical success if its ratio of
horse-power to tonnage is insignificant. A flying machine that would fly
at a speed of 50 miles an hour with engines of 1,000 horse-power would
not be upheld by its wings at all at a speed of less than 25 miles an
hour, and nothing less than 500 horse-power could drive it at this
speed. But a boat which could make 40 miles per hour with engines of
1,000 horse-power would still move four miles an hour even if the
engines were reduced to one horse-power. The problems of land and water
travel were solved in the nineteenth century, because it was possible to
begin with small achievements and gradually work up to our present
success. The flying problem was left over to the twentieth century,
because in this case the art must be highly developed before any flight
of any considerable duration at all can be obtained.
[Illustration]
However, there is another way of flying which requires no artificial
motor, and many workers believe that success will first come by this
road. I refer to the soaring flight, by which the machine is permanently
sustained in the air by the same means that are employed by soaring
birds. They spread their wings to the wind, and sail by the hour, with
no perceptible exertion beyond that required to balance and steer
themselves. What sustains them is not definitely known, though it is
almost certain that it is a rising current of air. But whether it be a
rising current or something else, it is as well able to support a flying
machine as a bird, if man once learns the art of utilizing it. In
gliding experiments it has long been known that the rate of vertical
descent is very much retarded, and the duration of the flight greatly
prolonged, if a strong wind blows up the face of the hill parallel to
its surface. Our machine, when gliding in still air, has a rate of
vertical descent of nearly six feet per second, while in a wind blowing
26 miles per hour up a steep hill we made glides in which the rate of
descent was less than two feet per second. And during the larger part of
this time, while the machine remained exactly in the rising current,
there was no descent at all, but even a slight rise. If the operator had
had sufficient skill to keep himself from passing beyond the rising
current he would have been sustained indefinitely at a higher point than
that from which he started.
* * * * *
[Illustration]
In looking over our experiments of the past two years, with models and
full-size machines, the following points stand out with clearness:--
1. That the lifting power of a large machine, held stationary in a wind
at a small distance from the earth, is much less than the Lilienthal
table and our own laboratory experiments would lead us to expect. When
the machine is moved through the air, as in gliding, the discrepancy
seems much less marked.
2. That the ratio of drift to lift in well-balanced surfaces is less at
angles of incidence of five degrees to 12 degrees than at an angle of
three degrees.
3. That in arched surfaces the center of pressure at 90 degrees is near
the center of the surface, but moves slowly forward as the angle
becomes less, till a critical angle varying with the shape and depth of
the curve is reached, after which it moves rapidly toward the rear till
the angle of no lift is found.
4. That with similar conditions large surfaces may be controlled with
not much greater difficulty than small ones, if the control is effected
by manipulation of the surfaces themselves, rather than by a movement of
the body of the operator.
5. That the head resistances of the framing can be brought to a point
much below that usually estimated as necessary.
6. That tails, both vertical and horizontal, may with safety be
eliminated in gliding and other flying experiments.
7. That a horizontal position of the operator's body may be assumed
without excessive danger, and thus the head resistance reduced to about
one-fifth that of the upright position.
8. That a pair of superposed, or tandem, surfaces has less lift in
proportion to drift than either surface separately, even after making
allowance for weight and head resistance of the connections.
[Illustration]
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|Transcriber's Note: |
| |
| |
|On page 15: |
| |
|Wilbur, who was at one end, seized it in front, Mr. Daniels and I,|
|who were behind, tried to stop it behind, tried to stop it by |
|holding to the rear uprights. |
| |
|has been changed to |
| |
|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.|
| |
| |
|On page 21: |
| |
|Lilienthal was the first to discover this exceedingly though our |
|measurements differ considerably from those of Lilienthal. While |
|important fact, which is fully set forth in his book, "Bird |
|Flight the Basis of the Flying Art," but owing to some errors in |
|the methods he used in making measurements, question was raised |
|by other investigators not only as to the accuracy of his |
|figures, but even as to the existence of any tangential force at |
|all. Our experiments confirm the existence of this force, at Kitty|
|Hawk we spent much time in measuring the horizontal pressure on |
|our unloaded machine at various angles of incidence. |
| |
|has been changed to |
| |
|While our measurements differ considerably from those of |
|Lilienthal, Lilienthal was the first to discover this exceedingly |
|important fact, which is fully set forth in his book, "Bird |
|Flight the Basis of the Flying Art," but owing to some errors in |
|the methods he used in making measurements, question was raised |
|by other investigators not only as to the accuracy of his |
|figures, but even as to the existence of any tangential force at |
|all. Our experiments confirm the existence of this force. At |
|Kitty Hawk we spent much time in measuring the horizontal |
|pressure on our unloaded machine at various angles of incidence. |
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