EQUILIBRIUM AND CONTROL
In an aerodrome it is essential not only that its component parts
shall be so disposed that the initial equilibrium is correct and
highly stable, but also that some efficient means be provided for
quickly and accurately restoring the equilibrium, if for any reason
it is disturbed. If the aerodrome is of sufficient size and power to
carry a human being it is, of course, possible merely to supply an
efficient means of controlling the lateral and horizontal equilibrium
of the machine and depend upon the intelligence and skill of the
operator, as developed by practice and experience, to maintain the
proper equilibrium of the machine while in the air. This method,
however, is open to the objection that no matter how skilled the
aviator may be there remains the probability of a serious if not
fatal accident as the result of any momentary lapse or diversion
of attention until the “sense of equilibrium” has been developed.
One of the chief problems, therefore, which had impressed itself
from the beginning of the work, was to devise some means by which
the equilibrium of the aerodrome would be automatically maintained
under the varying conditions of flight, so as to leave the aviator
free, as far as possible, to control the direction of flight and
to devote his attention to other important matters connected with
the proper functioning of the various parts of the aerodrome. In
the development of the models it had been absolutely necessary to
develop some efficient automatic control, as they were far too small
to carry an aviator, and the conditions of flight in the open air,
even on the calmest day, were such that constant readjustments of the
equilibrium were necessary. The success attained in the automatic
control of the equilibrium of the models had been so great, and so
much time would have been required for an aviator to acquire skill
sufficient to control a machine without such automatic equilibrium,
that it was considered both expedient and safe to embody in the large
aerodrome the plans which had proved so successful in the models.
It was necessary, however, to provide in addition in the large
machine means whereby the aviator could quickly and accurately either
modify the action of the automatic devices or, if desired, entirely
supersede the automatic control by purely manual control. Three
distinct problems were, therefore, encountered in connection with the
equilibrium and control of the large aerodrome. In the first place,
the machine as a whole had to be so designed, and its component parts
so disposed as to secure a highly stable initial equilibrium; second,
automatic means had to be provided for [p208] maintaining this
equilibrium under the varying conditions of flight and for restoring
it if for any reason it was disturbed, and, finally, provision had
to be made for the quick and accurate control of the flight by the
aviator. These problems, while intimately related, had to be met one
by one and solved separately.
The general type of machine adopted was that which had been developed
in the years of experiment with the steam-driven models. From
the very first consideration of the large aerodrome, it seemed
advisable to follow this type, which not only had shown itself to
be distinguished by remarkable longitudinal and lateral stability
in the tests, but was actually the only type in the world which had
at that time shown any possibility of successful flight. There was,
of course, a question whether single surface or superposed wings
would be used, and in spite of the negative results obtained in the
tests of the models with the superposed wings, it was felt that
a considerable field for development was open in this direction.
However, in spite of the advantages which theoretical considerations
showed might be obtained through the introduction of this and various
other modifications of the original type, the whole teaching of past
experience in the construction of the model aerodromes had been that
success was more certain to be achieved by following the course in
which genuine practical results had been achieved. It was decided,
therefore, that in the construction of the large aerodrome the design
should follow as closely as constructional conditions would permit
the lines of the successful model Aerodromes Nos. 5 and 6, which have
already been fully described.
The longitudinal stability of an aerodrome is largely dependent
upon the relation of three chief factors; the center of pressure,
the center of gravity and the line of thrust. For an aerodrome of
the “Langley” type, the relative positions of these which give the
greatest degree of stability had been determined as far as possible
through the years of experiment with the models. However, while it
is the usual experience in designing machinery, or even scientific
apparatus, that what appears theoretically to be the best plan has
to be considerably modified for constructional reasons, yet in the
design of an aerodrome this is particularly true, for not only must
all the various parts function properly, both separately and as
a whole, but this result must be secured for the very minimum of
weight. Experience alone can enable one to appreciate thoroughly how
seriously this consideration of weight complicates the problem.
In making the original designs for the large aerodrome it had been
recognized that the relative positions of the line of thrust, center
of pressure, and center of gravity were much better in model No. 6
than in model No. 5. From Data Sheet No. 1, for Aerodrome No. 5 when
it made its flight on May 6, 1896, it will be noted that the line
of thrust being assumed to be at the point 1500,[43] [p209] the
center of gravity was at the point 1497, and that, assuming the rear
wings to have two-thirds of the lifting effect of the front ones,
the center of pressure was calculated to be at the point 1498, or
one centimetre in front of the center of gravity, measured in the
horizontal plane. In the vertical plane the center of pressure was
calculated to be at the point 2536, and the center of gravity was
found by test to be at the point 2501, when the line of thrust was
assumed to be at the point 2500, the center of gravity being actually
one centimetre above the line of thrust.
From the data sheet of Aerodrome No. 6, for its flight of November
28, 1896, it will be noted that the line of thrust being at the point
1500 the center of pressure was at the point 1487, and the center
of gravity at the point 1484; that is, the center of pressure was
three centimeters in front of the center of gravity, measured in the
horizontal plane. In the vertical plane, taking the line of thrust at
the point 2500, the center of pressure was at the point 2525, and the
center of gravity at the point 2486, the center of gravity being 14
centimetres ‹below› the line of thrust and 39 centimetres below the
center of pressure, the distance from the center of pressure to the
line of thrust being, therefore, 64 per cent of the distance between
the center of pressure and the center of gravity.
As has been explained in Part I, while it is not desirable that the
center of gravity be a great distance below the center of pressure,
as such a relation tends to produce a special kind of rolling and
pitching in varying currents of air, it is highly desirable that the
center of gravity should lie some distance below the line of thrust
in order that the three forces may be balanced. In a machine like
model No. 5, where the center of gravity was actually, though very
slightly above the line of thrust, there is a constant tendency to
produce rotation of the aerodrome, if for any reason its equilibrium
is disturbed, which is corrected in practice by the action of the
Pénaud tail. In model No. 6, on the other hand, the disposition of
the three factors was such that they tended to maintain, rather than
to destroy, the initial equilibrium of the machine.
These desirable relative positions had been made possible in model
No. 6 by the fact that the center of gravity and line of thrust could
be located at practically any desired point, since with the use of
steam the power plant consists of two separable parts, the boiler,
with its fuel and water tanks, and the engine. These parts can,
therefore, be placed in any part of the aerodrome that constructional
or theoretical reasons demand. Furthermore, the engine constitutes
such a relatively small portion of the weight of the entire machine
that, if for any reason it is desirable to place the engine in the
same plane as the line of thrust, its weight is not sufficient to
alter materially the position of the center of gravity, since the
boiler, water and fuel tanks can be placed as low as desirable and
connected with the engine by suitable pipes. [p210]
With a gasoline engine, however, the conditions are very greatly
altered. Here the engine constitutes practically the entire weight
of the power plant, only such accessories as the ignition coil,
batteries, and carburetor being available for lowering the center of
gravity, unless the fuel, cooling water tanks and radiator be placed
below the engine and the liquids forced up by means of a pump. In
making the first designs for the large aerodrome, therefore, it was
found that it would be practically impossible to make the relative
positions of the center of gravity and line of thrust the same as had
existed in model No. 6, however desirable it might be. The center of
gravity could be brought appreciably lower than the line of thrust
only by placing the gasoline engine in a plane considerably below
that of the propellers, and this necessitated the addition of at
least two more sets of gears with heavy bearings and braces. Besides
this almost prohibitive factor of weight, it was also foreseen that
great difficulty would be experienced in keeping even the two sets
of bevel gears already necessary aligned and in proper condition for
efficiently transmitting the power to the propellers unless the frame
and other parts were made prohibitively heavy. It was, therefore,
found necessary to bring the center of gravity practically in the
same plane with the line of thrust, which made its general features
as regards equilibrium more nearly resemble those of model No. 5 than
of No. 6.
The weight of the aviator, it is true, constituted an appreciable
part of the flying weight of the large machine, and it at first
seemed possible to lower the center of gravity by placing him at a
considerable distance below the line of thrust. But it was recognized
from the beginning that the aviator would probably have to give a
great deal of attention to any form of engine in order to insure its
working properly, and his position must, therefore, be selected with
a view to the proper supervision of the engine and without regard to
its effect on the center of gravity.
Although the repeated successful flights of model No. 5 under varying
conditions of wind and power inspired the belief that the minor
adjustments, as well as the general plan of the large aerodrome,
were such as to give highly stable equilibrium, nevertheless, more
direct corroboration of this opinion was desired, and it was largely
for this reason that the quarter-size model was constructed. In it
every detail of the larger machine which in any way affected its
equilibrium was exactly reproduced to scale, and the greatest care
was taken that the same relative positions of the center of pressure,
the center of gravity and the line of thrust which it was proposed
to employ for the large aerodrome should be used on the model in
its flight of August 8, 1903, which is later described. The entire
success of this flight, so far as the balancing was concerned, in
spite of the fact that the engine worked erratically and that the
launching speed was much less than it should have been, removed every
doubt [p211] that the equilibrium of the large aerodrome would be
satisfactory under normal conditions.
The second problem encountered in connection with the balancing and
control of the large aerodrome was that of providing an efficient
means for maintaining the equilibrium under varying atmospheric
conditions. Although much had been done toward the solution of
this problem in the development of the models, the whole question
was reopened and thoroughly reconsidered in designing the large
aerodrome. The Pénaud tail, when made elastic or when more or less
rigid, but attached to the frame through an elastic connection,
and normally set at a negative angle, furnishes a means of
automatically controlling the equilibrium, which is sufficiently
sensitive and accurate to enable a machine to fly for a considerable
distance, at least in moderately calm weather, as is evidenced by
the various flights of the model aerodromes, where there was no
human intelligence to control them. But owing to the principle of
action of the Pénaud tail, the flight of an aerodrome controlled
by it must of necessity be more or less undulatory in its course.
Furthermore, the tests with the models had indicated that, while the
Pénaud tail served remarkably well as a means of controlling the
equilibrium of the machine, provided the balancing had been rather
accurately determined, and, further, provided nothing happened to
affect seriously the equilibrium of the machine, it was limited in
its effectiveness by its narrow range of action. It was thought
that a control mechanism which should be more sensitive and at the
same time should act more powerfully to prevent the upsetting of
the equilibrium when the machine was subjected to rather strong
disturbing forces was desirable for any machine which was to
transport a human being and, therefore, involved the risk of a fatal
accident.
In the earlier period of the work and before the correct application
of the Pénaud tail to the model aerodromes had been found, Mr.
Langley had planned a large number of different forms of automatic
control for preserving the equilibrium of the machines. The more
frequently recurring of these were devices for changing the angle of
the wings or tail, and others for shifting the wings or tail bodily
so as to shift the position of the center of pressure with respect
to the center of gravity, the motive power for operating the devices
being in some cases that derived from a gyroscope or a pendulum, and
in others small electric motor apparatus controlled by a pendulum
or a gyroscope. Most of these, however, never reached the stage
of development where they were actually tried on the machines in
flight, as the tests of some of them in the shop showed that they
were unreliable, while others were abandoned either when partly
built or when only the drawings for them had been made. Among the
better-preserved models of devices for this purpose which were in
existence when the writer became associated with the work are those
shown in Plate 68, [p212] where the piece at the top is a pendulum
(inverted or direct) which controls the movement of the horizontal
tail by means of the cords and apparatus shown, actuating these
through the small electro magnets and apparatus attached. Just below
the rod, which represents a piece of the midrod, are three parts,
the first of which is a group of six little batteries clustered in a
circle, while next to it is a system of needles hung in gymbals, with
electro-steering apparatus in cups which itself turns on a graduated
base, these electric connections, together with the battery,
controlling the vertical rudder. On the right of this is another
piece of apparatus for actuating windlass cylinders which turn one
way or the other as the contact is made by one side or the other of
the pendulum or the needle. At the bottom, on the two rods, is a
tail-piece which automatically throws the center of pressure forward
or backward according as the aerodrome departs one way or the other
from the horizontal.
In spite of the fact that all the early attempts of Mr. Langley
to devise such a mechanical control had been very unsatisfactory,
the idea that something of this kind was necessary had never
really been abandoned by him. Here was to be seen one of his chief
characteristics, which was never to abandon any idea that seemed
valuable until it was brought to a successful issue or some very
strong proof was developed that the idea was impracticable. While
on a trip abroad during the summer of 1899, and especially while
resting at Vallombrosa, Italy, Mr. Langley’s mind again turned to
this problem, and he wrote a number of very interesting letters
emphasizing the importance of devising such a mechanism which should
be controlled by gravity. When he returned to the Institution in the
fall he insisted upon the same idea.
[Illustration: PL. 68
AUTOMATIC EQUILIBRIUM DEVICES]
[Illustration: PL. 69
FIG. 1
FIG. 2
MECHANISM OF CONTROL]
A mechanism which had been devised by the writer for another, but
somewhat similar, purpose seemed to be well adapted to this end,
and it was accordingly decided to construct a small model of such a
size as would be suitable for use on one of the steam-driven models.
The plan of control which it was proposed to follow was to have
some mechanism which would control the angle of the tail through
the action of gravity on a pendulum bob. Since it would require an
exceedingly heavy pendulum should the deflections of it be directly
utilized to produce corresponding movements of the tail, the most
feasible plan seemed to be to have a light pendulum, which, while
free to move under the action of gravity, would nevertheless by
its movement cause some outside force to produce corresponding and
simultaneous movements of the tail. The general scheme of arrangement
is shown in Plate 69, Figs. 1 and 2. This device consists essentially
of a cylinder (1) in which is mounted a piston with the piston rod
(3) passing through the cylinder head and connected to the cord (5)
which passes over the pulley (6), fastened to the tube (2), which is
slidably mounted on the midrod (7), whence it is carried over the
pulley (8) on the guy-post (9). From here it is connected to the
spring (10) which is fastened by the [p213] bridle (11) to the
upper side of the Pénaud tail (12). The other end of the piston rod
(3) passes through the head in the other end of the cylinder, and
has connected to it a cord (14) which passes over the pulley (15)
fastened to the tube (2), whence it is continued over the pulley (16)
and is joined to the spring (17), which is connected by the bridle
(18) to the lower side of the tail. Mounted on top of the cylinder
(1) is a valve chamber (20) having ports leading to the two ends
of the cylinder. Mounted in the valve chamber is a rocking valve
surrounded by a bushing having ports in it, and to which is fastened
a rod (25) which passes through the said valve and the head of the
valve chamber. Fastened to the rod (25) of the bushing is a lever
(26), which by means of the link (27) is connected to the piston rod
(3). Fastened to the rocking valve is a rod (28) which telescopes
over the rod (25) and also passes through the same head of the valve
chamber, and carries at its outer end a pendulum (29) on the lower
end of which is the bob (30).
If steam or any other fluid under pressure is furnished to the valve
chamber through the pipe (31), none will be admitted to the cylinder
so long as the pendulum is vertical or at right angles to the axis
of the cylinder; and the tail will be in its normal position, which
we will suppose to be an upward inclination of five degrees. If,
now, the front of the machine be depressed, thereby causing the
pendulum to move to the right, such movement of the pendulum will
cause the valve to open, admitting fluid to the left-hand end of
the cylinder. This, acting on the piston, will force it towards
the right, which, by means of the cord, will cause the angle of
the tail to be increased, thereby causing the rear of the machine
to be depressed and the front to be raised. But as soon as the
piston begins to move under the action of the fluid pressure it
simultaneously moves the bushing which surrounds the valve by means
of the connecting links and levers, so that as soon as the piston
has moved a distance proportional to the amount that the valve has
been opened by the pendulum, it causes the bushing to shut off the
port and thus prevents further fluid entering the cylinder. As soon
as the aerodrome responds to the action of the tail the pendulum
will, of course, begin to move back to its normal position of
perpendicularity to the cylinder, and will then open the valve to
the other port, thereby causing fluid to pass into the opposite end
of the cylinder. This fluid acting on the piston will move it in the
opposite direction and thereby cause the tail to be drawn back to its
normal position at the same time that the pendulum gradually reaches
its normal position, owing to the return of the aerodrome to its
normal position. In the explanation given above it was assumed that
the slidable tube (2) was in a fixed position. It was planned to have
the equilibrium normally maintained automatically and at the same
time permit the operator to modify the automatic control and even to
assume full manual control. To secure this, the slidable tube (2) was
connected at each end to an [p214] endless cord (20) which after
passing over suitable pulleys was connected to the control wheel (51)
at the aviator’s car.
A model of this device was constructed in the spring of 1900 and was
tested with steam pressure in the shop. The test showed that the
device acted immediately and with precision, the piston performing
movements simultaneously and in exact accordance with the pendulum.
The device, however, was never tried in a flight of any of the
aerodromes owing to the lack of time necessary to properly install
it on the machine. Furthermore, it was thought probable that the
rapid acceleration of the aerodrome at the moment of launching would
so disturb the pendulum as to cause it to be in a very different
position from that of vertical, and also that the motion of the
aerodrome through the air would itself be a somewhat disturbing
factor.
Because of the difficulties involved in this or any other mechanical
device for controlling the equilibrium, it was in every way advisable
to retain in the large machine the Pénaud system, which, though
itself imperfect in many ways, had been thoroughly tested in actual
flight. In the models, it will be remembered, the combined Pénaud
tail and rudder controlled the longitudinal equilibrium by movement
in the vertical plane under the combined influence of its initial
negative angle and the elasticity of its connection with the frame,
the flight being kept as nearly as possible in a straight line by the
vertical surfaces of the tail. Although it was necessary that the
large aerodrome should be capable of being steered in a horizontal
direction, it was felt to be unwise to give the combined Pénaud
tail and rudder motion in the horizontal plane in order to attain
this end, since the use of it for such a double function might
very seriously interfere with its proper action in preserving the
longitudinal stability. It was, therefore, at first thought best to
dissociate the rudder and tail so that the rudder might be used for
horizontal steering without in any way interfering with the proper
functioning of the tail. But, as the main desideratum was to obtain
a flight of the large machine as soon as possible, and perfection
of steering control seemed secondary, it was decided, after further
consideration, in order not to risk the unpredictable effects that
might result from small changes, to duplicate on the large machine
the combined Pénaud tail and rudder of the model, and to add
another rudder for steering in the horizontal plane. Constructional
requirements determined as the only available position for this
rudder a rather disadvantageous one. As will be seen from Plate 53,
its efficiency was diminished by its being only about half as far
from the center of gravity as the combined Pénaud tail and rudder,
and by being located in the lee of a considerable portion of the
frame, where it would be subject to the cross-currents of air created
by the forward motion of the frame. [p215]
For the preservation of the equilibrium of the aerodrome, though the
aviator might assist by such slight movements as he was able to make
in the limited space of the aviator’s car, the main reliance was upon
the Pénaud tail. But, in the absence of any data for determining
the effect produced in passing from the model to the large machine,
it could not be certain that calculations based upon the balancing
of the model would accurately determine the proper balancing of the
large machine. It was therefore decided to provide such attachment
for the Pénaud tail that, while it would always have elastic
connection with the main frame, yet its angle could be appreciably
changed without affecting in any way the degree of elasticity of this
connection. After many changes in plans for securing this result,
it was finally decided to arrange it in the manner shown in the
drawings. Referring to the general plans in Plates 53 and 54, and to
the details in Fig. 1 of Plate 56, the main stem of the Pénaud tail
is seen to be connected by a pin to the horn (17), which is brazed
to the clamping thimble, by which it is mounted on the vertical tube
(16), suitably connected and braced to the rear end of the midrod,
the horn (17) being larger than the stem of the tail and set at an
angle to the vertical tube (16), the pin connection permitting the
tail to swing up and down. The bridle (40), connected to the center
of the tail on its upper side, passes upward where it is connected
to the spring (41), the other end of which is connected to a single
wire rope (42), which passes over the pulley mounted on the top of
the post (43), which is guyed to the upper guy-post by the wire (44).
The wire rope (42), after passing over the pulley, is connected to
the spring (45), around the two ends of which it forms a loop, and
from there it passes down to the plane of the main frame and through
suitable pulley blocks to the aviator’s control wheel (50), which is
mounted on the starboard side of the main frame, convenient to the
aviator’s right hand when he is facing forward. From this point the
wire rope passes through the various pulley blocks towards the rear
of the machine, and through the pulley block (46) mounted on the side
and near the bottom of the rear lower guy-post. At a short distance
beyond this pulley it is connected to a weaker spring (47), the other
end of which is connected by a second bridle (48) to the under side
of the Pénaud tail at its center. In order to prevent the springs
(41), (45) and (47), which furnish the elasticity for the Pénaud-tail
connection, from being strained beyond their elastic limit, either
by a sudden gust of wind or by the aviator attempting to move so
large an area of surface too suddenly, the wire rope (42) was made
continuous around the springs, the portion between the points where
it was joined to the two ends of the springs being made of such a
length as to take the entire strain should the strain on the cord
become greater than sufficient to stretch the springs 50 per cent of
their original length. [p216]
In the construction of the equilibrium control wheel it was decided
that some arrangement must be secured whereby the wheel would
normally be inactive and maintain whatever position it had been set
to, and at the same time could be moved by the aviator with one
hand, the mere act of grasping it rendering it free to be moved, and
whereby it must automatically lock itself in any position in which it
might be when the aviator removed his hand from it. The multiplicity
of things requiring the attention of the aviator made it desirable
that his attention to any one of the important details, whether the
engine, the equilibrium, or the steering, should never require more
than one hand, thus leaving the other hand free either to hold on to
the machine or to control some other detail at the same time. While
an irreversible wheel, such as would be secured by the use of a worm
and worm-wheel, at first seemed likely to answer the purpose, yet the
movement of a worm-wheel by means of a worm is necessarily very slow
if it is irreversible, and it here seemed desirable to so arrange the
wheel that in case of emergency, of for rising or descending, the
aviator could swing the Pénaud tail from its extreme upper position
to its extreme lower one by a small motion of his hand, and thus
small or large adjustments of the Pénaud tail could be intuitively
felt to have been produced without the aviator having to remember how
many turns he had made of the wheel.
The control of the steering rudder was effected by a steering wheel
(51) similar in construction to the equilibrium control wheel (50), a
continuous cord (52) passing from the steering wheel through suitable
pulleys to either side of the steering rudder (‹r›), springs being
interposed in loops in the cord on either side of the steering rudder
to give some elasticity to the control apparatus in order to prevent
possible danger from the aviator attempting to move the rudder too
suddenly. This steering rope passed directly through the steering
rudder at the points where it was joined to it; so that, should one
side of the cord in any way become entangled with the frame or with
its pulleys, the strain produced by the aviator in attempting to
move it in the opposite direction would be taken up by the cord and
thereby avoid the possibility of destroying the rudder. For even
should the cord become entangled on one side, the rudder could be
given a slight amount of adjustment through the elasticity of the
coiled springs.
The design of the combined Pénaud tail and rudder followed very
closely that which had been used for the models, and its area
of ninety-five square feet on the horizontal surface with a
corresponding area of vertical surface bore the same relation to the
area of the tail and rudder of the models that the area of the wings
of the large machine bore to that of the wings of its prototype.
While the provisions for automatic equilibrium and manual control
were not entirely ideal, even for the quiet atmospheric conditions
under which it [p217] was proposed to make the first tests,
nevertheless it was and still is believed that the provisions for
such conditions were sufficient to enable a successful flight of a
few miles to be obtained. It was thought to be very certain that,
once a successful flight could be made, the funds for the further
prosecution of the work would be readily forthcoming, and that when
these funds were obtained the many problems of control, rising and
alighting, could be undertaken.
[p218]