CONSTRUCTION AND TESTS OF THE LARGE ENGINE
The main requirement in an engine for an aerodrome--aside from
reliability and smoothness of operation, which are necessary in an
engine for any kind of locomotion—is that it shall develop the
greatest amount of power for the least weight. It is, therefore,
desirable to reduce the weight and number of parts of the engine to
the very minimum, so far as this can be done without sacrificing
reliability and smoothness of running. Furthermore, since the
strongest metal for its weight is steel, and since the greatest
strength of steel is utilized when the stress acting on it is one
of tension, it is advisable to design the engine so that the parts
which sustain the greatest strains shall be of steel and, as far as
possible, meet with strains which are purely tensional ones.
In designing the new engine for the large aerodrome it was,
therefore, planned to make it entirely of steel, as far as this was
possible. The only parts which were not of steel were the bronze
bushings for the bearings, the cast-iron pistons, and cast-iron
liners of the cylinders. Previous experience had shown that, while
it is possible to use a cast-iron piston in a steel cylinder or
even a steel piston in a steel cylinder, provided the lubrication
be kept exactly adjusted, yet the proper lubrication of the piston
and cylinder of a gas engine is difficult even under the most
favorable conditions, owing to the fact that excessive lubrication
causes trouble from the surplus oil interfering with the sparking
apparatus. It was, therefore, determined not to risk serious trouble
by attempting to have the pistons bear directly on the steel walls of
the cylinders.
While visiting the French engine builders in the summer of 1900 in
the attempt to find one willing to undertake the construction of a
suitable engine for the aerodrome, it was pointed out to them that
the great amount of weight which they claimed to be necessary for the
cylinders, and which they stated made it impossible for them to build
an engine which would meet the requirements as to power and weight,
could be very greatly reduced by making the cylinders in the form of
thin steel shells having cast-iron linings. All, however, to whom
this suggestion was made declared that it was impossible to build
satisfactory cylinders in this way; some of them even stated that
they had tried it and found it impossible to keep the thin liners
tight in the steel shells. The difficulty which they had encountered
is due to the difference in expansion of the steel and the iron when
raised to a rather high temperature by the heat of the explosions,
if the cylinders are not well jacketed with water; and if the steel
[p235] shells are water jacketed they then do not expand as much
as the cast-iron liners, and this causes the latter to become “out
of round” because of the compression strains produced in them when
trying to expand more than the steel shells. As past experience had
shown, however, that it was possible to keep the liners tight in
small cylinders, it was believed that by taking proper care in the
construction there would be no difficulty in this respect with the
cylinders of this larger engine.
In carrying out these plans, however, of making the cylinders of
steel, numerous constructional difficulties were encountered which
could not be foreseen when the design was made. Had they been
foreseen, provision for obviating them could easily have been made.
As will be seen from the drawing, Plate 78, the engine cylinders
consisted primarily of a main outer shell of steel one-sixteenth
of an inch thick, near the bottom end of which was screwed and
brazed a suitable flange, by which it was bolted to the supporting
drum or crank chamber. These shells, which were seamless, with the
heads formed integral, were designed to be of sufficient strength
to withstand the force of the explosion in them, and, in order to
provide a suitable wearing surface for the piston, a cast-iron
liner one-sixteenth of an inch thick was carefully shrunk into
them. Entering the side of the cylinder near the top, was the
combustion chamber, machined out of a solid steel forging, which
also formed the port which entered the cylinder and was fastened to
it by brazing. The water jackets, which were formed of sheet steel
.020 inch thick, were also fastened to the cylinder by brazing,
and it was in connection with the brazing of these water jackets
that the first serious difficulty was met in the construction of
the engine. In the first place, as the jackets were of an irregular
shape and of a different thickness of metal from the walls of the
cylinder to which they were joined, the expansion and contraction
due to the extreme heat necessary for properly brazing the joints
caused such serious strains in various and unexpected directions
that it was only by exercising the very greatest care and patience
that a completely tight joint at all points of the jacket could be
secured. In the second place, the size of the cylinders and the
consequently large extent of water-jacket surface, complicated the
problem. The maintenance over this large surface of the extreme
heat necessary for brazing involved discomfort and, indeed, actual
suffering to the person engaged in the work, and much care and skill
were demanded in so distributing the heat that the temperature
of the surface of the jackets would be uniform enough to prevent
serious strains from expansion and contraction. As no workman could
be found either competent to do the work or willing to undergo the
personal discomfort, the writer was obliged to do all this brazing
work himself. Besides the difficulties due to the expansion and
contraction of the jackets while they were being brazed, the greatest
care had to be exercised to avoid heating the cylinders so hot as to
weaken the [p236] joint where the explosion chambers were joined to
the cylinders, which, of course, had been brazed before the jackets
were fitted to them preparatory to brazing them.
Another great difficulty was that the ring which encircled the
cylinder near the middle of its length, and which formed the bottom
part of the water jacket, expanded very much more than the cylinder
itself, so that, if it was brazed to the cylinder before the jacket
was brazed to it, the heat of brazing the jacket to the ring would
cause the ring to break loose from the cylinder; while if the ring
was not previously brazed to the cylinder, but was brazed after the
jacket had been brazed to it, the very much greater heat required
for brazing the ring to the cylinder caused the spelter to burn
out of the joint between the jacket and the ring. Furthermore, it
was found very difficult to braze the two joints at the same time,
since in brazing the ring to the cylinder it was best to have the
cylinder in an inverted vertical position, so that the spelter could
be made to flow evenly around the ring and form a fillet against the
wall of the cylinder, while in brazing the jackets to the ring it
was best to have the cylinder in the reverse vertical position or
lying on its side so that the spelter could properly flow into this
joint. Finally, however, after what proved to be most exasperating
and tedious work, the five cylinders necessary for the engine were
completed and a series of tests was immediately made. During the
course of these tests the water circulation became obstructed in
several instances, and the consequent high temperature to which
the cylinders and jackets were raised caused severe strains in the
jackets which, in turn, produced breaks in the brazed joints. These
breaks had to be rebrazed, and in brazing them it was necessary
in almost every case to remove the cast-iron liners and rebraze
the entire jackets from start to finish, as the application of the
intense heat necessary for brazing at any one point produced such
severe strains that before the break which was being repaired could
be completed other breaks developed at various points of the jacket.
It was, therefore, necessary to get the whole jacket up to a fairly
uniform heat and complete the brazing while it was in this condition,
and then keep the whole cylinder at a uniform but gradually
decreasing temperature until it had sufficiently cooled off.
On account of these troubles with the water jackets and the cylinders,
it was decided to build some extra cylinders, not only because past
experience had suggested improvements in detail in the construction of
the jackets, which would prevent to a large extent the great troubles
which had been met with in the brazed joints, but also to insure
having sufficient cylinders to enable the engine to be always in
working condition, even though several of the cylinders might be out of
commission from slight imperfections in the jackets or at other points.
While the construction of these new cylinders involved a repetition
of the arduous task of brazing, yet the minor improvements which
were introduced [p237] proved eminently successful in providing
against future troubles from leaky jackets.
[Illustration: PL. 78
ENGINE OF AERODROME A. SECTION THROUGH CYLINDER AND DRUM]
[Illustration: PL. 79
ENGINE OF AERODROME A. END ELEVATION, PORT SIDE]
[Illustration: PL. 80
ENGINE OF AERODROME A. TOP PLAN]
[Illustration: PL. 81
ENGINE OF AERODROME A. ELEVATION STARBOARD BED PLATE, SPARKING
MECHANISM]
The general form of construction of the engine with the improved
cylinders will be readily understood from the drawings, Plates 78–81,
in which Plate 78 is a detail sectional view, previously referred to,
through one of the cylinders; Plate 79 is an end elevation of the
port side, Plate 80 is a plan view, and Plate 81 is an elevation of
the starboard bed plate which supports that side of the engine, and
by which it was fastened to the aerodrome frame, this view showing
particularly the sparking apparatus which was mounted on the bed
plate. The engine consists primarily of a single crank shaft provided
with a single crank pin, the shaft having bearings in a drum which
consists essentially of two heads. Arranged around the crank shaft
and attached at equidistant points of the drum are five cylinders.
Mounted on the port side of the crank shaft and close to the crank
arm is a small gear, which through suitable gears mounted on the port
head of the drum drives a double-pointed cam which has a bearing on
the exterior of the hub of the drum. The ratio of these gears is such
that the cam is driven at one-quarter the speed of the crank shaft,
and in the reverse direction. Mounted on the exterior side of the
port head of the drum are five punch rods, the upper ends of which
are within a sixty-fourth of an inch of being in contact with the
exhaust-valve stems of the cylinders, and on the lower end of these
rods are hardened-steel rollers which rest on the double-pointed
cam--this one cam thus serving to operate the exhaust valves of all
five of the cylinders. The port head of the drum is connected to
the port bed plate, by which it is supported, by means of a flanged
bushing in which are formed tongues and grooves which fit into
corresponding grooves and tongues formed in the hub of the drum, it
being necessary to have a certain amount of space between this bed
plate and the head of the drum to provide room for the exhaust-valve
cam and its co-acting punch rods. The starboard bed plate is fastened
to the starboard head of the drum by bolts which draw the web of the
bed plate against the face of the drum. The sparking gears are driven
by means of a gear formed on a sleeve which telescopes over the hub
of the starboard drum, and has a bearing thereon, the end of the
sleeve terminating in a ring which is fastened to the crank shaft.
Since the five connecting rods must center on the one crank pin, the
bronze shoes in which they terminate can occupy only a portion of the
circumference of the pin, and with the relative proportions which
here existed between the length of stroke of crank and the length of
the connecting rod, the circumferential width of the connecting-rod
shoes was slightly less than sixty degrees, thus leaving uncovered
a crank space of about one-sixth of the circumference, which it was
necessary to have in order to provide room for the change in relative
position of the shoes due to the angularity of the connecting rods.
In [p238] the experimental engine the connecting-rod shoes were
all given their bearing directly on the crank pin, as heretofore
described, being held in contact therewith by means of cone nuts,
which were screw-threaded to the crank pin, the taper of the cones
permitting adjustment for wear. This method of connecting these
parts to the crank pin is the usual plan of connecting three or
more connecting rods to one crank pin. So much trouble had been
experienced with the water jackets and with minor defects in the
experimental engine that no long runs had been possible with it,
and consequently no trouble had been experienced because of the
small amount of bearing area provided by this method of joining the
connecting rods to the crank pin. When, however, the new engine
was completed it was found that after working at high power for a
few minutes the connecting-rod shoes heated so rapidly that it was
impossible to run the engine for more than ten or twelve minutes, the
excessive heating of the shoes causing a great diminution in power
besides the danger of serious damage if the tests were continued
longer. At first this defect seemed almost fatal, as there appeared
to be no way of providing sufficient bearing area for the five
connecting rods on one crank pin. Happily, however, the writer was
able to overcome this defect by an improved design which enables all
five connecting rods to operate on the one crank pin, and at the
same time provides each with the full amount of bearing area which
it would have were it the only connecting rod operating on the crank
pin. This arrangement consists essentially of a main connecting rod
formed of a steel forging terminating in a sleeve which encircles
the crank pin and is provided with a bronze lining for giving a
proper bearing surface between the connecting rod and the crank pin,
both the steel sleeve and the bronze lining being split, but at
right angles to each other, to permit assembling them on the crank
pin. This steel sleeve, the upper half of which is formed integral
with the main connecting rod is rounded off to a true circle on its
exterior circumference, except at the point where the rod joins it.
The other four connecting rods terminating in bronze shoes are then
caused to bear on the exterior of this sleeve, being held in contact
therewith, and permitted to have a sliding motion thereon sufficient
to take care of the variation in angularity of the connecting rods,
by means of the cone nuts which are screw-threaded to the sleeve and
locked thereto by means of the jam nuts, as shown in the drawings.
The main connecting rod, of course, acts in the same way as in the
ordinary case where each cylinder has its separate crank pin. The
other four connecting rods deliver their effort to the crank pin
through the sleeve in which the first connecting rod terminates, and
they, therefore, do not receive any of the rubbing effect due to
the rotation of the crank pin, except that of slipping a very short
distance over the circumference of the sleeve during each revolution,
the amount of slipping depending on the angularity of the connecting
rod. This improved type of bearing was successful from the time of
its first trial, and even in later [p239] tests in which the engine
was run for ten consecutive hours at full power it showed no signs
whatever of overheating. As this new form of connecting-rod bearing
for the crank pin had never been tried before, the precaution was
taken to leave the threads on the crank pin for the cone nuts, so
that if this new bearing should not prove successful the old plan of
having the connecting-rod shoes bear directly on the crank pin could
be reverted to. These threads are clearly seen in Plate 78 and were
never removed from the crank pin, though their removal would have
added considerably to the area of the bearing surface of the main
connecting rod, had more bearing surface seemed necessary.
The lubrication of the main crank-shaft bearing and of the crank
pin was effected by means of a small oil cup, fastened to the port
bed plate, which fed oil through a hole in the hub of the drum to a
circular groove formed in the bronze bushing in the hub. The crank
shaft being hollow, a hole was drilled through it in line with the
groove in the bushing, and the oil was then led from the interior
of the crank shaft through a pipe connected to the plug in the end
thereof, and through a hole drilled in the crank arm to the hollow
crank pin. Small holes through the crank pin permitted oil to pass to
the exterior thereof and thus oil the bearing of the main connecting
rod. Small holes through the sleeve and bushing of the main
connecting rod fed oil under the shoes of the other four connecting
rods, the small holes being placed in oil grooves formed in the
interior of the bronze bushing. The lubrication of the pistons was
effected by means of small crescent-shaped oil cups fastened to the
outer wall of the cylinders, which distributed the oil equidistantly
around the circumference of the pistons, through small tubes which
projected through corresponding holes drilled in the cylinder wall.
These oil cups for the cylinders were, while small, of sufficient
size to furnish a supply for approximately one hour, and were so
positioned on each cylinder as to have a gravity feed. It may be
mentioned here that while there were many parts of the engine which
were of unprecedented lightness there was nothing which excelled
these oil cups in this respect, as they were made of sheet steel .003
of an inch thick, riveted and soldered up. The crank-shaft bearing in
the starboard drum was oiled from an oil cup mounted on the outside
of the bed plate and connected by a pipe to a hole in the inner wall
of the drum, which was connected to the oil grooves in the bronze
bushing in the hub of the drum.
The first set of pistons for this engine were similar in design to
those shown in the assembled drawings, except that they had side
walls and heads which were twice as thick as those shown. These
lighter pistons were constructed later, and were just as good as
the earlier and heavier ones. It will be noted that the pistons
have two deep but thin ribs reinforcing the head. The pistons were
slightly tapered from the middle, where they were .005 inch smaller
than the cylinder bore, toward the outer end, where they were .0075
inch smaller [p240] than the bore. The outer piston ring was .0035
inch narrower than its groove, the second one .003 inch, the third
.0025 inch, and the inner one .002 inch narrower than its groove.
The rings were bored one-sixteenth inch off center with the exterior
surface, and had one-eighth inch diameter of spring. They were of
the lap-joint type, with the sides of the laps carefully fitted and
only one-sixty-fourth-inch clearance at the ends of the laps to allow
for thermal expansion. As no grinding facilities were obtainable in
Washington, the cylinders were carefully bored smooth and free from
taper, and the pistons were worn in to a perfect fit by running them
in by a belt for twenty-four hours, with copious oil supply.
The main connecting rod was 7/8-inch diameter and solid, while
the other four were of the same diameter but with a 5/8-inch hole
in them. The gudgeon pins in the pistons were hollow steel tubes
7/8-inch diameter and case-hardened, and were oiled entirely by the
oil thrown off by centrifugal force from the crank-pin bearing, the
oil running along the connecting rods and through suitable holes at
the heads into oil grooves in the bronze bushings in these heads.
Since on an engine for an aerodrome the best plan for releasing
the exhaust gases from the engine is to get rid of them as soon
as possible, so long as they are released behind the aviator and
do not interfere with his view in the direction of motion, it was
decided to have the gases exhaust immediately from the combustion
chambers; but in order to prevent their playing on and heating the
main bearing of the crank shaft in the port drum the combustion
chambers were each provided with a chamber below the exhaust-valve
seat, with a side outlet therefrom. The manifold pipe through which
the gaseous mixture was supplied to the inlet valves of the engine
consisted of a tube bent to a circle and having five branch tubes,
each leading to one of the automatic inlet valves, which fitted
removable cast-iron seats fastened by a nut in the upper part of
each combustion chamber. The very small amount of clearance between
the engine and the frame necessitated that this pipe be cut in
three places and joined by flanges in order to properly assemble
it on the engine when the latter was mounted in the frame. The
carburetor, which was placed near the rear of the aviator’s car,
was connected through suitable pipes to this circular inlet pipe,
at a point horizontally in line with the center of the shaft. The
auxiliary air valve consisted of a sleeve rotatably mounted on the
vertical pipe leading from the carburetor to the manifold, holes in
the sleeve being brought to coincide more or less with holes in the
vertical pipe, by the operator, when more or less air was required
or when he wished to vary the speed of the engine. The cooling water
for the jackets of the cylinders was led to them through a circular
manifold pipe on the starboard side connected by a vertical pipe
with the centrifugal pump situated at the lower point of the lower
pyramid of the aerodrome frame. The heated water was led from the
jackets through another [p241] circular manifold pipe on the port
side, through two connections to the radiating tubes at the front and
rear, respectively, of the cross-frame. These radiating tubes, which
were provided with thin radiating ribs soldered to them, finally led
the cooled water to the tank situated in the extreme rear of the
aviator’s car, a suitable pipe from the bottom of this tank being
connected to the inlet side of the centrifugal pump. The centrifugal
pump was driven by means of a vertical shaft connected to the crank
shaft through a set of bevel gears which drove it at three times the
speed of the engine. The bearings through which these gears were
connected were mounted on the port bed plate, and in order to allow
for a certain amount of vibration between the engine and the pump
this vertical connecting shaft had a telescoping section connected
through suitable splines.
The sparking apparatus comprised, first, a primary sparker similar
to the simplest form of such devices which have since come into
common use, where a cam driven by the engine co-acts with a pawl
on the end of a spring, but in this case, as this sparker was used
for all five cylinders, the cam was driven at a speed of two and
one-half times that of the engine shaft, thus making and breaking the
primary circuit five times in each two revolutions of the engine.
Second, a spark coil, the primary terminals of which were connected
to the primary sparker and to a set of dry batteries. Third, a
secondary distributor consisting of a disc carrying a contact brush
and driven at a speed one-half that of the engine, this brush being
constantly connected through a contact ring to one of the terminals
of the high-tension side of the spark coil and running over the
face of a five-section commutator, each of the sections of which
was connected to a spark plug, the other high-tension terminal of
the spark coil being, of course, grounded on the engine frame. This
sparking apparatus was first constructed by using blocks of red
fibre for insulation. After the engine was completed and was being
tested difficulties were met with in the sparking apparatus which
at that time appeared inexplicable. After a great deal of annoyance
and loss of time it was finally discovered that the red fibre was
not as good an insulating medium as it was supposed to be, owing
to the zinc oxide used in making it. In damp weather the sparking
apparatus absolutely refused to work, and it was found that the
moisture in the air caused the zinc oxide in the fibre to nullify its
insulating qualities. This trouble, after being located, was cured by
substituting hard rubber for the red fibre.
At the time when this engine was built, as well as earlier when the
experimental engine was built, it was impossible to procure any wire
which had been properly insulated to withstand the high voltages
necessary for the connections between the high-tension side of the
spark coil and the secondary distributor, and from the secondary
distributor to the spark plugs in the cylinders. While at this time
this appears a very simple matter, yet the trouble experienced and
[p242] the delays caused by the lack of such small accessories
which are now so easily procurable were very exasperating, and it
was finally necessary to insulate these wires by covering them with
several thicknesses of ordinary rubber tube of different diameters
telescoped over each other.
In the early tests of this new engine, which were made with it
mounted on a special testing frame and delivering its power to the
water-absorption dynamometers, the engine was operated without any
fly wheels, and, so far as its smoothness of operation was concerned
and its ability to generate its maximum power, it did not require any.
After the completion of the tests on the testing frame the engine
was assembled in the aerodrome frame, which was first mounted on the
floor of the launching car. The car itself was mounted on a short
track in the shop, which arrangement provided a smoothly rolling
carriage which could be utilized for measuring the thrust of the
propellers by merely attaching a spring balance between the rear of
the car and a proper holding strap on the track. In the first tests
of the engine under these conditions, it was found that while the
engine itself did not require any fly wheels, yet the lack of them
caused trouble with the transmission and propeller shafts, which,
while it had never been anticipated, was easily understood when it
was encountered. This difficulty was caused by the “reverse torque,”
which fluctuated from a maximum to a minimum five times during
each double revolution of the engine, and which set up fluctuating
torsional strains of such magnitude in the transmission and propeller
shafts that the shafts themselves became exceedingly hot after a few
minutes operation of the engine, and under more prolonged periods
of operation these fluctuating torsional strains caused a permanent
twisting and bending of the shafts. The transmission and propeller
shafts were at first made of tubing one-sixteenth of an inch thick,
but these were abandoned both on account of the necessity of
abandoning the screw-thread method of attaching the flange couplings
and gears, and also because these shafts had been designed when it
was expected to transmit only twelve horse-power to each propeller,
while the increase of power in the large engine necessarily required
much stronger shafts. The first shafts which were actually tested
in the frame were, therefore, one and one-half inches in diameter
by three-thirty-seconds of an inch thick, the tubing having been
one-thirty-second of an inch larger originally and turned down to
this size to insure a straight shaft. When these shafts twisted under
the action of the reverse torque of the engine, a very much heavier
set, practically twice as thick, were constructed. When used in the
tests these heavier shafts, while much stronger, still showed a large
amount of heating due to the fluctuating torsional strains.
Upon calculation it was found that by providing specially light fly
wheels the major portion of this reverse torque could be eliminated
for a less increase [p243] in weight than would be occasioned
by sufficiently increasing the thickness of the transmission and
propeller shafts to safely stand it. Since it was desired to
concentrate as much as possible of the weight of the fly wheels
in the rims, the idea at once suggested itself of building them
up like a bicycle wheel by means of tangent spokes. Two steel
automobile-wheel rims were therefore procured thirty-three inches in
diameter, and these were provided with tangent spokes connected to
special steel hubs fitted to the crank shaft of the engine. The rims
themselves not being quite heavy enough, and constructional reasons
necessitating their being at different distances from the center of
length of the crank pin, the extra weight which it was desired to
give to these rims was provided by means of steel wire wound tightly
around and fastened to the rims, the weight of each rim being made
inversely proportional to its distance from the center of the crank
pin. The first spokes which were used for these wheels were standard
bicycle spokes three-thirty-seconds of an inch in diameter, but these
were soon found to be entirely too weak to withstand the sudden
strains due to the rapid starting of the engine. They were therefore
replaced by standard spokes one-eighth of an inch in diameter, but
these also proved too weak and were later replaced with special
spokes made in the shop out of No. 10 coppered-steel wire, which by
test was found to have a tensional strength of 2192 pounds. As these
steel rims were only one-sixteenth of an inch thick and had not been
made exactly true, but had been straightened before being used, it
was found that they very quickly went out of shape under the strain
due to the centrifugal force at high speeds, and also when the engine
was suddenly accelerated. As long as they did stay true, however, it
was found that they were sufficiently heavy to provide all of the
fly-wheel effect it was necessary to have in order to eliminate all
trouble from the reverse torque.
After further consideration, it was decided that the only means of
constructing a fly wheel which would have a stiff rim and at the
same time would not be heavier than the steel ones, which had been
found adequate, was by perpetrating what would at first sight appear
to be an absurdity. A new set of rims for the fly wheels was made
by constructing them of an aluminum casting, the section of the rim
being U-shaped. After machining these rims and assembling the fly
wheels with them, it was found that they were many times stiffer
than the previous steel ones of the same weight, and after this
change no further trouble was experienced in keeping the fly wheels
perfectly true, even under the most severe strains. In fact, on one
occasion when the engine broke loose from the propellers, it ran to a
speed, which, while not exactly known, yet reached the limit of the
tachometer, which was 2000 R. P. M., without injury to the fly wheels.
It will be recalled that in starting up the engine on the
quarter-size model, the initial “cranking” necessary with a gasoline
engine was accomplished by [p244] having two of the mechanics turn
the propellers. While this same plan might have been followed in the
case of the large aerodrome, yet it would have involved some danger
to the mechanics and would also have left the aviator without any
means of restarting the engine should it for any reason stop while
in the air. Believing it to be very important to provide means for
enabling the aviator to restart the engine in case it stopped in the
air, the writer devised the starting mechanism shown in the drawings,
Plates 78 to 80. Fastened by tongues and grooves to the port side
of the engine crank shaft, just outside of the bed plate, is a worm
wheel, on the hub of which is mounted the bevel gear which drives
the water-circulation pump through the bevel pinion, as already
described. Mounted on the web of the bed plate are two brackets,
in which the shaft for the starting crank is journaled, this shaft
passing forward and downward through the front of the cross-frame
of the aerodrome, where it is journaled in a bracket secured to the
brace tubes thereof. At the front or lower end of the shaft a crank
handle is connected thereto by a ratchet mechanism. The upper end
of the starting shaft, between the bearings of the two supporting
brackets, is tongued and grooved, and slidably mounted thereon with
co-acting grooves and tongues is a worm screw which, in the position
shown in Plates 79 and 80, is in gear with the worm wheel just
described. However, when the worm screw is slid along on the shaft
until it is against the upper bracket it is out of gear with the
worm wheel. Mounted in the interior of the tubular starting shaft is
a spring-pressed pawl plug, not shown, but which projects through
one of the tongues on the shaft near the upper bracket. If the worm
screw is slid up against this upper bracket, this pawl catches in a
radial hole in the worm screw and holds it in this position out of
gear with the worm wheel. Connected to this pawl plug and passing
longitudinally through the center of the shaft is a wire which
terminates in a button just at the end thereof. By pulling on this
button the operator may release the worm and thus permit it to slide
downward so that when the starting crank is turned in a clockwise
direction the worm will screw itself into gear with the worm wheel,
and any further turning of the starting crank will cause the worm to
force the worm wheel, and, consequently, the engine shaft, around in
a clockwise direction. As soon as the engine gets an explosion the
worm wheel slides the worm along against the upper bracket, where the
spring pawl catches and holds it till it is again released by the
operator as before.
This starting mechanism was a success from the first, and the engine
was never started up in any other way. With an aerodrome having the
qualities of automatic equilibrium, which the Langley machines have,
it was felt very certain that by this mechanism the engine could
be easily restarted while in the air, in case it was inadvertently
stopped. [p245]
The reason for building the engine with five cylinders instead of
some other number, and for arranging them radially on a central drum
using only one crank pin may not appear quite obvious. The advantages
gained by such a construction, however, are very great, and may be
briefly summed up as follows:
First, since in a gas engine of the four-cycle type there is only one
explosion in each cylinder every two revolutions, and the crank shaft
and crank pin therefore are loaded only one-quarter of the time for
each cylinder, it is obvious that by having four cylinders arranged
radially around a central drum the load on the bearings of a single
crank shaft and crank pin may be kept very uniform. However, with
four cylinders thus arranged it is impossible to have the cylinders
explode and exert their effort on the crank at uniform intervals
in the cycle, it being necessary to have the cylinders explode in
the order of 1, 3, 4, 2, 1, etc., thus giving intervals between
explosions of 180 degrees, 90 degrees, 180 degrees, 270 degrees,
etc., or to have them explode in the order of 1, 3, 2, 4, 1, etc.,
thus giving intervals of 180 degrees, 270 degrees, 180 degrees, 90
degrees, etc. On the other hand, with any odd number of cylinders the
explosions will occur at equal intervals in the cycle. With three
cylinders they will explode in the order of 1, 3, 2, 1, etc., or at
equal intervals of 240 degrees, while with five cylinders they will
explode in the order of 1, 3, 5, 2, 4, 1, etc., or at equal intervals
of 144 degrees. It is therefore seen that there is a great advantage
in smoothness of operation and uniformity of torque of the engine
through having an odd number of cylinders instead of an even number.
Second, it is readily apparent that the greater the number of
cylinders, provided the number is an odd one, the more uniform the
torque will be, and it would seem at first that seven cylinders
would therefore be better than five, since the uniform intervals
between explosions with seven cylinders would be only 103
degrees (approximately). The advantage gained, however, through
seven cylinders instead of five is largely, if not completely,
counterbalanced by the added number of parts and the difficulty of
providing sufficient circumferencial width for the connecting-rod
shoes on the crank-pin bearing, even with the improved construction
of this bearing already described. There is considerable fluctuation
of the torque in each revolution of the engine with five cylinders,
but this fluctuation of torque is more easily smoothed out by the use
of very light fly wheels than by increasing the number of cylinders,
and thus adding to the complication of the engine.
Third, the strongest point in favor of the radially arranged
cylinders is the reduction in weight and complication which it
permits. The crank shaft is reduced to the very minimum, there being
only one crank pin with two main bearings which can, without any
difficulty whatever, be kept absolutely in line with each other
and thus prevent binding and loss of power. Again, the use of a
single-throw crank not only reduces the cost and weight of the crank
[p246] itself, but makes it very much less liable to damage; long
crank shafts with several crank pins being frequently twisted by
improper explosions in the cylinders. The supporting drum or crank
chamber is likewise reduced to the very minimum, both in weight
and simplicity, the drums being perfectly symmetrical with no lost
space either inside of them or on their exteriors. The cam mechanism
for operating the valves is reduced to a simple ring carrying (for
a five-cylinder engine) a double-pointed cam and journaled on the
exterior of the hub of one of the drums, the cam being driven by a
train of gears journaled on studs mounted on the drum, and co-acting
with a gear fastened to the crank shaft against the crank arm.
The radial arrangement of the cylinders is thus seen to give not
only an engine with the smallest number of parts, each of which is
as far as possible worked to a uniform amount during each complete
revolution of the crank shaft, but it also gives a very compact and
readily accessible mechanism with its center of gravity coincident
with its center of figure, and with the liability of damage to it,
in case of a smash of the vehicle on which it is used, reduced to
the minimum from the fact that the greatest weight is located at the
strongest part.
Fourth, and of almost as great importance as the reduction in
weight which the five-cylinder radial arrangement permits, is its
unusual qualities as regards vibration. Since these five-cylinder
engines were built by the writer a very thorough treatment of their
properties as regards balancing has been given in a treatise on
the balancing of engines,[44] so no discussion of the mathematical
formulæ involved in a study of the question of the inherent balancing
properties of these engines will be here given. It is sufficient to
call attention to the fact that in an engine having five cylinders
arranged radially, all of the reciprocating parts are balanced for
all forces of the first, second and third orders. As it is only the
reciprocating parts which give any trouble in balancing any engine,
the unbalanced rotating parts being readily balanced by placing an
equal weight at an equal distance from the center of rotation, and
on the opposite side thereof, it is readily seen that the properties
of balancing which are inherent in this type of engine are unusual.
A six-cylinder engine having a six-throw crank shaft is not nearly
so thoroughly balanced as this type having its five cylinders
radially arranged, for in the latter case all the moving parts are
in one plane, while in the former case the moving parts are in six
separate and parallel planes, and there is consequently considerable
longitudinal vibration which can never be overcome. While this is
true as regards the vibration due to moving masses, it is still more
impressively true as regards vibration due to reaction arising from
the force of the explosions in the engine cylinders, especially when
the engine is running slowly and having heavy explosions.
The usual practice in balancing the rotating parts of an engine is to
attach [p247] balance weights to the crank arms which are prolonged
beyond the center of the crank shaft and on the opposite side from
the crank pin; the radius of rotation of these balance weights being
made approximately equal to the radius of the crank pin. But aside
from the constructional difficulties which would be introduced, it
was seen that if this plan was followed in this engine it would
require a very large additional weight. Since the amount of this
weight could be diminished in exact proportion to the increase of the
radius of rotation of the balance weights, it was at first decided
to attach the weights to the rims of the fly wheels, the relative
amount of weight attached to each wheel being inversely proportional
to its actual longitudinal distance from the crank-pin center. It
was very soon found that the attachment of these balance weights to
the fly wheel caused excessive strains on the rims of the wheels,
thereby causing them to go out of line. In order, therefore, to keep
the amount of balance weight small by carrying it at a considerable
distance from the center of the shaft, the weights were finally
arranged as clearly shown in the drawings, Plates 78 to 80. There
it is seen that the main portion of each of the balance weights
consists of a flat arm bolted between the flanges which couple the
transmission shafts to the engine shafts. The flat arm terminates in
a lozenge-shaped lug, additional weight being provided by a plate
fastened to one end of a tube, the other end of which terminates in a
collar fastened around the transmission shaft. The tube is inclined
at an angle of about thirty degrees with the flat balance arm, thus
acting as a brace to prevent the balance arm from wobbling, the plate
on the bracing tube being fastened to the lozenge-shaped lug by means
of small bolts.
The tabulated statement of the weight of this large engine is given
below. From this it will be readily seen that the net weight of
the engine proper is 124.17 pounds. The fly wheels were in no way
necessary to the engine itself, but were used solely for the purpose
of smoothing out the torque of the engine so that the transmission
shafts and propeller shafts might be kept down to the very minimum in
weight. Including the two fly wheels, the weight is 140 pounds.
Including the 20 pounds of cooling water the total weight of the
power plant is 207.47 pounds. Without flywheels the total weight is
191.64 pounds.
The construction of this large engine was completed in December,
1901, and the first tests of it were made in January, 1902. As
already stated, these first tests were made with the engine
mounted on a special testing frame and delivering its power to two
water-absorption dynamometers, no fly wheels being used, as none were
required. Later, when it became necessary either to use fly wheels
or to greatly increase the weight of the transmission and propeller
shafts, in order to overcome the reverse torque, the two light fly
wheels were added, and another series of tests was made of the engine
on its testing frame. The arrangement of the engine, dynamometers,
and accessory [p248] apparatus is clearly shown in Plates 82, 83
and 84. The engine ran in a clockwise direction, as viewed in Plate
82. ‹AA› are the fly wheels; ‹BB› the balance weights; ‹CC› the
dynamometer shafts, on which are fastened the rotor plates which
revolve inside of the dynamometer drums, ‹DD›, between stator plates
fastened therein. The drums have a hub on either side, by which
they are supported, these hubs being journaled on ball-bearings in
the pedestals resting on the wooden framework. The rotor plates do
not touch the stator plates in the dynamometers, but drag on the
water with which the drums are partially filled, and thus tend to
cause the drums to revolve around with them. The torque on each
drum is measured by means of a rope, not shown, fastened into the
hook at the top of the drum, the rope being given a partial coil
around the drum and passing off tangent thereto at the horizontal
diameter is fastened to a pair of spring scales hung from the ceiling
vertically above the point of tangency. The scales and ropes were
unfortunately not in position when these photographs were taken, but
the arrangement of them should be readily understood. As the friction
of the rotor plates on the water heats it in exact proportion to the
amount of power absorbed, the small amount of water in the drums
would be soon converted into steam unless continually renewed or
cooled. When the rotor plates are revolving the centrifugal force
keeps the water pressed toward the circumference of the drum, and the
friction at any speed is dependent on the area of the rotor plates
in contact with the water. The horse-power required to revolve the
plates at any definite speed can therefore be controlled by having an
outlet for the water at the proper radial distance from the shaft.
The water from the water mains is led through the upper vertical pipe
and allowed to flow into the funnel, and thence into the drum near
the center where the centrifugal force throws it to the circumference
of the drum. The lower vertical pipe is connected to the drum at a
suitable radial distance from the center, and the heated water thus
passes through this pipe and into the lower funnel connected to
the sewer. By the use of the funnels the drums are allowed to rock
sufficiently to exert their pull on the spring scales without being
affected by the supply and exhaust of water.
[Illustration: PL. 82
DYNAMOMETER TESTS OF LARGE ENGINE]
[Illustration: PL. 83
DYNAMOMETER TESTS OF LARGE ENGINE]
[Illustration: PL. 84
DYNAMOMETER TESTS OF LARGE ENGINE]
The water for cooling the cylinders is led from the bottom of the
tank ‹E› to the circulating pump ‹F›, supported in a discarded lower
pyramid of the aerodrome frame, the pump being driven by the small
vertical shaft, as already described. The water, after passing
through the pump and the engine cylinders, is led back to the upper
part of the tank. By suitable connections to the water mains and
sewer, the water in the tank is kept at any desired temperature. The
gasoline supply tank is seen on the left-hand side of the testing
frame, as viewed in Plate 82, the carburetor being placed below it
and the gas supply pipe from the carburetor passing through the
gasoline tank. Instead of jacketing the carburetor, a grid formed
of thin copper tubes is supported just above the multitude of small
air pipes leading into the carburetor, and some of [p249] the
hot water from the engine is by-passed through this grid and thus
warms the air as it passes into the carburetor. The small pipe that
by-passes this water through the grid is seen connected to the outlet
water pipe just above the cylinders, a small butterfly valve in the
outlet pipe enabling the amount of heated water passing through
the grid to be controlled. The return from the grid is by means
of the small pipe leading to the top of the large water tank. The
tachometer, which gives instantaneous readings of the speed of the
engine, is seen at ‹G›, where it is at all times in full view of the
operator.
These dynamometers proved to be excellently suited for the testing
work, and far ahead of anything else the writer has ever found for
engine testing. Since the power required to rotate the rotor plates,
with a uniform amount of water in the drums, varies as the cube of
the speed, it is readily seen that it is impossible for the engine to
race or injure itself by running away, as frequently happens where
there is no engine governor and Prony brakes are used to measure the
power.
In the early tests the engine was never allowed to develop more than
40 horse-power, as it was feared that by letting it develop more,
which it was clearly seen to be capable of, it might be injured and
cause a delay in the tests of the aerodrome. In the second series of
tests it was allowed to develop 51 horse-power at 935 R. P. M., but
it was not thought to be advisable to let it run at maximum power
for more than an hour, for the same reason as before. In the summer
of 1904, after it was seen that there was no immediate possibility
of securing funds for continuing the tests of the aerodrome, it
was planned to enter the engine in the competitive tests at the
St. Louis Exposition, where a prize of $2500 was offered for the
lightest engine for its power. As the conditions specified in this
competition required that the engine run at its maximum power for one
hour, and that this be followed by a durability test of ten hours’
continuous running, it was decided to make some durability tests of
the engine before taking it to St. Louis. In these tests, the engine
was run on three separate trials for a period of ten hours[45] with
a constant load of 52.4 horse-power at 950 R. P. M. Even in these
long durability tests the engine and the dynamometers both worked
so smoothly and evenly that the engine did not vary its speed more
than ten revolutions per minute, and the pull on the spring scales
varied less than ten pounds in the entire ten hours. Considerable
correspondence was had with the officials of the St. Louis Exposition
regarding the entrance of the engine in the competition, in order
to make sure that suitable facilities for conducting the tests had
been provided. After receiving assurance that everything necessary
had been provided, the engine and its testing dynamometers were
boxed for shipment to St. Louis and arrangements were just being
completed for their transportation when the following telegram was
[p250] received from the director in charge of the aeronautical
department of the Exposition: “On account of lack of competition
engine tests abandoned.” As the main object of entering the engine
in the competition was to insure for it an unquestioned record of
its performance it was decided to reassemble it in the testing frame
in Washington and invite some engineers of prominence to witness and
certify to its performance, but on account of the lack of funds for
meeting the expenses incident to such a series of tests as it was
planned to make this was never done.
In the tests which were witnessed on April 26, 1902, by Captain I.
N. Lewis, Recorder of the Board of Ordnance and Fortification, the
engine was held down to a pull of 200 pounds on a 13-inch lever, when
running at 1000 revolutions per minute. In the later tests in May,
1903, which were witnessed by Captain Gibson, who was then Recorder
of the Board of Ordnance and Fortification, and Mr. G. H. Powell, the
Secretary of the Board, the engine was allowed to work at a pull of
265 pounds on the 13-inch lever arm at a speed of 950 revolutions per
minute. In the tests made in August, 1904, the engine was run for ten
consecutive hours[46] at a pull which varied from 263 to 271 pounds,
or an average of 267 pounds, on a 13-inch lever, with the speed
varying from 945 to 955 revolutions per minute, thus showing 52.4
horse-power at the average speed of 950 R. P. M.
DETAILED WEIGHT OF NEW LARGE ENGINE.
Name of Weight in
part. grammes.
Crank shaft 5,225
Connecting rods (total) 5,005
Pistons--
No. 1 1,652
No. 2 1,647
No. 3 1,655
No. 4 1,660
No. 5 1,646
Cylinders--
No. 1 (including exhaust and inlet valves, oil
cups, etc.) 4,768
No. 2 (including exhaust and inlet valves, oil
cups, etc.) 4,685
No. 3 (including exhaust and inlet valves, oil
cups, etc.) 4,638
No. 4 (including exhaust and inlet valves, oil
cups, etc.) 4,637
No. 5 (including exhaust and inlet valves, oil
cups, etc.) 4,796
Port crank chamber drum, including cam, cam
gears, punch rods, etc. 5,225
Starboard crank chamber drum 3,440
Spark plugs (5) 450
Outlet water pipe 450
Inlet water pipe 360
Inlet gas manifold 1,700
Primary and secondary sparkers and wires 512
Balance arm with braces for same--starboard 1,040
Balance arm with braces for same--port 1,067
------
Total 56,323 = 124.17 lbs.
Starboard fly wheel 3,946
Port fly wheel 3,234
------
Total weight of engine and fly wheels 63,503 = 140.00 lbs.
Spark coil and batteries 6,800
Carburetor 3,751
Inlet gas pipe from carburetor to manifold 756
Gasoline tank 1,004
Water tank 717
Water circulating pump and shaft 807
Radiator 7,700
------
Total weight of power plant 85,038 = 187.47 lbs.
[p251]