records are not complete, but with one exception there is no
indication of any chronic or even occasional crankshaft failure. This
would seem to show that it apparently never happened that any of their
designs came out such that the frequency of a vibrating force of any
magnitude occurred at the natural frequency of the shaft. Much later,
when this type of vibration became understood, it was found virtually
impossible, with power outputs of any magnitude, to design an
undampened shaft, within the space and weight limitations existing in
an ordinary engine, strong enough to withstand the stress generated
when the frequency of the imposed vibration approximated the natural
frequency of the shaft. The vibratory forces were mostly relatively
small in their engines, so that forced vibration probably was not
encountered, and the operating speed range of the engines was so
limited that the natural frequency always fell outside this range.
The flywheel was about the least complex of any of their engine parts
and required little studied consideration, although they did have to
balance its weight against the magnitude of the explosion forces which
would reach the power transmission chains, with their complete lack of
rigidity, a problem about which they were particularly concerned. The
flywheel was made of cast iron and was both keyed to and shrunk on the
shaft.
Some doubt still exists about the exact method of lubricating the
first engine. The unit presently in the airplane has a gear-type oil
pump driven by the crankshaft through a worm gear and cross shaft, and
the Appendix to the _Papers_ states that it was lubricated by a small
pump; nevertheless Baker says, after careful research, that despite
this evidence, it was not. Also, the drawings prepared by Christman
(they were commenced under the supervision of Orville Wright) do not
show the oil pump. In March 1905 Wilbur Wright wrote to Chanute,
"However we have added oiling and feeding devices to the engine ...";
but this could possibly have referred to something other than an oil
pump. But even if a pump was not included originally, its presence in
the present engine is easily explained. Breakage of the crankcase
casting caused the retirement of this engine, which was not rebuilt
until much later, and the pattern for this part had no doubt long
since been altered to incorporate a pump. It was therefore easier in
rebuilding to include than to omit the pump, even though this required
the addition of a cross shaft and worm gear combination. On later
engines, when the pump was used, oil was carried to a small pipe,
running along the inside of the case, which had four small drill holes
so located as to throw the oil in a jet on the higher, thrust-loaded
side of each cylinder. The rods had a sharp scupper on the outside of
the big end so placed as also to throw the oil on this same thrust
face. Some scuppers were drilled through to carry oil to the rod
bearing and some were not.
The first engine was finished and assembled in February 1903 and given
its first operating test on 22 February. The Wrights were quite
pleased with its operation, and particularly with its smoothness.
Their father, Bishop Wright, was the recorder of their satisfaction
over its initial performance, but what he noted was probably the
afterglow of the ineffable feeling of deep satisfaction that is the
reward that comes to every maker of a new engine when it first comes
to life and then throbs. They obtained 13 hp originally: later figures
went as high as almost 16, but as different engine speeds were
utilized it is rather difficult to settle on any single power figure.
The most realistic is probably that given in the _Papers_ as having
been attained later, after an accurate check had been made of the
power required to turn a set of propellers at a given rpm. This came
out at approximately 12 hp, the design goal having been 8. Following
exactly the procedure that exists to this day, the engine went through
an extended development period, and it was the end of September 1903
before it was taken, with the airplane, to Kitty Hawk where the
historic flights, which have had such a profound effect on the lives
of all men, were made on 17 December 1903.
The Engines With Which They Mastered The Art of Flying
Two more engines of this first general design were built but they
differed somewhat from each other as well as from the original.
Together with a third 8-cylinder engine these were begun right after
the first of the year in 1904, shortly after the Wrights' return from
Kitty Hawk. In planning the 8-cylinder engine they were again only
being forehanded, but considerably so, in providing more power for
increased airplane performance beyond that which might possibly be
obtained from the 4-cylinder units. Progress with the 4-cylinder
engines was such that they fairly quickly concluded that the
8-cylinder size would not be necessary, and it was abandoned before
completion. Exactly how far it was carried is not known. The record
contains only a single note covering the final scrapping of the parts
that had been completed; and apparently there were no drawings, so
that even its intended appearance is not known with any exactness. It
was probably a 90 deg. V-type using their original basic cylinder
construction.
The changes carried through in the two 4-cylinder engines were not
major. The water-cooled area of the cylinder barrel was increased by
nearly ten percent but the head remained only partially cooled. In
hindsight, this consistent avoidance of complete cylinder-head cooling
presents the one most inexplicable of the more important design
decisions they made, as it does not appear logical. In the original
engine, where the factors of time and simplicity were of paramount
importance, this made sense, but now they were contemplating
considerably increased power requirements, knowing the effect of
temperature on both the cylinder and the weight of cylinder charge,
and knowing that valve failure was one of their most troublesome
service problems. Nor does it seem that they could have been avoiding
complete cylinder cooling through fear of the slightly increased
complexity or the difficulty of keeping the water connections and
joints tight, for they had faced a much more severe problem in their
first engine, where their basic design required that three joints be
kept tight with only two sets of threads, and had rather easily
mastered it; so there must have been some much more major but not
easily discernible factor which governed, for they still continued to
use the poorly cooled head, even carrying it over to their next engine
series. Very probably they did not know the effect on detonation of a
high-temperature fuel-charge.
One of the new engines was intended for use in their future
experimental flying and has become known as _No. 2._ It had a bore of
4-1/8 in., incorporated an oil pump, and at some time shortly after
its construction a fuel pump was added. The fuel pump was undoubtedly
intended to provide a metering system responsive to engine speed and
possibly also to eliminate the small inherent variation in flow of the
original gravity system.
This engine incorporated a cylinder compression release device not on
the original. The exact reason or reasons for the application of the
compression release have not been determined, although the record
shows it to have been utilized for several different purposes under
different operating conditions. Whatever the motivation for its
initial application, it was apparently useful, as it was retained in
one form or another in subsequent engine models up to the last
6-cylinder design. Essentially it was a manually controlled mechanism
whereby all the exhaust valves could be held open as long as desired,
thus preventing any normal charge intake or compression in the
cylinder. Its one certain and common use was to facilitate starting,
the open exhaust valves easing the task of turning the engine over by
hand and making priming easy. In flight, its operation had the effect
of completely shutting off the power. The propellers would then
"windmill" and keep the engine revolving. One advantage stated for
this method of operation was that when power was required and the
control released, the engine would be at fairly high speed, so that
full power was delivered immediately fuel reached the engine. It is
also reported to have been used both in making normal landings and in
emergencies, when an instant power shutdown was desired. Although it
is not clear whether the fuel shutoff cock was intended to be
manipulated when the compression release was used for any of these
reasons, over the many years of its availability, undoubtedly at one
time or another every conceivable combination of operating conditions
of the various elements was tried. Because of the pumping power
required with at least one valve open during every stroke, the
windmilling speed of the engine was probably less than with any other
method of completely stopping power output, but whether this
difference was large enough to be noticeable, or was even considered,
is doubtful.
Since a simple ignition switch was all that was required to stop the
power output, regardless of whether a fuel-control valve or a
spark-advance control was used, it must be concluded that the primary
function of the compression release was to facilitate starting, and
any other useful result was something obtained at no cost. The
compression release was later generally abandoned, and until the
advent of the mechanical starter during the 1920s, starting an engine
by "pulling the propeller through" could be a difficult task. With the
Wrights' demonstrated belief that frugality was a first principle of
design, it is hardly conceivable that they would have accepted for any
other reason the complication of the compression-release mechanism if
a simple ignition switch would have sufficed.
The compression-release mechanism was kept relatively simple,
considering what it was required to accomplish. A small non-revolving
shaft was located directly under the rocker arm rollers that actuated
the exhaust valves. Four slidable stops were placed on this shaft,
each in the proper location, so that at one extreme of their travel
they would be directly underneath the rocker roller and at the other
extreme completely in the clear. They were positioned along the shaft
by a spring forcing them in one direction against a shoulder integral
with the shaft, and the shaft was slidable in its bearings, its
position being determined by a manually controlled lever. When the
lever was moved in one direction the spring pressure then imposed on
the stops would cause each of them to move under the corresponding
rocker roller as the exhaust valve opened, thus holding the exhaust
valve in the open position. When the shaft was moved in the other
direction the collar on the shaft would mechanically move the stop
from underneath the roller, allowing the valve to return to normal
operation.
[Illustration: _Figure 8._--Development engine No. 3, 1904-1906,
showing auxiliary exhaust port, separate one-piece water-jacket block.
(Photo by author.)]
If the 1903 engine is the most significant of all that the Wrights
built and flew, then certainly the _No. 2_ unit was the most useful,
for it was their sole power source during all their flying of 1904 and
1905 and, as they affirmed, it was during this period that they
perfected the art, progressing from a short straightaway flight of 59
seconds to a flight controllable in all directions with the duration
limited only by the fuel supply. It is to be greatly regretted that no
complete log or record was kept of this engine.
The Wrights again exhibited their engineering mastery of a novel basic
situation when, starting out to make flight a practical thing, they
provided engine _No. 3_ to be used for experimental purposes. In so
doing they initiated a system which continues to be fundamental in the
art of providing serviceable aircraft engines to this day--one that is
expensive and time consuming, but for which no substitute has yet been
found. Their two objectives were: improvement in performance and
improvement in reliability, and the engine was operated rather
continuously from early 1904 until well into 1906. Unfortunately,
again, no complete record exists of the many changes made and the
ideas tested, although occasional notes are scattered through the
diaries and notebooks.
In its present form--it is on exhibition at the Engineers Club in
Dayton, Ohio--the _No. 3_ engine embodies one feature which became
standard construction on all the Wright 4-cylinder models. This was
the addition of a number of holes in a line part way around the
circumference of the cylinder barrel so that they were uncovered by
the piston at the end of its stroke toward the shaft, thus becoming
exhaust ports (see Figure 9). This arrangement, although not entirely
novel, was just beginning to come into use, and in its original form
the ports exhausted into a separate chamber, which in turn was
evacuated by means of a mechanically operated valve, so that two
exhaust valves were needed per cylinder. Elimination of this chamber
and the valve arrangement is typical of the Wrights' simplifying
procedure, and it would seem that they were among the very first to
use this form.[14]
[Footnote 14: Rankin Kennedy, _Flying Machines--Practice and Design_,
1909.]
The primary purpose of the scheme was to reduce, by this early release
and consequent pressure and temperature drop, the temperature of the
exhaust gases passing the exhaust valve, this valve being one of their
main sources of mechanical trouble. It is probable that with the
automatic intake valves being used there was also a slight effect in
the direction of increasing the inlet charge, although with the small
area of the ports and the short time of opening, the amount of this
was certainly minor. With the original one-piece crankcase and
cylinder jacket construction, the incorporation of this auxiliary
porting was not easy, but this difficulty was overcome in the
development engine by making different castings for the crankcase
itself and for the cylinder jacket and separating them by several
inches, so that room was provided between the two for the ports.
This engine demonstrated the most power of any of the flat 4s,
eventually reaching an output of approximately 25 hp, which was even
somewhat more than that developed by the slightly larger
4-1/8-in.-bore flight engine, with which 21 hp was not exceeded.
Indicative of the development that had taken place, the performance of
the _No. 3_ engine was twice the utilized output of the original
engine of the same size, an increase that was accomplished in a period
of less than three years.
The Wrights were only twice charged with having plagiarized others'
work, a somewhat unusual record in view of their successes, and both
times apparently entirely without foundation. A statement was
published that the 1903 flight engine was a reworked Pope Toledo
automobile unit, and it was repeated in an English lecture on the
Wright brothers. This was adequately refuted by McFarland but
additionally, it must be noted, there was no Pope Toledo company or
car when the Wright engine was built. This company, an outgrowth of
another which had previously manufactured one-and two-cylinder
automobiles, was formed, or reformed, and a Pope license arrangement
entered into during the year 1903.
The other incident was connected with Whitehead's activities and
designs. Whitehead was an early experimenter in flying, about the time
of the Wrights, whose rather extraordinary claims of successful flight
were published in the 1901-1903 period but received little attention
until very much later. His first engines were designed by a clever
engineer, Anton Pruckner, who left at the end of 1901, after which
Whitehead himself became solely responsible for them. It was stated
that the Wrights visited the Whitehead plant in Bridgeport,
Connecticut, and that Wilbur remained for several days, spending his
time in their machine shop. This was not only categorically denied by
Orville Wright when he heard of it but it is quite obvious that the
1903 or any other of the Wright engine designs bears little
resemblance to Pruckner's work. In fact, its principal design features
are just the opposite of Pruckner's, who utilized vertical cylinders,
the 2-stroke cycle, and air-cooling, which Whitehead at some point
changed to water-cooling.[15]
[Footnote 15: Considerable doubt surrounds Whitehead's actual flight
accomplishments, but Pruckner's engines were certainly used, as
several were sold to early pioneers, including Charles Wittemann. It
is probable that the specific power output was not very great, for the
air-cooled art of this time was not very advanced and Pruckner had a
rather poor fin design. But the change to water cooling eliminated
this trouble, and the engines were most simple, should have been
relatively quite light, and with enough development could probably
have been made into sufficiently satisfactory flying units for that
period.]
The Four-Cylinder Vertical Demonstration Engine and the First
Production Engine
In 1906, while still doing general development work on the flat
experimental engine, the Wrights started two new engines, and for the
first time the brothers engaged in separate efforts. One was "a
modification of the old ones" by Wilbur and the other, "an entirely
new pattern" by Orville. There is no record of any of the features of
Wilbur's project or what was done in connection with it. Two months
after the experimental operation of the two designs began, an entry in
Wilbur's diary gives some weight and performance figures for the "4" x
4" rebuilt horizontal," and since Orville's design was vertical the
data clearly refer to Wilbur's; but since the output is given only in
test-fan rpm it does not serve to indicate what had been accomplished
and there is no further mention of it.
Orville's design became the most used of any model they produced. It
saw them through the years from 1906 to 1911 or 1912, which included
the crucial European and United States Army demonstrations, and more
engines of this model were manufactured than any of their others
including their later 6-cylinder. Although its ancestry is traceable
to the original 1903 engine, the design form, particularly the
external configuration, was considerably altered. Along with many
individual parts it retained the basic conception of four medium-size
cylinders positioned in line and driving the propellers through two
sprocket wheels. From the general tenor of the record it would seem,
despite there being no specific indication, that from this time on
Orville served as the leader in engine design, although this occurred
with no effect whatsoever on their finely balanced, exactly equal
partnership which endured until Wilbur's death in 1912.
The first major change from the 1903 design, putting the engine in an
upright instead of flat position, was probably done primarily to
provide for a minimum variation in the location of the center of
gravity with and without a passenger. Whether or not it had any
influence, the vertical cylinder arrangement was becoming predominant
in automobile powerplants by this time, and the Wright engines now
began to resemble this prevailing form of the internal combustion
engine--a basic form that, in a wide variety of uses, was to endure
for a long time.
Over the years, the Wrights seem to have made many changes in the
engine: the bore was varied at different times, rod assembly methods
were altered, and rod ends were changed from bronze to steel.
Chenoweth states that on later engines an oil-control ring was added
on the bottom of the piston, necessitating a considerable increase in
the length of the cylinder barrel. This arrangement could not have
been considered successful, as it apparently was applied to only a
limited number of units and was not carried over to the later
6-cylinder engine model. There was much experimentation with cam
shapes and most probably variations of these got into production.
With the crankcase, they did not go all the way to the modern
two-piece form but instead retained the one-piece construction.
Assembly was effected through the ends and a detachable plate was
provided on one side for access to the interior. It is clear that they
regarded this ability to get at the interior of the case without major
disassembly as a valuable characteristic, and later featured it in
their sales literature. They were apparently willing to accept the
resultant weakening of the case and continued the construction through
their last engine model. The integrally cast cylinder water jackets
were abandoned and the top of the crankcase was machined flat to
provide a mounting deck for individual cylinders. The use of aluminum
alloy was continued, and the interior of the case was provided with
strengthening webs of considerable thickness, together with supporting
ribs. The cam shaft was supported directly in the case.
The individual cylinder design was of extreme simplicity, a single
iron casting embodying everything except the water jacket. The valves
seated directly on the cast-iron cylinder head and the guides and
ports were all contained in an integral boss on top of the head. The
exhaust valve location on the side of the engine opposite the pilot
was a decided advantage over that of the 1903 design, where the
exhaust was toward the pilot. A four-cornered flange near the bottom
of the cylinder provided for fastening it to the crankcase, and a
threaded hole in the top of the head received a vertical eyebolt which
served as the rocker-arm support. The cylinder was machined all over;
two flanges, one at the bottom and the other about two-thirds of the
way down provided the surfaces against which the water jacket was
shrunk. The jacket was an aluminum casting incorporating the necessary
bosses and double shrunk on the barrel; that is, the jacket itself was
shrunk on the cylinder-barrel flanges and then steel rings were shrunk
on the ends of the jacket over the flanges. The jacket thickness was
reduced by machining at the ends, making a semigroove into which the
steel shrink rings fitted. These rings insured the maintenance of a
tight joint despite the tendency of the aluminum jacket to expand away
from the cast-iron barrel.
[Illustration: _Figure 9._--4-Cylinder vertical engine: a, magneto
side; b, valve port side with intake manifold removed; c, flywheel end
of engine at Carillon Park Museum, Dayton, Ohio; d, magneto side with
crankcase cover removed. (Photos: a, Smithsonian A-3773; b, d, Pratt &
Whitney D-15003, 15007; c, by A. L. Rockwell.)]
Why the one-piece crankcase and cylinder jacket combination of the
1903 engine was abandoned for the individual cylinder construction can
only be surmised. The difference in weight was probably slight, as the
inherent weight advantage of the original crankcase casting was
largely offset by the relatively heavy valve boxes, and the difference
in the total amount of machining required, because of the separate
valve boxes, cages, and attaching parts, also was probably slight.
Although the crankcase had shown itself to be structurally weak, this
could have been cared for by proper strengthening. The 1903 design did
have some fundamental disadvantages: it required a fairly complex
pattern and expensive casting, plus some difficult machining, part of
which had to be very accurate in order to maintain both gas and water
joints tight; and the failure of any one cylinder that affected the
jacket meant a complete crankcase replacement.