utilized, possibly three. One, and most probably the original,
consisted of a duplicate of the injection pump of the 4-cylinder
fitted to a manifold which ran the length of the engine, with three
takeoffs, each of which then split into two, one for each cylinder. Of
this arrangement they tried at least two variations involving changes
in the location and method of injecting the fuel into the manifold;
and there seems to have been an intermediate manifold arrangement,
using fuel-pump injection at the middle of the straight side, or
gallery, manifold, which was fed additional air at both ends through
short auxiliary inlet pipes. This would indicate that with the
original arrangement, the end cylinders were receiving too rich a
mixture, when the fuel in the manifold was not properly vaporized.
Although the exhaust was on the same side of the engine as the inlet
system, no attempt was made to heat the incoming charge at any point
in its travel. An entirely different system adopted at the time of the
complete redesign in 1913 consisted of two float-feed Zenith
carburetors each feeding a conventional three-outlet manifold. This
carburetor was one of the first of the plain-tube type, that is, with
the airflow through a straight venturi without any spring-loaded or
auxiliary air valves, and was the simplest that could be devised. When
properly fitted to the engine, it gave a quite good approximation of
the correct fuel and air mixture ratio over the speed-load running
range, although it is considerably more than doubtful that this was
maintained at altitude, as is stated in one of the best descriptions
of the engine published at the time the carburetors were applied.
The compression ratio of this engine was lowered by almost 20 percent
from that of the vertical 4. This, in combination with the low
bore-to-stroke ratio, the unheated charge, and the later mechanically
operated inlet valve, indicates that the Wrights were now attempting
for the first time to secure from an engine something approaching the
maximum output of which it was capable.
As the engine originally came out, it continued to utilize only one
spark plug in each cylinder. The high-tension magneto had a wide range
of spark advance adjustment, which again provided the only control of
the engine when equipped with the original fuel pump injection.
The location of the valves and pushrods was similar to that in the 4,
so that the cams were immediately adjacent to the camshaft bearings,
which were carried in the crankcase ends and in the heavy webs. The
camshaft was gear-driven and the cam shape was similar to that of the
last 4s, with a quite rapid opening and closing and a long dwell,
leaving the valve opening accelerations and seating velocities still
quite high.
The crankshaft was a continuation of their basic design of rather
light construction, particularly in the webs. The cheeks were even
thinner (by 1/4 in.) than those of the 4 although the width was
increased by 1/8 in. (see Figure 13e). For the first time they went to
a forging, the rough contour type of the time, and utilized a
chrome-nickel alloy steel.
Lubrication was by means of the usual gear pump, and the piston and
rod bearings continued to be splash-fed. The rod big-end bearing
carried a small sharp undrilled boss at the point where, on the other
engines, had been located scuppers whose purpose was apparently still
to throw lubricating oil on the cylinder wall carrying the more highly
loaded side of the piston. The rod big-end bearing was lubricated by a
hole on the top of the big-end boss catching some of the crankcase
splash, which was then carried to the bearing by a groove.
When the 6-cylinder engine was completely redesigned in 1913 this led
to the introduction in late fall of that year of a new model called
the 6-60, the 60 designating the rating in horsepower. There is little
in the Wright records to show why such a radical revision was thought
necessary, but the general history of the period gives a rather clear
indication. The competition had caught up to the Wrights in
powerplants. Other engines were being installed in Wright airplanes,
and Navy log books show these other engines being used interchangeably
with those of the Wrights.
Most of the descriptions of the new model published at the time it was
introduced concentrate on the addition of the two carburetors and the
mechanical operation of the inlet valves, but these were only two of
many major changes. The cylinder was completely revised, the intake
being moved to the camshaft side of the engine from its position
adjacent to the exhaust, so that the two ports were now on opposite
sides of the cylinder. By proper positioning of the rocker-arm
supports and choice of their length and angles, all valves were made
operable from a single camshaft. The shrunk-on steel water jacket
cylinder was retained, but the water connections were repositioned so
that the water entered at the bottom and came out at the top of the
cylinder. Over the life of the 6-cylinder engine several different
valve types were used but the published specifications for the model
6-60 called for "cast iron heads"--the old two-piece construction. The
piston pins were case hardened and ground and the crankshaft pins and
journals were heat treated and ground.
The fuel and oil pumps were removed from the side of the crankcase and
a different ignition system was applied, although still of the
high-tension spark-plug type which by this time had become general
practice on all so-called high-speed internal-combustion engines. A
second threaded spark-plug hole was provided in the cylinder head and
despite its more common use for other purposes, it is evident that the
intention was to provide two-plug ignition. It is doubtful that at the
specific output of this engine any power difference would be found
between one-and two-plug operation, so that the objective was clearly
to provide a reserve unit in case of plug failure. However, it was
also used for the installation of a priming cock for starting and
because of the prevalence of single-wire ignition systems on existing
and illustrated engines, it seems to have been used mostly in this
manner, even though dual-ignition systems later became an unvarying
standard for aircraft engines.
Viewed externally, the only part of the engine that appears the same
as the original 6 is the small lower portion of the crankcase; but
what is more visually striking is the beauty of the new lines and
extreme cleanness of the exterior design (see Figures 14 and 15). Many
of their individual parts had shown the beauty of the sparse design of
pure utility but it was now in evidence in the whole. Despite the
proven practical value of their other models, this is the only one
that can be called a good-looking engine, instantly appealing to the
aesthetic sense, even though the vertical 4 is not an ugly engine. The
appearance of their final effort, in a field they were originally
reluctant to enter and concerning which they always deprecated the
results of their own work, was a thing of which a technically trained
professional engine designer could be proud.
The 6-60 was continued in production and development until it became
the 6-70, and indications are that it eventually approached an output
of 80 horsepower.
[Illustration: _Figure 14._--6-Cylinder 6-60 and 6-70 engine, right
rear intake side. (Pratt & Whitney photo.)]
[Illustration: _Figure 15._--6-Cylinder 6-70 engine, incorporating
flexible flywheel drive, exhaust side. (Smithsonian photo A-54381.)]
Minor Design Details and Performance of the Wright Engines
In the Wright brothers' various models were many minor design items
which altogether required a great deal of consideration, but which did
not materially affect overall engine performance. The results
generally could all be classed as good practice; however, one of these
utilized in the 4-cylinder vertical engine was rather unorthodox and
consisted of offsetting the cylinders with relation to the crankshaft.
This arrangement, which can be seen in the drawing (Figure 11) was
apparently an attempt to reduce the maximum side load on the piston
during the power stroke, but since the peak gas loading usually occurs
at about 10 to 15 percent of the power stroke, this probably did not
have much effect, and it was not carried over to the 6-cylinder
design.
All engine bearings were of the plain sleeve type and, except for the
bronze and steel bearings in the connecting rod, were of babbit. The
advantages of babbit for bearings were discovered very early in the
development of the mechanical arts, and apparently the Wrights never
encountered a bearing loading sufficiently high to cause a structural
breakdown in this relatively weak material.
Valve openings show no variation through the successive production
engines, although the Wrights most probably experimented with
different amounts. The 1903 engine and the vertical 4-and 6-cylinder
all had lifts of 5/16 in., but the valve-seat angles varied somewhat;
the records show included angles of 110 deg. to 90 deg.--not a large
difference.
The valve-operating mechanism was the same from the first vertical 4
onward. The high side thrust caused by the cam shape required for the
very rapid valve opening they chose was, no doubt, the reason for the
use of the hinged cam follower, and since the same general cam design
was used in their last engine, the 6-cylinder, the same method of
operation which had apparently proved very serviceable was continued.
How satisfactory was the considerably simpler substitute used in the
Bariquand et Marre version of the 4-cylinder engine is not known.
Possibly it was one of the alterations in the Wrights' design that
Wilbur Wright objected to, although in principle it more closely
conforms to the later fairly standard combination valve tappet and
roller construction: The available drawings do indicate, however, that
the cam of the Bariquand et Marre engine was also altered to give a
considerably less abrupt valve opening than the Wright design, so that
there was less side thrust. For the Wright 6-cylinder engine their
4-cylinder cam was slightly altered to provide a rounding off near the
top of the lobe, thus providing some reduction in the velocity before
maximum opening was reached. All their cam designs indicate a somewhat
greater fear of the effect of seating velocities than of opening
accelerations.
Since the range of cylinder diameters utilized did not vary greatly,
the valve sizes were correspondingly fairly uniform. The diameter of
the valves for the original 4-in.-bore cylinder was 2 in., while that
for the 4-3/8-in. bore used in the 6-cylinder engine was actually
slightly smaller, 1-7/8 in. Possibly the Wrights clung too long to the
automatic inlet valve, although it did serve them well; but possibly,
as has been previously noted, there were valid reasons for continuing
its use despite the inherently low volumetric efficiency this
entailed.
The inherent weakness in the joints of the three-piece connecting rod
has been pointed out, but aside from this, the design was excellent,
for all the materials and manufacturing methods required were readily
available, and structurally it was very sound. Tubular rods were still
in use in aircraft engines in the 1920s.
The Wrights had a surprisingly thorough grasp of the metallurgy of the
time, and their choice of materials could hardly have been improved
upon. Generally they relied upon the more simple and commonly used
metals even though more sophisticated and technically better alloys
and combinations were available.[17] Case hardening was in widespread
use in this period but their only utilization of it was in some parts
of the drive chains purchased completely assembled and in the piston
pins of their last engine. The treatment of the crankshafts of all
their engines except the final 6-cylinder was typical of their
uncomplicated procedure: the particular material was chosen on the
basis of many years of experience with it, hardening was a very simple
process, and the expedient of carrying this to a point just below the
non-machinable range gave them bearing surfaces that were sufficiently
hard, yet at the same time it eliminated the possibility--present in a
heat-treating operation--of warping the finished piece.
[Footnote 17: Baker states that the first crankshaft was made from a
slab of armor plate and if this is correct the alloy was a rather
complex one of approximately .30-.35 carbon, .30-.80 manganese, .10
silicon, .04 phosphorus, .02 sulphur, 3.25-3.50 nickel, 0.00-1.90
chromium; however, all the rest of the evidence, including Orville
Wright's statement to Dr. Gough, would seem to show that it was made
of what was called tool steel (approximately 1.0 carbon).]
In the entire 1903 engine only five basic materials--excepting those
in the purchased "magneto" and the platinum facing on the
ignition-system firing points--were used: steel, cast iron, aluminum,
phosphor bronze, and babbit. The steels were all plain carbon types
with the exception of the sheet manifold, which contained manganese,
and no doubt this was used because the sheet available came in a
standard alloy of the time.
Overall, the Wright engines performed well, and in every case met or
exceeded the existing requirements. Even though aircraft engines then
were simpler than they became later and the design-development time
much shorter, their performance stands as remarkable. As a result, the
Wrights never lacked for a suitable powerplant despite the rapid
growth in airplane size and performance, and the continual demand for
increased power and endurance.
Few service records dating from before 1911, when the military
services started keeping log books, have been found. Some of those for
the period toward the end of their active era have been preserved, but
for that momentous period spanning the first few years when the
Wrights had the only engines in actual continuous flight operation,
there seems to be essentially nothing--perhaps because there were no
standard development methods or routines to follow, no requirements to
be met with respect to pre-flight demonstrations or the keeping of
service records. Beginning in 1904, however, and continuing as long as
they were actively in business, they apparently had in progress work
on one or more developmental or experimental engines. This policy, in
combination with the basic simplicity of design of these engines,
accounted in large measure for their ability to conduct both
demonstrations and routine flying essentially whenever they chose.
Time between engine overhauls obviously varied. In mid 1906 an engine
was "rebuilt after running about 12 hours." This is comparatively
quite a good performance, particularly when it is remembered that
essentially all the "running" was at full power output. It was
considerably after 1920 before the Liberty engine was redesigned and
developed to the stage where it was capable of operating 100 hours
between overhauls, even though it was being used at cruising, or less
than full, power for most of this time.
The Wrights of course met with troubles and failures, but it is
difficult, from the limited information available, to evaluate these
and judge their relative severity. Lubrication seems to have been a
rather constant problem, particularly in the early years. Although
some bearing lubrication troubles were encountered from time to time,
this was not of major proportions, and they never had to resort to
force-feed lubrication of the main or rod big-end bearings. The piston
and cylinder-barrel bearing surfaces seem to have given them the most
trouble by far, and examination of almost any used early Wright engine
will usually show one or more pistons with evidence of scuffing in
varying degrees, and this is also apparent in the photographs in the
record. This is a little difficult to understand inasmuch as most of
the time they had the very favorable operating condition of cast iron
on cast iron. Many references to piston seizure or incipient seizure,
indicated by a loss of power, occur, and this trouble may have been
aggravated by the very small piston clearances utilized. Why these
small clearances were continued is also not readily explainable,
except that with no combination of true oil-scraper rings, which was
the basic reason why the final form of aviation piston engine was able
to reach its unbelievably low oil consumptions, their large and rather
weak compression rings were probably not doing an adequate job of oil
control, and they were attempting to overcome this with a quite tight
piston fit.[18] In any event, they did encounter scuffing or seizing
pistons and cylinder over-oiling at the same time. As late as 4 May
1908 in the Wright _Papers_ there appears the notation: "The only
important change has been in the oiling. The engine now feeds entirely
by splash...."
[Footnote 18: Their intended piston ring tension is not known.
Measurements of samples from the 4-and 6-cylinder vertical engines
vary greatly, ranging from less than 1/2 lb per sq in. to almost 1-1/4
lb. The validity of these data is very questionable as they apply to
parts with unknown length of service and amount of wear. It seems
quite certain, however, that even when new the unit tension figure
with their wide rings was only a small fraction of that of the modern
aircraft piston engine.]
Their troubles tended to concentrate in the cylinder-piston
combination, as has been true of almost all piston engines. References
to broken cylinders are frequent. These were quite obviously cylinder
barrels, as replacement was common, and this again is not readily
explainable. The material itself, according to Orville Wright, had a
very high tensile strength, and in the 1903 engine more than ample
material was provided, as the barrel all the way down to well below
the attachment to the case was 7/32 in. thick. The exact location of
the point of failure was never recorded, but in its design are many
square corners serving as points of stress concentration. Also, of
course, no method was then available for determining a faulty casting,
except by visual observation of imperfections on the surface, and this
was probably the more common cause. It is interesting, however, that
the engine finally assembled in 1928 for installation in the 1903
airplane sent to England has a cracked cylinder barrel, the crack
originating at a sharp corner in the slot provided at the bottom of
the barrel for screwing it in place.
Valve failures were also a continuing problem, and Chenoweth reports
that a large proportion of the operating time of the 1904-1906
development engine was concentrated on attempts to remedy this
trouble. None of their cams, including those of the 6-cylinder engine,
evidence any attempt to effect a major reduction in seating
velocities. United States Navy log books of 1912 and 1913 record many
instances of inlet valves "broken at the weld," indicating that some
of the earlier 6-cylinder engines were fitted with valves of welded
construction.
For the engineer particularly, the fascination of the Wrights' engine
story lies in its delineation of the essentially perfect engineering
achievement by the classic definition of engineering--to utilize the
available art and science to accomplish the desired end with a minimum
expenditure of time, energy, and material. Light weight and
operability were the guiding considerations; these could be obtained
only through constant striving for the utmost simplicity. Always
modest, the Wrights seem to have been even more so in connection with
their engine accomplishments. Although the analogy is somewhat
inexact, the situation is reminiscent of the truism often heard in the
aircraft propulsion business--few people know the name of Paul
Revere's horse. Yet, as McFarland has pointed out, "The engine was in
fact far from their meanest achievement." With hardly any experience
in this field and only a meagerly equipped machine shop, they designed
and assembled an internal combustion engine that exceeded the
specifications they had laid down as necessary for flight and had it
operating in a period of about two months elapsed time. The basic form
they evolved during this unequalled performance carried them through
two years of such successful evolutionary flight development that
their flying progressed from a hop to mastery of the art. And the
overall record of their powerplants shows them to have been remarkably
reliable in view of the state of the internal combustion engine at
that time.
Appendix
Characteristics of the Wright Flight Engines
-------------------------------------------------------------------------
_1903 _1904-1905 _1908-1911 _1911-1915
First flight Experimental Demonstrations service_
engine[a]_ flights_ and
service_
-------------------------------------------------------------------------
Cyl./Form 4/flat 4/flat 4/vertical 6/vertical
Bore and stroke (in.) 4x4 4-1/8x4 4-3/8x4 4-3/8x4-1/2
Displacement (cu. in.) 201 214 240 406
Horsepower 8.25-16 15-21 28-42 50-75
RPM 670-1200 1070-1360 1325-1500 1400-1560
MEP 49-53 52-57 70-87 70-94
Weight (lb) 140-180 160-170 160-180 265-300
-------------------------------------------------------------------------
[Footnote a: Concurrently with the Wrights' first engine work, Manly
was developing the engine for the Langley Aerodrome, and a comparison
of the Wrights' engine development with that of Manly is immediately
suggested, but no meaningful comparison of the two efforts can be
drawn. Beyond the objective of producing a power unit to accomplish
human flight and the fact that all three individuals were superb
mechanics, the two efforts had nothing in common. The Wrights' goal
was an operable and reasonably lightweight unit to be obtained quickly
and cheaply. Manly's task was to obtain what was for the time an
inordinately light engine and, although the originally specified power
was considerably greater than that of the Wrights, it was still
reasonable even though Manly himself apparently increased it on the
assumption that Langley would need more power than he thought. The
cost and time required were very much greater than the Wrights
expended. He ended up with an engine of extraordinary performance for
its time, containing many features utilized in much later important
service engines. His weight per horsepower was not improved upon for
many years. The Wrights' engine proved its practicability in actual
service. The Manly engine never had this opportunity but its
successful ground tests indicated an equal potential in this respect.
A description of the Langley-Manly engine and the history of its
development is contained in _Smithsonian Annals of Flight_ number 6,
"Langley's Aero Engine of 1903," by Robert B. Meyer (xi+193 pages, 44
figures; Smithsonian Institution Press, 1971)]
It is not possible to state the exact quantities of each engine that
the Wrights produced up to the time that their factory ceased
operation in 1915. Chenoweth gives an estimate, based on the
recollection of their test foreman, of 100 vertical 4s and 50 6s. My
estimate (see page 2) places the total of all engines at close to 200.
Original Wright-built engines of all four of these basic designs are
in existence, although they are rather widely scattered. The
Smithsonian's National Air and Space Museum has examples of them all,
including, of course, the unique first-flight engine. Their condition
varies, but many are operable, or could easily be made so. Among the
best are the first-flight engine and the last vertical 6, at the
Smithsonian, the first vertical 6, at the United States Air Force
Museum, and the vertical 4, at the Carillon Park Museum.