The engine that made
barnstorming possible
The Curtiss OX-5 has the
distinction of being the first mass-produced
aircraft engine in the United States. First
available in 1915, it served in trainers through
World War I, continued to be produced by Curtiss
as well as other licensees until 1918 or 1919, and
then powered a veritable explosion in general
aviation after the end of the war and for a decade
to follow. In 1929, 2,510 of 6,631 licensed
airplanes (38 percent) were powered by OX-5s. In
the eleven months ending March 1, 1930, 4,574
planes were licensed. Of these, 1,359 were powered
by war-surplus OX-5s and only 1,016 by brand-new
state-of-the-art Wright Whirlwinds. Though in 1930
almost all aircraft engines were better, few could
compete with the cost of war-surplus engines (some
sold new in the crate for a mere $20.00). The
story of the OX-5 is one of long evolution, and in
many ways, the story of Glenn Curtiss, one of the
truly great pioneers of aviation in America.
Figure 1. Curtiss OX-5
History
Glenn Hammond Curtis was born
May 21, 1878 in Hammondsport, New York, the son of
a harness maker. Though Curtiss showed an early
aptitude for mechanical things, he disliked formal
schooling and dropped out of high school at the
age of fourteen. While working as a camera
assembler at Eastman Kodak, Curtiss became
interested in racing bicycles, eventually opening
several bicycle shops. In 1901, after seeing a
.Thomas Auto-Bi. motorcycle at the Pan-American
exposition in Buffalo, New York, he got the idea
of attaching an engine to his bicycle. He built
his first engine from .Auto-Bi. rough castings
purchased through Scientific American. His
first carburettor was a tomato can stuffed with
steel wool. Unsatisfied with the performance of
engines designed by others, he was soon producing
motorcycles with engines of his own design, and
started the Curtiss Company in 1902. Development
of motorcycles led to his January 1907 World
Record of 136.36 mph in Ormond Beach, Florida,
making him the fastest man on earth for the next
seven years.
The first Curtiss aviation
engine was a modified 5-hp motorcycle engine sold
in 1904 to Captain Thomas Baldwin to power the
first successful U.S. dirigible, the California
Arrow. Curtiss discovered he could sell
aircraft engines for the same price as complete
motorcycle, though he continued to produce
motorcycles and motorcycle engines until 1917. By
1906, Curtiss had developed V-8 air-cooled engines
used to power aircraft of the Aerial Experiment
Association, led by Dr. Alexander Graham Bell.
Curtiss made his first flight
on his 30th birthday--May 21, 1908--in White
Wing, a design of the Aerial Experiment
Association. White Wing was the first plane in
America to be controlled by ailerons instead of
the wing warping used by the Wrights. It was also
the first plane on wheels this side of the
Atlantic.
Curtiss had big years from 1908
through 1910. He won the Scientific American
magazine trophy, the Gordon Bennett Trophy plus a
$5,000 prize at the first international air meet
at Reims, France, and the $10,000 New York Times
prize for the first flight between Albany and New
York City. These accomplishments were primarily
due to Curtiss always having the engine edge in
his aircraft.
Curtiss deserves credit for
pioneering the design of the floatplane, the
amphibian, and the flying boat. It was a Curtiss
plane flown by Eugene Ely, a company exhibition
pilot that made the first successful takeoff from
a Navy ship in 1910. Another Curtiss plane, the
NC-4, made the first crossing of the Atlantic in
1919. Curtiss built the first U.S. Navy aircraft,
called the Triad, and trained the first two naval
pilots. He received the Collier Trophy and the
Aero Club Gold Medal for the greatest
accomplishment in aviation during 1911.
The American entrance into
World War I produced huge orders for the Curtiss
Company, and made Glenn Curtiss a rich man. The
company continued to improve its line of engines
through the 1920s but fell on hard times in the
late 1920s due to its involvement in the Army Air
Corps. misguided attempts to build
high-temperature liquid-cooled engines. The
Curtiss Company merged with Wright in 1929 to
become Curtiss-Wright. Glenn Curtiss died in July
of 1930 from complications of appendicitis
Design and Development
Early Curtiss aircraft engines
were variants of his air-cooled V-twin motorcycle
engine. The design philosophy espoused minimal
weight consistent with acceptable reliability,
advanced mechanical features, and careful
machining. These early engines showed remarkable
insight in their crankshaft design by
incorporating roller main and big end rod
bearings. Engines were enlarged by bolting the
V-twins together until finally a V-8 was appeared
in 1906. The cylinders and valve gear were
unremarkable F-head suction-intake designs with
cast-iron cylinders and pistons that overheated on
anything but the shortest flights. By 1908,
engines had become large enough so that
air-cooling gave way to water cooling in the form
of Monel (a nickel-copper alloy) water jackets
brazed onto cast-iron cylinders. Later engines
used brazed-on steel water jackets. Each cylinder
was secured to the aluminium wet-sump crankcase by
four nickel-steel studs, contributing to engine
lightness. Curtiss spared little expense, using
the best materials and considerable machining and
assembly labour. By 1909, cross-flow overhead
valves with double-acting cam, a pull tube
actuating the intake valve and a push rod
actuating the exhaust valve were introduced. This
was refined into the first OX series of engines,
introduced in 1912, the result of a U.S. Navy
power requirement for the A-1 amphibian.
Curtiss was able to attract and
afford some of the best engineering talent
available at the time. This included Henry
Kleckler, Charles Kirkham, and Charles Manly.
Kleckler had come up through the ranks at Curtiss
after beginning as shop foreman in 1906, and
handled many of the day-to-day shop issues.
Charles Kirkham, chief of engine design, was the
son of John Kirkham, whose machine shop had built
Curtiss. early engines. Kirkham had manufactured a
line of both motorcycle and aircraft engines of
his own before coming to Curtiss. Charles Manly
was a former SAE president and had built the 1903
Manly-Balzer 5-cylinder radial to power the
Langley Aerodrome. This team refined the
OX-5 pistons, valve gear, intake manifold, magneto
drive, and carburettor in 1915, improving the
power and reliability of the engine.
A few of the aircraft using
OX-5 engines were the Laird Swallow, Travel Air
2000, Waco 9 and 10, the American Eagle, and some
models of the ubiquitous Curtiss JN-4 .Jenny.. It
is interesting that these large, fairly heavy
aircraft performed as well as they did with only
90 hp (though the .Jenny's climb has been
described as 'imperceptible'). Some authors have
attributed this to the engine having great torque,
which in their mind made up for horsepower.
Horsepower, however, is nothing but torque times
RPM. Though the OX-5 developed its maximum torque
at a low 1,000 RPM, two factors were responsible
for its better-than-expected performance. First,
its rating of 90 hp was at 1,400 RPM, though it
was capable of running as fast as 1,800 RPM for
short periods, and producing nearly 105 hp. This
slow engine speed, especially when compared to
modern direct-drive aircraft engines, allowed
propellers to be much larger and more efficient,
producing greater thrust.
Service
When the OX-5 was introduced it
was as light, efficient and reliable as any other
aircraft engine. By the end of its use nearly
thirty years later, it was considered heavy and
unreliable. Since it had been used largely in
training and barnstorming aircraft, some think it
contributed to the death of quite a few students.
A number of design and production problems gave
rise to these reliability difficulties. As with
all separate cylinder engines, the crankcase had
to be extremely heavy in order to be sufficiently
rigid to avoid flexing. Though the OX-5 bottom end
was reasonably good, typically having a
time-between-overhaul (TBO) of a few hundred
hours, the valve gear was both fragile and
troublesome. It had no provisions for lubrication
other than grease and oil applied by hand. Fifty
hours was the typical limit of the valve train.
The single ignition typically powered by a Dixie
or Berling magneto did not stand up well in
service. In fairness to the design, Bosche
magnetos were originally specified, but were
unavailable due to the war. Later Scintilla units
fared better. Other problems resulted from
vibration-induced cooling leaks. The large volume
of oil in conjunction with long intake manifolds
made cold weather starting difficult. Pilots would
often drain the oil from the engine and keep it
warm overnight to aid cold starting. Since the
engine had been widely licensed, Curtiss had no
control over the quality. Though Curtiss-produced
engines were of high quality, many of the licensed
units exhibited appalling quality. During the
1920.s a thriving after-market of improved top-end
overhaul components (roller rockers, lubrication
upgrades, dual ignition, etc.) appeared.
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