How two brothers from Dayton added a new twist to
By Robert L. Ash, Colin P. Britcher, and Kenneth W. Hyde
had mastered aircraft control, the Wright brothers believed that propelling
one would be a minor challenge, a mere matter of attaching an engine and
screws to a winged machine. By December 1902, their main concern in progressing
to powered flight was the weight penalty of the propulsion system. They
believed that they could purchase a light gasoline engine and then apply
the principles of ship propeller design to the design of airplane propellers.
None of the dozen or so gasoline engine manufacturers they contacted could
quote the engine they were looking for: an 8- or 9-horsepower model that
weighed less than 20 pounds per horsepower. When they couldn't
buy the engine, the Wright brothers and their mechanic, Charlie Taylor,
were forced to build one themselves. They needed to keep the weight of
their powered flyer below 700 lbs., including the pilot. Together with
Taylor, the Wrights designed and built their first engine in six weeks,
beginning in January 1903.
Steamships had been plying the Atlantic since 1846. By 1900, numerous
books on screw propeller design had been published. From this literature,
Wilbur Wright learned that the design process was more empirical than
theoretical. Ship propeller design theory did not apply directly to air
screws. For one thing, water's higher density helps marine props
produce thrust through changing momentum. For another, the possibility
of cavitation precludes a lifting-surface approach to marine propulsion.
Aeronautical researchers of the day were doing no better than the Wrights.
Those researchers concerned themselves mostly with the aerodynamics of
flat plate, circular arc, and parabolic section airfoilsall of
which generated lift via momentum changes.
The Wright brothers did not realize it at first, but their wind tunnel
testing program, completed in December 1901, had given them a key to revolutionizing
Arguing About Design Approaches
Wilbur referred to Rankine's momentum theory and to Froude's basic work
on propeller design. Those theories predicted only the relationship between
thrust and wake velocity; they didn't provide guidance on the optimal
geometry of a practical airplane propeller.
The brothers argued for many hours over their design approach. Often,
they'd switch sides. They drew numerous vector diagrams. They finally
visualized propellers as rotating wing sections moving in air that had
been accelerated by the propeller disc before meeting the blade section.
|From top: The 1903 Wright Flyer
reproduction prepares for tests in the Langley tunnel; end view of
the reproduction 1903 propeller; Wright propeller mounted on a 15-foot
test stand; Larry Parks using period tools to carve the propeller.
Using their newly acquired wind tunnel data, the Wrights selected an
airfoil section that they believed would maximize thrust (lift) and minimize
the torque (local "drift" or drag multiplied by local radius)
required to drive the blade. Even though their wind tunnel models were
so small that they didn't yield realistic Reynolds number scaling or precise
information on wing cross sections, the airfoils did supply the data they
needed to design their propellers.
The Wrights' realization, that a propeller was a rotating lifting device
that could be treated like a wing section, was not obvious to others.
They also knew that propeller performance in flight would be different
from propeller performance produced by a static flying machine because
of the different relationship between the approaching air and the rotating
The Wrights never explained the process that they used to choose their
airfoil section, although their "Airfoil Number 9" (a circular
arc with a 1/20 camber) was the most efficient airfoil, in terms of the
ratio of lift to drag, for the angle of attack range their propellers
would see. Their notebooks suggest that it was their basic propeller design
element. How they came to specify the blade width distribution as a function
of radius, or why Wilbur decided to use a reference propeller design position,
what they called its center of pressure, located at approximately 5/6
of the overall radius both remain mysteries.
The Wright brothers knew mathematics only as far as algebra and trigonometry.
They made all of their calculations by hand.
Wright Flying Machine Propellers of 1903
In December 1902, Wilbur and Orville tested their first propeller model
based on their wind tunnel airfoil tests and Wilbur's propeller theory.
Their 2-hp shop engine powered the small-scale, 28-inch-diameter propeller.
A brake loaded the propeller and measured shaft power. The brothers appeared
to have surveyed the wake in order to calculate thrust. Those tests showed
that the propeller thrust varied quadratically with propeller speed.
The Wright brothers completed construction of their first full-scale propeller
in February 1903 and tested it at 245 rpm on the shop motor. They may
have measured the thrust with a spring balance system similar to what
they employed at Kitty Hawk the previous November. Evidently, they were
confident enough in the test results to enable Wilbur, in his notebook
entry of March 6, 1903, to predict that the efficiency of their full-scale
propellers would be 66 percent.
By early fall, they had built the propellers that would carry them aloft.
The Wright brothers did not have the instrumentation to measure actual
propeller performance in flight. However, the Wright notebooks contain
static thrust measurements recorded for the actual propeller pair (on
Nov. 21, 28, and Dec. 17, 1903). To make these measurements, the Wrights
placed the Flyer in a shed and perched it on its launch carriage. They
restrained one of the lower wing tips and attached the other wing tip
to a line. That line connected to "50 pounds of sand" and then
to a grocer's scale, letting the brothers measure the restraining force
while the machine was powered in the shed. Since the Wright Flyer had
no throttle, the engine speed could not be controlled, but the notebook
entries indicate that the average measured static thrust was between 132
and 136 pounds (that is, each propeller produced a thrust of between 66
and 68 pounds) when the propellers were powered at a nominal rotational
speed of 350 rpm.
Unfortunately, the atmospheric conditions were not recorded when the thrust
measurements were made. (The actual times of day were not recorded.) We
do not know whether propeller speed was measured or estimated using their
revolution counter. Furthermore, we cannot assess the exact locations
of the wing tip restraint or the line that was connected to the grocer's
scale (the Wright Flyer is not symmetrical, either), nor do we know the
calibration accuracy of the static thrust test stand or the grocery scale.
|Measurement variation of thrust
coefficient, power coefficient, and efficiency functions of advance
ratio for 1903 Wright Flyer propeller replica.
The operating characteristics of the Wright Flyer engine changed rapidly
as it warmed up (and then overheated), making the measuring of static
thrust difficult. However, it is fair to say that the Wright Flyer propellers
produced a thrust of approximately 67 pounds when rotated at 350 rpm in
A wind gust destroyed the 1903 Flyer shortly after its fourth flight on
December 17. Although the mounting tabs for the engine block were broken
off in the crash, the original propellers remained intact to be used again
on the 1904 Flyer II. These propellers broke on May 26, 1904, when the
Wright brothers crashed the 1904 Flyer during a press demonstration.
Large pieces of the original propellers survived and were placed in storage
in Dayton. Unfortunately, floodwaters got to them in 1913. Only fragments
exist today. The National Air and Space Museum holds parts from one propeller
in addition to its reproductions of the originals built under Orville's
The National Park Service, which holds fragments from the other original
propeller, allowed The Wright Experience of Warrenton, Va., to examine
the artifacts. Constructed from laminated spruce, the pieces have deteriorated
over the years. However, by assuming that the spruce boards from which
the propellers were constructed were once straight, The Wright Experience
has been able to correct for much of the distortion using a digital database.
The Wright Experience, a nonprofit organization, has been chartered to
restore the technological legacy of the Wrights.
Starting with digital representations of the propeller blade surfaces,
the surface profiles were "deformed" until the lamination planes
were flat. From the corrected digital representations of the broken blade
section, a complete digital propeller was constructed by invoking blade
symmetry conditions in order to recreate the missing portion of the propeller.
That digital data was then used to construct templates at selected blade
The propeller reproductions were constructed by laminating spruce boards
of the same dimensions as the originals, then roughing out the propeller
shape. Expert woodworker Larry Parks, on loan from BAE Systems, carved
the propeller reproductions with tools similar to what the Wrights had
used. Digitally produced propeller section templates aided The Wright
Experience in reproducing the original propeller geometry to submillimeter
|Samuel P. Langley's Great Aerodrome
propellers, circa 1903 (top). Gustav Whitehead, leaning against his
1901 flying machine propellers (middle). Sir Hiram Maxim's 1896 propellers
measured 18 feet, 6 inches in diameter (bottom).
In 1999, Old Dominion University and NASA Langley Research Center began
working with The Wright Experience to test the performance of the Wright
Flyer propellers. So far, reproductions of the 1903 Flyer propellers,
the 1904 Flyer propellers used at LeMans in 1908, and the "bent-end"
propellers have been manufactured and tested in the Langley Full Scale
The Full Scale Tunnel, operated by Old Dominion University since 1997
under a Space Act Cooperative Agreement, was built by the National Advisory
Committee for Aeronautics in 1930.
The propeller reproductions were mounted on a test tower, with the propeller
drive axis located in the central flow zone of the open test section.
A 25-hp variable speed Teco Westinghouse motor drove the propellers. A
calibrated thrust torque balance (provided by NASA Langley Research Center)
connected between the propeller hub and the driveshaft for measuring thrust
Static thrust measurements require no wind tunnel flow. However, operating
the propeller on the test stand without activating the wind tunnel only
approximated that condition, since a measurable wind tunnel velocity resulted.
Furthermore, the thrust coefficient used to characterize propeller performance
was not defined until after World War I. Now, it's known that thrust (T)
is related to propeller diameter (D), propeller speed (n)
(revs/sec.), and air density (r), according
to the formula
T = CT[rn2D4],
where CT º T/[rn2D4]
= Thrust Coefficient.
The Wright brothers could measure their propeller diameter to within
0.1 inch, and that would not have contributed substantially to any error.
However, the accuracies of their propeller speed and thrust measurements
are not known and the atmospheric conditions (air density) at the time
of their thrust measurements were not recorded. We measured the (near)
static thrust of the propeller reproduction to be 69 lbs. at 380 rpm.
Our estimate of the static thrust coefficient for the original Wright
propeller was 0.16 ± 0.02, whereas the static thrust coefficient
for the propeller reproduction was 0.139 ± 0.003. The two coefficient
intervals overlapbarelybut it is not possible to isolate experimental
error from physical differences.
Unlike the Wright brothers, we can measure the thrust and torque and,
by varying wind tunnel speed, characterize the propeller performance over
the possible range of flight conditions.
The propeller performance, at different combinations of propeller speed
(n) and forward speed (U) using the advance ratio,
J= U/nD can be characterized in terms of thrust coefficient
(CT), power coefficient, CP = 2pQ/(rn2D5),
and efficiency, h = J CT/CP.
These data show that the 1903 Wright propeller had a maximum efficiency
of 82 percent.
Based on Wilbur Wright's notes on the fourth flight of Dec. 17, 1903,
the Flyer had an estimated forward speed of 31 mph during the steady flight
portion of its path and the propellers were turning at 379 rpm, which
yields an advance ratio of 0.85. Hence, the 1903 Wright propellers were
operating at a mechanical efficiency of slightly over 75 percent during
This was a remarkable feat, considering the state of propeller knowledge
prior to World War I.
|Greg Cone of The Wright Experience starts
the Wright brothers' engine No. 20 during operational testing.
Since Wilbur estimated their propeller performance to be 66 percent in
March of 1903, we found the results of our experimental tests to be quite
surprising. Using Wright bent-end propeller reproductions as our reference
test case (there are several well-preserved sets in existence), we have
subjected these propellers to multiple wind tunnel tests. We recalibrated
the instrumentation used in the propeller tests and we subjected the bent-end
geometry propellers to a full Navier-Stokes equation computational fluid
dynamics analysis in order to affirm our test results. The bent-end propellers
had peak efficiencies of nearly 87 percent. The overall comparisons between
the numerical predictions and the test results agreed. To our surprise,
we learned that the Wrights' bent-end propeller twist distribution (a
variation of pitch angle with radius) was in nearly exact agreement with
modern computer-based designs over the outer two-thirds of the propeller
It is apparent that the Wright brothers believed that slowly rotating,
large-diameter propellers, with low blade loading, similar to the propellers
that are used on modern-day human-powered aircraft, achieved better performance
than smaller propellers rotating at higher speeds. Their approach of extracting
maximum propeller performance from their very marginal power plant enabled
them to achieve controlled powered flight in 1903. (Samuel P. Langley's
1903 Great Aerodrome was powered by a five-cylinder 53-hp gasoline engine
that weighed only 125 pounds.) In fact, their propeller designs were just
as critical as their flight control system in their overall system engineering
approach. Their use of chain drives with sprockets to power the propellers
on their flying machines is another manifestation of that genius. They
knew that the propeller speed and performance had to be matched with their
engine performance, even though theirs varied with time and conditions.
The Wright brothers could adjust their propeller speed to their engine
performance under various flight conditions simply by changing out sprockets.
They definitely understood the balance between engine power and propulsive
power, as influenced by propeller performance.
Robert L. Ash is interim vice president for research at Old Dominion University
in Norfolk, Va.. He is a professor of aerospace engineering and is designated
as an Eminent Scholar. Colin P. Britcher is a professor of aerospace engineering
at Old Dominion and directs the university's NASA Center for Experimental
Aeronautics. Kenneth W. Hyde is the president of The Wright Experience,
which is based in Warrenton, Va.
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