Prop-wrights: how two brothers from Dayton added a new twist to airplane propulsion.
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 airfoils--all 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 propeller design.
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.
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 propeller.
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.
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 the winter.
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, flood-waters 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 supervision.
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 locations.
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 surface accuracy.
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 Tunnel.
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 and torque.
Static thrust measurement 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 ([rho]), according to the formula
T = [C.sub.T] * [[rho][n.sup.2][D.sup.4]],
where [C.sub.T] [equivalent to] T/[[rho][n.sup.2][D.sup.4]] = 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 [+ or -] 0.02, whereas the static thrust coefficient for the propeller reproduction was 0.139 [+ or -] 0.003. The two coefficient intervals overlap--barely--but it is not possible to isolate experimented 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 ([C.sub.T]), power coefficient, [C.sub.p] = 2[pi]Q/([rho][n.sup.2][D.sup.5]), and efficiency, [eta] = J [C.sub.T]/[C.sub.p].
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 steady flight.
This was a remarkable feat, considering the state of propeller knowledge prior to World War I.
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 blade.
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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|Title Annotation:||the beginnings of airplane propeller design|
|Author:||Ash, Robert L.; Britcher, Colin P.; Hyde, Kenneth W.|
|Date:||Dec 1, 2003|
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