Articles

The Wright Propeller

In Uncategorized on January 7, 2016 by hillermuseum

Wrights propellerOpening the World of Flight

By Jon Welte

The Hiller Aviation Museum displays a full scale model of the first airplane flown by Wilbur and Orville Wright. The novelty of many of its features command the attention of many visitors: the enormous biplane wings, the forward placement of the horizontal stabilizer, the unorthodox flying position of the pilot, the bicycle-inspired chain linkage between the engine and the propellers. In contrast, the propellers themselves are so unremarkable to visitors as to often go unnoticed—and in that respect, they are perhaps the 1903 Flyer’s most remarkable feature of all.

As early as 1809, Sir George Cayley concluded that human-powered flying machines were impractical and that an engine would be required. The widespread development of steam engines later in the 19th century provided a potential, albeit heavy, power source. The question then became how to turn steam power into thrust, the force that carries a powered aircraft forward.

Steam had already been harnessed as a means of marine propulsion. Steam engines created thrust in ships by turning paddlewheels. This was inefficient, however, as at any time more than half of the turning wheel was out of the water and not able to contribute to thrust. By 1838 the Archimedes became the first ocean-going vessel to use screw propulsion.

By the late 1870s, aspiring inventors including Hiriam Maxim and Clement Ader had adapted the shapes of marine propellers to (hoped-for) aerial use. These oddly-shaped contrivances created thrust, but were grossly inefficient. One of Ader’s devices managed to briefly leave the ground in 1890, but the combination of heavy steam engines and inadequate propellers doomed these efforts to failure.

The lack of success met by 19th-century propeller designers stemmed from a fundamental difference between marine and aviation propellers. Marine propellers are subject to cavitation, which occurs when moving propeller blades cause a drop in pressure that creates bubbles. Cavitation interferes with the screw’s ability to displace water, reducing thrust. Consequently, marine screw design sought to minimize pressure changes to avoid cavitation.

Unlike water, air is compressible and pressure changes are fundamental to flying an aircraft. In 1901, a series of unsuccessful glider tests inspired the Wrights to begin a large-scale series of laboratory experiments to better characterize how wing shapes change air pressure and create lift. Their 1902 glider, designed and built in accordance with these wind tunnel tests, flew spectacularly better than any winged aircraft yet built.

Ready to proceed to construction of a powered aircraft, the Wrights reviewed existing research on marine propellers and found it wanting—a feeling not shared by contemporaries such as Samuel Langley, who insisted that “…there is considerably analogy between the best form of aerial and of marine propellers.” Instead, the Wrights determined that a propeller was nothing more than a rotating wing, and the design of a propeller could be modelled with their wind tunnel data.

The Wrights found the challenge bracing. Wilbur noted that “…nothing about a propeller, or the medium in which it acts, stands still for a moment.” Nevertheless, the Wrights developed a mathematical model that translated their best shaped airfoil into an efficient propeller. The propeller’s cross section formed a cambered airfoil. The blades were angled to produce an appropriate angle of attack as they sliced through the air at the airplane’s design airspeed, and the blade angle varied to adjust for the higher speed experienced by the tips of the propeller blades compared to the hub. The Wrights tested a scale model of their propeller in their Dayton shop in December 1902, and were delighted to measure thrust almost exactly as predicted. As Orville later noted, “All the propellers built heretofore are all wrong.”

The Wrights had good reason to develop an exceptional propeller. While many features of the original Wright Flyer—its high wing aspect ratio, modest wing camber, and three axis control system among them—were far beyond the contemporary state of the art, its engine is characterized by the Smithsonian Institution as “…a bit crude, even by the standards of the day.” The internal combustion engine had been invented half a century earlier, and was widely used in industry by 1903. Wilbur Wright had requested quotes from many engine manufacturers for construction of a powerplant suitable for the Flyer. None responded, due more to the expense and impracticability of a custom-built engine than its technical feasibility. The Wrights turned to Charlie Taylor, their bicycle mechanic, to construct one in. Taylor had never built such an engine, and in a heroic effort constructed one weighing 170 lbs. and producing some 12 horsepower. An engine of comparable power today would weigh less than half as much.

Encumbered by its unremarkable engine, only the exceptional performance of its propellers allowed the 1903 Flyer to sustain powered flight. By the fall of 1903, the Wrights had fabricated a pair of full-scale propellers designed to wring the most possible thrust out of their modest engine. Retested using modern techniques, these propellers were found to be nearly 70% efficient—that is, 70% of the engine’s power was translated into thrust—not far below the 80% seen in the very best wooden propellers of the modern day. On December 17th, 1903, those propellers thrust the Wright Flyer into the pages of history.

The Wrights’ successful flights in 1903 were the culmination of four years of scientific research and engineering design work spanning a wide range of aeronautical disciplines. On Saturday, October 24th, the Hiller Aviation Museum hosts its annual Aero Design Challenge for children Grades 4-8. This year’s problem challenges participants to design, build and test a propeller able to accomplish a particular mission. The propeller was one of the keys used by the Wrights to unlock the realm of aerial transportation to the world. By designing and building a propeller of their own, this year’s Aero Design Challenge participants will follow in the footsteps of the original pioneers of flight.

Resources

http://airandspace.si.edu/exhibitions/wright-brothers/online/fly/1903/engine.cfm, 7 August 2015

http://airandspace.si.edu/exhibitions/wright-brothers/online/fly/1903/propellers.cfm , 7 August 2015

http://www.wrightexperience.com/pdfs/props.pdf, 6 August 2015

http://wrightstories.com/ , 6 August 2015

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One Response to “The Wright Propeller”

  1. Very interesting post about the Wright Brothers propeller. I sure would like to visit the museum…someday soon I will.

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