A century ago the debut of propeller-driven aircraft kicked off a global aerospace technology boom that continues to this day. But since the emergence of the jet aircraft engine during World War II, research into propeller-powered flight has often taken a backseat to the turbofan technology that carries jetliners faster and farther. Speed and range come at a cost, however, and both rising fuel prices and increased demand for regional air travel have changed the economics of flight over the past 10 years. Now airlines are once again looking to smaller, more efficient turboprop planes to handle shorter routes, driving the development of a new generation of prop-driven aircraft technologies poised to take wing by the end of the decade.
Turboprop planes accounted for roughly half of the 20- to 99-seat passenger aircraft delivered to airlines in 2013, according to market research conducted by Canadian plane maker Bombardier— parity that has not existed since the 1990s. Demand has risen because on flights less than about 500 nautical miles, turboprops are far more fuel-efficient than turbofans, which fly most efficiently only after they have made the long climb to their much higher cruising altitudes. But in exchange for their efficiency, traditional turboprops sacrifice airspeed and generate noise and vibrations that compromise passenger comfort. For airlines competing over customer experience as much as price (and acutely aware of passengers’ perceptions of propeller-driven aircraft as passé), the propeller technology of the last century will not do.
Among those paving the way for a new generation of turboprops, General Electric Aviation’s Dowty Propellers is exploring anew the interactive effects among the propeller, engine nacelle and aircraft wing. Using computational fluid dynamics tools that were not available even a few years ago, engineers at the Gloucester, England–based firm are not only designing blades with new efficiency-enhancing shapes but rethinking the layout of the propeller as a whole.
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“The computational power that’s avail- able now has really made the difference,” says Dowty’s Jonathan Chestney, noting that researchers can analyze data on an individual-blade basis. “It’s an exciting time for us,” he remarks. “We’re able to see much more detail, like a scientist who just got a microscope for the first time.”
Dowty engineers are currently exploring two novel spacing ideas for eight- blade propellers. One positions the blades unequally around the circumference of the propeller hub; the other staggers the blades axially, with four blades mounted farther forward on the hub than the others. These spacing schemes break up and change the audible frequencies created in flight. Dowty is in the midst of testing the corresponding cabin sounds on volunteers to see which ones they prefer.
Dowty’s research is not taking place in a vacuum. Advanced propellers will appear in the next-generation helicopters that the U.S. Department of Defense wants and in upcoming unmanned aerial vehicles, says aerospace engineer Lakshmi Sankar of the Georgia Institute of Technology. As such, research is taking place across the industry and even across disciplines. Computational fluid dynamics research on propellers con- ducted at places like the NASA Glenn Research Center and Georgia Tech are feeding into designs coming out of suppliers including Dowty and Charlotte-based UTC Aerospace Systems.
Novel designs are not far from the tarmac. Says Dowty’s Chestney, “We expect to see some key players going public with new aircraft designs in the next couple of years.”
FURTHER READINGS AND CITATIONSScientificAmerican.com/nov2014/advances
