New Twist on an Old Idea : Future Airliners May Have Propellers

SAN DIEGO —  The next wave of commercial airliners may be powered by a propeller-driven propulsion system that’s now in the early stages of testing, researchers said.

Model and ground testing of the propeller, called a propfan, shows it will cut fuel consumption of today’s turbofan-powered jets by as much as one-third, said Keith Sievers, chief of the NASA Lewis Research Center’s Advanced Turboprop Project Office.

“People always ask, ‘Why are you looking at propellers? Isn’t that a step back?’ But we consider this more of a revolutionary step forward,” Sievers said.

NASA has earmarked $200 million through its aeronautical division for a Propfan Test Assessment program. The propfan has eight thin, curved blades that are reinforced with an aluminum alloy, instead of the usual two to four blades on standard propellers.

The additional blades produce more thrust than propellers of the same size, allowing a plane’s engine to either operate under reduced strain or to cut down the runway space needed by about 30%, Sievers said.

Longer Flying Time

The shorter takeoff space required and longer flying time allowed by slower fuel consumption is particularly appealing to the military, Sievers said.

The propfan, which has withstood speed tests of up to nearly 600 m.p.h. at altitudes up to 30,000 feet, also could be used on cruise missiles to double their current range, he said.

The military favors the use of propfans on the wings of aircraft to cut down on the amount of runway space needed, said Allen Novick, chief product engineer of the Allison Gas Turbine Division of General Motors Corp.

Commercial airline officials favor the use of propfans on the rear of their planes to cut down on noise, Novick said.

The ground testing of propfan blade sturdiness that started two weeks ago at the Rohr Turbine Engine Test Facility in San Diego is scheduled to end later this month. After evaluating the data, flight testing is to begin in Georgia next February using a Gulfstream II business jet.

Unanswered Questions

Questions to be answered are whether the propfan blades will be sturdy enough to withstand constant commercial use, and whether they will operate quietly enough to satisfy passengers and local airport noise requirements.

NASA contracted with Lockheed-Georgia Co. of Marietta, Ga., to design, build and flight-test the propfan system. Lockheed has subcontracted with Allison Gas Turbine of Indianapolis; Rohr Industries Inc., of Chula Vista, Calif.; Hamilton Standard of Windsor Locks, Conn., and Gulfstream Aerospace Corp. of Savannah, Ga., to provide parts and to test various aspects of the system.

“We’re not building a prototype aircraft, we’re providing the technology to the people that will build the aircraft,” NASA’s Sievers said. “We want the technology for them because it’s going to take anywhere from $3 billion to $6 billion of private capital for the engine, aircraft and propeller people to bring these things to market through certification.”

The nine-foot diameter propfan being tested was built to fit a 6,000-horsepower Allison gas turbine engine. But it would take a 10,000-horsepower engine to get a 100-passenger plane in the air. Such an engine is now being designed now by Allison researchers in Indianapolis.

Ground testing of that propfan is set for next summer, with flight testing scheduled for December, 1987, Novick said.