Can airplanes be made that use less fuel and create fewer emissions? And can these airplanes also be significantly quieter than the planes currently flying through the skies?
As aeronautics technology continues to evolve, researchers from government agencies and companies around the globe are trying to answer these questions. Developing and testing technology for planes that produce significantly less noise and have a decreased environmental impact are two of NASA's key research goals.
The counter rotating test rig at NASA's Glenn Research Center enables unique testing, such as the current collaborative testing of open rotor technology by Glenn and General Electric Co., or GE. Image Credit: NASA
Several NASA centers, including the Glenn Research Center in Cleveland, are involved in a multitude of initiatives regarding new aeronautics technology. Glenn scientists, engineers and researchers are working on several NASA projects and collaborating with the aerospace industry to investigate and test new technologies.
Through a Space Act Agreement, Glenn is working with General Electric Co., or GE, to test new technology for a jet engine with two high-speed propellers on the outside, called an open rotor. This effort, initiated under the Subsonic Fixed Wing Project, and now supported by the Environmentally Responsible Aviation Project, of NASA's Aeronautics Research Mission Directorate in Washington, includes testing of the open rotor technology in a Glenn wind tunnel with a test rig designed and built specifically for open rotor testing.
The test rig allows the one propeller to spin in one direction while the propeller directly behind it spins in the opposite direction. This counter-rotating rig is a special piece of equipment. Very few exist, and none are available commercially in the United States.
"We bring the drive rig and the test capability and the test facility, the 9'X 15' wind tunnel. GE brings the design capability and fabrication capability for the open rotor systems," says Brian Fite, the chief of the Acoustics Branch at Glenn. "It is a good fit of the capabilities that each institution has to offer."
The unique shape and design of the high-speed propeller allows airplanes to fly at speeds close to that of airplanes with jet engines. Jet airplanes currently fly much faster than airplanes with standard propellers. Counter rotating propellers increase fuel efficiency but increase the amount of noise produced. Careful design of the open rotor blades is required to reduce the noise both outside and inside the passenger cabin.
"Our goal is to validate noise reduction with an open rotor system while still getting a good fuel burn performance metric," Fite says. Quieting the noise to make open rotors acceptable to the flying public is a major technical challenge.
NASA studied an earlier form of open rotor technology, called a prop fan, in the late 1980s and early 1990s. The Glenn team began this current testing by reevaluating the center's data from earlier studies to compare it to the new data derived from the new experiments. Some Glenn personnel from the original tests still work at Glenn, and they have provided invaluable input.
Glenn stored the counter rotating test rig from the original experiments, and so Glenn was able to restore it for new use with this testing.
"It had been so long since this kind of technology had been looked at; everyone else got rid of their experimental test capability for it. Glenn stored our rig at Plum Brook, and we were able to refurbish it," says Dale Van Zante, the Environmentally Responsible Aviation Project's open rotor research team lead. "With the run-up in fuel prices over the last few years and other environmental issues, engine companies and air framers are again looking at the open rotor technology. Potentially, it gives a big decrease in fuel burn -- gas mileage gets a lot better with that kind of system."
On this open rotor, one high-speed propeller spins in one direction while another high-speed propeller directly behind it spins in the other direction. Image Credit: NASA
The open rotor collaboration began with the signing of a Space Act Agreement between NASA and GE in 2008. GE designed and manufactured the rotors that would be tested, while Glenn refurbished, tested and fine-tuned the counter rotating rig. GE then brought its hardware to Glenn, and began testing last summer. The acoustic and performance data generated will help engineers determine how the open rotor concept could be refined.
The testing has been taking place in the 9'x 15' Low-Speed Wind Tunnel at Glenn, with the testing conditions set up to simulate takeoff and landing. Low speed testing is scheduled to be completed in spring 2010. Testing to measure cruise performance, a major contributor to potential fuel burn reduction, is scheduled to be completed by the end of 2010.
The GE blade design is mounted on Glenn’s counter rotating test rig. For each test, the blades' operation is assessed for acoustic and aerodynamic performance. Variables are explored, including the angle of each blade and the geometric relationship between the rows of blades.
Testing will continue with diagnostic assessments such as using pressure sensitive paint on the propeller surface to explore how the aerodynamic pressure, or loading, is distributed on the blades. Glenn purchased a special phased array, or series of microphones, to measure acoustics in this test. The phased array displays sound as color and requires expert application and data processing to get meaningful results.
Tony Opalski of GE installs hardware that will support detailed blade and flow field measurement systems during the test. Image Credit: NASA
The project has been intensive -- the experiments in the tunnel have been running in two shifts, meaning testing is going on daily from 9 a.m. to 11 p.m. A crew of about 30 NASA employees has been working with about five GE employees onsite.
"We have had very few issues with the hardware or the data systems. Since we've been up and running, things have gone smoothly with the test," Van Zante says."It's been both fun and challenging, and very dynamic."
Early results are promising, and there is a possibility of testing second generation blade designs with GE after company researchers have analyzed their data. Airplane manufacturers also have expressed interest in using the Glenn wind tunnel and counter rotating rig to test the blade technology with an influence model, which explores changes to the noise and fuel burn reduction when the open rotor is installed on the fuselage of an aircraft.
Both the current tests and the 1980s experiments have measured primarily takeoff and landing noise with minimal cruise noise data. Additional testing planned in the 8' x 6' Supersonic Wind Tunnel at Glenn, where conditions can simulate cruising velocity, will provide data on the high-speed performance of open rotors operating at cruise speeds.
Whatever exciting next steps the inquiry into open rotor calls for, Glenn will be ready with its unique blend of experienced personnel and specialized equipment.
"This drive rig is a unique contribution by NASA Glenn," Brian Fite says. "It's the only drive rig in the country that can do this testing."