Air pollution from commercial aircraft exhaust has long been recognized as a environmental problem, but what about noise pollution?
Dr. Ian Waitz's group in aeronautics and astronautics has come up with an idea that could significantly reduce the noise from jet engines by injecting air from the trailing edges of the rotating blades within the engine.
To aircraft manufacturers and operators, noise is a problem as serious as emissions. "It significantly impacts the economics," said Dr. Waitz, an assistant professor and director of the Aero-Environmental Research Laboratory, which also has a program addressing emissions from aircraft engines.
Noise from planes flying over residential areas impairs people's ability to work, learn in school and sleep, and consequently also results in lowered property values in affected areas. As passenger volume increases and new and larger airports are built, noise is becoming even more of a concern.
Measures to control noise production include Federal Aviation Administration certification standards for new airplanes, restricted flight paths, flight curfews and ticket taxes. NASA, which is funding Dr. Waitz's research, has set a goal of reducing aircraft engine noise by 6 EPNdB (effective perceived noise decibels). Successful design innovations may eventually be adopted as requirements by the FAA and the Environmental Protection Agency.
Much of the noise from gas turbine engines comes from air flowing back through the rapidly spinning fan blades at the front of the engine, Dr. Waitz said. Behind each blade is a wake, or an area of lower-speed air, much like the calmer water behind a rock sticking out of a stream. When these wakes move over stationary blades called stators, which are located downstream of the rotating blades, they produce strong, unsteady pressures. This unsteady pressure field is what causes most of the sound associated with aircraft engines, Dr. Waitz explained.
Most noise-control measures, such as acoustic liners in the engines, have focused on reducing the amplitude of the sound after it is produced. But Dr. Waitz and his colleagues are working on a method of cutting down on noise at the source. Their idea is to "fill in" the wake behind each rotor blade by pushing air through the trailing edges of the rotating blades.
This recently became feasible with the advent of newer engines having fan blades that are larger than ever before at three to four feet high. At this size, the thin blades can be made hollow (indeed, Pratt & Whitney and Rolls-Royce are manufacturing hollow titanium fan blades, which are used in the engines for the new Boeing 777, Dr. Waitz said). It is therefore possible to blow air through the inside of each blade and expel it from the trailing edge, where it mixes with the air flowing around the blade and makes the flow into the stator more uniform.
The new technique is made possible by the trend toward larger and larger fan blades in engines, a trend which is driven by the desire for higher fuel efficiencies, Dr. Waitz said.
Dr. Waitz believes that the wake-management approach will prove more successful than "active control." Behind this competing technique is the fact that sounds of a given tone produce a unique sound-wave pattern; when a sound with a wave pattern that is a mirror image of the undesirable sound is produced, the two cancel out. However, this technique works best with pure tones rather than the broadband, multitonal "white noise" that engines produce, Dr. Waitz said.
The wake-management technique has been tested on a simplified scale model with good results, Dr. Waitz said. Within the next year, it will be tried with a model of a complete gas turbine engine fan in a lab; if that is successful, it may be tested the following year at NASA's Lewis Research Center. Preliminary results have shown that the technique could indeed meet the NASA goal of reducing engine noise by 6 EPNdB, he said.
A version of this article appeared in MIT Tech Talk on October 25, 1995.