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RESEARCH AND DEVELOPMENT
V. L. Blumenthal
W. S. Huntington
J. M. Streckenbach
BOEING COMMERCIAL AIRPLANE COMPANY
NOISE REDUCTION RESEARCH AND DEVELOPMENT, 1972 PROGRESS
V. L. Blumenthal, W. S. Huntington, and J. M. Streckenbach
aviation industry. Currently, over three-quarters of the commercial airlines seats are on airplanes powered by JT8D or JT3D engines. There is no way open for these airplanes to be replaced with new aircraft or be retrofitted with new engines in the near future, even if funds were available. The current wide-body jets with new high-bypass engines are too large for many airline routes, both present and future. In addition, no high-bypass-ratio replacement engines in the 20,000- to 30,000-pound thrust class are in production or likely to be in production in the near future.
The JT3D and JT8D engines powering the current low-bypass-ratio fanjet fleet of commercial airliners have been continually improved and uprated, such that today they are well-proven engines that incorporate significant technical advances over their predecessors. Although these engines may be noisier than desired, their well-established reputation for reliability deserves careful consideration of means by which they can be quieted.
The expenditure of over $60 million in Boeing funds alone over the past 8 years on noise reduction research and production development is indicative of the importance the company has placed on improving community noise. During the same period of time, this expenditure has been supplemented with over $30 million in government funding. Our earlier paper(1) went into considerable detail in discussing the specific noise problems of each of the JT3D/JT8D powered airplanes in the low-bypass turbofan fleet, and reported progress made up to the end of the 1971 calendar year. The following discussion will serve to update these efforts through 1972 for each airplane type.
The 727 Airplane
Early tests on the 727 airplane and its JT8D engines showed that noise reduction was dependent on lowering both jet noise roar and the higher frequency rotating machinery noise. The first immediate step taken was to reduce the fan-generated noise levels as far as possible with sound-absorbent linings without waiting for a solution to the more complex and less-understood jet noise problem. This effort resulted in development of the production quiet nacelle illustrated in Figure 3, which has been certified by the FAA as fully meeting the Appendix C noise levels of Federal Air Regulation Part 36 specified for new airplane type designs.
Figure 4 is an indication of both the continuing popularity of the 727 for future domestic and foreign airline routes, and the importance placed by the airlines on purchasing quieter aircraft. Since mid-1971, when the quiet nacelle was first offered as an option, 185 model 727 airplanes have been ordered. The airlines have voluntarily ordered the majority of these with quiet nacelles. This is significant, since there is currently no regulation that requires older aircraft types such as the 707, 727, and 737 to meet the noise levels required of new types.
A lengthy Boeing program aimed at the development of a JT8D jet noise suppression concept for the 727 was reported previously. (1) A subsequent FAA/Boeing co-funded program culminated in flight testing in late 1972 of the configuration illustrated schematically in Figure 5. Two views of the flight test ejector-suppressor installation are shown in Figures 6 and 7. The concept included a 20-lobe jet noise suppressor enclosed in an acoustically lined ejector shroud, in combination with an acoustically treated, ringed inlet to suppress forward-radiated fan noise, and fan duct treatment to reduce aft-radiated fan noise. The objective was a balanced design that would work equally well in reducing jet-dominated takeoff noise and fan-dominated approach noise.
Although the final flight test results confirmed earlier predictions that significant reductions in community noise were attainable using this configuration, the performance losses were much greater than was expected or considered reasonable. Figure 8 summarizes the noise and airplane performance values of the in-production quiet nacelle (fan noise treatment only) and the ejector-suppressor concept (fan treatment and jet suppression), both related to the baseline 727 airplane.
Figure 9 illustrates the range penalties associated with operating ejector-suppressor-equipped airplanes from limited field lengths and high-altitude airports. A detailed discussion of the ejector-suppressor development program is covered in References 2 and 3.
An alternate means of reducing noise is to increase the bypass ratio of current engines in a direction toward the new high-bypass engines powering the 747, DC-10, and L-1011 airplanes. This bypass ratio increase results in a reduction of jet velocity, thereby reducing jet noise. Studies started by Boeing, and under NASA sponsorship since August 1972, are aimed at determining the feasibility of noise reduction on JT3D and JT8D powered airplanes by replacing the two-stage fans with larger diameter single-stage fans. The new fans extract more work from the turbine, resulting in reduced jet velocity and jet noise. Pratt & Whitney and McDonnell-Douglas are also participating in the NASA-sponsored program. Work accomplished to date indicates that this fan replacement concept is potentially very attractive.
Along with reduced primary jet velocity, the refanned engines also experience improvements in specific fuel consumption, and in takeoff and cruise thrust.
Figure 10 is a schematic comparison of the refanned and production JT8D engines and their treated nacelles.
The data comparisons shown in Figure 8 indicate considerable noise reduction for the refanned engine installation (next to the last column), but the weight of the installation results in a high range loss when operating at the basic airplane gross weight. By taking advantage of the higher thrust rating of the refanned engine, allowing the takeoff weight to increase so as