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Figure 10.-Production and New Front Fan JT8D Engine
to retain the same field length as the basic airplane (last column of Figure 8), the range penalty is reduced to about 6 percent. That same airplane, at a gross weight of 182,500 pounds, offers a 150-mile increase over the basic airplane, provided that a 630-foot increase in field length is acceptable.
Figure 11 compares all three concepts with the baseline airplane in the form of community noise footprint contour areas and noise measured under the airplane flightpath during approach and takeoff. These comparisons give an indication of the relative noise improvements of the various concepts in terms of square miles of the community (and therefore of the number of people) subjected to a given level of noise, in this case, 90 EPNdB.
The 707 Airplane
Following extensive Boeing-funded research and development on 707 noise reduction concepts, and a NASA-funded
flight test program, (1) Boeing joined with the FAA in a contract to further develop the concept. The FAA program includes flight demonstration of production-quality hardware. The concept illustrated schematically in Figure 13 was ground tested in late 1972 at Boeing's Wichita plant, and is scheduled for flight testing in March to May of this year. Figure 14 is a photograph of the nacelle configuration on the ground test rig.
The FAA nacelle, based on ground test results to date, is expected to demonstrate compliance with FAR 36 Appendix C noise levels. Figure 15 compares the NASA flight test results and the full-scale ground test data for the FAA nacelle with the baseline 707-320B airplane. It shows improvements in both noise reduction and airplane performance for the FAA nacelle, but flight test verification is required.
90 EPNdB CONTOUR
As a portion of the overall NASA JT8D/JT3D refan program, Boeing, McDonnell-Douglas, and Pratt & Whitney have conducted studies to assess the advantages of installing refanned JT3D engines on 707 and DC-8 aircraft. These studies have indicated that further reductions in 707 noise are attainable through replacing the current JT3D two-stage fan with a larger diameter single stage. Figure 16 is a schematic comparison of the two engines with their respective treated nacelles. The bypass ratio for one concept, the JT3D-9, would increase from the present 1.36 to 2.25, with substantial decrease in primary jet velocity. Enclosing this refanned engine in a suitable acoustically treated nacelle is predicted to result in the noise numbers and performance values shown in Figure 15.
Although the extra noise reduction, relative to the FAA fan treatment, is considerable, the airplane is seen to experience some degradation in range, due mainly to the weight of the installation, which requires fuel offload to stay within the 333,600-pound takeoff gross weight. However, the higher thrust rating of the refanned engine and the load-relieving effect on the wings of the heavier installation indicate the possibility of recovering the range loss by a slight increase in takeoff gross
weight. A typical example, shown in the last column of Figure 15, was based on increasing the takeoff gross weight to 339,400 pounds, which is within the current structural capabilities and fuel capacity of the airplane. This resulted in a range increase of 125 nautical miles over that of the basic airplane, while retaining essentially the same noise values as the 333,600pound refanned airplane. Thus, the airplane will be capable of serving the same route structure as the existing airplanes, with significant reductions in noise.
Although the JT3D refan contract was cancelled in January 1973 due to lack of funds, Boeing knows of no critical technical defects in the concept, based on work completed to date. Boeing would have preferred to continue this work, at least through full-scale ground test, in order to fully assess the potential for noise reduction and performance improvement.
Availability schedules for either the fan noise treatment or the refanned engine plus acoustic treatment, as currently envisioned, are shown in Figure 17. The fan noise treatment schedule reflects current Boeing planning, whereas the refan schedule assumes adequate and timely funding.
The 737 Airplane
In a manner similar to that described for the 727, the JT8D-powered 737 airplane was studied from the standpoint of first reducing fan-generated noise levels as far as possible with sound-absorbent linings. The configuration shown in Figures 18 and 19 was designed and constructed, and FAR 36 certification flight testing was completed, in March 1972. The data were approved by the FAA as satisfying Appendix C requirements. Boeing has since received firm orders for nine 737-200 airplanes with FAR 36 compliant quiet nacelles. It is expected, as in the case of the 727, that the majority of future 737 purchases by the airlines will specify the quiet nacelle option. The 737 production line has adopted a redesigned inlet that can be easily modified to the treated configuration. Retrofit kits for quieting the current 737 fleet could be delivered in approximately 15 months from the date of order.
Figure 19.-Flight Test Installation of 737 Quiet Nacelle
Looking to the future, a research effort was deemed necessary to determine the potential for further noise reduction. Accordingly, a joint noise reduction program was entered into with P&WA during 1972. Since evaluation of the 747/JT9D noise sources indicated that forward-radiated noise was a very significant component at approach power, the joint program during 1972 was oriented toward control of inlet-propagated noise. To achieve suitable inlet acoustic-geometric relationships, inlet splitters have been tested. A complete assessment of performance, weight, and other installation penalties has not yet been accomplished; preliminary results indicate that the penalties may be less severe than was originally anticipated.
An analytical assessment of the effect of splitter ring diameter on noise reduction indicated a weak dependence on diameter, provided the splitter is at least 65% of the fan rotor diameter. To verify this, splitters were built in two diameter ratios for full scale test. Figure 22 is a photograph of one of the splitter installations. The noise reduction results obtained are shown in Figure 23. These results appear to confirm previous analyses.
Figure 22.-747 Inlet Single Splitter Installation
A further investigation of inlet noise control led to the design and fabrication of a set of splitters which could be tested either as a double splitter inlet or, with the innermost splitter removed, as a single splitter inlet. In Figure 24 the double splitter version is shown on a JT9D engine at the Boeing Commercial Airplane Company's Tulalip test site. Figure 25 compares the attenuation of the single and double splitter versions at approach power. These results demonstrate that the double splitter configuration was more effective in attenuating forward-radiated noise than a single splitter; however, both configurations provide significant reductions in engine noise in