Nozzle systems with the maximum perceived noise level suppression were selected for further full-scale testing on a J-75 engine. The systems selected demonstrated jet noise reductions in the order of 6 PNdB related to the General Electric GE4/J5P turbojet engine selected to power the SST.

Later studies, related to the Boeing 10- to 20-PNdB noise reduction goal, centered on suppressor configurations having a igh number of elements. Some concepts achieved as much as 17 PNdB reduction in jet noise. Typical scale models are shown in figure 33. Although full-scale tests of the more attractive tubular concepts were conducted by both Boeing and General Electric, further work was initiated to explore the benefits possible by surrounding tubular suppressors with lined ejector sirouds. Figure 34 shows one of the model-scale ejectorsuppressors tested. This concept resulted in noise suppression of more than 15 PNdB (figure 35), but had thrust losses greater than desired.

Figure 36. NSC-119B Ejector-Suppressor Model

Not all the SST engine noise emanated from the jet exhaust. Various means were investigated for suppressing noise radiating forward from the engine inlets. Figure 37 shows a Boeing-designed inlet mounted on a GE4 engine. Figure 38 is a schematic of the inlet, illustrating its principal features. By moving the centerbody to the aft position, a sonic flow is created in the throat that prevents emanation of most of the high-frequency sound from the engine compressor components. Figure 39 shows the magnitude of noise suppression at various frequencies. The choked position of the inlet would be used during ground operations and subsonic flights over airport communities.

Subsequent to the SST program termination, negotiations were carried out with the Department of Transportation to complete the testing and analysis of the NSC-119B ejectorsuppressor previously discussed, and to document the suppressor development conducted during the SST program.

During studies of lift engine systems, considerable research was conducted on suppressing jet noise. Many configurations of ejector-suppressor type devices were tested in model scale and full scale, including 40-, 48-, and 60-lobe nozzles enclosed in ejector shrouds of various area ratios, and of single and double stages. Some were similar to those described earlier for the 727 However, differences in airplane design constraints between conventional-takeoff-and-landing (CTOL) aircraft and STOL transports make concepts feasible in one application unsuited to the other.

A 48-lobe configuration that has demonstrated significant noise suppression is shown in figure 22. Acoustic data for this 48-lobe nozzle concept are shown in figure 41, comparing the standard engine tailpipe to variations of the suppressor in both model-scale tests and full-scale J-75 engine tests. The effect of single- and two-stage lined ejector shrouds is shown relative to the more nominal attenuation of the bare 48-lobe nozzle Furthermore, whereas the bare lobed nozzle caused a high loss in engine static thrust, the single- and dual-stage ejectors gained back the thrust lost by the bare nozzle (figure 42).

In 1970, Boeing started an intensive effort on the augmentor wing concept (figure 43), wherein all the fan air from the engine would be directed into the augmentor in the wing trailing edge flap area. The engine cycle would be chosen to produce a low-velocity primary exhaust with very low noise It was also believed that the engine inlet noise could be reduced to very low levels by a combination of engine design and

choked inlets. The augmentor itself had all the characteristics of an excellent ejector-suppressor with unusually large surfaces for high ejector performance and large areas for installation of acoustic treatment. Based on all these considerations, it appeared that this concept had high noise reduction potential compared to other STOL airplane concepts, and perhaps had a reasonable chance of meeting a tentative target of 95 PNdB sideline noise at 500 feet. Using a turbofan engine having a by pass ratio of about 3.0 and a fan pressure ratio of about 2.7, the primary exhaust velocity is consistent with desired low jet noise. Further detailed studies evolved a wing ducting system and augmentor nozzles that had the potential of being acceptable from both noise and performance standpoints.

Augmentor wing studies led to award of a contract to Boeing in 1971 (NASA Ames Contract NAS2-6344), “Design Integration and Noise Study for a Large STOL Augmentor Wing Transport." The objective of the program, scheduled for completion in April 1972, is to develop, through analysis, design, experimental ground rig testing, wind tunnel testing, and design integration studies, an augmentor wing jet flap configuration for a jet STOL transport aircaft having maximum propulsive and aerodynamic performance with minimum noise generation. The program is primarily a technology development effort, consisting of three major tasks: (1) studies of various nozzle design parameters for noise suppression of slot-type nozzles, (2) studies of the entire augmentor wing jet flap, with acoustic treatment of various segments of the flap for noise suppression, and (3) aerodynamic studies at forward speed to assess the effects of noise suppression modifications on the performance of the flaps.

Work to date indicates that noise suppression will meet or better the initial estimates, consistent with the contractual goal of 95 PNdB noise level at a 500-ft sideline distance for a STOL transport capable of carrying 100 to 150 passengers.

VTOL Aircraft

A community noise reduction program for helicopters was initiated by Boeing in 1955 with an engine noise quieting program for the Model 44 helicopter. This reciprocating-enginepowered aircraft was a commercial version of the Army Model H-21. By redesigning and retuning the exhaust collector ring, and directing the exhaust ports upward, a noise reduction of about 10 dB was effected at 500 ft altitude.

A brief summary of other programs that have been conducted include noise reduction by blade tip design, and by rotor-to-rotor separation through cyclic trim optimization and increased height of the aft pylon. Analytical studies have been made to understand the basic mechanisms of rotor noise generation, including studies of rotor wake geometry and tip vortex control. Several programs funded by Boeing, the Army, and the Navy were aimed at measurement of the tip vortex path in flight for correlation with the analytical methods being developed. Contracts with NASA in 1967 and 1970 supported studies of the subjective response to noise of several types of V/STOL aircraft, providing guidelines of community acceptance for future aircraft designs. A continuing effort is being maintained in various areas of helicopter noise reduction.

A specific example of the VTOL studies is in the area of blade tip design. More than 12 rotor blade tip designs were evaluated on a tie-down helicopter, and six were tested in the wind tunnel. One of the latter concepts, a porous tip shown in figure 44, was tested in flight; it reduced rotor hovering noise by about 10 dB. However, little noise reduction was obtained in forward flight. This fact, as well as performance penalties experienced in forward flight, indicated that further development would be required before such technology could be considered for incorporation in service.

The combined results of many of the V/STOL studies (other than porous tips) have been incorporated into the design of the Model 347 helicopter. A sound pressure level comparison of the external noise of this helicopter with other Boeing models having lesser speed and payload capabilities is illustrated in figure 45, which shows the significant community noise benefits of this aircraft over earlier models. It can be seen in figure 46 that in hovering, the Model 347 meets the interim guideline for STOL aircraft of 95 PNdB at a 500-ft sideline distance. Although not shown in the figure, the perceived noise level in forward flight also meets this criterion.

Noise Abatement Operating Procedures

All the discussions to this point have related to means of relieving community noise through reduction of the noise at its source-the engine and the engine installation on the aircraft. Another potential for significant relief of the community noise problem at relatively low cost lies in several areas of airplane operation in the vicinity of airports. Such operations, either separately or in combination with acoustic treatment concepts, permit a more all-inclusive consideration of the noise problem. Boeing has been studying noise reduction through operating procedures, including flight tests using various aircraft, for several years.

CH-47A

CH-47C

347

CH-47A

CH-47C

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CH-47A

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Two-segment approaches

HOVER

60 KN

In general, any action taken to increase the height of aircraft over a community will reduce the noise exposure. Many complaints in the past have been based on aircraft flying at low altitude for miles over the community during landing approach. Increased holding and maneuver altitudes over suburban areas. and preferential routing of approaching and departing aircraft over areas of low population density are means available to the FAA and local airport authorities for considerable reduction in community noise. Furthermore, whereas 3° glide slopes are generally accepted and are standard at many airports today. approximately 30% of present glide slopes at major U.S. airports are as low as 2.5°. The 3.5° glide slope at the Berlin Tempelhof Airport has been in existence for years, with no known accidents that could be attributed to the glide slope angle.

Figure 47 illustrates the trades between glide slope angle and noise for the 727-200 airplane at various distances from the runway threshold. Noise reductions on the order of 5 to 7 EPNdB are shown for a 1° increase in glide slope. Similar benefits are available with other aircraft.

Selection of glide slope intercept altitudes of about 3,000 ft over the community instead of 1,500 ft also produces significant reductions in community noise. The 7-EPNdB noise reduction shown in figure 48 results from the higher altitude alone. This is an example of the type of benefits currently being attained through implementation of FAA Order 7110.22, the "keep 'em high" order, of September 19, 1970.