minimizes the area of the airport community that is exposed to objectionable noise.

Research studies and some preliminary experience has suggested, for example, that it may be of overall benefit to exchange a tolerance for a higher noise level close to the airport for a major reduction in noise level over greater areas at a distance from the airport. Aircraft designs which have excess thrust available under takeoff and initial climbout conditions, as is becoming more prevalent in some current high-performance aircraft, can utilize this capability for a more rapid takeoff and steeper climb than has been possible in the past. While this procedure increases the noise level close to the airport, the rapid attainment of an altitude where the power setting can be safely reduced, minimizes the exposure of more distant areas to adverse climbout noise. The usefulness of this technique, of course, is dependent on the acceptability of higher noise levels close to the airport, and varies with individual airport communities. It is expected, however, that this capability will tend to increase in future transport designs, and the possibility for further extenisons of this capability with takeoff thrust-augmentation systems, particularly by the burning of additional fuel in the bypass flow of turbofan engines, is recognized. For many current aircraft designs, however, the incorporation of takeoff thrust-augmentation systems may be technically impracticable, or economically prohibitive.

Currently, the noise levels on the ground under aircraft flight paths are highest during the landing approach. This occurs because of the shallow descent slopes to the runway and high power settings that have been selected to provide ease of control and safety for the aircraft in the transition to the touchdown or waveoff maneuvers under instrument approach condtions.

Because the landing approach operation has been a major and continuing offender, the NASA research problem on operations noise has primarily focused on this problem. Since it is apparent that a steeper descent slope to the runway would increase the distance of the aircraft from ground observers under the flight path, and permits the use of lower power settings, an investigation of the feasibility of such a change in procedure has been underway. Preliminary results of flight tests of several different classes of aircraft under simulated instrument landing conditions have demonstrated that its may be operationally acceptable to increase the descent slopes, but have also shown specific problems that will limit the extent to which the slopes can be increased and the power settings can be decreased. Because of the high promise of this technique, however, it is planned to expand this research program on an expedited basis to cover a wide variety of other types of contemporary aircraft, to facilitate establishment of uniform limits for all routine operations under instrument flight rules.

It is also recognized that the testing and preflight runup of aircraft engines at airports is a noise problem of increasing importance. Some use is already being made of ground noise-suppressor installations to alleviate this problem, and it is expected that their application will be expanded as more powerful engines are introduced. Fortunately, such suppressors are not subject to the weight considerations of inflight aircraft noise suppressors, and can better be designed with the advancing technology. During aircraft taxiing and takeoff ground roll at airports, this approach is not applicable, of course, but atmospheric conditions may focus sounds in such a way that selection of the proper runway will minimize community noise exposure. A study will therefore be undertaken to define a minimum system for atmospheric monitoring that will permit selection of the optimum runway from the noise standpoint.

ANNOYANCE FACTORS OF AIRCRAFT NOISE AND COMPATIBILITY OF AIRCRAFT

COMMUNITY ARRANGEMENTS

As has already been indicated, it is highly important that efforts to improve the airport community noise problem be guided by a rational understanding of the specific manner in which the community population responds to various noise experiences. This is a complex problem since the subjective evaluation of the degree of annoyance is dependent not only on the overall intensity of the noise, but also on the specific distribution of sound energy in various frequency ranges as it relates to the sensitivities of the human ear and the masking of other sound communication efforts, on the duration and repetition of the noise disturbance, on the suddenness of onset and rate of change of the sound character and its familiarity, on the types of activities that are being pursued in the area as it particularly affects work efficiency or sleep, on the degree of structural shielding from aircraft noise exposure or the masking by other industrial and

commercial noise sources, and on individual acclimatization to the disturbance and economic involvement in its causes or effects.

The NASA interests in this problem are primarily concerned with the development of techniques that will aid prediction of the usefulness of specific changes in radiated aircraft noise intensities, frequency distributions, and time sequences, on the subjective response of ground observers. The "perceived noise level" (PNdB) method that is widely used in assessment of the acceptability of noise, offers a useful basis for such a technique. Because the method does not adequately account for many situations that are pertinent to the airport community noise problem, however, a contract research program is underway to extend the concept to cover the important variables of aircraft noise.

It is obvious that a major factor in the provision of a long-range solution to the airport community noise problem, is the development of practicable methods for the control and adjustment of the uses of community property in certain critical areas around an airport. Optimum applications of noise reduction techniques to advanced aircraft, and the use of steeper climb and landing ap-. proach techniques can substantially reduce the critical areas, but it is not likely that they can be completely eliminated. Decisions regarding the development or redevelopment of such land to bring about compatibility are frequently very difficult for existing airport communities because of the high value of the property in question. The needs for such control in the development of new airports should be obvious.

The NASA program in this area has only attempted to define some important considerations that are involved in the adjustments of property usage near airports. The results of these contract studies should be a useful guide for community assessments of prospective measures for the achievement of compatibility. SECTION II. TECHNICAL DETAILS OF THE NASA RESEARCH PROGRAM ON THE AIRPORT-COMMUNITY NOISE PROBLEM

CURRENT RESEARCH PROGRAMS

AIRCRAFT NOISE GENERATION AND REDUCTION AT THE SOURCE

Jet exhaust noise

1. An investigation of unstable shear flows and their relation to aerodynamic noise. This study being conducted at the Massachusetts Institute of Technology under a NASA contract has involved very meticulous measurements with advanced instrumentation and techniques for the purpose of correlating the detailed aerodynamic flow phenomena of the jet with its radiated noise. Such quantities as the jet mean velocity profiles, turbulent velocity fluctuations, space-time correlations, and spectra have been measured precisely and have been correlated with acoustic measurements and high resolution shadowgraph pictures of the flow. The results of such studies, which have been under the supervision of Dr. Erik Mollo-Christensen of Massachusetts Institute of Technology, have shown that small changes in the mean flow structure of a jet may significantly change its noise emission.

2. Noise investigations with impinging jet flows.-The interactions of lowenergy control jets with main jet flows are being studied under NASA contract at Syracuse University under the supervision of Dr. D. S. Dosanjh. The objective is to document, by measurements and theoretical studies, the beneficial effects on the radiated noise of jet flow interactions for a wide range of jet operating conditions. Optical data will be obtained by interferometric. shadowgraphic, and schlieren techniques for correlation with acoustic and aerodynamic data. Such information is of interest for possible application to some advanced types of V/STOL aircraft. Much of the effort to date has related to facility and equipment developments, and hence the research work is still in its initial phases.

3. Mach wave radiation from jets.-This work is under the supervision of Dr. J. E. Ffowcs-Williams of Imperial College, London, England, and is being performed under a NASA contract with Bolt, Beranek & Newman, Inc., of Cambridge, Mass. The overall objective of the study is to develop a unified theory for jet noise generation. Particular attention is paid to the mach wave radiation problem since this is a significant source of noise under certain operating conditions of a jet. This source of noise, which has only recently been shown to be present in jets, is judged to be important for all high performance

jet and rocket engines having exhaust velocities exceeding about 2,000 feet per second. For such aircraft as the supersonic transport, mach wave radiation may be significant particularly during the ground runup and takeoff operations. 4. Measurement of correlation functions in jet noise fields. In order to evaluate some of the flow quantities involved in Dr. Ffowcs-Williams' theoretical work in the mach wave radiation problem, a series of time delay correlation measurements will shortly be made in both the near and far noise fields of a high velocity jet. From analyses of such data, the eddy convection velocities at various locations in the shear layer of the jet can be determined as well as the locations of the sources of the noise in the jet. These data will be correlated with high resolution shadowgraph pictures of the noise field and extensive noise pressure surveys. These experiments will be conducted in the Noise Research Laboratory of the Langley Research Center, and will be a collaborative effort between NASA-Langley personnel and those of Bolt, Beranek & Newman, Inc., of Los Angeles, Calif.

5. Parametric study of noise of bypass jets.-This is a continuing study involving Langley in-house facilities and equipment. (See fig. 29.) Results of the first series of studies involving a range of secondary jet flow conditions (up to bypass flow ratios of 8) have recently been published and have shown that bypass jets provide a means of generating less noise for a given amount of thrust. This result strongly suggests that the high bypass ratio engine cycle is an attractive one for future commercial applications where community noise exposures are an important consideration.

6. Refraction of sound from a point source placed in an air jet.—This study is part of a basic research program on jet noise generation being carried out under a NASA research grant to the University of Toronto, with Dr. Herbert Ribner as principal investigator. The objective of the work is to document experimentally to what extent refraction of sound may account for the characteristic heart-shaped noise radiation pattern of jets. Experiments to date have involved the operation of a small, specially developed noise source in the potential flow core of the jet mixing region to study its radiation pattern in the presence of the jet. Initial experiments have indicated that there is a refraction effect of the jet on the above superposed discrete frequencies, and that this is greater at higher frequencies and higher jet velocities. Advanced procedures involving very narrow filters and correlation techniques are being used to separate the discrete test noise signals from the background random noise of the jet.

Jet inlet noise

1. Effects of inlet geometry on radiated noise from inlet.-Experiments have been completed on a large, axial-flow research compressor in an outdoor setup to define the basic compressor noise radiation patterns and to study the effects of inlet geometry on these patterns. The discrete frequency noise signals were found to be amplitude modulated under all operating conditions and, in this regard, behaved similarly to those measured on other occasions from engines. It was noted that the discrete frequency noise from the compressor exhibited a lobed radiation pattern, and that reductions of the radiated noise occur for longer inlet ducts and particularly for peripheral resonators located upstream of the compressor.

2. Effects of rotor-stator interactions on radiated noise from inlets.—A series of experiments have recently been completed for a single-stage model compressor operated in the Noise Research Laboratory test cell (see fig. 30) for studying rotor-stator interactions. It was found that for this configuration, the discrete frequency compressor noise was more intense for a small separation of the rotor and the upstream stator. Sizable noise reductions, with only small aerodynamic performance deficiencies, were noted for large separation distances (up to about six blade chord lengths). This result suggests that the elimination of inlet guide vanes or the use of proper separation distance would be beneficial in reducing the noise generated by the first stage of an axial flow compressor. 3. Effects of choking on noise radiated from supersonic inlet.-Operational studies have been made of a supersonic inlet on an actual engine to determine its acoustic performance, aerodynamic performance, and the engine performance while the inlet was in the choked condition at partial engine power. The results indicated that for some operating conditions, sizable reductions were obtained in the discrete frequency noise radiation from the inlet. Since the results were not always beneficial, there is need for further studies to optimize the acoustic performance of such an inlet.

4. Procurement of three-stage transonic research compressor.-A three-stage research compressor having provision for a variety of geometric changes over a speed range through transonic is being built under contract with the Continental Engine Co. It will be installed in the Noise Research Laboratory at Langley for the purpose of continuing the above rotor-stator interaction studies to transonic speeds and to study the effects of inlet geometry changes on radiated noise. One of these investigations will include means of altering flow velocities within and ahead of the compressor, particularly by the use of high inlet velocity front stages, as an aid to substantially reducing the forward propagation of compressor noise.

5. Analytical studies of noise propagation from a duct.-The purpose of this study is to simplify the rather complex analytical expressions available for the far field noise radiation pattern from an open ended duct of semi-infinite length.

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These relations will then be useful for studying the effects of duct geometry on the radiation patterns of noise from ducted fans and compressors. Measurements of aircraft noise

1. Aircraft noise survey project.-A complex of microphone stations and associated recording equipment installed at the NASA Flight Research Center will be used to obtain measurements of aircraft noise. These measurements which will be made using representative aircraft, including the B-70, will assist in establishing standards of correlation for the evaluation of acoustic design criteria affecting present and future aerodynamic vehicles. The noise characteristics of various V/STOL aircraft such as the XV-5 and the XC-142 will also be studied as part of future programs.

AIRCRAFT OPERATIONAL PROCEDURES THAT MINIMIZE EXPOSURE OF COMMUNITIES TO OBJECTIONABLE NOISES

1. Aircraft steep approaches to the runway.-Under instrument landing conditions, approaches to the runway are normally made at glide slopes of 2.5° to 3.0°. A continuing flight program is underway to determine the various characteristics of aircraft that may limit increases in the steepness of the approach. Tests have thus far been completed on C-47, T-33, TF-102, and the Boeing 367-80 aircraft under simulated instrument conditions (pilot under a hood) and are now underway for a DC-8F.

The maximum operational glide slope was found to be about 6° for both the C-47 and T-33. For the TF-102, the maximum glide slope was at least 7°-the limit of the tests. The limiting factors for the C-47 were found to be the inability to increase drag without approaching the propeller-windmilling condition and for the T-33 the inability to reduce thrust without encountering engine flameout or appreciably increasing the engine response time for waveoff. The maximum operational glide slope of the Boeing 367-80 airplane was 7° and was limited primarily by the inability to increase drag. Tests made thus far with the DC-8F have indicated a capability of flying a glide slope of at least 6°. For all aircraft tested, the pilots could fly the steeper approaches about as precisely as the 3° approaches although the pilot's workload was appreciably greater. The workload, however, was reduced considerably and the task sim