Engineering topics
Publications and source records attributed to Hubbard, H. H..
The effects of primary and secondary sonic boom carpets are reviewed with reference to waveform characteristics and audibility, role of the atmosphere, source characteristics, carpet measurements and predictions, effects of aircraft maneuvers, and sonic boom minimization. It is shown that primary booms which involve only propagation in the lower atmosphere are well predicted by means of current methods and are amendable to minimization.
Far field computations were performed for a large wind turbine to evaluate the effects of non-uniform aerodynamic loading over the rotor disk. A modified version of the Farassat/Nystrom propeller noise prediction program was applied to account for the variations in loading due to inflow interruption by the upstream support tower. The computations indicate that for the uniform inflow case, relatively low noise levels are generated and the first rotational harmonic dominated the spectrum. For cases representing wake flow deficiences due to the tower structure, subtantially increased noise levels for all harmonics are indicated, the greatest increases being associated with the higher order harmonics.
It is pointed out that a need exists for the orderly development of technology and engineering methods for noise control of all types of aircraft. The nature and scope of aircraft noise problems are reviewed, and a description is provided of noise control progress made to date. The most serious aircraft noise problems confronting communities in the past two decades have been associated with the subsonic air carrier jet transport. Operational trends related to traffic growth and operational constraints are examined, and noise level trends are considered, taking into account engine cycle developments, the role of Federal noise certification, future noise exposures, and multiple noise sources. Advanced source noise reduction technology developments are discussed along with a noise impact assessment, and advanced operating procedures. Attention is also given to engine noise and sonic boom exposures in connection with supersonic air carrier aircraft, and exterior and interior noise control related to propeller/rotor aircraft.
The status of sonic boom technology with emphasis on the recent research results is summarized. Included are definitions of the boom carpets, both primary and secondary, a discussion of existing experience with primary booms including the status of overpressure predictions and boom minimization methodology through airplane design, an indication of the boom waveforms and audibility, and a discussion of focus booms resulting from aircraft maneuvers as well as the effect of abnormal atmospheric conditions on these maneuver booms.
A broad perspective of needs in helicopter exterior and interior control is presented. Emphasis is given to those items which support noise certification of civil helicopters and which result in reduced environmental noise impact to community residents as well as to helicopter passengers. The activities described are related to the Langley responsibilities for helicopter acoustics as defined by NASA roles and missions.
Guidelines are presented for the preparation of illustrated talks which are audience oriented and which are aimed at the efficient transfer of technical information. Early decisions concerning the required number of slides are helpful in initial planning for a good-quality talk. Detailed considerations are the establishment of limited objectives, selection of appropriate slide material, development of a text which is well coordinated with the slides, and accurate timing.
Guidelines are presented for the preparation of illustrated talks which are audience oriented and which are aimed at the efficient transfer of technical information. Early decisions concerning the required number of slides are helpful in initial planning for a good quality talk. Detailed considerations are: the establishment of limited objectives, selection of appropriate slide material, development of a text which is well coordinated with the slides, and accurate timing.
In order to evaluate reaction of people to sonic booms of varying overpressures and time durations, a series of closely controlled and systematic flight tests/studies were conducted from June 3 to June 23, 1966. The dynamic responses of several building structures were measured, with emphasis on a two-story residence structure. Sample acceleration and strain recordings from F-104, B-58, and XB-70 sonic boom exposures are included, along with tabulations of the maximum acceleration and strain values measured for each one of about 140 flight tests. These data are compared with similar measurements for engine noise exposures of the building during simulated landing approaches and takeoffs of KC-135 aircraft.
A study was undertaken by the NASA Langley Research Center to determine the noise reduction potential of the U-10 airplane in order to reduce its aural detection distance. Static and flyover noise measurements were made to document the basic airplane noise signature. Two modifications to the airplane configuration are suggested as having the best potential for substantially reducing aural detection distance with small penalty to airplane performance or stability and control. These modifications include changing the present 3-blade propeller to a 5-blade propeller, changing the propeller diameter, and changing the propeller gear ratio, along with the use of an engine exhaust muffler. The aural detection distance corresponding to normal cruising flight at an altitude of 1,000 ft over grassy terrain is reduced from 28,000 ft (5.3 miles) to about 50 percent of that value for modification 1, and to about 25 percent for modification 2. For the aircraft operating at an altitude of 300 ft, the analysis indicates that relatively straightforward modifications could reduce the aural detection distance to approximately 0.9 mile. Operation of the aircraft at greatly reduced engine speed (1650 rpm) with a 1.3-cu-ft muffler provides aural detection distances slightly lower than modification 1.
A study was undertaken to determine the noise reduction potential of the A-6 airplane in order to reduce its aural detection distance. Static and flyby noise measurements were taken to document the basic airplane signature. The low-frequency noise which is generally most critical for aural detection was found to be broad-band in nature from this airplane, and its source is the turbojet engine exhaust. High-frequency compressor noise, which is characteristic of turbojet powerplants, and which is prominent at close range for this airplane, has no measurable effect on aural detection distance. The use of fluted-engine exhaust nozzles to change the far-field noise spectra is suggested as a possible means for reducing the aural detection distances. Detection distances associated with eight-lobe and four-lobe nozzles are estimated for a 1,000-foot altitude and grassy terrain to decrease from 4 miles to about 3 miles, and from 3 miles to about 2 miles for a 300-foot altitude and grassy terrain.
The factors affecting noise from small propeller driven airplanes are reviewed to quantify their affects where possible, and to indicate the potential for noise reduction. The main sources of external noise are noted to be the propellers and engines; the airframe being of less importance for both aural detection and community annoyance. Propeller noise is a strong function of tip speed and is affected adversely by nonuniform inflows. Reciprocating engine exhausts are noisier than those of comparably rated turboshaft engines, but their noise can be reduced by the use of flight certified exhaust mufflers. Presently, there are no generally accepted engineering methods for development of optimized propellers and exhaust muffler designs from weight and performance penalty standpoints. Flight demonstration results, however, suggest that required noise reductions for future certification should be possible with potentially small penalties.
Flight vehicles are characterized according to their manner of operation and type of propulsion system; and their associated sources of noise are identified. Available noise reduction technology as it relates to engine cycle design and to powerplant component design is summarized. Such components as exhaust jets, fans, propellers, rotors, blown flaps, and reciprocating-engine exhausts are discussed, along with their noise reduction potentials. Significant aircraft noise reductions are noted to have been accomplished by the application of available technology in support of noise certification rules. Further noise reductions to meet more stringent future noise regulations will require substantial additional technology developments. Improved analytical prediction methods, and well-controlled validation experiments supported by advanced-design aeroacoustic facilities, are required as a basis for an effective integrated systems approach to aircraft noise control.
Flight vehicles are characterized according to their manner of operation and type of propulsion system, and their associated sources of noise are identified. Available noise reduction technology as it relates to engine cycle design and to power plant component design is summarized. Such components as exhaust jets, fans, propellers, rotors, airflow-surface interactions, and reciprocating engine exhausts are discussed, along with their noise reduction potentials. Significant aircraft noise reductions are noted to have been accomplished by the application of available technology in support of noise certification rules. Improved analytical prediction methods, and well controlled validation experiments supported by advanced design aeroacoustic facilities, are required as a basis for an effective integrated systems approach to aircraft noise control.
Progress report on the problem of community noise control for jet-powered STOL aircraft. Noise goals are discussed, along with noise control approaches for meeting these goals. Such significant noise control factors as the basic engine cycle, engine-airframe integration varieties, and certain details of the lift augmentation system are given special attention.
Research on aircraft-generated sonic booms has led to the development of special techniques to generate controlled sonic-boom-type disturbances without the complications and expense of supersonic flight operations. This paper contains brief descriptions of several of these techniques along with the significant hardware items involved and indicates the advantages and disadvantages of each in research applications. Included are wind tunnels, ballistic ranges, spark discharges, piston phones, shock tubes, high-speed valve systems, and shaped explosive charges. Specialized applications include sonic-boom generation and propagation studies and the responses of structures, terrain, people, and animals. Situations for which simulators are applicable are shown to include both small-scale and large-scale laboratory tests and full-scale field tests. Although no one approach to simulation is ideal, the various techniques available generally complement each other to provide desired capability for a broad range of sonic-boom studies.
Attempt to identify the main sources of noise in various types of power plants, and to indicate trends in design and operations that are beneficial in noise control. Information is presented for such noise sources as jet exhausts, fans, high-lift devices, and unducted rotors. It is shown that lower noise levels are generally associated with lower airfoil velocities and lower pressure ratios.
The nature of aerospace noise-induced vibration problems is summarized, with emphasis placed on reviewing and categorizing types and sources of noise which are significant in such problems. Physical characteristics of representative noise loads associated with propellers, turbojet engine exhausts, and sonic boom waves are presented. Examples of noise-induced structural vibrations are discussed.
Jet aircraft noise sources and reduction from gas turbine engines including jet exhausts, fans, lift devices, and unducted rotors