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Showing papers on "Turbofan published in 1981"


Patent
27 Nov 1981
TL;DR: In this paper, a convertible aircraft engine with turboshaft and/or turbofan modes of operation is provided in a configuration that combines certain components to permit variation in bypass flow to match engine airflows with the mode of engine operation.
Abstract: A convertible aircraft engine, capable of turboshaft and/or turbofan modes of operation, is provided in a configuration that combines certain components to permit variation in bypass flow to match engine airflows with the mode of engine operation. The provided components permit the engine to decrease the power requirements of the engine's forward fan when the engine is operated in the turboshaft mode. To permit a decrease in fan power requirements, the fan is split into an outer portion and an inner portion separated by a rotating shroud. Airflow into the fan's outer portion is controlled with a part span inlet guide vane and an outlet guide vane. The guide vanes can be used to lessen the load on the outer portion of the fan while the inner portion continues to accelerate and compress a normal airflow into an engine compressor for supplying the engine's core. The power normally used to drive the fan outer portion is used instead to power a transfer shaft in the turboshaft mode.

64 citations


Journal ArticleDOI
TL;DR: In this article, a mixed flow turbofan is analyzed from the point of view of i.r. emission characteristics, and the influence of cycle parameters is assessed using an /zth power dependence of radiation on temperature and a simple mixing model.
Abstract: A mixed flow turbofan is analyzed from the point of view of i.r. emission characteristics. A simple absorption coefficient model of the core flow gas displays the potential effectiveness of high aspect ratio nozzles as a variable in the design of aircraft engine installations for low i.r. signature. The simplicity of the model described and the limited view perspectives used to assess the signature restrict the usefulness of the results to that of guiding preliminary design. In general, more precise solutions are complex and depend on specification of a relatively large number of independent variables to describe the source—seeker geometry, weather, etc. The level of effort to carry out more accurate analysis may be inconsistent with the preliminary design process where simple criteria such as those described here may be sufficient to select the best of several candidate designs. The influence of cycle parameters is assessed using an /zth power dependence of radiation on temperature and a simple mixing model to estimate the core length. The analysis shows that cycle parameters which improve cycle efficiency and thus fuel consumption also reduce i.r. emissions. Bypass ratio near unity gives low i.r. signature for both optically thin and thick spectral regions.

44 citations


Patent
29 Sep 1981
TL;DR: In this paper, a two-spool turbofan engine with variable bypass passageways downstream of the compressor is described, where the bypass flow is controlled for low and high power settings.
Abstract: A supersonic two spool turbofan engine capable not only of developing sufficient power to accelerate up to supersonic cruise and maintain efficient operation at supersonic cruise, but also arranged to cruise at subsonic velocities with a relatively low specific fuel consumption. The engine is provided with a variable bypass passageway downstream of the compressor. Flow into the bypass passageway is controlled so that during low power setting the bypass passageway is closed so that all the gaseous flow is directed through the turbine. During higher power settings, the bypass passageway is opened to the extent that a selected portion of the gaseous flow is directed through the bypass passageway to bypass the first stage of the turbine section so that the corrected flow to the first turbine stage remains substantially constant for high and low power setting of the engine. The bypass flow is then directed into the area between the first and second turbine to combine with the gaseous flow passing through the first turbine and pass through the second turbine.

35 citations


01 Feb 1981
TL;DR: In this article, the feasibility of the propfan relative to the turbofan is summarized, using the Douglas DC-9 Super 80 (DS-8000) as the actual operational base aircraft.
Abstract: The feasibility of the propfan relative to the turbofan is summarized, using the Douglas DC-9 Super 80 (DS-8000) as the actual operational base aircraft. The 155 passenger economy class aircraft (31,775 lb 14,413 kg payload), cruise Mach at 0.80 at 31,000 ft (8,450 m) initial altitude, and an operational capability in 1985 was considered. Three propfan arrangements, wing mounted, conventional horizontal tail aft mounted, and aft fuselage pylon mounted are selected for comparison with the DC-9 Super 80 P&WA JT8D-209 turbofan powered aircraft. The configuration feasibility, aerodynamics, propulsion, structural loads, structural dynamics, sonic fatigue, acoustics, weight maintainability, performance, rough order of magnitude economics, and airline coordination are examined. The effects of alternate cruise Mach number, mission stage lengths, and propfan design characteristics are considered. Recommendations for further study, ground testing, and flight testing are included.

29 citations


Proceedings ArticleDOI
01 Jul 1981
TL;DR: In this article, a new technique is presented for directly measuring the core noise levels from gas turbine aircraft engines, which requires that fluctuating pressures be measured in the far-field and at two locations within the engine core.
Abstract: A new technique is presented for directly measuring the core noise levels from gas turbine aircraft engines. The technique requires that fluctuating pressures be measured in the far-field and at two locations within the engine core. The cross-spectra of these measurements are used to determine the levels of the far-field noise that propagated from the engine core. The technique makes it possible to measure core noise levels even when other noise sources dominate. The technique was applied to signals measured from an AVCO Lycoming YF102 turbofan engine. Core noise levels as a function of frequency and radiation angle were measured and are presented over a range of power settings.

24 citations



Proceedings ArticleDOI
01 Jul 1981
TL;DR: In this article, the authors presented the aerodynamic performance characteristics and experimental results obtained from both the core engine and full engine tests, as well as a scale demonstrator version of this engine.
Abstract: The Core Driven Fan Stage (CDFS) Variable Cycle Engine (VCE) has been identified as a leading candidate for advanced supersonic cruise aircraft. A scale demonstrator version of this engine has been designed and tested. This testbed engine features a split fan with double bypass capability, variable forward and aft mixers, and a variable area low pressure turbine nozzle to permit exploration and optimization of the cycle in both single and double bypass modes. This paper presents the aerodynamic performance characteristics and experimental results obtained from both the core engine and full engine tests.

14 citations


Proceedings ArticleDOI
01 Jan 1981
TL;DR: In this paper, the effects of varying engine rpm, changing the angle-of-attack of the engine inlet to tunnel flow and mounting an aircraft wing to simulate an installation condition on an actual aircraft are presented.
Abstract: Tests have been conducted on a JT15D-1 turbofan engine both statically and at simulated forward speed in the Ames 12 x 24 Meter Wind Tunnel. Both far-field acoustic data and unsteady pressure data from transducers mounted on the fan blades were acquired. Results showed a sound power reduction of about 10 dB in the far-field acoustic levels with simulated forward speed over those measured without forward speed. Blade mounted transducer results showed rotor-turbulence interaction dominated the noise field at very low speeds while an interaction between the rotor and internal struts dominated at higher speeds. Results are presented to show the effects of varying engine rpm, changing the angle-of-attack of the engine inlet to tunnel flow and mounting an aircraft wing to simulate an installation condition on an actual aircraft.

13 citations


01 Jan 1981
TL;DR: In this paper, the performance of a conventional engine/inlet installation, in which inlet and engine flow field interaction occurs, was compared to the same inlet remote coupled to the engine, and the results indicated that engine interaction allows the inlet to operate with lower distortion levels at and beyond the separation angle of attack experienced without engine interaction.
Abstract: The performance of a conventional engine/inlet installation, in which inlet and engine flow field interaction occurs, was compared to the performance of the same inlet remote coupled to the engine. The remote coupled inlet configuration decouples the influence of the engine on the inlet flow field and simulates current small scale inlet test techniques in which inlet airflow is provided by a vacuum source or coupled engine. The investigation was conducted in the NASA-Ames 40- by 80-foot wind tunnel using a General Electric TF-34 turbofan engine and a subsonic inlet having an average inlet contraction ratio of 1.26. Test results indicated that engine interaction allows the inlet to operate with lower distortion levels at and beyond the separation angle-of-attack experienced without engine interaction.

12 citations


Proceedings ArticleDOI
01 Jul 1981
TL;DR: In this article, the ability of a variable inlet guide vane (VIGV) to modulate the thrust of a high bypass turbofan engine was evaluated at altitude/Mach number conditions of 4572 m/0.6 and 9144 m/ 0.93.
Abstract: The ability of a part span variable inlet guide vane (VIGV) to modulate the thrust of a high bypass turbofan engine was evaluated at altitude/Mach number conditions of 4572 m/0.6 and 9144 m/0.93. Fan tip, gas generator and supercharger performance were also determined, both on operating lines and during fan duct throttling. The evaluation was repeated with the bypass splitter extended forward to near the fan blade trailing edge. Gross thrust attentuation of over 50 percent was achieved with 50 degree VIGV closure at 100 percent corrected fan speed. Gas generator supercharger performance fell off with VIGV closure, but this loss was reduced when a splitter extension was added. The effect of VIVG closure on gas generator performance was minimal.

9 citations


Proceedings ArticleDOI
01 Jan 1981
TL;DR: In this article, the design, fabrication, and initial testing of energy efficient engine combustors, developed for the next generation of turbofan engines for commercial aircraft, are described, which utilize an annular configuration with two zone combustion for low emissions, advanced liners for improved durability, and short, curved-wall, dump prediffusers for compactness.
Abstract: The design, fabrication, and initial testing of energy efficient engine combustors, developed for the next generation of turbofan engines for commercial aircraft, are described. The combustor designs utilize an annular configuration with two zone combustion for low emissions, advanced liners for improved durability, and short, curved-wall, dump prediffusers for compactness. Advanced cooling techniques and segmented construction characterize the advanced liners. Linear segments are made from castable, turbine-type materials.

Proceedings ArticleDOI
17 Mar 1981
TL;DR: A systematic, computer-aided, self-documenting methodology for developing hybrid computer simulations of turbofan engines and comparisons between hybrid simulation and specified engine performance data are presented.
Abstract: This paper offers a systematic, computer-aided, self-documenting methodology for developing hybrid computer simulations of turbofan engines. The methodology that is presented makes use of a host program that can run on a large digital computer and a machine-dependent target (hybrid) program. The host program performs all of the calculations and data manipulations that are needed to transform user-supplied engine design information to a form suitable for the hybrid computer. The host program also trims the self-contained engine model to match specified design point information. A test case is described and comparisons between hybrid simulation and specified engine performance data are presented.

01 Jan 1981
TL;DR: In this article, an investigation of the effects of mistuning on flutter and forced response of a cascade in subsonic and supersonic flows is presented, where the aerodynamic and structural coupling between the bending and torsional motions and the turbine blades are included.
Abstract: An investigation of the effects of mistuning on flutter and forced response of a cascade in subsonic and supersonic flows is presented. The aerodynamic and structural coupling between the bending and torsional motions and the aerodynamic coupling between the blades are included. It is shown that frequency mistuning always has a beneficial effect on flutter. Additionally, the results indicate that frequency mistuning may have either a beneficial or an adverse effect on forced response, depending on the engine order of the excitation and Mach number.

Patent
28 Dec 1981
TL;DR: In this article, an electronic determination of the normal linear relationship of fan speed with temperature at a fixed pressure or at a series of fixed pressures is presented, in terms of a function having a negative slope with increasing pressure, and an electronic circuit arrangement for shifting from one characteristic to the other to avoid damage to the turbofan engine by running it at above-specification rotational speed.
Abstract: The maximum speed of a turbofan engine varies in accordance with moderately complex relationships involving the temperature, the pressure or altitude, and whether or not the subordinate systems such as the anti-ice systems are operative The maximum allowable rotational speed for the fan section of a turbofan engine increases with increased altitude or reduced pressure, and normally increases as the ambient temperature increases up to speeds limited by the centrifugal and other forces acting on the turbine blades However, there is a maximum fan rotational speed above which the engine should not be operated, and there is a limit which decreases with increasing temperature above which the fan should not be operated regardless of the pressure In addition, when the anti-ice or other subordinate systems are operative, these limits are reduced substantially The present invention involves the electronic determination of the normal linear relationship of fan speed with temperature at a fixed pressure or at a series of fixed pressures, a determination of the maximum fan speed limit for each temperature, in terms of a function having a negative slope with increasing pressure, an electronic circuit arrangement for shifting from one characteristic to the other to avoid damage to the turbofan engine by running it at above-specification rotational speed; and also includes switching arrangements for changing operational limits upon operation of the de-icing or similar equipment

01 Apr 1981
TL;DR: The DYGABCD program as mentioned in this paper generates linear state-space models for simulating turbofan and turbojet engines over complete range of power settings and flight conditions using FORTRAN IV for batch execution and is implemented on IBM 360-series computer.
Abstract: Digital program DYGABCD generates linear state-space models for simulating turbofan and turbojet engines over complete range of power settings and flight conditions. Program is written in FORTRAN IV for batch execution and is implemented on IBM 360-series computer.

01 Jan 1981
TL;DR: A simplified gross thrust calculation technique was evaluated in flight tests on an F-15 aircraft using prototype F100-PW-100 engines as mentioned in this paper, which relies on afterburner duct pressure measurements and empirical corrections to an ideal one-dimensional analysis to determine thrust.
Abstract: A simplified gross thrust calculation technique was evaluated in flight tests on an F-15 aircraft using prototype F100-PW-100 engines. The technique relies on afterburner duct pressure measurements and empirical corrections to an ideal one-dimensional analysis to determine thrust. In-flight gross thrust calculated by the simplified method is compared to gross thrust calculated by the engine manufacturer's gas generator model. The evaluation was conducted at Mach numbers from 0.6 to 1.5 and at altitudes from 6000 meters to 13,700 meters. The flight evaluation shows that the simplified gross thrust method and the gas generator method agreed within plus or minus 3 percent. The discrepancies between the data generally fell within an uncertainty band derived from instrumentation errors and recording system resolution.

01 Dec 1981
TL;DR: The effects of steady-state and time-dependent fan inlet total temperature disturbances on the stability of a TF30-P-3 turbofan engine were determined in this paper.
Abstract: The effects of steady-state and time-dependent fan inlet total temperature disturbances on the stability of a TF30-P-3 turbofan engine were determined. Disturbances were induced by a gaseous-hydrogen-fueled burner system installed upstream of the fan inlet. Data were obtained at a fan inlet Reynolds number index of 0.50 and at a low-pressure-rotor corrected speed of 90 percent of military speed. All tests were conducted with a 90 deg extent of the fan inlet circumference exposed to above-average temperatures.


Journal ArticleDOI
TL;DR: In this paper, a control system designed under the multivariable control synthesis (MVCS) program for the F100 turbofan engine is described, and the engine altitude test phase of the MVCS program, and engine responses in a variety of test operating points and power transitions are presented.
Abstract: The control system designed under the Multivariable Control Synthesis (MVCS) program for the F100 turbofan engine is described. The MVCS program, applied the linear quadratic regulator (LQR) synthesis methods in the design of a multivariable engine control system to obtain enhanced performance from cross-coupled controls, maximum use of engine variable geometry, and a systematic design procedure that can be applied efficiently to new engine systems. Basic components of the control system, a reference value generator for deriving a desired equilibrium state and an approximate control vector, a transition model to produce compatible reference point trajectories during gross transients, gain schedules for producing feedback terms appropriate to the flight condition, and integral switching logic to produce acceptable steady-state performance without engine operating limit exceedance are described and the details of the F100 implementation presented. The engine altitude test phase of the MVCS program, and engine responses in a variety of test operating points and power transitions are presented.

Proceedings ArticleDOI
01 Dec 1981
TL;DR: In this paper, the authors studied the application of two tilt propulsion concept VTOL aircraft to the business/executive transport mission, and compared the total trip cost of a conventional helicopter/business jet combination for a typical executive transport mission.
Abstract: As part of defining the needs and technology requirements for VTOL aircraft research and development, the objective of this paper is to study the application of two tilt propulsion concept VTOL aircraft to the business/executive transport mission. The two concepts selected for study are the tilt jet concept utilizing rotating turbofan engines for both vertical lift and cruise thrust, and the tilt rotor concept using relatively low disc loading propellers for hover and cruise. Overall mission costs, including the time-value cost of the executives, was computed for a selected range of mission distances, up to the design mission range of 750 nm (1400 km). The total trip cost was also compared to that of a conventional helicopter/business jet combination for a typical executive transport mission.

Proceedings ArticleDOI
12 Jan 1981
TL;DR: In this paper, a subsonic tilt-nacelle V/STOL aircraft configuration is described which is powered by two turbofan engines, mounted on a single carry-through structure that is designed to maintain the thrust axis close to the center of gravity at nacelle incidences of 5-95 deg.
Abstract: A subsonic tilt-nacelle V/STOL aircraft configuration is described which is powered by two turbofan engines, mounted on a single carry-through structure that is designed to maintain the thrust axis close to the center of gravity at nacelle incidences of 5-95 deg. Control during V/STOL operation, often from small ship platforms, is achieved by means of a vane assembly that is immersed in each turbofan's exhaust. Wind tunnel test data have been obtained with an 11.2-m wing span model for the vertical-to-horizontal flight transition capability of the concept. It is found that the aircraft can operate over a broad transition corridor, with sufficient maneuver capability about the trim points. The control vane exhibited linear response characteristics over a large deflection range, with little influence from power, angle of attack or ground proximity in hover. The model induced a positive ground effect in hover that increased as wheel height was approached and strake angle increased.

Proceedings ArticleDOI
01 Nov 1981
TL;DR: In this article, the effect of engine/inlet flowfield interaction on inlet performance near flow separation was determined by comparing the performance of a large-scale subsonic inlet (CR = 1.26) close-coupled and remote-couple to a TF-34 turbofan engine, and the results indicated that engine interaction allows the inlet to operate with lower distortion levels at and beyond the separation angle of attack attained without engine interaction.
Abstract: A low-speed wind tunnel test was conducted in the NASA/Ames 40-x-80-foot wind tunnel to investigate the effect of engine/inlet flowfield interaction on inlet performance near flow separation. The effect of engine/inlet flowfield interaction was determined by comparing the performance of a large-scale subsonic inlet (CR = 1.26) close-coupled and remote-coupled to a TF-34 turbofan engine. The remote coupled inlet configuration removes the influence of the engine on the inlet flowfield and further, typifies conventional small-scale inlet test techniques which generally provide no simulation of turbomachinery effects. Test results indicated that engine interaction allows the inlet to operate with lower distortion levels at and beyond the separation angle-of-attack attained without engine interaction.

Journal ArticleDOI
TL;DR: In this paper, the effect of shielding provided by the wing/flap surface on aft-radiated turbofan engine noise was also studied and some attempts were made to reduce USB noise.
Abstract: Acoustic characteristics of the external upper surface blowing (USB) concept of a propulsive-lift configuration were studied with a full-scale model static experiment. Test components included a FJR710 turbofan engine with an acoustically treated nacelle and a USB wing/flap assembly. These were utilized in conjunction with ground verification testing of the propulsive systems of the National Aerospace Laboratory's Quiet STOL Research Aircraft. Results were compared with the previous 8% scale cold-flow model data. The effect of shielding provided by the wing/flap surface on aft-radiated turbofan engine noise was also studied and some attempts were made to reduce USB noise.

Journal ArticleDOI
TL;DR: In this paper, the effects of advanced propeller (propfan) characteristics on aircraft direct operating costs, fuel consumption, and noiseprints were evaluated using parametric studies.
Abstract: Parametric studies were conducted to define the effects of advanced propeller (propfan) characteristics on aircraft direct operating costs, fuel consumption, and noiseprints. Selected propfan aircraft realized 21-percent fuel savings and 15-percent lower DOCs relative to advanced turbofan aircraft. While both the propfan and turbofan aircraft satisfied current federal noise regulations, the propfan aircraft had smaller noiseprints at 90-EPNdB noise levels but larger noiseprints at lower noise levels. Several techniques for reducing the propfan aircraft noiseprints were explored; some of these contribute substantial reductions in noiseprint areas. Also, a propfan aircraft for the C-X role was studied.

Proceedings ArticleDOI
01 Jan 1981
TL;DR: In this article, the thrust, specific fuel consumption, and relative merits of the tandem fan and the dual reverse flow front fan propulsion systems for a supersonic V/STOL aircraft are discussed.
Abstract: The thrust, specific fuel consumption, and relative merits of the tandem fan and the dual reverse flow front fan propulsion systems for a supersonic V/STOL aircraft are discussed. Consideration is given to: fan pressure ratio, fan air burning, and variable core supercharging. The special propulsion system components required are described, namely: the deflecting front inlet/nozzle, the aft subsonic inlet, the reverse pitch fan, the variable core supercharger and the low pressure forward burner. The potential benefits for these unconventional systems are indicated.

01 Jul 1981
TL;DR: In this article, a computerized aircraft synthesis program was used to examine the feasibility and capability of a V/STOL aircraft based on the Navy S-3A aircraft, and two major airframe modifications were considered: replacement of the wing, and substitution of deflected thrust turbofan engines similar to the Pegasus engine.
Abstract: A computerized aircraft synthesis program was used to examine the feasibility and capability of a V/STOL aircraft based on the Navy S-3A aircraft. Two major airframe modifications are considered: replacement of the wing, and substitution of deflected thrust turbofan engines similar to the Pegasus engine. Three planform configurations for the all composite wing were investigated: an unconstrained span design, a design with the span constrained to 64 feet, and an unconstrained span oblique wing design. Each design was optimized using the same design variables, and performance and control analyses were performed. The oblique wing configuration was found to have the greatest potential in this application. The mission performance of these V/STOL aircraft compares favorably with that of the CTOL S-3A.

Patent
17 Sep 1981
TL;DR: The basic gas turbine engine hydromechanical fuel control is adaptable to different engine configurations such as turbofan, turboprop and turboshaft engines by incorporating in the main housing those elements having a commonality to all engine configurations and providing a removable block for each configuration having the necessary control elements and flow passages required for that particular configuration as discussed by the authors.
Abstract: Fuel Control For Gas Turbine Engines Abstract The basic gas turbine engine hydromechanical fuel control is adaptable to different engine configurations such as turbofan, turboprop and turboshaft engines by incorporating in the main housing those elements having a commonality to all engine configurations and providing a removable block for each configuration having the necessary control elements and flow passages required for that particular configuration That is to say, a block with the elements peculiar to a turbofan engine could be replaced by a mating block that includes those elements peculiar to a turboshaft engine in adapting the control for a turboshaft configuration Similarly another block with those elements peculiar to a turbo-prop engine could replace any of the other blocks in adapting the control to a turboprop configuration Obviously the basic control has the necessary flow passages terminating at the interface with the block and these flow passages mate with corresponding passages in the block

01 Jan 1981
TL;DR: A TF34 turbofan engine was modified to produce shaft power from an output coupling on the fan disk when variable inlet guide vanes are closed to reduce fan airflow as mentioned in this paper.
Abstract: A TF34 turbofan engine is being modified to produce shaft power from an output coupling on the fan disk when variable inlet guide vanes are closed to reduce fan airflow The engine, called a convertible engine, could be used on advanced rotorcraft such as X-wing, ABC (Advanced Blade Concept), and Folding Tilt Rotor, and on V/STOL craft in which two engines are cross-coupled The engine will be tested on an outdoor static test stand at NASA Lewis Research Center Steady-state tests will be made to measure performance in turbofan, turboshaft, and combined power output modes Transient tests will be made to determine the response to the engine and a new digital engine control system for several types of rapid changes in thrust and shaft loads The paper describes the engine modifications, the test facility equipment, proposed testing techniques for several types of tests, and typical test results predicted from engine performance computer programs

Patent
24 Feb 1981
TL;DR: In this article, a torque converter is interconnected between the fan and core rotor of a turbofan engine such that the speed of the core rotor can be boosted from windmill speed to a speed sufficient to allow an in-flight start by selectively extracting power from the windmilling fan during the start sequence.
Abstract: 13DV-6914 METHOD AND APPARATUS FOR WINDMILL STARTS A torque converter is interconnected between the fan and core rotor of a turbofan engine such that the speed of the core rotor can be boosted from windmill speed to a speed sufficient to allow an in-flight start by selectively extracting power from the windmilling fan during the start sequence of the engine. Control logic is included to automatically drain the oil from, and thereby unload, the torque converter whenever the engine is in the operating speed region. Further assurance is provided by an overrunning clutch which decouples the system when the core rotational speed exceeds the converter output.

Journal ArticleDOI
TL;DR: An analytical study of the effects of wind tunnel turbulence on turbofan rotor noise was carried out to evaluate the effectiveness of the NASA Ames 40 by 80-foot wind tunnel in simulating flight levels of fan noise as mentioned in this paper.
Abstract: An analytical study of the effects of wind tunnel turbulence on turbofan rotor noise was carried out to evaluate the effectiveness of the NASA Ames 40 by 80-foot wind tunnel in simulating flight levels of fan noise. A previously developed theory for predicting rotor/turbulence interaction noise, refined and extended to include first-order effects of inlet turbulence anisotropy, was employed to carry out a parametric study of the effects of fan size, blade number, and operating line for outdoor test stand, NASA Ames wind tunnel, and flight inlet turbulence conditions. A major result of this study is that although wind tunnel rotor/turbulence noise levels are not as low as flight levels, they are substantially lower than the outdoor test stand levels and do not mask other sources of fan noise.