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NASA NTRS1978

Bounds on thickness and loading noise of rotating blades and the favorable effect of blade sweep on noise reduction

The maxima of amplitudes of thickness and loading noise harmonics are established when the radial distribution of blade chord, thickness ratio, and lift coefficient is specified. It is first shown that only airfoils with thickness distribution and chordwise loading distributions which are symmetric with respect to midchord need be considered for finding the absolute maxima of thickness and loading noise. The resulting chordwise thickness and load distributions for these maximum noise conditions require infinite slope at some points along the chord but otherwise are uniform. It is shown that sweeping the blades reduces the thickness and loading noise, but there is no optimum sweep which generates the lowest noise.

Farassat, F.↗

NASA NTRS1983

Limits on the prediction of helicopter rotor noise using thickness and loading sources: Validation of helicopter noise prediction techniques

The techniques of helicopter rotor noise prediction attempt to describe precisely the details of the noise field and remove the empiricisms and restrictions inherent in previous methods. These techniques require detailed inputs of the rotor geometry, operating conditions, and blade surface pressure distribution. The Farassat noise prediction techniques was studied, and high speed helicopter noise prediction using more detailed representations of the thickness and loading noise sources was investigated. These predictions were based on the measured blade surface pressures on an AH-1G rotor and compared to the measured sound field. Although refinements in the representation of the thickness and loading noise sources improve the calculation, there are still discrepancies between the measured and predicted sound field. Analysis of the blade surface pressure data indicates shocks on the blades, which are probably responsible for these discrepancies.

Succi, G. P.↗

NASA NTRSDate not supplied

A Synthesis Plug-in for Steady and Unsteady Loading and Thickness Noise Auralization

This paper describes the development and architecture of a plugin for the NASA Auralization Framework that synthesizes rotor sound pressures sample by sample for simulated propagation to auralize rotorcraft flyovers. The main component of the plugin is a preprocessor that synthesizes sound pressures before propagation methods are called. Rotor blade loadings, motion, and geometry data from various tools serve as input to the plugin. Farassat formulation 1A, a solution to the Ffowcs Williams-Hawkings equation, is used for the sound pressure calculations. Use of the plugin is demonstrated with two examples of steady periodic sound synthesis, but the synthesis method and preprocessor architecture described in this paper are also applicable to unsteady periodic and aperiodic sound syntheses.

Siddhartha Krishnamurthy↗

NASA NTRS1987

Comments on the aeroacoustics of helicopter rotors

Some techniques and procedures used in rotor aeroacoustics are examined. The advantages and disadvantages of using the acoustic analogy for noise prediction of helicopter rotors are discussed. The time domain and frequency domain approaches to rotor noise predictions are described. Consideration is given to steady and unsteady thickness noise, loading noise, and quadrupole noise.

Farassat, F.↗

NASA NTRSDate not supplied

A Synthesis Method to Auralize Rotor Noise During Transitional Flight

This paper describes a new capability to synthesize Urban Air Mobility (UAM) vehicle rotor/propeller loading and thickness noise and broadband self noise for transitional flight auralization. A vehicle smoothly transitioning between flight states is approximated here as transitioning in time through a series of trimmed steady-state flight conditions. To generate information that helps synthesize loading and thickness noise for transitional flight, this paper describes the development of a tool that combines blade loading, motion, and geometry data from a series of separate trimmed flight states. This paper also describes a method to combine broadband self noise acoustic predictions, each representing a separate trimmed flight state, to synthesize broadband self noise for transitional flight. Different flight profiles of the six-passenger NASA UAM Quadrotor reference vehicle demonstrate these capabilities, with each profile specified by a series of trimmed flight states.

Auralization↗

NASA NTRS1992

Helicopter noise prediction - The current status and future direction

The paper takes stock of the progress, assesses the current prediction capabilities, and forecasts the direction of future helicopter noise prediction research. The acoustic analogy approach, specifically, theories based on the Ffowcs Williams-Hawkings equations, are the most widely used for deterministic noise sources. Thickness and loading noise can be routinely predicted given good plane motion and blade loading inputs. Blade-vortex interaction noise can also be predicted well with measured input data, but prediction of airloads with the high spatial and temporal resolution required for BVI is still difficult. Current semiempirical broadband noise predictions are useful and reasonably accurate. New prediction methods based on a Kirchhoff formula and direct computation appear to be very promising, but are currently very demanding computationally.

Brentner, Kenneth S.↗

NASA NTRS1995

Acoustic design of rotor blades using a genetic algorithm

A genetic algorithm coupled with a simplified acoustic analysis was used to generate low-noise rotor blade designs. The model includes thickness, steady loading and blade-vortex interaction noise estimates. The paper presents solutions for several variations in the fitness function, including thickness noise only, loading noise only, and combinations of the noise types. Preliminary results indicate that the analysis provides reasonable assessments of the noise produced, and that genetic algorithm successfully searches for "good" designs. The results show that, for a given required thrust coefficient, proper blade design can noticeably reduce the noise produced at some expense to the power requirements.

V. L. Wells↗

NASA NTRSDate not supplied

Prediction-Based Auralization of a Multirotor Urban Air Mobility Vehicle

Recent advances in auralization methods applicable to rotary wing vehicles have made it possible to undertake a prediction-based auralization of a representative multirotor urban air mobility vehicle. These advances include a new capability for synthesizing loading and thickness noise directly from the prediction method and a new capability for predicting and synthesizing modulating broadband self noise within a unified system noise prediction-auralization framework. These capabilities are demonstrated for a six passenger quadrotor reference vehicle design using collective-pitch control. Propagation of the source noise to a ground observer completes the auralization process. The demonstrated capability serves as the basis for future work directed at perception-influenced design of low noise urban air mobility vehicles.

Auralization↗

NASA NTRSDate not supplied

Prediction of Quadrotor Acoustics Using RVLT Toolchain

The NASA Revolutionary Vertical Lift Technology (RVLT) Project toolchain is used to predict the acoustics of the NASA 6-passenger quadrotor concept vehicle. A qualitative study of the blade vertical loading to understand noise trends with tip speed and number of blades was performed. Trends of the individual rotor noise sources (thickness, loading, and broadband) are studied. Noise is predicted and analyzed for three flight conditions: approach (6-deg descent), level flight flyover, and takeoff. Three quadrotor designs are considered: tip speed=700 ft/sec, 3 blades (“700/3”), “550/3”, and “375/7”. The qualitative study shows that during approach multiple, strong BVIs occur on the advancing and retreating sides for “700/3” and “550/3”, but for “375/7” only weak BVIs are present. Consistent with this BVI scenario, the quadrotor noise results show that “700/3” has the highest loading noise (and EPNL) and “375/7” the lowest loading noise (and EPNL), with “550/3” falling in between. Broadband noise during approach and flyover is roughly the same for all three designs. Takeoff broadband noise is much lower, especially for “375/7”. The bulk of the results have been obtained with a rigid uniform blade model, with a flap hinge and a pitch bearing. For “550/3”, noise predictions with elastic nonuniform blades have been initiated and are ongoing.

Quadrotor↗

NASA NTRS1984

Helicopter noise as predicted by three-dimensional monopole and quasi-steady full-potential dipole sources

A three-dimensional quasi-steady full-potential rotor-flow-analysis program, called ROT22 developed at NASA Ames Research Center is run in conjunction with Farassat's (1981) helicopter-noise-prediction code to assess the thickness and loading noises made by a rotor blade. As a model example, the case of a 1/7th UH-1H NACA-0012-profile straight nonlifting rotor blade in hover is studied. Results for tip Mach numbers ranging from .4 to .962 are presented. Also presented is the effect of blade-tip loading on the overall noise and of profile curvature on the thickness noise. The study confirms that the blade volume displacement is a dominant source of helicopter noise and further concludes that the blade tip makes only a small noise contribution that steadily decreases with increasing tip Mach numbers, and that the inclusion of blade profile curvature tends to improve the negative peak amplitudes overpredicted by Schmitz and Yu (1983).

Aggarwal, H. R.↗

NASA NTRS1991

Acoustic predictions using measured pressures from a model rotor in the DNW

A contemporary design, 4-bladed United Technologies model rotor with pressure-instrumented blades was tested in the Duits-Nederslandse Windtunnel. Simultaneous acoustic and pressure measurements were made for a wide range of operating conditions. Microphones were optimally positioned at a number of locations in the flow forward of the rotor to measure rotor thickness noise, high-speed impulsive noise (both in the rotor plane), and blade-vortex interaction noise (forward and 25 deg below the rotor plane). The blade surface pressure data are used as aerodynamic input to WOPWOP, which is a state-of-the-art rotor noise prediction program that predicts rotor thickness and loading noise. The predicted results using WOPWOP are compared to the measured noise levels for cases where either thickness noise, blade-vortex interaction noise, or high-speed impulsive noise is the dominant noise mechanism. The comparisons show regions of good agreement, as well as areas where further improvement is necessary.

Visintainer, Joseph A.↗

NASA NTRS1994

Sensitivity of acoustic predictions to variation of input parameters

Rotor noise prediction codes predict the thickness and loading noise produced by a helicopter rotor, given the blade motion, rotor operating conditions, and fluctuating force distribution over the blade surface. However, the criticality of these various inputs, and their respective effects on the predicted acoustic field, have never been fully addressed. This paper examines the importance of these inputs, and the sensitivity of the acoustic predicitions to a variation of each parameter. The effects of collective and cyclic pitch, as well as coning and cyclic flapping, are presented. Blade loading inputs are examined to determine the necessary spatial and temporal resolution, as well as the importance of the chordwise distribution. The acoustic predictions show regions in the acoustic field where significant errors occur when simplified blade motions or blade loadings are used. An assessment of the variation in the predicted acoustic field is balanced by a consideration of Central Processing Unit (CPU) time necessary for the various approximations.

Brentner, Kenneth S.↗

NASA NTRS2013

An Update on Phased Array Results Obtained on the GE Counter-Rotating Open Rotor Model

Beamform maps have been generated from 1) simulated data generated by the LINPROP code and 2) actual experimental phased array data obtained on the GE Counter-rotating open rotor model. The beamform maps show that many of the tones in the experimental data come from their corresponding Mach radius. If the phased array points to the Mach radius associated with a tone then it is likely that the tone is a result of the loading and thickness noise on the blades. In this case, the phased array correctly points to where the noise is coming from and indicates the axial location of the loudest source in the image but not necessarily the correct vertical location. If the phased array does not point to the Mach radius associated with a tone then some mechanism other than loading and thickness noise may control the amplitude of the tone. In this case, the phased array may or may not point to the actual source. If the source is not rotating it is likely that the phased array points to the source. If the source is rotating it is likely that the phased array indicates the axial location of the loudest source but not necessarily the correct vertical location. These results indicate that you have to be careful in how you interpret phased array data obtained on an open rotor since they may show the tones coming from a location other than the source location. With a subsonic tip speed open rotor the tones can come form locations outboard of the blade tips. This has implications regarding noise shielding.

Podboy, Gary↗

NASA NTRS1991

Sensitivity of acoustic predictions to variation of input parameters

The noise prediction code WOPWOP predicts the thickness and loading noise produced by a helicopter rotor, given the blade motion, rotor operating conditions, and fluctuating force distribution over the blade surface. However, the criticality of these various inputs, and their respective effects on the predicted acoustic field, have never been fully addressed. This paper examines the importance of these inputs, and the sensitivity of the acoustic predictions to a variation of each parameter. The effects of collective and cyclic pitch, as well as coning and flapping, are presented. Blade loading inputs are examined to determine the necessary spatial and temporal resolution, as well as the importance of the cordwise distribution. The acoustic predictions show regions in the acoustic field where significant errors occur when simplified blade motions or blade loadings are used. An assessment of the variation in the predicted acoustic field is balanced by a consideration of CPU time necessary for the various approximations.

Brentner, Kenneth S.↗

NASA NTRS1980

Noise from a vibrating propeller

This paper is concerned with an analytical study of the noise from a vibrating propeller. The influence of airfoil thickness and of steady loading are also included to provide a basis for comparison. The analysis was based on the concept of distributing sources and doublets on the surface of the blade, which were multiplied by their appropriate strength factors. The noise in the plane of the propeller was dominated by the thickness noise. Moving the observer's position to the propeller axis, the thickness noise and loading noise were zero, and a pure sinusoidal noise was found, caused by the vibrations of the propeller.

Runyan, H. L.↗

NASA NTRS1989

Acoustic design considerations: Review of rotor acoustic sources

It is not sufficient to optimize a rotor design in terms of a single noise level calculated for a single flight condition and a single measurement location. The various noise sources, their frequency content, amplitude, and directivity as a function of operating condition must be considered. A summary of the frequency ranges, directivity patterns and the most important operational and design parameters for major rotor noise sources is presented. It is difficult to generalize design requirements for rotor noise because the acoustic output varies so widely depending on the noise source, flight condition, measurement location, and frequency range. However, assuming the rotor must lift a fixed nominal payload and operate over a wide range of flight conditions, three general design guidelines can be stated: (1) minimize tip Mach number; (2) minimize blade thickness in the tip region; and (3) minimize gradients in the spanwise lift distribution in the tip region. Constraints on blade thickness, maximum values for hover tip Mach number, advancing tip Mach number and spanwise lift coefficient gradient will be specified during the aerodynamic, dynamic and structural optimization process. The rotor noise sources to be considered include the low frequency loading and thickness noise, and the higher frequency noise due to blade-vortex interactions (BVI). The analyses to be employed will include the comprehensive rotor analysis and design program CAMRAD and the rotor noise prediction program WOPWOP.

Martin, Ruth M.↗

NASA NTRS2018

Tonal Noise Prediction of a Distributed Propulsion Unmanned Aerial Vehicle

Noise is expected to be a major barrier of unmanned aerial vehicles (UAVs) to public acceptance. A noise prediction scheme is introduced in this paper and applied to a specific vehicle configuration, namely, the Greased Lightning-10. Results herein will be used to demonstrate the feasibility of incorporating a noise constraint within the flight control system of a distributed electric propulsion vehicle by modifying commands for low-noise operation over sensitive areas, e.g., communities, schools, etc. Steady loading and thickness noise signatures of single propellers are computed using the Propeller Analysis System of the NASA Aircraft NOise Prediction Program. The individual signatures are then superposed at far field observers after applying corrections for spherical spreading and phase delays based on straight ray propagation. Two-propeller source fields are verified using analytical directivity patterns of monopoles. Notional effects of rotation rate, rotation direction, and relative propeller phase are given. Under ideal circumstances and equivalent RPM, random phasing, which occurs in most small UAVs, can produce up to 20 decibels uncertainty in the tonal sound pressure level at a given ground observer. Additionally, directivity modification via relative propeller phase control is shown to have great potential as a noise reduction technique. This paper will focus on the forward flight mode but will also briefly discuss the vertical flight mode.

Pascioni, Kyle A.↗

NASA NTRS1990

The radiation of sound from a propeller at angle of attack

The mechanism by which the noise generated at the blade passing frequency by a propeller is altered when the propeller axis is at an angle of attack to the freestream is examined. The measured noise field is distinctly non axially symmetric under such conditions with far field sound pressure levels both diminished and increased relative to the axially symmetric values produced with the propeller at zero angle of attack. Attempts have been made to explain this non axially symmetric sound field based on the unsteady (once per rev) loading experienced by the propeller blades when the propeller axis is at non zero angle of attack. A calculation based on this notion appears to greatly underestimate the measured azimuthal asymmetry of noise for high tip speed, highly loaded propellers. A new mechanism is proposed; namely, that at angle of attack, there is a non axially symmetric modulation of the radiative efficiency of the steady loading and thickness noise which is the primary cause of the non axially symmetric sound field at angle of attack for high tip speed, heavily loaded propellers with a large number of blades. A calculation of this effect to first order in the crossflow Mach number (component of freestream Mach number normal to the propeller axis) is carried out and shows much better agreement with measured noise data on the angle of attack effect.

Mani, Ramani↗