Blade Slap Research Articles

Flow‐induced sound

After a brief historical account introducing some lasting concepts, the origins and main features of aerodynamically generated sound are described in terms of the melding of some pertinent elements of fluid dynamics and of acoustics. These are, for example, flow separation, flow instability, and vortices on the one hand, coupled with the hydrodynamic flows of acoustic monopoles, dipoles, and quadrupoles on the other. With emphasis on the vortex theory of aerodynamic sound, various theoretical approaches are generally described in physical terms and are illustrated by a variety of sound‐generating flows. These may be classified as free flows with no solid surfaces present (spinning vortices, turbulent jet noise, supersonic jet screech), flows over rigid surfaces (boundary layer noise, whistling telephone wires, edge tones, pipe tones, vocal fricatives, and whistling, organ pipes), interaction with steadily moving surfaces (helicopter blade slap, fan blade interaction, but not including the rotational noise of fans and propellers), flow with excited surfaces (interior aircraft boundary layer noise, aeolian tones, reed and lip‐driven musical instruments, vocal chord vibration, snoring and raspberries). Some of these involved no resonance at all, while others have flow resonance, acoustic resonance or mechanical vibration or resonance.

Heliport noise contours with and without a penalty for induced building rattle

The debate over whether a penalty should be added to the A-weighted noise level of helicopters to account for the added annoyance of building rattles induced by blade slap is long-standing. Recent evidence suggests that the decision to add a penalty should depend on the type of construction in the area subjected to the noise. An implication for community planners is that noise contour maps should be presented in two forms: with and without a penalty, thus allowing the planners to decide whether building codes to reduce rattle should be enforced for new development. Examples from two Army airfields are provided to demonstrate the advantages and disadvantages of this process. On-site noise monitoring data are also presented to confirm the accuracy of the unpenalized noise contours.

Extraction of helicopter-radiated noise by frequency domain processing

Real data observations of farfield radiated noise from an approaching UH-1 helicopter revealed that the received time-domain radiated noise contained impulses. The cited literature attributes these observed impulses to blade slap which is caused by, among other mechanisms, blade vortex interactions. This paper is concerned exclusively with extracting these measured impulses by a frequency-domain processing method. As is well known, the total radiated noise of a helicopter is quite complex. No attempt is made in the paper to discuss the full complexity of the radiated noise field. However, references are cited in the text which support our findings and give a more general treatment to helicopter-radiated noise. The method for extracting impulses generated by an approaching helicopter utilizes a nonlinearity implemented in the frequency domain. It is shown with real helicopter-radiated noise data that autocorrelation estimates of a signal, interfered with by additive helicopter-radiated noise, can be improved by extracting some of the interfering components. The extracted components are also available as an enhanced time-domain representation.

Comparison of model helicopter rotor primary and secondary blade/vortex interaction blade slap

A study of the relative importance of blade/vortex interactions which occur on the retreating side of a model helicopter rotor disk is described Some of the salient characteristics of this phenomenon are presented and discussed It is shown that the resulting secondary blade slap may be of equal or greater intensity than the ad vancing side (primary) blade slap Instrumented model helicopter rotor data are presented which reveal the nature of the retreating blade/vortex interaction The importance of secondary blade slap as it applies to predictive techniques or approaches is discussed When secondary blade slap occurs it acts to enlarge the window of operating conditions for which blade slap exists Nomenclature B = number of blades c =chord CT = thrust coefficient thrust/pIIK2 (ttR)2 CT/a = blade loading R = rotor radius V = tunnel speed x/c = nondimensional chordwise position ots = shaft angle Os = pitch at tip =0T A = vortex/blade interaction angle fj, = advance ratio V/SIR p = density of air a = rotor solidity Bc/HR Q = angular velocity

Dynamic Surface Measurements on a Model Helicopter Rotor During Blade Slap at High Angles of Attack

The modern helicopter offers a unique operational capability to both the public and private sectors. However, the use of the helicopter may become severely limited due to the radiated noise generated by the rotor system. A description is presented of some of the experimental results obtained with a model helicopter rotor in an anechoic wind tunnel with regard to blade stall as a source mechanism of blade slap. Attention is given to dynamic rotor blade surface phenomena and the resulting far field impulsive noise from the model helicopter rotor at high angles of attack and low tip speed. The results of the investigation strongly implicates the boundary layer as playing an important role in blade slap due to blade/vortex interaction (BVI) in a highly loaded rotor. Intermittent stall cannot be ruled out as a possible source mechanism for blade slap. This implies that blade surface characteristics, airfoil shape and local Reynolds number may now be used as tools to reduce the resultant far-field sound pressure levels in helicopters.

Research on Model Helicopter Rotor Blade Slap at Moderate Tip Speeds

Research on Model Helicopter Rotor Blade Slap at Moderate Tip Speeds

Model helicopter rotor impulsive noise

An investigation of helicopter rotor blade slap was performed on a 1·27 m diameter model rotor. The rotor blades were a NACA 0012 section with a −8° linear twist. The experiments were conducted in the M.I.T. Anechoic Wind Tunnel. The effects of number of blades, blade pitch, advance ratio, and shaft angle on the generation and intensity of blade slap have been investigated. The effect of each parameter was determined by varying the parameter of interest while keeping the other constant. The rotor tip speeds of the tests performed in this study were limited to approximately Mach 0·4. The directivity associated with the blade slap signature was measured. The results of this investigation are based on a subjective definition of blade slap.

Effect of tip vortex structure on helicopter noise due to blade-vortex interaction

A potential cause of helicopter impulsive noise, commonly called blade slap, is the unsteady lift fluctuation on a rotor blade due to interaction with the vortex trailed from another blade. The relationship between vortex structure and the intensity of the acoustic signal is investigated. The analysis is based on a theoretical model for blade/vortex interaction. Unsteady lift on the blades due to blade/vortex interaction is calculated using linear unsteady aerodynamic theory, and expressions are derived for the directivity, frequency spectrum, and transient signal of the radiated noise. An inviscid rollup model is used to calculate the velocity profile in the trailing vortex from the spanwise distribution of blade tip loading. A few cases of tip loading are investigated, and numerical results are presented for the unsteady lift and acoustic signal due to blade/vortex interaction. The intensity of the acoustic signal is shown to be quite sensitive to changes in tip vortex structure.

Annoyance scales for simulated VTOL and CTOL overflights

Annoyance of simulated VTOL and CTOL overflights was measured by magnitude estimation scaling. Forty subjects were presented individually with a series of overflight sounds consisting of a mix of aircraft. L‐1011 arrivals and departures at each of three intensity levels were interleaved with Boeing‐Vertol B‐347 overflights with flight parameters adjusted to produce three subjectively varying levels of “blade slap.” Each of the “blade slap” variations was also presented at three intensity levels. In addition the L‐1011 arrivals and departures were octave filtered (10 dB) at each of three center frequencies: 315, 800, and 1600 Hz. These six “treated” spectra were also presented at three intensity levels. An L‐1011 arrival at 89 dbA was used as the standard called “100.” Each of the 33 stimuli was presented in a different random order to each subject for their judgments. Plotted against dBA helicopters were more annoying than arrivals which were more annoying than departures. Plotted against EPNdB arrivals and departures showed equivalent annoyance, whereas helicopters continued more annoying. Helicopters with varying “blade slap” showed differential annoyance against dbA but were conformed by EPNL. [Work supported by DOT‐FAA and NASA.]

In-Flight Far-Field Measurement of Helicopter Impulsive Noise

Abstract : An in-flight technique for measuring UH-1H helicopter impulsive noise (sometimes called 'blade slap') by stationkeeping with a quiet instrumented lead aircraft was found to be highly successful. Far-field quantitative acoustic waveforms and radiation patterns were easily obtained over a wide, continuous range of UH-1H flight conditions, including several areas known to produce annoying acoustic radiation. The data collected using this technique were not (to any significant degree) contaminated by transmission path distortions that have hindered measurement efforts in the past. The two major finding of this initial measurement program were: (1) Judging the occurrence and severity of a helicopter's radiated impulsive noise signature from cabin-based noise measurements can be misleading. For the UH-1H helicopter, reduction in cabin audible impulsive noise levels may constitute a necessary but certainly not sufficient to indicate that far-field impulsive noise radiation has been reduced. The following three distinct types of impulsive noise are radiated by the Un-1H helicopter while flying between 80 and 115 knots at descent rates from zero to 1000 ft/min: (a) A series of positive pressure pulses believed to be related to blade-tip vortex interaction. These pulses are responsible for the crisp popping sound of the radiated noise; (b) A negative pressure disturbance that rapidly increases in amplitude with forward velocity, becoming quite intense and sawtoothed in shape at 115 knots IAS; and (c) A narrow positive pressure spike that closely follows that sawtooth-shaped negative pressure pulse at high airspeeds (115 knots).

Wave forms for a supersonic rotor

The geometry of the wave forms of a supersonic rotor is described, based on a ray theory. The waves formed by a non-translating supersonic rotor are found to be limited by a hyperboloid, causing a cone of silence centred on the axis where only radiation from conventional noise sources may be observed. The wave region is limited by cusps which imply local focusing of the radiated energy. These fundamental features apply also to cases with hub motion but their geometry becomes complex. The approach is one in which non-linear effects necessarily are ignored, but the results appear to have value in delineating potential regions of shock formation in the free field. Applications include the shock radiation from supersonic fans in jet engines and the advancing blade slap problem in helicopters.

Vortex induced helicopter blade loads and noise

The fluctuating lift on a helicopter rotor blade passing close to a tip vortex shed from a preceding blade may generate an intense cyclic banging noise, called blade slap, which is one of the most offensive of all helicopter noises. Such blade/vortex interactions are modeled by a finite aspect ratio wing flying at uniform speed over a carpet of equally spaced, infinitely long, line vortices. By using linearized lifting surface theory the harmonic blade loads are expressed as a Fourier series with coefficients involving the same “sinusoidal gust transfer function” that figures prominently in the analysis of aircraft response to atmospheric turbulence. An established theory ( Lowson and Ollerhead, 1969 ) can be used to calculate the radiated noise harmonics in terms of the Fourier coefficients. Consideration of the noise propagated to the far field of a blade-fixed reference frame indicates that blade slap noise is only weakly influenced by aspect ratio. For a blade passing over the vortices at fixed height, acoustic power generation is proportional to the inverse third power of the height and is efficient only if the vortex spacing is about five times the height.

Helicopter Noise due to Blade-Vortex Interaction

The generation of impulsive sound, commonly called blade slap, due to blade-vortex interaction for helicopter rotors is discussed. The unsteady lift on the blades is calculated using linear unsteady aerodynamic theory for an oblique gust model of the blade-vortex interaction. A theoretical model for the radiated sound due to the transient lift fluctuations is presented. Expressions for the directivity, frequency spectrum, transient signal, and total power of the acoustic signal are derived. Typical results are presented and discussed. Calculations of the transient signal are presented in comparison with recent experimental results. The agreement is very good.

A theoretical study of helicopter rotor noise

A comprehensive theoretical study of the problem of helicopter rotor noise radiation is presented. The theory includes blade slap, rotation noise and vortex noise effects. Peak spectral levels over the “vortex noise” region are shown to be due to the higher harmonics of the rotational noise. An exact theoretical expression for the noise radiation has been used as the basis for the development of a comphrehensive computer program to calculate helicopter noise at any field point, including all effects of fluctuating airloads and all possible rigid and flexible blade motions. Under very reasonable approximations an analytic expression has been found for the sound field far from the helicopter. Computations based on this expression have also been made. The results show that it is the very high harmonics of the loading which contribute to the important harmonics of the sound field. For instance, calculation of the tenth harmonic of a four-bladed rotor requries a knowledge of loading harmonics up to the sixtieth. Details of such loadings are not available from theory or experiment. Rotor aerodynamic loadings have therefore been reviewed in detail and empirical harmonic decay laws derived. Loading phases appear to be best described as random, and this introduces simplification in the theory, together with the necessity for definition of a correlation length. Results of a parameter study show trends in agreement with experiment, both for overall levels and for spectrum shape. Sound at the higher harmonics is basically proportional to thrust times disc loading times tip velocity squared. For the lower harmonics the dependence on tip velocity is to the 2B power where B is the number of blades. The effect of forward speed is to increase the sound radiated forward and decrease that radiated aft, causing a difference of as much as 20 dB for the second and third harmonics at a forward Mach number of 0·25. The overall sound directionality pattern is found to have a minimum slightly above the plane of the disc and a broad maximum about 20° below. The effects of both the near field and blade motion effects are found to be small. The theory generally shows fair agreement with experiment for overall levels and good agreement for trends, and should therefore be of direct use for design trade-off studies to minimize noise in future helicopters.

Helicopter blade slap

Blade slap is the sharp increase in helicopter rotor noise, at the blade passing frequency, that is characteristic of certain model helicopters during some régimes of flight. An investigation has been conducted to determine the nature of the phenomenon. This has included flight tests with the Wessex and Belvedere helicopters and a comprehensive operational survey, military and commercial, in the United States and United Kingdom. Blade slap appears to be caused by a blade passing through the tip vortex shed by another blade in its proximity. This has been simulated on a rotor whirl stand under controlled conditions and the effect of various parameters investigated. A theory has been developed to predict the noise generated during a blade slap condition. There is good correlation between the flight tests, model tests and theory. The paper discusses all aspects of the investigation.