Helicopter Rotor Noise Research Articles

Prediction of underwater transmission of helicopter rotor noise from measurements of noise during hovers over land

It is desirable, for such activities as helicopter-towed mine sweeping, to be able to make predictions concerning the noise transmitted into water from the helicopter. It is generally not feasible to make a “dry run” survey, nor is information readily available concerning rotor forces which would facilitate analytical predictions. A method is proposed for predicting the transmitted sound field by comparison measurements of the noise from a helicopter hovering over open terrain. The proposed technique incorporates the previously reported analysis of the transmission into water of the noise from a fluctuating force moving in a circular path. A similar analysis of the sound received at a plane surface with finite acoustic impedance is employed to relate land-based noise measurements to blade forces. The rotating forces are assumed to maintain a constant orientation with respect to the rotor path. The hover condition in the over-land measurements is deemed sufficient inasmuch as significant transmission to appreciable depths in water occurs only for a limited area under the helicopter rotor. (A simple ray-theory model indicates that total reflection results for angles of incidence greater than 11°.) It is found that contributions from horizontal (drag) blade forces are transmitted into water with negligible amplitude. [Work supported by Naval Coastal Systems Center.]

A possible unsteady thickness noise mechanism for helicopter rotors

An alternative theoretical approach to the low‐speed helicopter rotor noise problem is proposed. The traditional treatment of this problem is to model the situation by a system of unsteady thrust and drag dipoles. The dipole strengths are either estimated empirically, or related to the inflow distortion via an unsteady aerodynamic response analysis. An analysis is presented which suggests that an alternative model consists of a system of unsteady lift dipoles, plus a distribution of inplane quadrupoles. The strength of these quadrupoles is related directly to the product of the blade thickness and the distortion inplane velocity components. It is shown how this mechanism is likely to be the cause of the subjectively important higher harmonic rotational noise observed in or near the rotor plane. Some comparisons with experiment are presented.

A simplified Mach number scaling law for helicopter rotor noise

A simplified Mach number scaling law is obtained for rotational and high frequency broadband noise from helicopter rotors. These scaling laws are based on geometric parameters of the rotor. The existing theory of Lowson and Ollerhead is used in deriving the conventional M t 6 law for rotational noise of geometrically similar blades operating in similar flow environ. The effects of number of blades, forward speed and directionality are included in the scaling formula. The ambiguous state of the art regarding the origin and and nature of high frequency broadband noise does not permit such a straightforward scaling law for this frequency regime. The vortices are assumed to be shed at an unknown Strouhal frequency and the scaling law is derived by simply integrating the blade sectional velocity over the span. The resulting scaling law is a function of the spanwise correlation lengths, Mach number, advance ratio, and geometric parameters of the rotor. Use of stationary airfoil data for the correlation lengths leads to a M t 5·8 lawfor the sound pressure level of the high frequency broadband noise. Both scaling laws were experimentally verified by using a 4·17 foot model rotor operating in the M.I.T. 5 × 7 1 2 foot anechoic wind tunnel. A comparison of the peak frequency location of high frequency broadband noise for full scale rotors obtained by Leverton was made, with the predicted Strouhal frequency parameter being used, and the agreement was found to be highly favorable. However, the predicted sound pressure levels were higher than the measured full scale rotor data.

Subsonic fan noise

Helicopter rotor noise theories are used to determine the noise of subsonic fans produced solely by the rotor, which results from aerodynamic intake flow distortions. Extensions to the theories allow for both stationary, phase-related distortions and distortions which vary randomly with time and may therefore be considered to rotate with random speeds. It is shown that the aerodynamic interaction of intake distortions with the blades will result in both rotor order tones and broad-band noise. The dominance of the one over the other depends on the space-time correlation of the distortions and the magnitude of typical fan length and time scales. Modulation effects are considered. The use of in-duct measurements is discussed and results presented to show the relative importance of steady and unsteady distortions in a simple fan rig. The amplitudes of the resultant blade force distortions have been determined from on-axis acoustic data and used to predict the full forward radiation field.

Sound radiation from a point force in circular motion

This paper is a theoretical study of sound radiation from a time-varying point force in accelerative motion, where the acceleration arises from steady rotation in a circle. The study is prompted by the question of whether such effects are significant in fan or helicopter rotor noise at subsonic tip speeds. Closed-form expressions are found for the overall radiation at a point in the far field, and for the radiated sound power, showing the acceleration effect as an additive term in each case. The effect of rotation on the broad-band far-field spectrum is demonstrated by a series expansion for rotational frequencies small compared with the radiated frequency.

Helicopter Rotor Noise Prediction and Control

This paper summarizes the results of Sikorsky Aircraft's research in the prediction and control of helicopter rotor noise. The work to he discussed was partly funded by the U.S. Army Aviation Material Laboratories under Contracts DA 44‐177‐AMC‐141(T) and DA 44‐177‐AMC‐448(T). An improved procedure has been developed for the prediction of main rotor vortex noise under conditions of uniform inflow for a single rotor helicopter. Both the overall sound pressure level and the spectrum shape of the vortex noise from square tipped blades can be calculated as a function of tip speed, blade area, and thrust. The geometry of the blade tip can alter both levels and spectrum shape appreciably. The USAAVLABS contracts also resulted in two computerized analyses for rotational noise prediction. Both analyses extend the work of Gutin to include the noise produced by harmonics of airload acting on the rotor blades. These analyses have demonstrated the importance of the higher harmonics of airload and the chordwise distribution of loading for accurate rotational noise prediction.

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.

Investigation and prediction of helicopter rotor noise part I. Wessex whirl tower results

This report includes noise measurements taken from a three-bladed Wessex rotor of 56 ft diameter, mounted on a rotor whirl tower. The blades used for the measurements were of constant chord and made to a N.A.C.A. 0012 profile section. The range of noise measurements taken was concentrated in the speed range from 180 to 250 rev/min and at collective pitch angles (measured at the cuff) from 7° to 17°. These measurements have been compared with theoretical predictions of broad-band noise. The magnitude of the higher harmonics of the ordered noise found from the tests appears to be in good agreement with the narrow pressure pulse width acting on the blade surface. This is discussed in the report and illustrated in Figure 10. The broad-band or “vortex” noise has been found to obey a reasonable relationship with the sixth power of the blade tip velocity and also to be dependent upon C L 1·66, where C L is a tip-referred lift coefficient. The limitations on the test results imposed by the use of the whirl tower are discussed in section 6 and suggestions for further work on other rotor configurations are indicated.

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.

A Controlled Experiment on the Perception of Helicopter Rotor Noise

During a recent open week at the National Gas Turbine Establishment at Pyestock, an experiment involving just over 1,000 subjects was made to establish some measure of the disturbance value of such rhythmic or amplitude modulated noises as are generated by helicopter rotors. The results were (i) that, as may be implied from Niese's results with modulated tones, the subjective assessment of such a noise depends upon its peak sound pressure level (i.e. 82·5 db. in Fig. 3) rather than upon its r.m.s. value, (ii) that the effect of the modulation frequency is negligible in the range from 4 to 12 cycles/sec. and (iii) that these results are invariant with peak sound pressure level in the range from 85 to 95 db. re 2 x 10–4 dynes/cm2.