Fluid reaction surfaces (i.e. – impellers) – Method of operation
Reexamination Certificate
2001-01-10
2002-10-08
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Method of operation
C416S023000, C416S042000, C416S500000
Reexamination Certificate
active
06461106
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to a method for reducing the blade-vortex interaction noise—generally referred to as BVI noise in the art—generated by a rotary wing. It also concerns a rotary wing implementing such a method.
B. Description of Related Art
It is known that, during low-speed flights, in particular when descending before landing, the blades of a rotor of a helicopter or of a similar aircraft move in proximity to their own wake and interact with the vortices which they have shed in the vicinity of their tip and which are generally referred to as “tip vortices”. These interactions create abrupt variations in pressure on the blades; the more intense the vortex and the closer it passes to the blade, the greater the amplitude of these pressure variations. The latter are sources of loud noise in a particularly audible frequency band. Since the blade-vortex interaction noise radiates downward and forward, that is to say toward the environs of heliports, it constitutes one of the most penalizing acoustic nuisances in the development of helicopters. Its reduction is today a major industrial issue, especially on account of the strengthening of international standards relating to acoustic nuisances.
In order to reduce the intensity of BVI noise, essentially two actions are known, consisting respectively in decreasing the intensity of the blade tip vortices and in distancing said vortices from said blades. These actions may be amalgamated and implemented by passive or active means.
For example, the documents FR-A-2 636 593, EP-A-0 482 932 and WO 97/07019 describe passive means of this type, formed by geometries of blades, especially of tips of blades, intended to reduce the intensity of said vortices.
Passive means of this type are optimized for a given flight configuration. They have the advantage of a certain technological simplicity. On the other hand, their effectiveness may depend greatly on the flight configuration. Specifically, the conditions of the BVI noise change as a function of the flight speed and of the angle of descent of the helicopter. Moreover, the techniques proposed are unnecessary in flight configurations which do not generate BVI noise and then generally prove to be penalizing in respect of the aerodynamic performance of the rotor.
Active means of reducing BVI noise are for example described by the documents EP-A-0 689 990 and U.S. Pat. No. 5,588,800. The first of these documents employs air jets at the trailing edge and at the tip of the blades in order to reduce the intensity of the tip vortices and distance them from said blades. The second uses a trailing edge flap toward the tip of the blades, the deflection of said flap being periodic and defined so as to accelerate the vertical convection of the tip vortices and thus distance them from said blades.
Active means of this type are technically more complex than said passive means, but they can be adjusted as a function of the flight configuration of the aircraft. They may therefore be activated only when they are necessary and be optimized within a more extensive flight domain. On the other hand, in their manner of operation they do not comprise any continuous reactive relation between the flight configuration under actual flight conditions—and their control. Furthermore, their manner of operation requires significant power. In particular, in the case of the device of document U.S. Pat. No. 5,588,800, the rotary wing has to be provided with high power, able to make the aerodynamic flaps oscillate at frequencies of the order of 20 Hz with amplitudes of several degrees.
The object of the present invention is to remedy these drawbacks. It relates to an active means of reducing BVI noise of the type with trailing edge flaps, the deflection of which can be feedback-controlled in relation to the actual flight conditions and which requires only relatively low power for its operation.
To this end, according to the invention, the method for reducing the noise generated by the rotary wing of an aircraft, such as a helicopter, due to the fact that, in the course of the rotation of said rotary wing and of the advancing of said aircraft, each component blade of said rotary wing encounters the tip vortex generated by a previous blade, said method implementing at least one trailing edge flap disposed toward the outboard tip of each of said blades, is noteworthy in that:
at least one value &PSgr;o of the azimuth of the blades is determined, for which value said blades shed tip vortices responsible for a peak intensity of said noise;
the section of each blade around which the speed circulation is a maximum is determined;
said trailing edge flap is disposed on each blade in such a way that the former generates at least one auxiliary vortex, parallel to said tip vortex and attached to a section of said blade lying between said section around which the speed circulation is a maximum and the tip section of said blade; and
a deflection is applied to each of said flaps, its value being constant in azimuth, but such that, for said value &PSgr;o of the azimuth, the speed circulation around said blade section to which said auxiliary vortex is attached is a specified fraction of said maximum speed circulation.
The present invention is based on the fact that BVI noise is not generated uniformly around the axis of rotation of the rotary wing, and that, during each cycle of rotation, each blade of the rotary wing sheds vortices whose characteristics, such as location of shedding, dimension, intensity, etc. are related to the load and to the geometry of the blades. The applicant has in particular found that the tip vortices of blades responsible for dominant BVI noise are shed by the blades—regardless of how many blades go to make up the rotary wing—at a specified azimuth, generally lying between 120° and 150° and often roughly 130°, reckoned in the direction of rotation of the blades starting from the 0° azimuth, which corresponds to the rear part of the longitudinal axis of the aircraft. BVI noise therefore includes, during each cycle of rotation, at least one peak intensity corresponding to the interaction with the tip vortices shed at this specified azimuth.
The value &PSgr;o of the azimuth, as well as the section of each blade around which the speed circulation is a maximum can be determined by calculation or by trials.
Thus, by virtue of the present invention, at the azimuth &PSgr;o and at azimuths neighboring &PSgr;o, the blades of the rotary wing shed several weaker vortices than the single vortex which would be shed at blade tips in the absence of implementation of the invention. The latter therefore makes it possible to divide each of these single vortices into several vortices of lower speed circulation rather than to distance them from the following blades. This results in a reduction in a dominant part of the BVI noise.
Of course, in the case where it is necessary, it is possible to apply the method in accordance with the present invention to several values &PSgr;o
1
, &PSgr;o
2
, . . . &PSgr;on of the azimuth at which the tip vortices shed would be the cause of BVI noise rather than to a single privileged value &PSgr;o of the azimuth of the blades.
Likewise, in accordance with the present invention, it is possible to employ more than one trailing edge flap per blade, so as to divide the vortical shedding into a plurality of auxiliary vortices. However, in this case there is a risk that auxiliary vortices which are too close to one another may combine into a single vortex.
Also, in a preferred mode of implementation of the present invention:
a single trailing edge flap is employed per blade;
said section to which said auxiliary vortex is attached is located at least approximately midway between said section around which the speed circulation is a maximum and said tip section of said blade; and
the speed circulation around the blade section to which said auxiliary vortex is attached is at least approximately equal to half said maximum speed circulation.
Thus, each bl
Kershteyn Igor
ONERA (Office National d'Etudes et de Recherches Aerospatia
Stevens Davis Miller & Mosher LLP
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