Fluid reaction surfaces (i.e. – impellers) – With control means responsive to non-cyclic condition... – Pressure or altitude responsive
Reexamination Certificate
1998-12-23
2001-01-09
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With control means responsive to non-cyclic condition...
Pressure or altitude responsive
C415S119000, C244S017130, C244S199100, C416S061000
Reexamination Certificate
active
06171056
ABSTRACT:
The invention described herein was made in the performance of work under U.S. Navy Contract No. N00014-96-C-2079.
1. Technical Field
This invention relates to the control of blade vortex interaction (BVI) noise in rotorcraft, and more particularly to signals useful in the control of such BVI.
2. Background Art
In the realm of rotorcraft (e.g., helicopter and tilt rotor) design and operation, the phenomenon of blade-vortex interaction (BVI) noise has long been recognized as an undesirable characteristic. This phenomenon results from the vortices, shed by the rotor blades and blade tips as they advance, impacting a following rotor blade. The noise occasioned by BVI is most pronounced during descent of the rotorcraft, at low speed, typically upon approach to a landing site or field. Such noise may be particularly annoying to persons on the ground near the landing site and/or in the flight path. Moreover, the BVI noise may pose a security hazard to the rotorcraft under military conditions, because it is rendered more detectable to the human ear and/or to other acoustical sensing devices. For these reasons, considerable analysis has been conducted and a variety of techniques have previously been suggested or used in an effort to reduce the occurrence and/or intensity of BVI noise during rotorcraft operation.
Examples of analyses conducted and data obtained regarding BVI are contained in papers by the present inventor, Peter F. Lorber, entitled “Aerodynamic Results of a Pressure-Instrumented Model Rotor Test at the DNW”, presented in the Journal of the American Helicopter Society, 1990, and “Blade-Vortex Interaction Data Obtained from a Pressure-Instrumented Model Rotor at DNW”, presented in 1991. These materials disclose the instrumenting of rotor blades with numerous pressure sensors and obtaining blade pressure measurements occasioned by BVI under a variety of simulated operating conditions and at various azimuthal positions of the instrumented blades. This data revealed significant information about the physics and aerodynamics of BVI noise generation.
Efforts to diminish BVI noise have broadly involved passive means and active means. The passive means have typically involved structural modifications of or additions to the rotor blades, generally in the tip region which generates the vortices. An example of such a passive device is described in U.S. Pat. No. 5,788,199 to Wake et al. for “Half-Plow Vortex Generators for Rotor Blades for Reducing Blade-Vortex Interaction Noise,” assigned to the assignee of the present application. There, a supplemental structure is mounted on each main rotor bade to generate a vortex rotating in opposition to that naturally generated by the blade tip to thereby attenuate any resulting vortex.
Examples of active devices for reducing BVI noise also include the aforementioned U.S. Pat. No. 5,788,199 in that it also discloses selectively deploying and retracting the vortex generator
10
either as a function of general flight conditions, i.e., descent, or as a more rapid function of azimuthal position of the rotor blade during rotation of the rotor (column 6, line 65 to column 7, line 29). A further example of an active means for attenuating BVI noise is described in a paper entitled “Effects of a Trailing Edge Flap on the Aerodynamics and Acoustics of Rotor Blade-Vortex Interactions” by B. D. Charles, et al. at pages 153-161 of Vol. 1 of the Proceeding of DGLR/AIAA 14
th
Aeroacoustics Conference of May 11-14, 1992. That paper describes the active control of flaps on rotor blades as a technique for BVI noise attenuation. Deployment of the flaps to various angles during various angular or azimuthal segments of blade rotation were analyzed for optimum results. The principles of the aforementioned paper appear also in U.S. Pat. No. 5,711,651 for “Blade Vortex Interaction Noise Reduction Techniques for a Rotorcraft” by Charles et al., which describes an active device (flaps) selectively deployable during rotation of the rotor blades through predetermined regions of the rotor azimuth.
While the passive devices provide the advantage of reduced complexity and perhaps less cost and weight, they do not afford the flexibility of active devices to adapt to changing BVI conditions throughout the flight regime, as with changes in speed and descent rate. Both of the aforementioned U.S. Pat. Nos. 5,788,199 and 5,711,651 mention the advantages of deploying or actuating the active device only during the period in which the rotorcraft is operating in a flight condition wherein significant BVI noise is likely to be generated. As disclosed in the U.S. Pat. No. 5,711,651, a predetermining schedule may be used to deploy the active device during the relevant region, or regions, of the blade rotation azimuth.
Relative to a fixed passive system or device, the actuation of active devices in response to a predetermined schedule affords a greater degree of flexibility in attaining BVI noise abatement and reducing the penalties associated with the drag caused by continuous deployment of a device. However, inefficiencies remain because of the need to predetermine a schedule of actuation based solely on previously determined BVI conditions as a function of descent rate, flight speed, device geometry and characteristics, etc.
What is needed is a technique for increasing or optimizing the efficiency and effectiveness of an active system for the reduction of BVI noise associated with the rotor of a rotorcraft.
DISCLOSURE OF INVENTION
A principal object of the present invention is to provide a technique for improving the efficiency and/or effectiveness of controlling active devices for the reduction of BVI noise associated with rotorcraft.
According to the present invention, there is provided a technique for improving the efficiency and/or effectiveness of controlling active devices for reducing BVI noise associated with rotorcraft. More specifically, according to the present invention there is provided a method for providing a control signal representative of blade vortex interaction noise for a rotorcraft having a multiblade rotor, which signal is provided for use as a control variable in a rotorcraft control system for the active control of BVI noise. The signal is provided by measuring the fluid (air) pressure at one or more predetermined locations on a rotor blade during at least one predetermined azimuthal segment of blade rotation to provide respective pressure measurements, and processing the respective pressure measurements to provide a signal for use as a control variable.
The air pressure is measured at two or more locations on the blade between 65% and 95% of the radial length of the blade and within 10% blade chord length of the leading edge of the rotating blade. The pressure is measured preferably at least during an azimuthal segment in which the blade is advancing relative to forward flight of the rotorcraft, particularly within the quadrant measured angularly forward 90° from the tail of the rotorcraft. It may additionally be separately measured during an azimuthal segment in which the blade is retreating relative to forward flight of the rotorcraft.
The pressure measurements are processed by filtering to retain substantially only a frequency band commensurate with BVI sources, typically between 20 and 48 times the rotation frequency of the rotor. The filtered pressure measurements are further processed in accordance with:
Fn
1
,
n
2
=
1
n
2
-
n
1
+
1
∑
n
1
n
2
f
n
2
,
where
where
Fn
1
, n
2
is the signal for use as the control variable,
fn is the Fourier amplitude over a specified azimuthal segment of blade rotation, and
n
1
and n
2
are frequency limits based on n per full revolution.
The value of n
1
and n
2
are initially determined by wind tunnel measurements for a particular set of test conditions simulating descent and are subsequently correlated with BVI far field acoustic sound pressure levels for the same set of test conditions. Values of n
1
and n
2
being substantially
24
and
32
respectively have been determined for one rotorcraft s
Cummings Ronald G.
Lefort Brian D.
Look Edward K.
McDowell Liam
Schneeberger Stephen A.
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