Aeronautics and astronautics – Miscellaneous
Reexamination Certificate
1999-09-10
2001-09-04
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Miscellaneous
C244S017110, C244S017130, C244S199100, C244S207000
Reexamination Certificate
active
06283406
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the reduction of noise caused by the movement of aircraft rotor blades such as those found in helicopters. More particularly, the present invention is concerned with reducing high speed impulsive noise in aircraft rotor blades as well as the additional problem of noise caused by blade vortex interaction. It is desirable to reduce noise of these types from the human factors standpoint of reducing environmental noise pollution and possible damage to hearing. In addition, in surveillance operations or military combat situations, such noise impairs the ability to avoid detection by criminal suspects or hostile forces.
The primary sources of helicopter blade tonal noise are thickness noise, loading noise, blade vortex interaction noise (BVI) and high-speed impulsive noise (HSI). In addition to these tonal sources there are a wide variety of broad band noise sources, all of which are produced by random pressure fluctuations on the rotor blade surfaces. In hover, thickness noise and loading noise are the predominant sources. Thickness noise comes about because the blade displaces air as it rotates. The steady lift and drag forces on the blade generate loading noise. Broad band noise is generally less intense than tonal noise, and, of the tonal noise sources, BVI and HSI noise are the most intense and annoying when they occur.
HSI noise is related to thickness noise. HSI noise occurs when the forward speed of the blade tip relative to the air approaches sonic velocity. This typically only occurs on the side of the helicopter where the blade is moving in the same direction as the helicopter. As the air accelerates to move around the finite thickness of the blade shocks form that lead to a dramatic increase in impulsive noise. The result can be considered as two steady volume sources on the blade, an outward flowing source on the front half and an inward flowing source on the rear half of the blade. The fact that the steady sources are in motion produces sound. In forward flight the advancing blade may have regions where the flow is locally supersonic because of the finite thickness of the blade, even though the sum of the blade's forward velocity and the helicopter flight speed may be subsonic. These local regions of supersonic flow can lead to significant intensification of the sound that is typically referred to as HSI noise which occurs at high forward speed when the blade tip mach number on the advancing side approaches 0.85. The intensification of the noise appears to be associated with the appearance of shock near the blade tip where the flow becomes locally sonic. As already noted, HSI noise has many of the characteristics of thickness noise in that it radiates strongly in the plane of the rotor. However, HSI noise is considerably more intense than thickness noise and the resulting pressure pulses are more asymmetric in nature due to nonlinear effects.
The approach to reducing high speed impulsive noise in rotor blades has been to employ thin, tapered, or swept end blades. While these modifications can reduce HSI noise, they can have deleterious effects such as stall at lower angles of attack and unfavorable pitching—moment characteristics.
BVI noise occurs when a blade encounters the trailing edge vortex of a preceding blade, typically during low speed forward descending flight. The variations in the flow velocity in the vortex on interacting with the blade cause pressure fluctuations that radiate and create intense impulsive noise. The intensity of the noise depends on the strength of the vortex and the closeness of its approach to the helicopter blade. Consequently, modifying the velocity distribution in the vortex is a method by which BVI noise can be controlled. One approach for affecting the velocity distribution in the vortex is described in U.S. Pat. No. 5,791,875 to Ngo which describes a system for directing pressurized air out of the rotor blade tip to reduce blade vortex interaction. The Ngo approach, however, does not take into account all of the factors that contribute to the production of BVI noise, nor does it provide for the reduction of HSI noise. Furthermore, the Ngo approach of cancelling the blade tip vortices by injecting air at the tip can be counterproductive in that it can kill the lift of the blade, thus reducing helicopter performance.
Accordingly, there is a need for a system for reducing aircraft rotor noise, and, in particular, for reducing high speed impulsive noise, especially in combination with a system for also reducing blade vortex interaction noise.
SUMMARY OF THE INVENTION
The present invention is directed to a system and method for reducing aircraft rotor noise and especially high speed impulsive blade noise and noise caused by blade vortex interaction that substantially avoids one or more of the problems associated with the prior art.
To achieve the objects and advantages, and in accordance with the purpose of the invention, as embodied and broadly described herein, the present invention, in one of its embodiments, provides a system for reducing high speed impulsive blade noise in aircraft rotors comprising: a plurality of rotor blades extending radially in spaced relationship from a central hub, each of the blades having spaced apart upper and lower surfaces, an interior volume between the surfaces, and leading and trailing edges at the respective joined edges of the upper and lower surfaces. Air intake passages are disposed on the surfaces of each blade proximate the leading edge and the outer edge tip to controllably admit air into the interior volume; and air output passages are disposed on the surfaces of each blade proximate the trailing edge and the outer edge tip to controllably expel air from the interior volume of the blade. Admitting and expelling of air into and out of the interior volume of each blade is controlled in accordance with a determined condition of that blade or the environment associated with the blade.
The two distinct approaches have four distinct components: a flow device at the helicopter blade tip; a helicopter condition-monitoring system for determining when to turn on the flow device; a set of sensors for monitoring the noise reduction performance of the flow device; and a control system for adjusting the operating parameters of the flow device based on the noise reduction performance monitoring sensors.
In one aspect of the invention a condition that controls admitting or expelling of air is the position of the rotating blade, air being admitted and expelled only during essentially forward-rotary movement of the blade relative to the direction of flight of the aircraft.
In another aspect of the invention, admitting or expelling air is controlled in accordance with pressure fluctuations on the rotor blade, rotational speed of the blade, aircraft forward speed relative to the ground, or detected blade noise, as registered by sensors placed primarily on the rotor blades.
In an additional embodiment of the invention, the above-described system for reducing high speed impulsive blade noise is combined with a system for alleviating rotor blade vortex interaction which comprises providing pressurized air output passages at an outboard tip edge of each rotor blade to controllably expel pressurized air from the interior volume. The air output passage utilized and flow rate at which the air is expelled being dictated by the sensed pressure state of the blade.
For each noise mechanism here are two candidates for the noise reduction monitoring sensors: a blade mounted set of pressure sensors, and a fuselage-mounted set of microphones.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one several embodiments of the invention and together with the description, serve to explain the principles of the inventio
Coney William B.
Curtis Alan Robert Douglas
Remington Paul J.
Barefoot Galen L.
GTE Service Corporation
Leonard Charles Suchyta
Weixel James K.
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