System and method for suppressing noise produced by rotors

Disclosed is a system and method for suppressing rotor noise by distributing mass and momentum sources and sinks on the rotor blade. A source is located on the blade, and therefore has the directivity of moving, as opposed to a stationary source. Moreover, the motion of the blade with respect to the observer amplifies the sound from the source in a manner similar to the manner in which motion of the blade amplifies all other sources of noise associated with the rotating blade. Two sources can be used to cancel noise. The first source, created by ejecting air from or drawing air into the blade, is used to cancel the "load" portion of rotor noise. The second source, created by developing a radial force with a proplet, is used to cancel the "thickness" portion of rotor noise. More control can be achieved by time modulating the rate of suction or the amplitude of radial force developed by said proplet.

BACKGROUND OF THE INVENTION 
The present invention relates to a system and method for suppressing noise 
and more particularly to a system and method for suppressing noise 
produced by high-speed rotors of aircraft through the distribution of 
sources on the rotor blade and/or by developing a radial force. 
Noise generated by aircraft is a problem for both the passengers in the 
aircraft as well as the community over which the aircraft flies. One 
source of noise which is often dominant is the noise due to rotors. In a 
helicopter, there are two such rotors: the main rotor and the tail rotor. 
In transport aircraft, the source of noise is the propeller. 
The noise produced by rotors comes about by virtue of the movement of the 
blade through the air. Traditionally, the noise sources have been 
subdivided into three categories: loading, thickness, and quadrupole 
noise. Loading noise results from the lift and drag forces the rotor 
exerts on the air. If the forces on the air vary with time, then the noise 
is called unsteady loading noise. Thickness noise is produced by 
displacement of the air due to the finite volume of the rotor blade. 
Quadrupole noise is due to strong gradients in the fluid near the rotor 
blade surface. Quadrupole noise as used herein is a generic term which 
includes the noise from shocks, the Reynolds stresses, the boundary layer, 
and the trailing vortex sheet. For sound to be radiated from a rotor, it 
is not necessary for the magnitude of these sources to vary with time (in 
blade-fixed coordinates). The noise itself results from the rapidly 
changing position of the blade (volume, forces and quadrupoles) with 
respect to the observer. This effect determines both the time-dependence 
and amplitude of the noise generated. For subsonic motion, both the 
amplitude and directionality are proportional to (1-M.sub.r).sup.-n, where 
M.sub.r is the Mach-number at which the source approaches the observer and 
n is a positive number. The factor n influences the amplitude at which the 
blade radiates sound due to its motion. 
Several techniques have been used to reduce rotor noise. Most of these 
techniques have concentrated on changing the source characteristics of the 
noise. One such method discovered by H. Hubbard (see H. H. Hubbard, 
"Propeller-noise Charts for Transport Airplanes," N.A.C.A. TN 2968 (1953), 
and H. H. Hubbard and L. N. Lassiter, "Sound from a Two Blade Propeller at 
Supersonic Tip Speeds," N.A.C.A. Rep., 1079 (1952)), involves the 
reduction of the tip speed. The amplitude of radiated noise becomes very 
large for points on the blade, where M.sub.r.sup..about. 1, and Hubbard's 
method reduces noise by reducing the maximum value that M.sub.r can 
attain. M.sub.r is the velocity of the blade divided by the speed of sound 
in the direction from a point on the blade to the observer. The maximum 
velocity on the blade is at the tip. Thus, by reducing tip velocity, the 
maximum possible value of M.sub.r is reduced. 
Another method for reducing rotor noise which was first proposed by D. 
Bliss (see U.S. Pat. No. 3,989,406, Bliss, D. B., "Method of and Apparatus 
for Preventing Leading Edge Shocks and Shock-Related Noise in Transonic 
and Supersonic Blades and the Like") involves the sweeping of the blades. 
Normally, rotor blades are made by stacking the aeordynamic centers of the 
blade sections along a straight radial line. To sweep a blade means to 
stack the blade sections along a curved arc. This arc can be located in a 
plane of rotation called in-plane sweep, or on a helical surface. 
Initially, this method was used with ducted fan blades, but it has been 
utilized with unducted rotors as well. The sweep of the blades reduces the 
shock formed on the blade, and thereby reduces the source strength. At a 
later date, Hanson discovered that unducted swept blades show reduced 
noise by virtue of another effect. First, approximately all points on a 
straight (radial) blade attain their maximum value of M.sub.r at the same 
retarded time. The term "retarded time" describes the time at which the 
blade emitted the sound. If the blade is swept, the points attain their 
maximum value of M.sub.r at different retarded times. Hanson (see D. B. 
Hanson," The Influence of Propeller Design Parameters on Far Field 
Harmonic Noise in Forward Flight", 1979 American Institute of Aeronautics 
and Astronautics Paper 79-0609 and D. B. Hanson, "Study of Subsonic Fan 
Noise Sources", 1975 American Institute of Aeronautics and Astronautics 
Paper 75-468 recognized that sweeping the blade is equivalent to shifting 
the phase of the sound wave emitted by each such point on the blade. 
Another method for reducing noise, proposed by Succi, (see "Design of Quiet 
Efficient Propellers", SAE paper 790584, Business Aircraft Meeting, April, 
1979) involves the changing of the number of blades. At any instant, there 
is only one blade that approaches the observer at its fastest rate, and it 
is this blade that is the greatest noise source. By reducing the strength 
of the source, by distributing the loading and thickness over a greater 
number of blades, noise is reduced. 
"Tip mass injection" has been proposed by R. White (see Pegg, R. J., 
"Insights into the Nature and Control of Rotor Noise," NASA Conference on 
Aircraft Safety and Operational Problems, NASA SP416) for reducing 
helicopter rotor noise. This noise is related to the interaction of the 
main rotor blade with its wake, and is known as blade/vortex interaction 
noise. White proposed injecting air into the tip vortex to change the 
character of the tip vortex and thereby change the blade/vortex 
interaction noise. 
Despite all of these attempted solutions, no known system satisfactorily 
reduces helicopter rotor noise. 
It is therefore a principal object of the present invention to provide a 
system and method for suppressing aircraft rotor noise more effectively 
than known systems and methods. 
Still another object of the present invention is to provide a system and 
method for suppressing aircraft rotor noise by separately cancelling rotor 
loading and thickness noise by using mass suction and/or radial forces. 
SUMMARY OF THE INVENTION 
Accordingly, the system and method of the present invention suppresses 
rotor noise by distributing mass sources on the rotor blade. These sources 
are two distinctly different types. The first source is a fluid mass 
source or sink. Such a source is implemented by exhausting (or drawing) 
air from a port on the rotor. In the simplest implementation, the port is 
located at the tip of the blade. More generally, the sources can be 
distributed at different points on the blade. The second source is a 
radial force. Such a source is implemented by placing a blade element, 
called a proplet, on the rotor blade so as to develop a force in the 
radial direction. In the simplest implementation, it can be done by having 
the blade element perpendicular to the rotor blade at the tips. More 
generally, such blade elements can be distributed at different points on 
the blade and at different angles of inclination. It is the combination of 
these two sources that provides significant noise reduction. The mass 
injection source is used to cancel loading noise; the radial force is used 
to cancel thickness noise. 
In the described embodiments the source is located on the blade and 
therefore has the directivity of a moving, as opposed to a stationary, 
source. Moreover, the motion of the blade with respect to the observer 
amplifies the sound from the source in a manner similar to the manner in 
which motion of the blade amplifies all other sources of noise associated 
with the rotating blade. The source can be created by ejecting air from or 
drawing air into the blade. In the simplest form of the invention, the 
mass is introduced or removed at a constant rate. More control can be 
achieved, if needed, by modulating the rate of injection. 
These and other features and objects of the present invention will be more 
fully understood from the following detailed description which should be 
read in light of the accompanying drawings in which corresponding 
reference numerals refer to corresponding parts throughout the several 
views.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The system and method of the present invention operates on the principle of 
cancelling sound by sound. The noise radiated by the rotating blade is 
cancelled by the addition of the noise radiated by rotating mass 
injection/suction devices or by a radial force. To simplify the 
terminology, the term "mass injection" will be used herein to cover both 
the cases of ejecting air from the blade or drawing air into the blade. 
To accomplish the noise reduction, sources are located on a rotating blade 
and thus have a directivity and amplitude similar to all other blade 
sources. This similarity is due to the factor (1-M.sub.r).sup.-n which 
appears in the expression for noise radiated by the mass source. To 
clarify this statement, the expression for pressure P(x,t) detected by an 
observer at position x and time t is as follows: 
##EQU1## 
Equation (1) states that the pressure p is due to the sum of all loading 
(P.sub.1) and thickness (P.sub.t) sources on the blade. In this 
expression, we assume that the blade is subdivided into many small pieces 
and that each piece is treated as a point noise source due to the forces 
(L) and volume (.psi.) occupied by the subdivision. For all points, except 
for M.sub.r .about.1, one can use equations 1 to 4 for the sound radiated 
by such a point. Here r represents the distance between the source and 
observer, t the time observer detects the sound, and .tau. the time the 
source emits the sound. Successive differentiation with respect to time 
generates the factors (1-M.sub.r).sup.-n which determine the directivity. 
Note that M.sub.r depends on both the motion of the source and the 
position of the observer. Consider, for example a hovering rotor. If the 
observer is on the axis of rotation, then M.sub.r =0 because the blades 
are neither approaching nor receding from the observer. If the observer is 
in the disk plane, then Mr varies periodically in time as the blade 
approaches and recedes from the observer. 
The use of mass injection will be discussed first. The principle of mass 
injection is to create a new source on the blade. Because the source is on 
the blade, it has a directivity pattern and amplitude similar to other 
moving points on the blade. The sound associated with mass injection or 
suction can be described as a variable or suction thickness source. The 
"load" comes from the transfer of momentum to the surrounding air. The 
"thickness" comes from the new mass added or subtracted to the surrounding 
air. Inspection of Equation (3b) shows that "thickness" noise is 
proportional to 
##EQU2## 
The term proportional to the volume (.psi.) is associated with rotor 
thickness noise. The next two terms are due to mass injection (d.psi./dt) 
and the time rate of change of mass injection (d.sup.2 .psi./ dt.sup.2). 
One embodiment of the present invention utilizing mass injection to cancel 
the sound emitted by a blade includes a rotor blade 14 having a channel 16 
completely housed within the rotor blade for drawing or pumping air 
through the blade 14. The channel 16 is terminated on the blade surface by 
a port 18 through which air is pumped or drawn. The channel 16 is 
terminated on its other end at the blade/hub intersection 20, where it is 
connected to a pump (not shown). In the simplest implementation shown in 
FIG. 1a, a single channel is used for each blade with an exit hole near 
the blade tip. In an alternate embodiment shown in FIG. 1b the rotor blade 
includes many channels connecting air ports on different portions of the 
blade. Alternatively, the rate at which air is pumped or drawn through the 
blade by modulation at the pump or at each port may be changed by valves. 
In certain rotor applications, such as the main rotor on a helicopter, the 
channel through which air is drawn can be connected to a companion channel 
through which air is pumped by pump 22 (FIG. 1c). By connecting channels 
in pairs, it is possible to minimize the energy expended in drawing air 
into the blade against the pressure gradient set up by the centrifugal 
forces. 
An alternate embodiment of the present invention shown in FIG. 2 utilizes 
proplets for creating a radial force and includes a rotor blade 14, having 
a proplet 23 located at the tip of the blade. The proplet is constructed 
so that the center line of the proplet is at an angle to the center line 
of the blade. For purposes of this description, the center line is defined 
as the line joining the aerodynamic centers of blade sections. Note that 
the proplet center line is at an angle .theta. to the blade center line. 
This angle is normally set at 90.degree.. In the alternate embodiment 
shown in FIG. 2b, this angle is set at -90.degree.. An array of proplets 
25 can also be used to distribute the radial loads as shown in FIG. 2c. 
A preferred embodiment of the invention is illustrated in FIG. 3. In this 
embodiment, both an orifice 18 for mass suction and a proplet 23 for 
radial force are located on the blade 14. The mass suction source is used 
principally to cancel rotor loading noise due to the drag dipole. The 
radial force developed by the proplet 23 is used to cancel the thickness 
noise of the rotor. 
In order to illustrate the mass injection/suction, the "thickness" effects 
of such mass injection will now be described. The "thickness"-like aspect 
can be used to cancel the "drag dipole" of a portion of the loading noise 
of a rotor. The radial load due to a proplet must be used to cancel the 
thickness noise. This load will be caused by injecting momentum in the 
radial direction so as not to alter the "drag" or "thrust" components of 
the rotor. 
Referring to FIG. 4, the following example illustrates the cancellation 
effect of the on-blade mass suction. The parameters for this example are 
as follows: the sound is radiated a single point force rotating at 2400 
RPM; the source is 1.082 meters from the center of rotation and has a 
strength of 1000 Newtons directed opposite from its angular velocity; the 
observer is located in the disk plane 5 meters from the center and there 
is no forward motion. One can cancel the sound from this "drag dipole" by 
drawing air into the blade at a rate of 0.312 kg/sec. The sound from the 
"drag dipole" is almost completely cancelled. (Recall, that in the disk 
plane, the drag dipole dominates the loading noise.) 
Referring to the example of FIG. 5, the observer is now 30.degree. below 
the disk plane with the source field remaining the same. Both the force 
term and the mass injection term are reduced in amplitude because the term 
M.sub.r is reduced by the same amount for each source. The details of the 
directivity pattern will differ because the description of sound radiation 
by a moving force is governed by a different equation than sound radiation 
from a mass sink. Nonetheless, the leading term is due to the factor 
(1-M.sub.r).sup.-n, which is the same for both sources. Thus, because the 
source used to cancel the rotor noise moves at the same rate as the rotor 
noise source, the directivity patterns are substantially the same. 
The "load" portion due to proplets will now be considered by illustrating 
the effect of the momentum transfer in the radial direction. In other 
words, rotor noise may be suppressed by distributing a radial force along 
the blade. Again, the source is located on the blade, and therefore, its 
directivity is dominated by the motion of the blade. In the simplest 
method, the radial force is constant, but dynamic (time-varying) control 
can be achieved by altering the radial force. 
In the example shown in FIG. 6, the sound is radiated by a simple point 
volume rotating at 2400 RPM, with the source located 1.052 meters from the 
center of rotation and having a volume of 0.001 cubic meters. The observer 
is located 5 meters from the center. To cancel the sound from this volume, 
one would create a radial force (directed outwardly) of strength 1579N. 
The negative pressure peak of this thickness noise is almost completely 
canceled. In FIG. 7, the same calculation is performed with the observer 
located 30.degree. below the disk plane. Again, each source is reduced in 
such a way so as to maintain sound cancellation. Note that the proplets 24 
are used to cancel thickness noise, and suction is used to cancel loading 
noise. 
While the foregoing invention has been described with reference to its 
preferred embodiments, various alterations and modifications will occur to 
those skilled in the art. All such variations and alterations are intended 
to fall within the scope of the appended claims.