CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to provisional application no.
61/605,041, filed February 29, 2012, the disclosure of which is hereby incorporated
by reference in its entirety.
BACKGROUND
The field of the present disclosure relates generally to rotary
machines, and more particularly to airfoils used with rotary machines.
At least some known rotary machines such as, gas turbine
engines used for aircraft propulsion, include a plurality of rotating blades that channel
air downstream. Each blade has a cross-sectional shape that defines an airfoil section.
Conventional single rotation turboprop engines provide high efficiency at low cruise
speeds (flight Mach number up to about 0.7), although some single rotation turboprop
engines have been considered for higher cruise speeds. Higher cruise speeds (Mach
0.7 to 0.9) are typically achieved using a ducted turbofan engine to produce the
relatively high thrust required.
Unducted, counter-rotating propeller engines, frequently
referred to as the unducted fan (UDF®), or open-rotor, have been developed to deliver
the high thrust required for high cruise speeds with higher efficiency than ducted
turbofans. Counter-rotating propellers for high cruise speed efficiency have strong
acoustic interactions (i.e., noise generation) at low flight speed, such as takeoff,
typically at flight Mach number of 0.3 or less. Counter-rotating propellers designed
for quiet operation at low flight speed tend to be inefficient at high cruise speeds.
Thus, a need exists for both single rotation and counter-rotating propellers that have
both good efficiency at high flight speed and low noise at low flight speed.
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To operate at a wide range of operating conditions, propeller
blades are typically attached to rotating hubs such that each blade setting angle, or
pitch, can be adjusted during flight. Although this adjustment of blade pitch angle
affects performance, because the blades are essentially rigid, the airfoil sections that
comprise a blade are shaped in a specific way to improve both efficiency at high
speed flight and reduce noise at low speed flight. Thus, a need exists for propellers
that have both high efficiency and low noise at high speed.
BRIEF DESCRIPTION
In one aspect, an airfoil section of a propeller for a propulsion
device includes a pressure surface and a suction surface, the pressure surface and
suction surface intersecting at a leading edge and a trailing edge. The airfoil section
has a meanline defined midway between the pressure surface and the suction surface
and a meanline angle is defined as an angle between a tangent to the meanline and a
centerline of the propeller. The blade has a meanline curvature defined as the slope of
a meanline angle with respect to chord fraction along the meanline, and at least a
portion of the meanline has meanline curvature that increases from between
approximately 0.1 chord fraction progressing toward the leading edge and at least
another portion of the meanline has meanline curvature decreases from between
approximately 0.1 chord fraction progressing toward the leading edge.
In another aspect, an airfoil section for a propeller for a
propulsion device includes a pressure surface and a suction surface, the pressure
surface and suction surface intersecting at a leading edge and a trailing edge. The
airfoil section has a meanline defined midway between the pressure surface and the
suction surface and a meanline angle is defined as an angle between a tangent to the
meanline and a centerline of the propeller. The airfoil section has a meanline
curvature defined as a slope of the meanline angle with respect to chord fraction along
the meanline, and a thickness of the airfoil section is defined as a distance measured
normal to the meanline between the pressure surface and the suction surface, and
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wherein the airfoil has a maximum thickness located between about 0.15 and about
0.25 chord fraction.
In yet another aspect, an open rotor propulsion device
includes a plurality of propeller blades, each of the propeller blades having at least
one airfoil section comprising a pressure surface and a suction surface. The pressure
surface and suction surface intersect at a leading edge and a trailing edge. The at least
one airfoil section has a meanline defined midway between the pressure surface and
the suction surface. A meanline angle is defined as an angle between a tangent to the
meanline and a centerline of the propeller blade, and the meanline has a meanline
curvature defined as the slope of a meanline angle with respect to chord fraction along
the meanline. The at least one airfoil section meets at least one of conditions (A) and
(B), wherein: (A) is at least a portion of the meanline has meanline curvature that
increases from between approximately 0.1 chord fraction progressing toward the
leading edge and at least another portion of the meanline has meanline curvature that
decreases from between approximately 0.1 chord fraction progressing toward the
leading edge; and (B) is a thickness of the airfoil is defined as a distance measured
normal to the meanline between the pressure surface and the suction surface, and
wherein the airfoil has a maximum thickness ratio located between about 0.15 to
about 0.25 chord fraction, and the thickness ratio is 0.8 or greater at approximately 0.1
chord fraction.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an illustration of an aircraft including an exemplary
propulsion device.
Fig. 2 is a side view of the exemplary propulsion device
shown in Fig. 1.
Fig. 3 shows a profile of an exemplary airfoil section of a
rotor blade of the propulsion device shown in Fig. 2.
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Fig. 4 is a plot of meanline angle as a function of a fraction of
chord length of a conventional rotor blade airfoil section and the rotor blade airfoil
section of Fig. 3.
Fig. 5 is a plot of a thickness distribution comparison of a
conventional rotor blade compared to the exemplary rotor blade of Fig. 3.
Fig. 6 is a plot of a meanline curvature comparison for a
conventional rotor blade and the exemplary rotor blade of Fig. 3.
DETAILED DESCRIPTION
Fig. 1 illustrates an exemplary aircraft 100 including a pair of
wings 102 and 104. Each wing 102 and 104 supports a rotary propulsion device 106
via a support 108. In other embodiments, one or more rotary propulsion devices 106
may be mounted to any suitable location on aircraft 100. In another embodiment,
propulsion device 106 is a counter-rotating propeller engine 110.
Fig. 2 illustrates a side view of counter-rotating propeller
engine 110. Counter-rotating propeller engine 110 has a longitudinal centerline 112.
In the exemplary embodiment, an engine cowling 114 is disposed co-axially with
centerline 112. Counter-rotating propeller engine 110 includes a core including a
compressor, a combustor and a turbine, which tnay be a multi-stage turbine.
In the exemplary embodiment, counter-rotating propeller
engine 110 includes an engine cowling 114 which houses a power generating rotary
machine (not shown). The rotary machine is coupled to a first set of rotor blades 116
and a second set of rotor blades 118. In operation, first set of rotor blades 116 and
second set of rotor blades 118 are in counter-rotation. First set of rotor blades 116
rotates about hub 120 and second set of rotor blades rotates about a second hub 122,
which are arranged co-axially with centerline 112. Each of first set of rotor blades 116
and second set of rotor blades 118 include a plurality of circumferentially spaced rotor
blades 124, 126.
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For a rotating propeller blade, a surface of the blade on an
advancing side thereof, due to rotation, is referred to as the pressure surface. A
surface on the retreating side of the blade, due to rotation, is called a suction surface.
The leading edge of a propeller blade is used herein to refer to a three-dimensional
curve at which the suction surface and pressure surface meet on an upstream edge of
the blade, based on the flight direction. A trailing edge refers to an intersection of the
same suction surface and pressure surface on the downstream edge of the blade. The
mean surface is used herein to refer to the imaginary surface connecting the leading
edge to trailing edge, which lies midway between the pressure surface and suction
surface.
Fig. 3 shows an airfoil cross section of rotor blade 124 (rotor
blade 126 may be similarly shaped) between a blade attachment point to hub 116
(shown in Fig. 1) and a tip of rotor blade 124 viewed radially downward toward
centerline 112. Rotation direction of blade 124 is indicated as a directional arrow in
Fig. 3. In Fig. 3, the blade surfaces appear as curves and the edges appear as points.
In the exemplary embodiment, blade 124 includes a pressure surface 134, a suction
surface 132, a leading edge 131, and a trailing edge 133 (although Fig. 3 is a 2-
dimensional illustration of blade 124, similar conventions are used for the threedimensional
blade). Meanline 130, which may also be referred to as the camber line,
is a two-dimensional view of the mean surface of blade 124.
In the exemplary embodiment, an airfoil section of blade 124
has a meanline angle 139, which refers to the angle between the tangent to meanline
130 and centerline 112. Meanline angle 139 can be measured at any location along
meanline 130, and is illustrated in Fig. 3 at approximately midway between leading
edge 131 and trailing edge 133. Thickness 136 is a distance measured normal to the
meanline between the pressure surface 134 and suction surface 132, which can be
measured at any location along meanline. Thickness 136 is illustrated in Fig. 3 as the
distance between two opposing arrows at a location approximately midway between
leading edge 131 and trailing edge 133. Chord is defined as a straight line distance
between leading edge 131 and trailing edge 133. A location along meanline 130 of
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either meanline angle 139 or thickness 136 may be approximated by a chord fraction.
As used herein, chord fraction refers to a distance of the location from leading edge
131 to a point of interest divided by chord. A maximum thickness 13 7 of the airfoil
section of blade 124 is represented by the diameter of an inscribed circle between
pressure surface 134 and suction surface 132. In one embodiment, maximum
thickness location 137 is located at approximately 0.2 chord fraction (i.e., 20 percent
of the total distance from leading edge 131 to trailing edge 13 3.
As used herein, camber is defined as a change in meanline
angle 139 between any two points along meanline 130. Curvature ofmeanline 130 is
calculated as the derivative, or slope, of meanline angle 139 with respect to chord
fraction along meanline 130. Typically, and as used herein, for a propeller airfoil
section in which the meanline angle generally decreases from leading edge to trailing
edge, camber is expressed as the meanline angle change from one specified point
along the meanline to another specified point closer to the leading edge (i.e., positive
camber is where the meanline angle increases progressing toward the leading edge).
Similarly, curvature is considered positive for an increasing meanline angle in a
direction toward the leading edge, although the slope of the meanline angle
distribution is mathematically negative for positive curvature.
Fig. 4 is a graph 140 illustrating meanline angle of two airfoil
sections across their respective chord fractions. Graph 140 includes a horizontal axis
142 graduated in units of chord fraction and a vertical axis 144 graduated in degrees.
A trace of meanline angle distribution for a conventional low noise airfoil section is
indicated as line 146, and a trace of meanline angle distribution for a low noise and
high speed efficiency airfoil section (e.g., such as within blade 124 or 126) is
indicated as line 148. The conventional low noise airfoil section's meanline angle
distribution 146 has an angle increase (i.e. camber) from 0.5 chord fraction to the
leading edge that is several degrees more than for a conventional design for high
speed efficiency (not shown). The higher camber of conventional low noise airfoil
146, relative to a conventional design for high speed efficiency, tailors the suction
surface of the airfoil to reduce flow separation near the leading edge, which would
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otherwise produce acoustic interactions (i.e., noise generation) with downstream
counter-rotating blades or other structures. As shown in Fig. 4, the conventional low
noise airfoil's meanline angle distribution 146 is substantially smooth and
monotonically increases from about 0.1 chord fraction progressing toward the leading
edge. It is noted that the leading edge is represented as 0.0 chord fraction. At high
flight speed, conventiona1low noise airfoil's meanline angle distribution 146 results in
flow losses (i.e., an efficiency penalty) near the leading edge thereof due to separated
airflow on the pressure surface thereof. In the exemplary embodiment, in region 14 7
progressing from approximately 0.1 chord fraction toward the leading edge (i.e.,
chord fraction 0,0), the meanline angle of the low noise and high speed efficiency
airfoil 148 initially increases compared to the conventional low noise airfoil's
meanline angle distribution 146. However, continuing toward leading edge and over
a short distance of approximately 0.05 chord fraction, the increase in meanline angle
148 is less than the increase in meanline angle 146
In one embodiment, the above described region 14 7, having
an increase followed by a decrease in slope of meanline angle distribution 148 as
compared to meanline angle distribution 146, is accompanied by a modification to the
thickness distribution along meanline 130 of an airfoil section within blade 124 that
shifts maximum thickness location 13 7 forward (toward leading edge 131) from
approximately 0.4 chord fraction to approximately 0.2 chord fraction. In the
exemplary embodiment, additional thickness is also added to an airfoil section within
blade 124 from approximately 0.0 to approximately 0.15 chord fraction, so that
suction surface 132 coincides closely to a suction surface of a conventional low noise
airfoil section and the thickness ratio greater than 0.8 at 0.1 chord fraction. The
resulting pressure surface 134 is thus farther from suction surface 132 than for a
conventional low noise airfoil section, thereby increasing a radius of curvature for the
airflow around pressure surface 134 near leading edge 131, as compared to a
conventional airfoil section, to reduce airflow separation and loss of efficiency in high
speed flight.
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Fig. 5 is a graph 150 illustrating thickness ratios of two airfoil
sections across their respective chord fractions. Graph 150 includes a horizontal axis
152 graduated in units of chord fraction and a vertical axis 154 expressed as thickness
ratio (i.e., airfoil section thickness at a point of interest divided by its maximum
thickness). The conventional low noise airfoil section thickness ratio 156 (as well as
for an airfoil section designed solely for high speed efficiency) peaks (i.e., is
maximum) at approximately 0.4 chord fraction. In contrast, for the airfoil section of
the exemplary embodiment designed for low noise and high efficiency (i.e., within
blade 124 or blade 126) thickness ratio 158 is substantially increased in between the
0.0 to 0.20 chord fraction range, as compared to the conventional airfoil section. In
one embodiment, a peak thickness 159 of the airfoil section of blade 124 is at
approximately 0.20 chord fraction.
Fig. 6 is a graph illustrating meanline curvature of two airfoil
sections across their respective chord fractions. Graph 160 includes a horizontal axis
162 graduated in units of chord fraction and a vert4cal axis 164 expressed as camber
per unit chord. A trace of curvature distribution 166 for a conventional low noise
airfoil section is plotted alongside a trace of curvature distribution 168 of an
exemplary low noise and high speed efficiency airfoil section (i.e., within blade 124
or blade 126) on graph 160. For conventional low noise curvature distribution 166,
the curvature increases or remains substantially constant from about 0.1 chord fraction
to the leading edge. For the exemplary low noise and high speed efficiency airfoil
section, the curvature distribution 168 increases then sharply decreases from about 0.1
chord fraction to the leading edge.
In one embodiment, the oscillation in curvature (i.e., the
curvature distribution 168 increases then sharply decreases from about 0.1 chord
fraction to the leading edge) occurs at least once between 0.1 and about 0.0 chord
fraction of blade 124 and is accompanied by thickness distribution that maintains
suction surface 132 to be suitable for a low noise airfoil. In one embodiment, the
curvature increase and decrease are each about 10 degrees per unit chord in
magnitude or greater, and each occurs over less than approximately 0.05 chord
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fraction. However, other curvature and thickness distributions along the meanline
may be used within the scope of the present disclosure.
This written description uses examples to disclose the
invention, including the best mode, and also to enable any person skilled in the art to
practice the invention, including making and using any devices or systems and
performing any incorporated methods. The patentable scope of the invention is
defined by the claims, and may include other examples that occur to those skilled in
the art. Such other examples are intended to be within the scope of the claims if they
have structural elements that do not differ from the literal language of the claims, or if
they include equivalent structural elements with insubstantial differences from the
literal languages of the claims.

WE CLAIM ;
1. An airfoil section of a propeller (124, 126) for a propulsion device
(106), comprising:
a pressure surface (134) and a suction surface (132), the pressure
surface and suction surface intersecting at a leading edge (131) and a trailing edge
(133);
wherein the airfoil section has a meanline (130) defined midway
between the pressure surface and the suction surface; a meanline angle (139) is
defined as an angle between a tangent to the meanline and a centerline of the propeller
^ ^ (124, 126); the blade has a meanline curvature defined as the slope of a meanline
angle with respect to chord fi-action along the meanline, and at least a portion of the
meanline has meanline curvature that increases fi-om between approximately 0.1
chord fi-action progressing toward the leading edge and at least another portion of the
meanline has meanline curvature that decreases fi-om between approximately 0.1
chord fraction progressing toward the leading edge.
2. The airfoil section according to claim 1, wherein the increase in
meanline curvature occurs over an extent of less than about 0.05 chord fraction and
the decrease in meanline curvature occurs over an extent of less than about 0.05 chord
fraction.
3. The airfoil section according to claim 2, wherein a magnitude of the
increase in meanline curvature and decrease in meanline curvature are 10 degrees per
^ ^ unit chord or greater.
4. The airfoil section according to claim 1, wherein a thickness (136)
of the airfoil section is defined as a distance measured normal to the meanline (130)
between the pressure surface (134) and the suction surface (132), and wherein the
thickness is a maximum between about 0.15 and about 0.25 chord fraction.
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5. The airfoil section according to claim 1, wherein a thickness (136)
of the airfoil section is defined as a distance measured normal to the meanline (130)
between the pressure surface (134) and the suction surface (132), a thickness ratio
(156) is defined as the thickness divided by a maximum thickness (137) of the airfoil
section, and the thickness ratio is 0.8 or greater at 0.1 chord fraction.
6. The airfoil section according to claim 1, wherein the airfoil section
is configured to operate at a flight speed of between about Mach 0.7 to about 0.9.
7. An airfoil section for a propeller (124, 126) for a propulsion device
(106), comprising:
^ ^ a pressure surface (134) and a suction surface (132), the pressure
surface and suction surface intersecting at a leading edge (131) and a trailing edge
(133);
wherein the airfoil section has a meanline (130) defined midway
between the pressure surface and the suction surface; a meanline angle (139) is
defined as an angle between a tangent to the meanline and a centerline of the propeller
(124, 126), the airfoil section has a meanline curvature defined as a slope of the
meanline angle with respect to chord fi-action along the meanline, and
wherein a thickness (136) of the airfoil section is defined as a distance
measured normal to the meanline between the pressure surface and the suction
surface, and wherein the airfoil has a maximum thickness (137) located between
about 0.15 and about 0.25 chord fraction.
8. An open rotor propulsion device (106), comprising:
a plurality of propeller blades (124, 126), each of the propeller blades
having at least one airfoil section comprising a pressure surface (134) and a suction
surface (132), the pressure surface and suction surface intersecting at a leading edge
(131) and a trailing edge (133); wherein the at least one airfoil section has a meanline
(130) defined midway between the pressure surface and the suction surface; a
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meanline angle is defined as an angle between a tangent to the meanline and a
centerline of the propeller blade; the meanline has a meanline curvature defined as the
slope of a meanline angle with respect to chord fraction along the meanline,
wherein the at least one airfoil section meets at least one of conditions
(A) and (B), wherein:
(A) at least a portion of the meanline has meanline curvature that
increases from between approximately 0.1 chord fraction progressing toward the
leading edge and at least another portion of the meanline has meanline curvature that
decreases from between approximately 0.1 chord fraction progressing toward the
leading edge; and
(B) a thickness of the airfoil is defined as a distance measured normal
to the meanline between the pressure surface and the suction surface, and wherein the
airfoil has a maximum thickness ratio located between about 0.15 to about 0.25 chord
fraction, and the thickness ratio is 0.8 or greater at approximately 0.1 chord fraction.
9. The propulsion device (106) according to claim 8, wherein all of the
propeller blades (124, 126) comprise an airfoil section that meets condition (A).
10. The propulsion device (106) according to claim 8, wherein all of
the propeller blades (124, 126) comprise an airfoil section that meets condition (B).