BACKGROUND OF THE INVENTION
The present invention relates generally to gas turbine engines, and,
more specifically, to fans and compressors thereof.
A turbofan gas turbine engine includes a fan followed in turn by a
multi-stage axial compressor each including a row of circumferentially
spaced apart rotor blades, typically cooperating with stator vanes. The
blades operate at rotational speeds which can result in subsonic through
supersonic flow of the air, with corresponding shock therefrom. Shock
introduces pressure losses and generates undesirable noise during operation.
In U.S. Patent 5,167,489 - Wadia et al, a forward swept rotor blade
is disclosed for reducing aerodynamic losses during operation including those
due to shock-boundary layer air interaction at blade tips.
However, fan and compressor airfoil design typically requires many
compromises for aerodynamic, mechanical, and aero-mechanical reasons.
An engine operates over various rotational speeds and the airfoils must be
designed for maximizing pumping of the airflow therethrough while also
maximizing compression efficiency. Rotational speed of the airfoils affects
their design and the desirable flow pumping and compression efficiency
thereof.
At high rotational speed, the flow Macbf numbers relative to the
airfoils are at their highest value, and the shock and boundary layer
interaction is the most severe. Mechanical airfoil constraints are also severe
at high rotor speed in which vibration and centrifugal stress have significant
affect. And, aero-mechanical constraints, including flow flutter, must also
be accommodated.
Accordingly, the prior art includes many fan and compressor blade
configurations which vary in aerodynamic sweep, stacking distributions,
twist, chord distributions, and design philosophies which attempt to improve
rotor efficiency. Some designs have good rotor flow capacity or pumping at
maximum speed with corresponding efficiency, and other designs effect
improved part-speed efficiency at cruise operation, for example, with
correspondingly lower flow pumping or capacity at maximum speed.
Accordingly, it is desired to provide an improved fan or compressor
airfoil having both improved efficiency at part-speed, such as cruise
operation, with high flow pumping or capacity at high speed, along with
good operability margins for stall and flutter.
BRIEF SUMMARY OF THE INVENTION
An airfoil includes a leading edge chord barrel between a root and a
tip, and forward aerodynamic sweep at the tip.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, in accordance with preferred and exemplary
embodiments, together with further objects and advantages thereof, is more
particularly described in the following detailed description taken in
conjunction with the accompanying drawings in which:

Figure 1 is an axial, side elevational projection view of a row of fan
blades in accordance with an exemplary embodiment of the present
invention.
Figure 2 is a forward-looking-aft radial view of a portion of the fan
illustrated in Figure 1 and taken along line 2-2.
Figure 3 is a top planiform view of the fan blades illustrated in Figure
2 and taken along line 3-3.
DETAILED DESCRIPTION OF THE INVENTION
Illustrated in Figure 1 is a fan 10 of an exemplary turbofan gas turbine
engine shown in part. The fan 10 is axisymmetrical about an axial centerline
axis 12.
The fan includes a row of circumferentially spaced apart airfoils 14 in
the exemplary form of fan rotor blades as illustrated in Figures 1-3. As
initially shown in Figure 3, each of the airfoils 14 includes a generally
concave, pressure side 16 and a circumferentially opposite, generally
convex, suction side 18 extending longitudinally or radially in span along
transverse or radial sections from a radially inner root 20 to a radially outer
tip 22. '-
As shown in Figure 1, each airfoil 14 extends radially outwardly along
a radial axis 24 along which the varying radial or transverse sections of the
airfoil may be defined. Each airfoil also includes axially or chordally spaced
apart leading and trailing edges 26,28 between which the pressure and
suction sides extend axially.
As shown in Figure 3, each radial or transverse section of the airfoil
has a chord represented by its length C measured between the leading and
trailing edges. The airfoil twists from root to tip for cooperating with the air
30 channeled thereover during operation. The section chords vary in twist
angle A from root to tip in a conventional manner.
As shown in Figures 1 and 3, the section chords of the airfoil increase
in length outboard from the root 20 outwardly toward the tip 22 to barrel
the airfoil above the root. In accordance with a preferred embodiment of the
present invention, the chord barreling is effected along the airfoil leading
edge 26 for extending in axial projection the leading edge upstream or
forward of a straight line extending between the root and tip at the leading
edge.
The airfoil or chord barrel has a maximum extent between the leading
and trailing edges 26,28 in axial or side projection of the pressure and
suction sides, as best illustrated in Figure 1. The maximum barreling occurs
at an intermediate transverse section 32 at a suitable radial position along
the span of the airfoil, which in the exemplary embodiment illustrated is just
below the mid-span or pitch section of the airfoil.
Preferably, the leading edge 26 in the barrel extends axially forward
of the root 20, and the trailing edge 28 is correspondingly barreled and
extends axially aft from the root 20. In this way, the airfoil barreling is
effected along both the leading and trailing edges 26,28 in side projection.
In accordance with another feature of the present invention as
illustrated in Figure 1, the airfoil includes forward, or negative, aerodynamic
sweep at its tip 22, as well as aft, or positive, aerodynamic sweep inboard
therefrom. Aerodynamic sweep is a conventional parameter represented by
a local sweep angle which is a function of the direction of the incoming air
and the orientation of the airfoil surface in both the axial, and circumferential
or tangential directions. The sweep angle is defined in detail in the above
referenced U.S. Patent 5,167,489, and is incorporated herein by reference.
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The aerodynamic sweep angle is represented by the upper case letter S
illustrated in Figure 1, for example, and has a negative value (-) for forward
sweep, and a positive value (+) for aft sweep.
As shown in Figure 1, the airfoil tip 22 preferably has forward sweep
(S) at both the leading and trailing edges at the tip 22.
Both the preferred chord barreling and sweep of the fan airfoils may
be obtained in a conventional manner by radially stacking the individual
transverse sections of the airfoil along a stacking axis which varies
correspondingly from a straight radial axis either axially, circumferentially, or
both, with a corresponding non-linear curvature. Furthermore, the airfoil is
additionally defined by the radial distribution of the chords at each of the
transverse sections including the chord length C and the twist angle A
depicted in Figure 3.
Chord barreling of the airfoil in conjunction with the forward tip
sweep has significant benefits. A major benefit is the increase in effective
area of the leading edge of the airfoil which correspondingly lowers the
average 'eading edge relative Mach number. Furthermore, the compression
process e ffected by the airfoil initiates or begins at a more upstream location
relative to that of an airfoil without leading edge barreling. Accordingly, the
airfoil is effective in increasing its flow capacity at high or maximum speed,
while also improving part speed efficiency and stability margin.
These advantages are particularly important for the airfoil 14 in the
form of the fan rotor blade as it rotates. However, corresponding
advantages may be obtained in fan or compressor stator vanes which do not
rotate. In the blade embodiment illustrated in Figure 1, an integral dovetail
34 conventionally mounts the airfoil to a supporting rotor disk or hub 36,
and discrete platforms 38 are mounted between adjacent airfoils at the
corresponding roots thereof to define the radially inner flowpath boundary
for the air 30. An outer casing 40 surrounds the row of blades and defines
the radially outer flowpath boundary for the air.
For the rotor blade configuration of the airfoil illustrated in Figures
1-3, the section chords C preferably increase in length from the root 20 all
the way to the tip 22, which has a maximum chord length. Barreling of the
airfoil is thusly effected by both the radial chord distribution and the varying
twist angles illustrated in Figure 3 for effecting the preferred axial projection
or side view illustrated in Figure 1.
As shown schematically in Figure 1, the tip forward sweep of the
airfoil is effected preferably at the trailing edge 28, as well as at the leading
edge 26. Forward sweep of the airfoil tip is desired to maintain part speed
compression efficiency and throttle stability margin. Forward sweep of the
trailing edge at the tip is most effective for ensuring that radially outwardly
migrating air will exit the trailing edge before migrating to the airfoil tip and
reduce tip boundary layer air and shock losses therein during operation.
Airflow at the airfoil tips also experiences a lower static pressure rise for a
given rotor average static pressure rise than that found in conventional
blades.
Forward swerp of the airfoil leading edge at the tip is also desirable
for promoting flow stability. And, preferably, the forward sweep at the
trailing edge 28 near the airfoil tip is greater than the forward sweep at the
leading edge 26 near the tip.
The forward sweep at the trailing edge 28 illustrated in Figure 1
preferably decreases from the tip to the root, with a maximum value at the
tip and decreasing in value to the maximum chord barrel at the intermediate
section 32. The trailing edge 28 should include forward sweep as far down
the span toward the root 20 as permitted by mechanical constraint, such as
acceptable centrifugal stress during operation. In the exemplary
embodiment illustrated in Figure 1, the trailing edge 28 includes aft sweep
radially inboard of the maximum barrel which transitions to the forward
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sweep radially outboard therefrom.
Since airfoil barreling is effected in combination with the desired
forward tip sweep of the airfoil, the leading edge 26 illustrated in Figure 1
has forward sweep which transitions from the tip 22 to aft sweep between
the tip and the maximum barrel at the intermediate section 32. The leading
edge aft sweep then transitions to forward sweep inboard of the maximum
barrel at the intermediate section 32. The inboard forward sweep of the
leading edge may continue down to the root 20.
However, in accordance with a preferred embodiment, the leading
edge 26 again transitions from forward to aft sweep outboard of the root 20
and inboard of the maximum barrel at the intermediate section 32. In this
way, the airfoil leading edge combines both chord barreling and forward tip
sweep to significantly improve aerodynamic performance at both part-speed
and full-speed.
Three dimensioial computational analysis has predicted that the
forward swept, barreled airfoil 14 disclosed above has leading edge effective
areas up to about one percent larger than conventional radially stacked fan
blades. This corresponds to a one percent increase in flow capacity at the
same or greater levels of compression efficiency.
Furthermore, part-speed or cruise efficiencies in the order of about
0.8 percent greater than conventional blades may also be achieved. A
significant portion of the part-speed efficiency benefit is attributable to the
forward tip sweep which reduces tip losses, and the aft sweep in the
intermediate span of the blade due to chord barreling which results in lower
shock strength and correspondingly reduced shock losses.
The modification of a fan blade for increasing effective frontal area
? through non-radial stacking of the transverse sections and chord barreling,
along with the local use of forward sweep at the blade tips has advantages
not only for fan blades, but may be applied to transonic fan stator vanes as
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well for improving flow capacity and reducing aerodynamic losses.
While there have been described herein what are considered to be
preferred and exemplary embodiments of the present invention, other
modifications of the invention shall be apparent to those skilled in the art
from the teachings herein, and it is, therefore, desired to be secured in the
appended claims all such modifications as fall within the true spirit and
scope of the invention.
WE CLAIM
1. An airfoil (14) comprising :
pressure and suction sides (16,18) extending in span along transverse
sections from root (20) to tip (22) and in section chords between leading
and trailing edges (26,28) with said chords increasing in length outboard
from said root to barrel said airfoil therefrom; and
said airfoil having forward aerodynamic sweep at said tip and aft
aerofynamic sweep inboard therefrom.
2. An airfoil as claimed in claim 1, wherein said tip forward sweep is effected
at said edge (28).
3. An airfoil as claimed in claim 2, wherein said tip forward sweep is effected
at said leading edge (26).
4. An airfoil as claimed in claim 3, wherein said section chords vary in twist
angle between said root (20) and tip (22), and said barrel has a maximum
extent between said leading and trailing edges (26,28) in axial projection
of said sides (18,20).
5. An airfoil as claimed in claim 4, wherein said leading edge (26) in said
barrel extends axially forward of said root (20), and said trailing edge (28)
in said barrel extends axially aft of said root.
6. An airfoil as claimed in claim 1, wherein said tip forward sweep is effected
at both said leading and trailing edges (26,28).
7. An airfoil as claimed in claim 6, wherein said leading edge (26) in said
barrel extends axially forward of said root (20) and said trailing edge (28)
in said barrel extends axially aft of said root.
8. An airfoil as claimed in claim 7, wherein said forward sweep at said trailing
edge (28) is greater than forward sweep at said leading edge (26).

This invention relates to an airfoil (14) comprising pressure and suction sides
(16,18) extending in span along transverse sections from root (20) to tip (22)
and in section chords between leading and trailing edges (26,28) with said
chords increasing in length outboard from said root to barrel said airfoil
therefrom; and said airfoil having forward aerodynamic sweep at said tip and aft
aerofynamic sweep inboard therefrom.