Apparatus for minimizing inlet airflow turbulence in a gas turbine engine

A seal, and method of using same, for an inlet guide vane in a gas turbine engine includes a longitudinally extending tubular portion that has a longitudinal axis defined therethrough, a dovetail portion including a rib portion and a retaining feature. The rib portion is integral with the tubular portion and extends parallel to the axis, and the rib portion extends in a direction radially outward from the axis. The retaining feature is integral with the rib portion and tapers toward the axis.

DESCRIPTION 
1. Field of the Invention 
This invention relates to gas turbine engines, and specifically to 
compressors thereof having selectively positionable inlet guide vanes for 
directing the flow of airflow entering the engine. 
2. Background of the Invention 
In aircraft gas turbines engines, such as the type shown in FIG. 1, 
compressors 10 are used to compress the air 12 entering the gas turbine 
engine 14. For maximum efficiency the airflow 12 entering the upstream end 
16 of the compressor 10 must impinge the fan blades 18 of the compressor 
10 at a precise orientation relative to the longitudinal axis 20 of the 
engine 14. The precise orientation is determined by the design of the 
engine 14 and the conditions at which the engine is operated. In order to 
optimize performance at a number of different operating conditions, many 
gas turbine engines include a mechanism for selectively controlling the 
angle at which the incoming air 12 impinges the first stage of compressor 
blades 18. 
As shown in FIG. 1, a gas turbine engine 14 includes one or more rotor 
shafts 22 which are suitably supported by annular frames. Typically, a fan 
18 is joined to a fan shaft 22 that is supported at its forward end in a 
fan inlet case. The fan inlet case includes an annular outer casing 24 and 
an inner shroud 26, and a plurality of circumferentially spaced apart 
struts 28 extending therebetween. The struts 28 are fixed relative to the 
inner shroud 26 and the casing 24. 
The struts 28 are aerodynamically shaped to direct air 12 entering the 
engine inlet 30 efficiently between adjacent struts 28 prior to reaching 
the fan 18. Such struts 28 are hereinafter referred to as "strut 
airfoils". Bearing 36, whose housing is attached to the inner shroud 26 of 
the fan inlet case supports the rotating fan shaft 22. The loads from the 
fan shaft 22 are transmitted through the inner shroud 26 and strut 
airfoils 28 to the annular casing 24. 
Since the air angle of impingement onto the fan blade varies as a function 
of the conditions at which the engine 14 is being operated, selectively 
positionable flaps 32 are positioned upstream of the compressor blades 18 
to direct the incoming air 12 to the fan stage 18 of the compressor. Such 
flaps 32 are hereinafter referred to as "flap airfoils". As shown in FIG. 
2, each flap airfoil 32 is positioned immediately downstream of one of the 
strut airfoils 28 relative to the incoming airflow 12. These flap airfoils 
32 are rotatable about a reference axis, and such rotation varies the 
angle at which the incoming airflow impinges the blades of the fan stage 
18 of the compressor 10. Together, the strut airfoil 28 and the flap 
airfoil 32 immediately downstream thereof form an inlet guide vane. 
For optimum engine operating efficiency and stability, it is important that 
a smooth airflow transition occurs between the strut airfoil 28 and the 
flap airfoil 32 immediately downstream thereof at all engine operating 
conditions. However, in some designs of the prior art, the inlet guide 
vanes introduce turbulence into the airflow at certain rotational 
positions of the flap airfoil. This turbulence is sufficient to cause a 
nonintegral rotor blade vibration known as "flutter" in the blades of the 
fan immediately downstream of the inlet guide vanes. In severe cases, the 
flutter may damage a fan blade, shorten its useable life, or restrict 
operation of the engine. As those skilled in the art will readily 
appreciate, any condition which could impair normal engine operation while 
the aircraft is in flight, should be avoided. 
What is needed is a means for minimizing the amount of turbulence 
introduced into the airflow as air flows through the inlet guide vanes to 
the fan. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a seal for an inlet guide 
vane that is located in the gap between the strut airfoil and the flap 
airfoil to prevent the flow of air through the gap. 
Another object of the present invention is to provide a method of 
preventing turbulence in airflow entering the fan of a gas turbine engine. 
Accordingly, the present invention provides a seal for an inlet guide vane 
which includes a longitudinally extending tubular portion, that has a 
longitudinal axis defined therethrough, a dovetail portion including a rib 
portion and a retaining feature. The rib portion is integral with the 
tubular portion and extends parallel to the axis, and the rib portion 
extends in a direction radially outward from the axis. The retaining 
feature is integral with the rib portion and tapers toward the axis. The 
invention further provides a method for using the seal to prevent flutter 
in the fan blades of a gas turbine engine.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
Air flowing past an inlet guide vane has a tendency to flow from the 
pressure side of the inlet guide vane to the suction side of the inlet 
guide vane. The inventors have discovered that this tendency is great 
enough to cause air flowing past an inlet guide vane to flow through the 
gap between the strut airfoil and the flap airfoil thereof. The inventors 
have further determined that this flow of air through the gap can cause 
separation of the airflow from the suction side of the inlet guide vane, 
which introduces turbulence into the airflow entering the fan blades. It 
is this turbulence that can cause the blades of the fan to flutter and 
ultimately fail at certain engine operating conditions. 
An inlet guide vane 100 of the present invention is shown in FIG. 3 in the 
inlet of a gas turbine engine. As those skilled in the art will readily 
appreciate, the inlet comprises an inner shroud 26 and an annular casing 
24 spaced radially outward from the inner shroud 26. A plurality of inlet 
guide vanes 100 extend radially from the inner shroud 26 to the casing 24. 
Each inlet guide vane includes both a strut airfoil 28 and a flap airfoil 
32. The strut airfoils 28, which are preferably spaced uniformly about the 
circumference of the inner shroud 26, structurally support the annular 
casing 24 in spaced relation to the inner shroud 26. Accordingly, each of 
the strut airfoils 28 is fixedly secured to the inner shroud 26 and the 
casing 24. As in the prior art, the strut airfoils 28 are aerodynamically 
shaped to deflect air entering the inlet 30 around the strut airfoil 28. 
Immediately downstream of each strut airfoil 28 is a flap airfoil 32, as 
shown in FIG. 3. 
Each of the strut airfoils 28 has a strut first end 42 adjacent the inner 
shroud 26, and a strut second end 44 in spaced relation to the first end 
42 and adjacent to the annular casing 24. Additionally, each of the strut 
airfoils 28 has a strut leading edge surface 46 and a strut trailing edge 
surface 48. As shown in FIGS. 3 and 4, first 50 and second 52 strut 
airfoil surfaces extend from the strut leading edge surface 46 to the 
strut trailing edge surface 48 and from the strut first end 42 to the 
strut second end 44 thereof, and as shown in FIG. 4, the first and second 
strut airfoil surfaces 50, 52 are in spaced relation to each other. 
Referring again to FIG. 3, each of the flap airfoils 32 has a flap first 
end 54 adjacent the inner shroud 26, and a flap second end 56 in spaced 
relation to the first end 54 and adjacent to the annular casing 24. 
Additionally, each of the flap airfoils 32 has a flap leading edge surface 
58 and a flap trailing edge surface 60. Referring to FIGS. 3 and 4, first 
62 and second 64 flap airfoil surfaces extend from the flap leading edge 
surface 58 to the flap trailing edge surface 60 and from the flap first 
end 54 to the flap second end 56 thereof, and as shown in FIG. 4, the 
first and second flap airfoil surfaces 62, 64 are in spaced relation to 
each other. 
Each flap airfoil 32 is rotatable about a reference axis 66 relative to the 
strut airfoil 28 immediately upstream thereof. This reference axis 66, or 
"trunion axis", is fixed relative to the strut airfoil 28. The leading 
edge surface 58 of each flap airfoil 32 is in spaced relation to the 
trailing edge surface 48 of the strut airfoil 28 immediately upstream 
thereof, defining a gap 70 therebetween. 
Located in the gap 70 of each inlet guide vane, and preferably secured to 
the trailing edge surface 48 of the strut airfoil 28 thereof, is an inlet 
guide vane seal 72, as shown in FIG. 4. The seal 72 of each inlet guide 
vane 100 extends from the trailing edge surface 48 of the strut airfoil 28 
to the leading edge surface 58 of the flap airfoil 32 immediately adjacent 
thereto, thereby preventing the flow of air through the gap 70. As shown 
in FIG. 5, the seal 72 has a longitudinally extending tubular portion 80 
and a dovetail portion 82, and a longitudinal axis 84 extends the length 
of the tubular portion 80. The tubular portion 80 includes first and 
second surfaces 86, 88, and both surfaces 86, 88 are in spaced relation to 
the longitudinal axis 84, as shown in FIG. 6. The second surface 88 is 
located radially outward from the first surface 86 relative to the 
longitudinal axis 84, and the second surface 88 is covered with a 
lubricious material, such as Teflon TM. As used herein, the term 
"lubricious material" means a material that having a coefficient of 
friction not greater than 0.04 when in contact with the flap airfoil 
leading edge surface 58. 
The dovetail portion 82 includes a rib portion 90 that is integral with the 
tubular portion 82, and a retaining feature 92 for securing the seal 72 to 
the trailing edge 48 of a strut airfoil 28, as shown in FIG. 7. 
Preferably, the retaining feature 92 is a tapered washer, as shown in FIG. 
5. The rib portion 90 extends along the tubular portion 80 parallel to the 
longitudinal axis 84, and the rib portion 90 also extends in a direction 
94 radially outward from the longitudinal axis 84. The tubular portion 80 
and the rib portion 90 are made of an elastomeric material, such as 
silicone rubber. For durability, the tubular portion 80 includes a 
reinforcing fiber mesh 96 embedded therein between the first and second 
surfaces 86, 88. As shown in FIGS. 5-7, the retaining feature 92 is 
integral with the rib portion 90 and tapers toward the longitudinal axis 
84. The retaining feature 92, which is made of a rigid material such as 
titanium or aluminum, is preferably embedded into the rib portion 90 of 
the seal 72. 
As shown in FIG. 8, the trailing edge 48 of the strut airfoil includes a 
retaining slot 102 that tapers in a direction away from the leading edge 
46 of the strut airfoil 28. The slot 102 extends the length of the 
trailing edge 48, and dovetail portion 82 must be slid into the slot from 
one of the ends 42, 44 of the strut airfoil 28. The width of the slot 102 
is only slightly greater than width of the retaining feature 92 so that 
once the dovetail portion 82 is received therein, the seal can only be 
detached from the strut airfoil 28 by sliding the retaining feature 92 out 
one of the ends 42, 44 of the strut airfoil 28. 
In operation, air flowing past the inlet guide vanes 100 is prevented from 
passing through the gap 70 between each flap airfoil 32 and the strut 
airfoil 28 immediately upstream thereof. Thus, the mechanism described 
above that causes separation on the suction side of the inlet guide vanes 
is eliminated, thereby preventing the turbulence that can result from such 
separation. Accordingly, the fan blades immediately downstream of the 
inlet guide vanes are not subjected to the type of airflow distortions 
that cause the flutter and associated engine damage described above. 
The method and seal of the present invention minimizes turbulence in the 
airflow entering the fan by eliminating the cause of airflow separation at 
the inlet guide vane. The present invention thus prevents the detrimental 
effect that flutter can have on the fan of gas turbine engines at certain 
engine operating conditions. 
Although the invention has been shown and described with respect to a 
preferred embodiment thereof it should be understood by those skilled in 
the art that other various changes and omissions in the form and detail of 
the invention may be made without departing from the spirit and scope 
thereof.