Damped turbine engine frame

A frame for a gas turbine engine includes an outer casing, an inner hub, and a plurality of circumferentially spaced apart struts extending radially therebetween. Each strut includes first and second sidewalls defining a radial passage therein, and an elongate damper is disposed in respective ones of the strut passages. Each damper is disposed in an interference fit laterally between the first and second sidewalls for maintaining contact therewith for effecting sliding friction to dampen vibration of the struts.

The present invention relates generally to gas turbine engines, and more 
specifically, to shaft-supporting frames thereof. 
BACKGROUND OF THE INVENTION 
A gas turbine engine includes one or more rotor shafts which are suitably 
supported by annular frames. In one exemplary engine, a fan is joined to a 
fan shaft which is supported at its forward end in a fan frame. The fan 
frame includes an outer casing, an inner hub, and a plurality of 
circumferentially spaced apart struts extending therebetween. The hub 
supports a bearing for the rotating fan shaft, with the loads therefrom 
being channeled through the hub and struts into the outer casing. The 
struts are airfoil shaped since inlet air to the engine first passes 
between the adjacent struts prior to reaching the fan. 
The struts are typically hollow members for reducing weight of the engine 
while also providing passages through which conduits may be disposed for 
carrying air or oil, for example, through the struts. Since the struts are 
hollow and support the rotating fan shaft and allow airflow between 
adjacent struts, they are subject to excitation forces from the rotating 
fan shaft and from the air which may cause vibration thereof. Where the 
natural frequencies of the frame approach the excitation frequencies from 
the rotating fan, conventionally known as (per)/rev frequencies, and from 
the inlet airflow, fundamental flexural and torsional modes of vibration, 
as well as panel modes of vibration may be excited in the individual 
struts. 
It is conventional to either design the frame to provide a suitable margin 
between the excitation frequencies and the natural resonance frequencies 
of the frame, or to provide suitable damping material for damping any 
vibrations which may result. For example, the hollow passages through the 
struts may be conventionally filled with a damping compound such as rubber 
for damping vibration of the struts themselves as well as damping 
vibration of the conduits therein. However, improved frame damping is 
desired especially where the frames are being made lighter in weight which 
decreases the margin between the excitation forces and the natural 
resonance frequencies thereof. 
SUMMARY OF THE INVENTION 
A frame for a gas turbine engine includes an outer casing, an inner hub, 
and a plurality of circumferentially spaced apart struts extending 
radially therebetween. Each strut includes first and second sidewalls 
defining a radial passage therein, and an elongate damper is disposed in 
respective ones of the strut passages. Each damper is disposed in an 
interference fit laterally between the first and second sidewalls for 
maintaining contact therewith for effecting sliding friction to dampen 
vibration of the struts.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Illustrated schematically in FIG. 1 is an exemplary augmented, turbofan gas 
turbine engine 10 for powering an aircraft in flight. The engine 10 
includes a conventional core engine 12, including compressor, combustor, 
and turbine, which rotates a fan shaft 14 from which extends a 
conventional fan 16. The forward end of the fan shaft 14 is supported by 
conventional bearings in a fan or front frame 18 in accordance with one 
embodiment of the present invention. The engine 10 includes a longitudinal 
centerline axis 20 about which the frame 18 is coaxially disposed, and 
conventional variable inlet guide vanes (VIGV's) 22 are disposed axially 
between the frame 18 and the fan 16 as is conventionally known. 
The engine 10 includes an inlet 24 for receiving ambient airflow 26 which 
is channeled through the frame 18 and VIGV's 22 and through the fan 16. A 
portion of the airflow 26 is conventionally channeled through the core 
engine 12 wherein it is mixed with fuel and ignited for generating 
combustion gases 28 from which energy is extracted for rotating the fan 
shaft 14 and in turn the fan 16, with the combustion gases 28 being 
discharged from the engine through a conventional variable area exhaust 
nozzle 30. 
The front frame 18 removed from the engine 10 is illustrated in more 
particularity in FIG. 2 and includes an annular, radially outer casing 32 
which is conventionally joined in the engine 10; and annular, radially 
inner hub 34 spaced radially inwardly from the outer casing 32; and a 
plurality of circumferentially spaced apart frame vanes or struts 36 which 
extend radially between the casing 32 and the hub 34. The outer and inner 
ends of the struts 36 are conventionally fixedly joined to the casing 32 
and the hub 34. 
FIG. 3 illustrates a portion of an exemplary one of the frame struts 36, 
with each strut 36 being hollow and having circumferentially spaced apart 
first and second sidewalls 38 and 40, respectively, defining a strut 
passage 42 extending radially between the casing 32 and the hub 34 for the 
full radial extent of the strut 36 as shown in FIG. 2. In accordance with 
the present invention, the frame 18 further includes a plurality of 
elongate damper members, or simply dampers, 44, with each damper 44 being 
disposed in a respective one of the strut passages 42. As additionally 
shown in FIG. 2, each damper 44 extends radially between the casing 32 and 
the hub 34 preferably for the full radial extent of each strut 36. Each 
damper 44 is preferably disposed in a conventional interference fit 
laterally, or circumferentially, between the strut first and second 
sidewalls 38 and 40 as shown in FIG. 3 for maintaining contact therewith 
for effecting sliding friction to dampen vibration of the struts 36. 
More specifically, each damper 44 has a longitudinal centerline axis 46 
which is disposed generally parallel to a radial axis of the frame 18 as 
shown in FIG. 2, and contacts the first and second sidewalls 38, 40 along 
at least two lines of contact extending substantially parallel to the 
damper centerline axis 46 as shown in FIG. 3. The damper 44 is illustrated 
in FIGS. 3 and 4 in accordance with a first embodiment wherein the damper 
44 is configured in the form of a tube 48 having an internal tubular 
passage 50. In the preferred embodiment of the invention, the damper 44 is 
a non-metal, preferably composite member which may be elastically 
compressed in an interference fit inside the strut passage 42 for damping 
vibration of the strut 36. In one exemplary embodiment as illustrated in 
FIG. 4, the damper 44 includes a plurality of concentric plies or layers 
52, three being shown, of structural fibers 54 joined together in a 
bonding matrix 56. The fibers 54 provide structural strength, in the hoop 
stress direction for example, and may be formed of conventional material 
such as carbon or graphite fibers in braided form such as that identified 
by the T-300 designation and available from the Amoco Company. The matrix 
56 is also conventional for suitably containing the fibers 54 and may be 
made from a polyimide resin such as that known under the PMR 15 
designation. 
The tubular form of the damper 44 provides inherent elasticity which, when 
compressed in an interference fit inside the strut passage 42, maintains 
two lines of contact with the opposing strut sidewalls 38, 40 for 
providing sliding friction damping of the strut 36. This tubular 
configuration of the damper 44 also allows the damper 44 to provide the 
additional function of a flow conduit for air or oil, for example, to be 
channeled through the struts 36 from or to conventional components 58 as 
shown in FIG. 1. Since the damper 44 itself provides the internal passage 
50 for flow therethrough of fluids such as oil or air, separate conduits 
therefor are not required. Furthermore, since the damper 44 contacts the 
strut first and second sides 38, 40, additional damping material such as 
rubber compounds need not be injected into the strut passage 42 for 
providing damping as is conventionally done. And, vibration of the damper 
44 itself is also dampened by its contact with the sidewalls 38, 40. 
In the embodiment of the damper 44 illustrated in FIGS. 3 and 4, the damper 
44 is cylindrical which results in the two lines of contact along the 
strut first and second sidewalls 38, 40. The damper 44 may take other 
suitable configurations as desired for providing at least damping of the 
struts 36, as well as providing a flow carrying function through the 
internal passage 50 if desired. 
As shown in FIG. 3, the damper 44 has a maximum thickness T, and the width 
of the strut passage 42 between the sidewalls 38, 40 is designated W, with 
the initial value of the damper thickness T being suitably larger than the 
passage width W so that when the damper 44 is inserted through the passage 
42 it is partially compressed to create an interference fit with the 
sidewalls 38, 40 generally along two opposing lines of contact. Since the 
damper 44 is preferably composite, and not metal, it may be readily pulled 
through the strut passage 42 into suitable position therein in the 
interference fit. 
Illustrated in FIGS. 5 and 6 is a second embodiment of the damper, 
designated 44a which includes a spring tab 60 formed integrally with the 
tube 48 which also extends radially between the casing 32 and the hub 34 
preferably for the entire radial extent of the tube 48 inside the strut 36 
like the damper 44 shown in FIG. 2. The spring tab 60 is configured on the 
tube 48 for contacting at least one of the strut first and second 
sidewalls 38, 40 in compression therewith inside the strut passage 42 for 
effecting sliding friction to dampen vibration of the strut 36 during 
operation of the engine 10. As shown more clearly in FIG. 6, the spring 
tab 60 preferably includes an arcuate base 60a integrally joined to the 
tube 48, by a conventional adhesive or by being integrally manufactured 
therewith for example, and the base 60a partially surrounds the tube 48 
for a portion of its circumference, which in the embodiment illustrated in 
FIGS. 5 and 6 is about 180.degree.. The tab 60 further includes integral 
first and second oppositely extending wings 60b and 60c, respectively, 
which extend radially away from the tube 48 relative to the damper 
centerline axis 46, with the first and second wings 60b, 60c being 
disposed in compression contact with at least one of the strut first and 
second sidewalls 38, 40 as shown in FIG. 5. 
In the embodiment illustrated in FIG. 5, the spring tab 60 is generally 
hat-shaped in transverse section and the spring tab first and second wings 
60b, 60c are both disposed in contact with only one of the strut first and 
second sidewalls 38, 40, for example the second sidewall 40. And, the 
spring tab base 60a is disposed in contact with the other of the first and 
second sidewalls, i.e. the first sidewall 38. In this way, at least three 
lines of contact are provided between the spring tab 60 and the first and 
second sidewalls 38, 40 for providing sliding friction to dampen vibration 
of the strut 36. As shown in FIG. 5, the bottom of the tube 48 itself may 
additionally contact the second sidewall 40 for providing yet another, or 
fourth, line of contact for promoting friction damping. 
Illustrated in FIGS. 7 and 8 is another, third embodiment of the damper, 
designated 44b, including a pair of substantially identical spring tabs 62 
each being generally W-shaped in transverse section, with an arcuate base 
62a and generally radially outwardly extending in transverse section, with 
an arcuate base 62a and generally radially outwardly extending first and 
second wings 62b and 62c. As shown in FIG. 7, each of the first and second 
wings 62b and 62c of the pair of spring tabs 62 is disposed in contact 
with the strut first and second sidewalls 38, 40, respectively. With the 
spring tabs 62 being disposed on opposite sides of the damper tube 48 
inside the strut passage 42, and the respective first and second wings 62b 
and 62c contacting the respective strut first and second sidewalls 38, 40, 
four lines of contact therewith are provided for effecting sliding 
friction damping during operation of the engine 10. 
In both the second and third embodiments of the damper 44a, and 44b, the 
respective tabs 60, 62 are integrally joined to the center tube 48 by 
suitable adhesive or other bond, for example. The tubes 48 in the second 
and third embodiments may be identical to the multi-ply tube illustrated 
in FIG. 4, with the spring tabs 60, 62 themselves also being formed of a 
composite material having fibers 54 in a matrix 56. The overall thickness 
T of the dampers 48a, and 48b as illustrated in FIGS. 5 and 7, 
respectively, have initial values which are suitably larger than the width 
W of the strut passage 42 so that the desired interference fit therein may 
be effected. In FIG. 5, the thickness T of the damper 44a is measured 
across its base 60a and the wings 60b and 60c. And in the damper 44b 
illustrated in FIG. 7, the thickness T is measured across the opposite 
first and second wings 62b and 62c of each damper 44b. 
FIGS. 4, 6, and 8 illustrate various configurations of the damper 44, and 
other variations are also within the scope of the present invention for 
ensuring effective elastic support and compression of the dampers 44 
between the strut sidewalls 38, 40 for ensuring continuous contact 
therebetween for effective sliding friction damping of the strut 36, as 
well as damping due to the inherent composite configuration of the damper 
44 itself. 
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.