Damped airfoil blade

The internal blade damper is an elongated member with a damping surface of discrete width in contact with the interior blade surfaces. Contact is continuous throughout a substantial length. The damper extends between 2.degree. and 30.degree. from the radial direction, producing a direction of contact having some radial component. Centrifugal force loads the damping surface.

TECHNICAL FIELD 
The invention relates to hollow blades for gas turbine engines and in 
particular to vibration damping of such blades. 
BACKGROUND OF THE INVENTION 
Airfoil blades in both compressors and turbines of gas turbine engines are 
subject to high, sometimes pulsating forces. Blades can experience high 
vibratory stresses resulting from resonance or flutter instabilities. This 
is particularly true for hollow blades which are used to reduce weight 
and/or permit internal air cooling. 
External restraints such as shrouds and platform dampers have been used to 
control the vibration problem. Internal dampers relying on impact or dry 
friction have also been suggested. These have packed the blades with 
particles or rods, or otherwise tended to wedge the dampers. This can 
overload and lock the damping action. 
Frictional damping inherently requires some slipping. Such slippage can be 
broken into macro slip and micro slip action. Macro slip is defined as 
substantially single point contact while micro slip is defined as a slip 
phenomena occurring over multiple points along the line of surface. In 
micro slip all points of contact are not necessarily stuck or slipping 
simultaneously. The pattern of local stick or slip depends on the local 
normal load and local deformation between the materials of the two contact 
surfaces. 
Both micro slip and macro slip theories indicate that the vibratory 
response is minimized when the damper stiffness is increased. In typical 
applications of turbine engines to ensure high stiffness with a 
functionally single point contact results in a heavy damper configuration. 
This heavy damper configuration tends to promote sticking of the damper 
because of excess loading. 
Those approaches which involve wedging of the damper against the surface 
tend to promote high loading leading to jamming or sticking of the damper 
rendering it ineffective. 
While dampers of the prior art may have had some micro slipping along with 
the macro slipping, the structure was selected based on macro slip 
concepts. Appreciation of the micro slip phenomena and the definition of 
new structure to take advantage of this phenomena provides a damper of 
light weight, less prone to locking, and more compatible with cooling air 
flow within a turbine blade. 
SUMMARY OF THE INVENTION 
A hollow airfoil blade is secured to a rotor disk either as a bonded blade 
or with a fir-tree type construction. The blade has interior surfaces and 
an effective radial length exposed to the gas flow through the gas turbine 
engine. 
The internal damper comprises an elongated member with a damping surface of 
discrete width in contact with an interior surface of the blade. This 
contact is continuous throughout a contact length greater than 50% of the 
effective radial length. The contact is in the direction having a radial 
component with respect to the axis of the rotor, preferably with the 
damper extending between 2.degree. and 30.degree. from the radial 
direction. This damping surface is the exclusive frictional contact 
between the damper and the blade. 
The damper cross-section is in the order of 0.2 inch by 0.06 inch with the 
major dimension being across the damping surface. This provides a damper 
stiffer in a direction parallel to the damping surface than in a direction 
perpendicular to the damping surface. Accordingly, the damper may readily 
conform to the wall to produce the continuous contact.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a gas turbine engine 10 with rotor 12 including a 
compressor disk 14. The compressor disk carries compressor airfoil blade 
16 located in the gas flow path 18. 
Also on the rotor is a turbine disk 20 carrying a plurality of turbine 
airfoil blades 22 located in the gas flow path 24. 
FIGS. 2, 3, 4 and 5 illustrate the use of the damper within a gas turbine 
airfoil blade 22. The airfoil blade is secured to the disk 20 by fir-tree 
26 and damper 28 is secured or restrained at an inboard location 30 on the 
blade by lug 33. The damper extends outboard from this location. Damping 
surface 32 of the damper is 0.20 inch wide and is in contact with interior 
surface 34 of the blade throughout the entire length of the blade beyond 
platform 36. The distance 38 from the blade platform to the tip of the 
blade is the portion of the blade in contact with the gas flow 24 and is 
considered the effective radial length of the blade since this is a major 
factor in the vibration of the blade. The damping surface 32 should be in 
contact with the inner surface 34 continuously throughout a length equal 
to at least 50% of the effective radial length 38 of the blade. 
The damper as illustrated here is 0.06 inch thick and 0.2 inch wide. This 
may be as low as 0.04 inch thick and 0.1 inch wide. In any event it is 
required that there be a discrete width of the damping surface in contact 
with the inner surface of the blade to provide a basis for the micro slip 
phenomena to occur. 0.1-0.2 inch is appropriate. 
When installed against the inner surface of the blade the direction of the 
damping surface 32 is indicated by line 40 which is at an angle 42 of 
3.degree. with respect to the radial line 44. The centrifugal force 
operating on the damper forces the damper against the internal blade 
surface so long as this damping surface has some radial component with 
respect to the axis of the rotor. An angle of less than 2.degree. will not 
provide sufficient loading against the surface while an angle exceeding 
30.degree. will produce too much loading leading to locking of the damper 
with loss of the energy dissipation capability. 
As best seen in FIG. 4, the damper is preferably set in a radial plane 
through the rotor axis. With this orientation the centrifugal force 
establishes no direct force on the damper in the direction which is 
perpendicular to the engine centerline direction 45. The only force in 
that direction would be a resultant force based on the loading of the 
damper against the internal surface of the blade. 
Turbine blade 22 also includes a plurality of internal cooling air passages 
48 for the passage of cooling air through the blade. In the conventional 
manner the flow passes serially through a number of these passages and 
exits through cooling holes in the blade structure. The damper 28 is 
located in one of these cooling flow paths. It is noted that this damper 
is sufficiently small that it may be installed without blockage of more 
than 25% of the passage on which it is located. This permits the use of 
the damper in an air cooled blade without unduly restricting the air 
cooling thereof. 
Flexural vibration of the blade is damped by longitudinal friction and 
slippage between the damper and the blade surface. Local micro-slipping 
will occur, with micro-slipping varying from a minimum near the damper 
support point to a maximum at the damper end. 
Support of the damper is not really required for the damping action itself 
It is required to locate the damper. Support at an inboard location in the 
blade is preferred. Support at an outboard location requires a stiffer 
damper, since the centrifugal force tends to buckle the damper.