Abstract:
A method and control for predicting the remaining useful life of an airfoil for a gas turbine engine includes the steps of monitoring conditions of the blade such as flutter, leaning, etc. A measured amount of deflection of the airfoil is compared to tabulated data to predict an expected crack length which is likely causing the deflection, etc. Once a predicted crack length has been identified, the amount of accumulated damage to the airfoil at the crack is monitored and stored. The amount of useful life for the blade can be predicted by compiling the accumulated damage over time. The useful life remaining can be displayed such that flight plans or maintenance schedules for the aircraft can be modified as appropriate.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to a system wherein movement, vibration, leaning or flutter of an airfoil in a turbine engine is monitored, and anomalies in the monitored condition are utilized to predict length of any crack that may be found in the airfoil. Once the crack length is determined, a “remaining life” is calculated given expected engine operating conditions. This expected life is to be utilized to plan flight schedules or missions and maintenance. 
     Gas turbine engines are provided with a number of functional sections, including a fan section, a compressor section, a combustion section, and a turbine section. Air and fuel are combusted in the combustion section. The products of the combustion move downstream, and pass over a series of turbine rotors, driving the rotors to create power. The turbine, in turn, drives rotors associated with the fan section and the compressor section. 
     The rotors associated with each of the above-mentioned sections (other than the combustion section) include removable blades. These blades have an airfoil shape, and are operable to move air (fan rotors), compress air (compressor rotors), and to be driven by the products of combustion (turbine rotors). 
     Cracks may form in airfoils, such as the blades. These cracks can result in a failure to the airfoil component over time. To date, no system has been able to successfully predict, detect and monitor the existence, and growth of a crack in an airfoil, which may lead toward failure, and predict the remaining life of an airfoil. 
     SUMMARY OF THE INVENTION 
     In the disclosed embodiment of this invention, movement of the blades in a rotor associated with a turbine engine is monitored. Vibration, flutter, leaning, etc. of each of the blades is monitored. As an example, if a leading edge of a blade reaches a position where a sensor can sense it earlier (or later) than it was expected, an indication can be made that the blade is vibrating, leaning or fluttering. 
     The present invention has identified certain conditions that are expected in the event that a crack has occurred in an airfoil. Thus, the condition as sensed is compared to stored information to detect a crack and predict its length when anomalies are found in the operation of the airfoil. Once a crack of a certain length has been detected, other stored information can be accessed which will predict remaining useful life of the particular airfoil under various system conditions. At this point, the remaining life can be utilized such as for flight scheduling, or to schedule maintenance. 
     As one example, if two aircrafts have engines wherein one of the engines has a blade with a remaining life that is relatively short compared to the other, the aircraft with the blade approaching the end of its useful life may be scheduled for less stressful operation. As for example, in a military application, the jet aircraft with the longer-predicted blade life can be utilized for more stressful missions such as air to ground missions, while the aircraft having a blade closer to the end of its useful life may be scheduled for less stressful operations such as air coverage, at which it is likely to be at a relatively stationary speed loitering. 
     These and other features of the present invention can be best understood from the following specifications and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a typical gas turbine engine. 
         FIG. 2  schematically shows a method according to this invention. 
         FIG. 3  shows a first table of information that allows the prediction of a crack of certain length in an airfoil. 
         FIG. 4  shows an alternative table of information for predicting a crack when based upon a second system condition. 
         FIG. 5  shows yet another alternative table for predicting a crack of certain length. 
         FIG. 6  shows a remaining life table based upon a crack length, and various stress levels which may be applied to the blade. 
         FIG. 7  shows a monitored stress condition that would be indicative of a failure in a gas turbine engine. 
         FIG. 8  is a flowchart of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a gas turbine engine  10 , such as a gas turbine used for power generation or propulsion, circumferentially disposed about an engine centerline, or axial centerline axis  12 . The engine  10  includes a fan  14 , a compressor  16 , a combustion section  18  and a turbine  11 . As is well known in the art, air compressed in the compressor  16  is mixed with fuel which is burned in the combustion section  18  and expanded in turbine  11 . The air compressed in the compressor and the fuel mixture expanded in the turbine  11  can both be referred to as a hot gas stream flow. The turbine  11  includes rotors  13  and  15  that, in response to the expansion, rotate, driving the compressor  16  and fan  14 . The turbine  11  comprises alternating rows of rotary blades  20  and static airfoils or vanes  19 .  FIG. 1  is a somewhat schematic representation, for illustrative purposes only, and is not a limitation of the instant invention, that may be employed on gas turbines used for power generation and aircraft propulsion. The compressor  16  and fan  14  also include rotors and removable blades. 
       FIG. 2  shows a method according to this invention in which remaining life for an airfoil such as turbine blade  30  is monitored. The invention extends to other blades, such as compressor, turbine or fan blades. A sensor  40  senses movement of blade  30 . Conditions such as the time at which the leading edge of the airfoil passes a predetermined point, compared to an expected time, can be monitored. If the leading edge actually passes a predetermined point at a time different from the expected time an indication can be made that there is some problem with the particular airfoil. As is clear, the sensor  40  is positioned remotely from the turbine blade  30 . The term “remotely,” as used in this application, merely means that the sensor  40  is not mounted on the turbine blade  30 , but rather is positioned such that the turbine blades move past the sensor  40 . 
     The present invention has developed transfer functions which associate a relative frequency change, or other changes, with growing length of a crack in the airfoil. Different modes of monitoring the airfoil can be taken at different locations at the airfoil and can be utilized to predict the location and length of the crack. The transfer function such as shown in  FIG. 2  can be determined experimentally and/or analytically, and are generally available to a worker of ordinary skill in this art. Over time, the damage to the airfoil will accumulate. Thus, a remaining life can be predicted given a particular crack length, and based upon the particular stresses on the airfoil in question. 
       FIG. 3  shows one embodiment of a table of information that associates a lean in the leading edge of the airfoil with a plurality of curves with different speeds of operation of the associated rotor. Now, a particular identified lean can be associated with the relative rotational speed, and in this manner a crack of certain length can be predicted. This information can be developed mathematically, and a worker of ordinary skill in the art would be able to develop the appropriate table. The Y axis is a measurement of blade deflection, or the “lean” of the leading edge measured in 1/1000 of an inch. 
       FIG. 4  shows another method of detecting a crack of certain length. Here, the tip of the leaning edge deflection is monitored. Again, the particular speed of operation is associated with a plurality of curves, and by finding the appropriate curve, and the appropriate amount of deflection, a prediction of a crack of certain length can be made. Again, the Y axis is measured as the leading edge deflection measured in 1/1000 of an inch. 
     Other deformations that can be measured include first bending mode, stiffwise bending mode, first torsion mode, chordwise bending mode, second leading edge bending mode, second bending mode, second torsion mode, chordwise second bending mode, and third trailing edge bending mode. 
     In general, each of these methods measure deformation of a position of a portion of the blade as the rotor and blade rotate. These deformations can then be associated with a crack length as mentioned above. 
       FIG. 5  shows yet another embodiment, where model frequency shift is calculated and associated with a plurality of distinct measurements. Again, this can be utilized to predict a crack of certain length, as shown in the formula found in  FIG. 5 . 
     Once a crack of certain length has been detected, another family of curves can be used to associate various stress levels on the airfoil with a remaining life. Examples of such curves are shown in  FIG. 6 . Each curve represents the effect of different stress levels. In this figure, the remaining life is defined in “mini-sweeps” or times when the engine is accelerated and de-accelerated across a resonance frequency for the airfoil. Once the number of “mini-sweeps” remaining can be identified, a prediction can be made for the remaining useful life before failure of a particular airfoil. Essentially, the particular airfoil closest to failure would be a limitation on the amount of useful life for the entire engine and would suggest maintenance before the useful life has expired. Another measurement of useful life remaining would be cycles or missions. A computer associated with the sensors stores information with regard to each of the airfoils which are experiencing apparent cracks. The amount of damage which has been accumulated to that airfoil is stored in the computer, such that the computer has a running total of the amount of useful life remaining. As can be appreciated from this figure, at different stress levels, the useful life remaining changes. Thus, the computer must store not only the crack length and how often the particular engine has been operated, but also the operating conditions. 
     Further, with this invention and due to the various effects of different stress levels, it is apparent that by planning a particular flight schedule for an aircraft holding a particular jet engine, the number of flights remaining can be optimized. For example, in military applications there are high stress and low stress flights. An air to ground attack mission might be a relatively high stress flight in that it could involve frequent accelerations and decelerations. On the other hand, air cover under which an aircraft tends to remain high in the air at a relatively constant speed should be relatively low stress. A field commander might assign a particular aircraft to one of these flight schedules based upon an indicated remaining life indicated by this invention. This can lengthen the time between necessary maintenance. 
     The information provided in this invention also can provide an indication of an apparent immediate failure. As an example,  FIG. 7  illustrates a series of mini-sweeps as each blade passes by the sensor. At points  1 - 2 - 3 , a dramatic drop occurs. This may be indicative of a blade that is bent so badly that it has contacted the sensor, etc. At any rate, such an indication might require immediate maintenance. 
       FIG. 8  is a basic flowchart of the present invention. The blade&#39;s rotation is monitored. A sensor and associated computer checks for flutter, etc. and determines that a particular blade has developed a crack. Once a crack has been detected, a crack length is determined. Once the crack length has been determined, a remaining life for the particular airfoil can be calculated. The computer then begins to store the actual conditions of operation for that airfoil such that a useful remaining life can be calculated in a continuous manner. The amount of remaining life can be utilized to schedule flights and maintenance, as mentioned above. 
     While the above embodiments of this invention are all disclosed utilizing a predicted crack length, other types of damage to a blade may also be utilized in connection with this invention. 
     While a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.