Abstract:
A rotary-wing aircraft rotor blade ( 23 ) includes (semi)conductive loops ( 10 ) extending along axes ( 19 A to  19 C,  19 W to  19 Z) that are oblique relative to the longitudinal axis ( 26 ) of the blade. Impedance changes in the loops are sensed that indicate a location of damage to the blade.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a system for monitoring damage to a rotor blade of a rotary-wing aircraft, to an aircraft rotor blade and to an aircraft fitted with such a system. 
   The technical field of the invention is that of manufacturing rotary-wing aircraft. 
   Various devices for monitoring damage to a blade of a rotor for providing a helicopter with lift and forward drive have been proposed. 
   Patents FR 2 119 182 and U.S. Pat. No. 3,744,300 describe a device for measuring damage due to fatigue in a helicopter blade, the device including a sensor comprising strain gauges connected in a Wheatstone bridge configuration. 
   U.S. Pat. No. 4,106,332 describes a device for detecting an interruption in a conductive loop applied to the surface of a helicopter rotor blade; the device includes a bistable serving, in flight, to record a state corresponding to detecting a fault that led to the conductive loop being interrupted; when the aircraft is on the ground, a unit can be connected to the blade in order to read the state of the bistable in order to test the blade while it is at rest. 
   A drawback with such systems is that they do not make it possible to determine the location and the extent of the fault affecting the blade. 
   U.S. Pat. No. 3,596,269 describes a system for detecting and monitoring structural faults in the skin of an airplane. That system comprises a group of thin elongate electrical conductors constituted by two metal sheets interconnected by a resistor; the conductors are connected in parallel and they are placed between two insulating sheets; the assembly is stuck to different points on the skin of an airplane. A switch enables an operator manually to select a group for measuring its resistance by means of an ohmmeter; a visual or audible alarm is provided. 
   A drawback with such systems is that they are adapted poorly or not at all to monitoring the defects that might appear on the surface or inside a blade made of composite material. 
   U.S. Pat. No. 4,524,620 describes apparatus for acoustically monitoring the noise emitted by a composite helicopter blade; the blade is fitted with one or more acoustic transducers and the signals therefrom are multiplexed where necessary, and then filtered and forwarded to a computer or to a recorder; a filter serves to detect and eliminate “normal” noise so as to pass only “critical” noise representative of excessive stresses in the blade and structural damage caused to the blade; the occurrence and the intensity of such critical noise are accumulated in the recorder or the computer in order to constitute a history of critical noise from a given blade. 
   Such a device is complex; furthermore locating a defect in the blade requires triangulation calculations to be performed, in deferred time, and on the ground, using an auxiliary computer. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to propose a rotary wing blade of composite material that is fitted with a simple and high-performance system for monitoring damage to the structure of the blade. 
   An object of the invention is to propose a system for acting while a rotary-wing aircraft is flying to monitor the appearance and the behavior in real time of damage to one or more blades of the wing, which system is improved and/or remedies the shortcomings and drawbacks of known systems, at least in part. 
   An object of the invention is also to propose a rotary-wing aircraft including such a monitoring system. 
   In a first aspect of the invention, there is provided a rotor blade for a rotary-wing aircraft, the blade presenting a longitudinal axis and including (semi)conductive loops extending along axes or directions that are oblique relative to the longitudinal axis. 
   In a preferred embodiment of the invention, a blade is provided for a main or auxiliary rotor of a rotary-wing aircraft, the blade comprising a root, a leading edge, a trailing edge, at least one spar, and a skin covering the spar(s); the blade includes a plurality of conductive or semiconductive loops extending under the skin and disposed obliquely or transversely relative to the longitudinal axis of the blade; the blade further includes a loop selector, and a member for measuring the resistance (or the impedance) of a loop, which selector and member are integrated in the root of the blade, and a plurality of electrical connections connecting the selector to each of the loops, respectively; this enables data or impedance measurements concerning each of the loops to be transmitted to a computer on board the aircraft while using only two conductors, or where appropriate, via a single-channel wireless link. 
   In another preferred embodiment of the invention, there are provided: a blade for an aircraft rotary wing, the blade including a plurality of loops each presenting an impedance; an impedance-measuring device designed and arranged to measure the impedance of a loop, and delivering a loop-impedance measurement signal; a scanner and connector device designed and arranged to connect each of the loops in the plurality of loops in succession to the impedance-measuring device, by scanning the loops of the plurality of loops regularly and in a determined order; an analog-to-digital converter designed and arranged to transform each measurement of the impedance-measuring device into digital impedance data; and transmission means designed and arranged to transmit the impedance data to an on-board computer. 
   In another aspect of the invention, there is a provided a blade for a main or auxiliary rotor of a rotary-wing aircraft, in particular a helicopter, the blade presenting a composite structure including fibers, in particular glass or carbon fibers, that are embedded in a thermoplastic or thermosetting matrix; the blade including a plurality of substantially conductive loops (possibly including semiconductor elements, where appropriate) each presenting an impedance; the loops extending parallel to some of the fibers so that the mechanical performance of the blade fitted with the loops is substantially identical to that of the same blade not having any loops, whereby inserting the loops in the structure of the blade does not significantly degrade or modify the mechanical and aerodynamic characteristics of the blade, and whereby manufacture of the blade is also facilitated. 
   In other preferred embodiments of the invention:
         the blade includes, on its pressure side and/or its suction side, a first group of substantially conducive loops extending in a first direction parallel to first fibers and/or to a first sheet of fibers of the structure of the composite blade; the blade further includes a second group of substantially conductive loops extending in a second direction parallel to second fibers and/or to a second sheet of fibers of the structure of the composite blade, the second direction being different from the first direction;   the blade includes a first array of first loops that are mutually parallel and disposed in a first layer, and a second array of second loops that are mutually parallel and that cross the first array;   the blade includes a layer of electrically insulating material separating two crossed and superposed arrays or layers of conductive loops;   each loop includes a plurality of bridges or conductive or semiconductive elements interconnecting the two strands of the loop (and/or connected in parallel);   each of these bridges or elements presents an impedance (in particular a resistance) that is substantially identical, e.g. close to a few hundreds or thousands of ohms;   each loop includes at least three bridges or (semi)conductive elements regularly spaced apart from one another along the axis of the loop;   the blade includes several tens or hundreds of bridges or (semi)conductive elements, and where appropriate several tens or hundreds of loops including such bridges or (semi)conductive elements;   the number of loops depends on the spacing or “pitch” between loops, and determines the size of damage that can be measured; this pitch may vary over the blade depending on the degree of vulnerability of the zone being monitored;   said first and second directions along which the series of loops extend are inclined relative to the longitudinal axis of the blade and relative to a transverse axis of the blade (orthogonal to the longitudinal axis), e.g. being inclined relative to these axes by an angle close to 45° (to within plus or minus 10°);   said first direction is substantially perpendicular to said second direction;   said loops are connected to the scanner or selector device by respective pairs of metal wires (or strips);   said bridges or (semi)conductive elements are surface-mounted components (SMCs);   the loops, the loop selector(s), the impedance-measuring device(s), and where appropriate the analog-to-digital converter(s) integrated in the blade are powered electrically by a supply of electrical energy integrated in the blade (an optionally-rechargeable battery), and/or by a circuit for supplying power for de-icing the blade;   the signals or data relating to the impedances of the loops are transmitted to an on-board calculator via a slip-ring assembly connecting a rotor bearing to the rotor hub, and/or via wireless transmission, e.g. radio transmission;   the conductive elements connecting the loops to the loop selector are grouped together in a bundle or cord extending close to the leading edge of the blade, the loops extending from said bundle or cord towards the trailing edge of the blade.       

   In particular, the cord is preferably located inside or in the immediate proximity of the spar of the blade. This disposition enables the cord to be protected by the spar which remains the last portion of the blade to be damaged after an impact. 
   By using an oblique configuration for disposing the loops that can be monitored for variations in impedance as a result of damage to a portion of the blade, it is possible to implement a system for monitoring the state of the blade in flight that is simple, compact, and of high performance. 
   In particular when the blade incorporates two crossed arrays of parallel loops, each loop including a plurality of same-impedance bridges, it is much easier to locate and evaluate damage that has caused electrical continuity to be broken in one or more loops. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention appear from the following description which refers to the accompanying drawings which show preferred and non-limiting embodiments of the invention. 
       FIG. 1  is an electrical circuit diagram of a loop having three bridges in a system of the invention. 
       FIG. 2  is a simplified diagram of the  FIG. 1  loop. 
       FIG. 3  is a diagrammatic plan view of a portion of a rotor blade showing how two superposed crossed arrays of loops are installed that are identical or similar to those of  FIGS. 1 and 2 . 
       FIG. 4  is a diagrammatic plan view of a helicopter rotor blade incorporating a monitoring system of the invention. 
       FIG. 5  is a simplified block diagram of means for sequentially measuring the impedances of the loops in a blade and for processing and transmitting the measurements to a computer on board an aircraft. 
       FIG. 6  is a diagrammatic cross-section view of a blade of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIG. 1  in particular, the loop  10  comprises two parallel conductor strands  11  and  12  that are interconnected by three identical resistors  13 ,  14 , and  15 . 
   When the resistance of the three resistive bridges  13 ,  14 ,  15  is identical and much greater than the resistance of the strands  11  and  12 , then the impedance of the loop  10  as “seen” from its poles  16  and  17  is equal to one-third of the resistance of each of the bridges, assuming there has been no damage; when damage to the blade causes at least one of the strands  11 ,  12  to be broken in an end zone such as the zone  12   c  at the end of the strand  12  (and of the loop  10 ), then the resistance of the loop becomes equal to half the resistance of each of the bridges; when damage to the blade breaks at least one of the strands  11 ,  12  in a middle zone such as the middle zone  12   b  of the strand  12  (and the loop  10 ), then the resistance of the loop becomes equal to the resistance of each of the bridges; and when damage to the blade breaks at least one of the strands  11 ,  12  in a head zone such as the zone  12   a  at the head of the strand  12  (and of the loop  10 ), then the resistance of the loop becomes substantially infinite, since the loop is then open-circuit. 
   When the bridges  13  to  15  are regularly distributed along the longitudinal axis  19  of the loop  10 , the respective lengths  18  of the zones  12   a ,  12   b ,  12   c  of the loop are equal. 
   With reference to  FIGS. 2 and 3  in particular, in order to simplify the drawings, and taking account of the fact that the length  20  of a loop  10  is very long compared with the distance  21  between the strands  11  and  12  of the loop, the bridges  13  to  15  are represented diagrammatically by black spots tangential to the two parallel strands  11  and  12  of each loop  10 . 
   With reference to  FIG. 3  in particular, the blade  23  extends along a longitudinal axis  26  and presents a leading edge  24  and a trailing edge  25  that are substantially parallel to the axis  26 . 
   The blade has a first array of loops comprising three substantially rectilinear loops  10   a ,  10   b , and  10   c  extending along respective mutually parallel axes  19 A,  19 B, and  19 C; these axes  19 A to  19 C are at an angle  22  close to 45° relative to the longitudinal axis  26  of the blade, which angle corresponds to the angle of inclination relative to the axis  26  of the fibers of a sheet of fibers (not shown) constituting the structure of the blade. 
   The blade  23  further includes a second array of loops comprising four substantially rectilinear loops  10 W,  10 X,  10 Y, and  10 Z extending along respective mutually-parallel axes  19 W,  19 X,  19 Y, and  19 Z; these axes are substantially perpendicular to the axes  19 A to  19 C. 
   Thus, the loops of the first array cross the loops of the second array, with the second array being superposed on the first array. 
   The loops of the first array are disposed in a first layer or sheet, while the loops of the second array are disposed in a second layer or sheet that is separated from the first layer or sheet by a layer (not shown) of an electrically insulating material, in particular the thermoplastic or thermosetting resin impregnating the fibers of the composite structure of the blade  23 . 
   As a result, no electrical contact exists at the intersections of two overlying loops, such as the loops  10 C and  10 X. 
   Since the loops  10 A to  10 C and  10 W to  10 Z are identical, and since their mutual spacing  27  measured along the axis  26  is much less than the product of their common length ( 20  in  FIG. 2 ) multiplied by √2, each loop of the first group or array crosses (in superposition) at least two loops of the second group or array of loops. 
   By meshing the arrays of loops and resistor bridges in this way, in the event of a break or an interruption occurring in the arrays in the shaded zone  28  of  FIG. 3 , the position and the approximate extent of the damage to the blade can easily be determined. 
   Supposing that all the resistor bridges present an impedance of 1 kilohm (kΩ), then such a break will open the loops  10 C and  10 Y in their middle zones, such that the impedances of these two loops will become 1 kΩ, whereas the impedances of the other loops will retain the “normal” value of about 333Ω. 
   As a function of the loop impedance measurements received from the system incorporated in the blade, an on-board computer controlled by a specific damage-monitoring program including data representative of the configuration of the meshes stored in a memory of the computer, can determine a maximum area  29  in which the damage must have occurred (and including the zone  28  that is actually damaged), corresponding to the impedance measurements; it is thus easy not only to determine the position of the damage, but also to determine an over-estimate of the extent of the damage. 
   With reference to  FIGS. 3 to 5 , each resistive loop is connected by a pair of conductors  30  to a unit  34  integrated in the blade in the vicinity of the root zone  33  whereby the blade is connected (hinged) to the rotor (not shown). 
   The conductors  30  are grouped together in a bundle  31  that extends longitudinally (parallel to the axis  26 ) close to the leading edge  24  of the blade  23 , and close to the spar  230  of the blade  23  (cf.  FIG. 6 ). 
   In order to reduce the number of conductors, a common power supply wire can be established for a plurality of adjacent loops or for all of the loops in an area, for example a pressure side or a suction side covering of the blade. 
   The loops  10  extend from the bundle to the vicinity of the trailing edge  25 . 
   With reference to  FIG. 4 , the bundle  31  presents an end portion  31   a  in the vicinity of the free end  32  of the blade  23 , which end portion follows the angle of inclination of the leading edge in this location. 
   In the vicinity of the root  33  of the blade  23 , resistive loops also extend between the bundle  31  and the leading edge  24 . 
   In order to avoid overcrowding  FIG. 3 , the conductors  30  are shown for loops  10 W to  10 Z; nevertheless, one pair of conductors (not shown) is also required for connecting each of the loops  10 A to  10 C to the unit  34  ( FIGS. 4 and 5 ); the unit includes a multiplexer  40  or the like for successively scanning all of the loops of the blade, an impedance-measuring device  41  for measuring the impedance of each loop, and an analog-to-digital converter  42  for converting the measurement signals delivered by the impedance-measuring device into digital data. 
   Connection means  43  (wired or wireless) serve to convey the impedance data delivered by the converter  42  to the computer  44  on board the aircraft, for each of the blades of the aircraft rotor. 
   Where appropriate, impedance data for the loops can be conveyed and the components housed in the unit on the blade can be powered via electrical conductors for delivering power and/or transporting data as already used by some other system incorporated in the blade, e.g. a system for de-icing the blade. 
   A battery (not shown) incorporated in the unit  34  serves to power the components  40  to  42 . 
   The invention is particularly applicable to composite blades as described in French patents Nos. 2 740 379, 2 699 498, and/or 2 699 499.