DETECTOR FOR CRACKS IN HOLLOW BLADES

Abstract: A system for indicating cracks in a hollow aircraft blade, such as a helicopter blade. A system wherein a pressure differential is maintained within the blade with respect to the ambient atmosphere and including a pressure sensor for mounting on the blade in communication with the interior of the blade and having a member which moves in response to pressure changes within the blade, a beta radiation source mounted on the member and disposed for radiation transmission to a detector, a beta radiation detector for mounting on a nonrotating portion of the aircraft for receiving radiation from the source, and circuitry having the output of the detector as an input and providing an output which varies as a function of the position of the pressure sensor member for indicating the pressure condition within the blade. An improved beta radiation detector housing which sub­ stantially eliminates detector sensitivity to radar transmissions while maintaining sensitivity to nuclear radiation in the beta range.

Description: This invention relates to the detection of cracks in hollow blades such as aircraft propeller and helicopter rotor blades. Such blades are subjected to severe stress and occasionally develop minute cracks. It is of critical importance that a crack in the blade be detected at an early time so that the blade may be replaced preventing an inflight accident. A wide variety of methods are available for detecting cracks, including the making of x-ray pictures, magnetic flux techniques, and detection of pressure changes within a sealed hollow blade. A method and apparatus utilizing a blade pressure sensor is disclosed in US patent 3,985,3l8 and the present invention is an improvement on that system. The prior art system is illustrated in Fig.l which shows a conventional helicopter with a plurality of rotor blades l0. A pressure sensor ll with a beta radiation source is mounted in each of the blades. A beta radiation detector l2 is mounted on a fixed portion of the aircraft adjacent the rotating blades. The system also includes a signal processor l3 containing electronic circuitry, and a warning indicator l4 for mounting on the instrument panel of the aircraft. The typical pressure sensor ll includes a member which moves as a function of the pressure within the blade on which the sensor is mounted. A beta radiation source is carried on the moving member and a radiation shield or window is provided in the sensor so that radiation from the source is blocked when the pressure on the blade is in an acceptable range, and radiation passes the shield or window when the pressure is in an unacceptable range, thereby indicating leakage through a crack in the blade. This prior art system is described in greater detail in the aforesaid U.S. Patent 3,985,318. The present invention is directed to a new and improved beta radiation detecting apparatus. The radiation detector is mounted on the aircraft in a position for receiving radiation from a sensor. However, modern day aircraft are exposed to many types of radiation including electromagnetic radiation in the microwave range produced by high power radar systems and the like. These extraneous radiation sources can adversely affect the operation of the crack detection system when received by the radiation detector. The present invention substantially eliminates detector sensitivity to radar transmissions and other transmissions in the electromagnetic region, while maintaining detector sensitivity to nuclear beta radiation. According to the invention, the beta radiation detecting apparatus has a housing which housing includes an enclosure of an electrical conducting material and having an open side, means for supporting the beta radiation detector therein, a cover for the open side and having a window for passage of electromagnetic and beta radiation, an electrical conducting sheet, preferably a metal wire screen, carried in a gasket sandwich having a first layer of metal foil with a window therein, a second layer comprising the screen, and a third layer of metal foil with a window therein. Preferably there is a fourth layer of a compressive sealant material for improved sealing of the housing. The apparatus of the invention also includes an RF filter compartment within the enclosure which compartment has electrical conducting walls with openings for a connector which provides electrical circuit connections between the exterior of the enclosure and the compartment and for electromagnetic interference filters, mounted inside the compartment, which provide circuit connections between the compartment and the interior of the enclosure. In the drawings: Fig. 1 illustrates a helicopter with the prior art blade crack indicator system installed therein; Fig. 2 is a top view of a beta radiation detector housing for use in the system of Fig. 1, and incorporating the presently preferred embodiment of the invention; Fig. 3 is a side view of the housing of Fig. 2, shown partly in section; Fig. 4 is a top view of a gasket for the housing of Figs. 2 and 3, and incorporating the presently preferred embodiment of the invention; and Fig. 5 is an exploded end view of the gasket of Fig. 4. DESCRIPTION OF THE PREFERRED EMBODIMENT The housing of Figs. 2 and 3 includes an enclosure 20 of electrical conducting material, typically steel or aluminum. The housing has an open side 21 which is closed by a cover 22 attached by a plurality of screws 23. Studs 24 may be provided on another side of the enclosure for mounting purposes. A beta radiation detector 26, typically a Geiger-Mueller tube, is carried within the enclosure, typically being mounted on an electronics package 27 which preferably is encased or potted in the conventional manner. The electronics package 27 is attached to the bottom side of the enclosure 20 by screws 28 with spacer washers 29. The cover 22 typically is a metal plate and has a window 31 which provides a radiation path to the detector 26. Preferably the window 31 is covered by another plate 32 which produces little or no attenuation of radiation in the microwave and beta ranges. Typically the plate 32 is sheet of plastic such as epoxy glass, which is cemented to the cover 22. A gasket 35 is inserted between the cover and the enclosure, and the presently preferred embodiment of the gasket is shown in Figs. 4 and 5. The gasket 35 preferably is a sandwich comprising layers 36, 37, 38 and 39. The layers 36 and 38 preferably are a metal foil with a window 40 therein. The layer 37 is an electrical conducting sheet with apertures therethrough, typically a woven metal wire screen. The layer 39 is of a compressive conductive sealant material, typically referred to as EM1 gasket material, with a window 40 therein. The windows in the layers 36, 38 and 39 are aligned with the window 31 in the cover 22 so as to provide direct line of sight from the exterior of the housing to the tube 26. Openings 43 are provided in all layers of the gasket sandwich for the cover mounting screws 23. An RF filter compartment 45 is provided within the enclosure 20. The walls of the compartment 45 are formed of electrical conducting material and typically of the same material as the enclosure 20, with the compartment being formed by welding or casting. A conventional electrical cable connector 47 is mounted in an opening 48 of the compartment 45, and provides electrical circuit paths between the exterior of the housing and the interior of the compartment. A plurality of insulated feed through type electromagnetic interference filters 49 are mounted on a plate 50 which in turn is attached to a wall 51 of the compartment 45 by screws 52, with the filters 49 passing through another opening 53 in the compartment 45. The filters 49 provide electrical circuit paths between the interior of the compartment 45 and the electronics unit 27 in the interior of the enclosure 20. With the RF filter compartment 45, the exterior electrical connections from the signal processor 13 are connected to the electronics package 27 through the compartment 45 and the filters 49, rather than being directly connected by a single connector as in the prior systems. The electrical conducting apertured sheet or screen is selected to have openings of a size which will not pass electromagnetic radiation in the microwave range. Typically each opening should have a width not greater than about 0.127 cm (.050 inches). At the same time, the sheet or screen with openings of this size readily passes beta particles. In operation, the beta particles from the source in the sensor are readily passed through the detector housing to the tube 26 when the sensor has its source exposed by the moving member and the blade carrying the sensor passes over the detector. However, electromagnetic radiation from radars and others sources in the vicinity of the aircraft is blocked from the detector 26 by being electrically grounded at the sheet 37. Another source of disturbance to the detector is electromagnetic interference transmitted via wire cable connections to the electronics in the housing. By utilizing the enclosed RF filter compartment which isolates the external electrical cable from the interior of the enclosure, along with the electromagnetic interference filters within the compartment, this type of disturbance is eliminated or greatly reduced.

Claims: A beta radiation detecting apparatus for indicating a crack in a hollow aircraft blade which uses a beta radiation source on the blade and a beta radiation detector on a non-rotating part of the aircraft, with a housing for the radiation detector, the housing comprising: an enclosure (20) of an electrical conducting material and having an open side (21) and means for supporting the beta radiation detector (26) therein to be exposed to radiation passing through said open side; a cover (22) for said open side (21), said cover having a window (31) for passage of electromagnetic and beta radiation; an electrical conducting sheet (37) entrapped between the cover and the enclosure wherein said electrical conducting sheet (37) is carried in a gasket (35) having a sandwich of a first layer of metal foil (36) with a window (40) therein, a second layer comprising said electrical conducting sheet (37), and a third layer of metal foil (38) with a window (40) therein, with said first and third layer windows aligned with the window (41) of the cover (22), said electrical conducting sheet having apertures therethrough, the size of the apertures being chosen so as to permit beta radiation particles to pass to the detector (26) while blocking electromagnetic radiation in the microwave range, and in that, to isolate external cable from the interior of the enclosure, the housing includes: a RF filter compartment (45) within said enclosure (22), said compartment having electrical conducting walls with first (48) and second (53) openings; connector means (47) mounted in said first opening (48) for providing electrical circuit connections between the exterior of said enclosure (20) and the interior of said compartment (45); and electromagnetic interference filter means (49) mounted within said second opening (53) for providing electrical circuit connections between the interior of said compartment (45) and the interior of said enclosure (20). A radiation detecting apparatus as claimed in claim 1 wherein said sandwich includes a fourth layer of a compressive sealant material (39) with a window (40) aligned with the window (41) of the cover (22). A radiation detecting apparatus as claimed in claim 1 or claim 2, wherein said electrical conducting sheet (37) is a metal wire screen. A system for detecting cracks in hollow aircraft blades wherein a pressure differential is maintained within the blade with respect to the ambient atmosphere and for operation in the presence of strong electromagnetic fields, the system including a pressure sensor adapted for mounting on the blade in communication with the interior of the blade and adapted to transmit radiation from a beta radiation source therein when predetermined pressure conditions are sensed, a beta radiation detecting apparatus for mounting on a non-rotating portion of the aircraft in a position to receive radiation from the source, and a circuit having the output of the detector as an input and providing an output varying as a function of the condition of the sensor for indicating the pressure condition within the blade, wherein the detecting apparatus is as claimed in any one of the preceding claims with the window positioned for receiving beta radiation from the source. A system according to claim 5 wherein the sensor includes a movable member mounting the source, the member moving to a position permitting radiation transmission when a loss of pressure is detected.

Inventors: BLINCOW,DONALD W, MAHONEY,JOHN J, MCCORMICK,JEROLD H
Applicants: GENERAL NUCLEONICS, INC.
Application year: 1987
Main classification: G01T1
IPC codes: G01T1, B64C27, G01L9, G21H5, G01Q90