Abstract:
A heat blanket for use in the repair of aircraft outer casings and method for using the heat blanket are provided. The heat blanket has an inner and outer layer of thermally conductive material. A series of heating elements are disposed between the inner and outer layers of materials and arranged into a plurality of heating zones. A control apparatus has a plurality of temperature sensors, each of which corresponds to one of the plurality of heating zones. The control apparatus provides power to the heating elements to control temperature for each respective zone as a function of sensed temperature. The blanket is shaped to cover a portion of the engine nacelle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In modern aircraft, much of the surface of the aircraft is composed of advanced composite structures. Composite structures are often made from two main components: filaments (or fibers) and resins. The composites in the aerospace industry contain fibers that are characterized by high length to diameter ratios and near crystal sized diameters. The fibers are used to transmit loads within the composite structure. The resin binds the fibers together, and is often referred to as the “matrix”. In addition to adding support for and transferring loads between the fibers, the resin also acts as an environmental protection for the structure(s) composed of the composite. The matrix has a lower density and strength than the fibers alone. The result of the matrix is a structural material that is characterized by high strength for its weight. 
         [0002]    During operation of the aircraft, damage is experienced by the composite structures. The damage may be the result of one or more of three categories: impact damage, thermal damage, and component failure. Impact damage results from contact of the composites with foreign objects. The foreign objects may be the result of normal operation of the aircraft (i.e., ice or dirt and gravel contact the engine structure due to operation), or from careless maintenance (i.e., dropped tools while repairing the aircraft, hitting the aircraft with another vehicle such as a forklift or luggage cart). Most impact damages cause structural degradation that leaves a mark or footprint that can be detected upon visual inspection. 
         [0003]    Excessive thermal exposure, especially to the engine case, also causes damage to composites. Over time, the composites of the engine casing will burn away the matrix leaving behind charred fibers or filaments. This can lead to delamination of the composites which can propagate to adjacent areas due to the weakened matrix. Thus, thermal damage repair is not limited to the specific area of visible damage. Damage may be inspected by radiographic (x-ray) inspection or ultrasonic inspection to find the extent of the damage. Such methods are non-destructive and thus minimize the need for additional repairs. 
         [0004]    Component failure can be due to catastrophic failure of the laminates of the composites. The failure is the result of numerous causes, such as improper design or improper use of the aircraft. Unanticipated or underestimated loads and stresses will result in component failure and require component replacement. This may be done by reinforcement of the component or remanufacture of the component. Again, the damage is found by non-destructive inspection. 
         [0005]    To repair damaged composites, heat sources are often used to decrease the required cure time of resins. Currently, small impact damages or areas of thermal damage are repaired by utilizing small localized heat blankets. For major repairs, aircraft components, such as an engine casing, are removed and placed into an autoclave. This requires much added expense from the extra labor for removal of components. Thus, there is a need in the art for a quick and less expensive repair method for aircraft components that eliminates the need for removal of the component from the aircraft. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In one embodiment, the invention is a heat blanket for use in the repair of aircraft outer casings. The heat blanket has an inner and outer layer of thermally conductive material. A series of heating elements are disposed between the inner and outer layers of materials and arranged into a plurality of heating zones. A control apparatus has a plurality of temperature regulating devices, each of which corresponds to one of the plurality of heating zones. Each of the regulating devices is capable of setting an exclusive temperature for each respective zone associated with the temperature regulating devices. The blanket is shaped to cover a portion of the engine nacelle. 
         [0007]    In another embodiment, the invention is a method of repairing an engine cowl. First, a section of a composite material containing a defect is removed and replaced with new material containing substantially the same properties as the removed section. Next, a heat blanket with a plurality of controllable zones, and which covers substantially the entire thrust reverser is supplied. At least one of the plurality of controllable zones of the heat blanket has its temperature is regulated from a common controller. The heat blanket is removed and a finishing process is applied to the replaced material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a plan view of a gas turbine engine casing. 
           [0009]      FIG. 2  is an exploded view of the gas turbine engine. 
           [0010]      FIG. 3  is a perspective view of a heat blanket for use in the repair of a damaged thrust reverser. 
           [0011]      FIG. 4  is a perspective view of another embodiment of a heat blanket for use in the repair of a damaged thrust reverser. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  is a plan view of a gas turbine engine  12 . Illustrated in  FIG. 1  is a nacelle  14  having a forward inlet portion  16 , middle cowl area  18 , and thrust reverser  20 . Nacelle  14  is constructed form a composite material. Also illustrated is exhaust nozzle  22 . Nacelle  14  protects, supports, and allows mounting of engine  12  to an aircraft, and is an outer casing for a gas turbine engine. The majority of the nacelle  14  is constructed from composite material due to the weight versus strength ratio of the material.  FIG. 2  is an exploded view of the gas turbine engine  12 . As shown, each portion of the nacelle has two individual halves, including inlet halves  16   a  and  16   b , cowl halves  18   a  and  18   b , and thrust reverser halves  20   a  and  20   b . Also illustrated are interior engine  24  and exhaust nozzle  22 . The individual components  16   a - 20   b  of nacelle  14  are secured together by fasteners. 
         [0013]    Forward inlet portion  16  is smoothly contoured with the front containing a generally toroidal shape, which directs air into the engine. Forward inlet portion is constructed from metal, or a combination of metal and composite. The forward edge is metal because of potential damage from ground vehicles or debris from engine operation during takeoff and landing. Middle cowl area  18  covers the fan, compressor, and combustion portions of the engine. The multipiece cowl area  18  may contain hinged attachments, access doors, or removable panels that allow for maintenance and inspection of the aforementioned engine components. Middle cowl area  18  is constructed from composite material. 
         [0014]    Thrust reverser  20  is generally conical, or contains a similar decreasing cross-sectional area towards the rear of the engine such as a quadratic surface. Thrust reverser  20  redirects the flow of exhaust from the engine back towards the engine to decelerate during landing of an aircraft. Thrust reverser  20  is constructed from composite. The composite is utilized for the nacelle components due to the light weight of the material compared to the strength of the material, as well as its resistance to high temperature. Thrust reverser  20  is constructed as a singular or two piece structure, compared to the multipiece structure of the middle cowl  18 . This assures a strong, continuous surface for redirecting airflow. 
         [0015]    Also illustrated in  FIG. 1  are damage defects  26 . Defects  26  cover a substantial portion of thrust reverser  20 . Repairing wire mesh composites of thrust reversers presently requires all attachment hardware be removed from the core cowl. The entire thrust reverser is repaired as a single unit to assure structural integrity. The thrust reverser is removed, epoxy and fillers are applied, a vacuum is drawn over the repair, and then the entire part is placed into an autoclave. The cost of the process ranges upward from $15,000.00 due to replacement parts and several weeks of labor. The current invention includes a heat blanket that covers the entire thrust reverser and/or the outer portion of the nacelle. The blanket eliminates the need for removal of the thrust reverser from the engine or aircraft. 
         [0016]      FIG. 3  is a perspective view of heat blanket  30  for use in the repair of a damaged thrust reverser  20  of nacelle  14 . Heat blanket  30  is constructed as is known by one of skill in the art. For example, heating blanket  30  may contain a series of flexible coil heating elements placed between thermally conductive layers, such as fiberglass reinforced silicone rubber sheets. Leads  32  extend from heating blanket  30  to control  34 . Control  34  allows for regulating the temperature of heating blanket  30 . Heat blanket  30  as illustrated is constructed to cover at least half of thrust reverser for example, thrust reversor half  20   a , to assure that any repair is completely covered by a singular heating source. Heat blanket  30  contains a contour to create a tight fit about the general conical shape of thrust reverser  20 . 
         [0017]      FIG. 4  is a perspective view of another embodiment of heat blanket  30 . Illustrated in  FIG. 4  are heat blanket  30  with leads  32   a  and  32   b  and controller  34 , thrust reverser  20 , and exhaust nozzle  22 . Heat blanket  30  contains an upper portion  30   a  and a lower portion  30   b , with each portion  30   a  and  30   b  containing separate leads  32   a  and  32   b . Leads  32   a  and  32   b  terminate at a common controller  34 . Common controller  34  allows for regulation of the entire heat blanket  30  from a single source. The two portions  30   a  and  30   b  of heat blanket  30  are affixed together by fasteners  36 . The two part construction promotes easy installation of blanket  30  about the contour of thrust reverser  20 . Fasteners  36  are any item that can temporarily hold the two portions  30   a  and  30   b  relative to one another that are common in the art, and may include snaps, zippers or similar items. Fasteners  36  may be located in sections  40   a  and  40   b  of blanket  30  that do not contain heating elements. 
         [0018]    Heat blanket  30  contains several separate heating zones  38 . Upper portion  30   a  contains eight individual zones  38   a - 38   h  as illustrated in  FIG. 4 . Each heating zone contains its own controllable heating elements and thermocouples. The number of thermocouples and heating elements for each zone may vary, and need not be a one to one relation. For example, heating zone  38   a  may contain twelve thermocouples  48  and twenty-four heating elements  50 . The thermocouples relay a temperature reading to controller  34  via leads  32 . Controller  34  will compare this reading to a setting entered by a user and adjust the current to the heating elements as required, e.g. increasing current to increase the temperature of the zone or decrease the current to reduce the temperature. Controller  34  contains a separate adjustment means for each heating zone  38   a - 38   h , as well as a master control for simultaneously controlling all zones to common temperature. Thus, if damage is located directly beneath zones  38   a - 38   b  and  38   g - 38   h , controller  34  set so all zone settings are set to a common temperature, or so that only zones  38   a - 38   b  and  38   g - 38   h  are heating. 
         [0019]    With the heating blanket  30  of the present invention, a repair can be done to fix damage  26  to the thrust reverser  20  of nacelle  14 . First, an inspection is done to determine the extent and location of damage. Next, the area near the damage or defect is removed as necessary for creating a more uniform area in which to place filler material. Any protrusions into the repair area are removed. After the repair area has been cleaned of any stray material, an filler epoxy is placed in the damaged area. This is allowed to set, and then the area is again cleaned, such as by abraiding, to obtain a flush surface. Alternatively, the filler epoxy is a resin that does not require hardening and finishing prior to application of a patch, but is inserted to create a flush outer surface on the repair part. In one embodiment, a release film is placed adjacent the repair area to prevent the resin from the repair from adhering to the undamaged composite structure adjacent the repair. 
         [0020]    After the repair area had been prepped as described, a patch of composite material is adhered to the area with a resin. The patch, is for example, fiber reinforced, such as with a wire mesh. The patch and resin together have essentially the same properties as the surrounding base material being repaired. Heating blanket  30  is then placed over thrust reverser  20 . 
         [0021]    A temperature is selected and set using controller  34  to apply heat over the repair area, and this is the temperature that heating blanket  30  must provide at its surface. Heating blanket  30  can be designed to provide a surface temperature of up to 250 degrees Celsius, with a deviation of plus or minus twenty degrees. To assure a proper cure and structurally sound repair, the temperature must be maintained. Often, the surface structure being repaired can at as a heat sink across the repair area. The contour design of heat blanket  30  assures that heat is provided to the entire surface covered by heat blanket  30 , thus eliminating the problem of standard heating blankets which will not accommodate complex or contoured surfaces. Further, with separate controllable zones, heat blanket  30  can auto adjust smaller areas to compensate for difference in heat transfer due to the structure of thrust reverser  20 . This counteracts the heat sink effect of any deviations in the surface of thrust reverser  20  to assure more even cure of the repair. 
         [0022]    The time required for curing of the resin varies upon the resin used and temperature utilized in the repair. For example, a resin may take four hours to cure at a temperature of 177 degrees Celsius. During the cure, heating blanket  30  can maintain a surface temperature on the patch of a deviation of plus or minus 1 degrees Celsius. When applying the heat, the temperature is ramped up to the required temperature. A rapid application of heat may create structural deficiencies. Similarly, upon completing the cure, the temperature is ramped down to prevent cracks or other defects. 
         [0023]    In an alternate embodiment, a vacuum is drawn over the repair are prior to heating. The vacuum drawn will provide a pressure difference over the repair. A flexible vacuum bag is positioned over the repair area, and sealed about its perimeter. A vacuum is then drawn to remove any air bubbles from the repaired area, and then heat is applied for curing the resin. Alternately, a vacuum bag is places around the entire thrust reverser and heating blanket. A vacuum is drawn prior to heating the repair area. Application of a vacuum assures that any bubbles are removed from the repair area to ensure a solid bond of the resin to the existing part. 
         [0024]    After repair area has been cured, heating blanket  30  is removed. A finishing process is done, if required, to achieve a smooth surface. Such finishing processes are known within the are including sanding or abrading of the repair area. The completed repair is inspected, tested using non-destructive techniques, and cleared for entry into service. 
         [0025]    Although the present invention has been described with reference to several defined embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.