Abstract:
Percussive impacts due to bird strikes upon the hollow fan blades  20  are a well known problem. These percussive impacts not only deform previous fan blades but also reduce their stiffness. In accordance with the present invention, hollow fan blades  20  incorporate a cavity  23  within which, a matrix  24  with embedded expandable elements  25 , is located. Thus, upon a percussive impact these expandable elements  25  are released in order to create an internal pressure within the cavity  25  which acts outwardly in order to relieve deformation and also stiffen the blade  20  as a result of the over pressure within the cavity  23.

Description:
FIELD OF THE INVENTION 
   The present invention relates to blades and more particularly to hollow fan blades used in gas turbine engines. 
   BACKGROUND OF THE INVENTION 
   Referring to  FIG. 1 , a gas turbine engine is generally indicated at  10  and comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high pressure compressor  14 , combustion equipment  15 , a high pressure turbine  16 , an intermediate pressure turbine  17 , a low pressure turbine  18  and an exhaust nozzle  19 . 
   The gas turbine engine  10  works in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  which produce two air flows: a first air flow into the intermediate pressure compressor  13  and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
   The compressed air exhausted from the high pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines  16 ,  17  and  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low pressure turbine  16 ,  17  and  18  respectively drive the high and intermediate pressure compressors  14  and  13 , and the fan  12  by suitable interconnecting shafts. 
   It will be understood that particularly with regard to aircraft engine installations, weight is an important consideration. In such circumstances, if turbine fan blades can be hollow there is a significant reduction in both individual and collective weight of the fan blades and the propulsive fan  12 . However, it will also be understood that in view of the significant air flow rates there is a high danger with regard to impacts, particularly upon the fan blades of the propulsive fan  12  at the air intake  11  end of the engine  10 . It will be understood that these impacts may be as a result of debris or bird strikes on the propulsive fan  12  and possibly as a result of secondary and cursory impacts through the engine. In such circumstances, the inherent reduced weakness and susceptibility to deformation of hollow fan blades is a significant problem. 
   It will be understood that the fan blade assembly in the engine must be balanced for appropriate operation and that each individual fan blade must remain sufficiently structurally strong for continued operation until repair is possible. Previously, it has been known to fill the hollow cavity within fan blades with elastomeric materials in order to provide principally vibration damping within the blade but also by implication some reinforcement of that blade. Nevertheless, it will be appreciated that these hollow cavity fillings being of an elastomeric nature will still become distorted with the fan blade when exposed to severe impact loads beyond the elastic deformation response of that fan blade in association with the filling. 
   As indicated above, the most dangerous impacts to fan blades relate to so-called bird strikes. Such bird strikes alter the run on behaviour of hollow fan blades due to alterations in the propulsive fan configuration which detrimentally affect both structural stiffness and natural frequency within the fan blade. These variations in structural stiffness and frequency combine with leading edge deformations in order to cause blade flutter and possibly further structural degradation due to vibrational work hardening etc. In view of these factors, it is generally accepted good practice to provide a more robust hollow fan blade than strictly necessary in order to resist initial deformation. Clearly, more robust fan blades necessitate greater blade mass for the same material type. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, there is provided a blade for a gas turbine engine, the blade having a hollow cavity and that hollow cavity incorporating releasable bias means to present outward pressure when subjected to significant percussive load. 
   Preferably, the releasable bias means comprises a matrix with incorporated rupturable elements which rupture in use upon percussive load to provide the outward pressure. 
   Typically, the matrix is an elastomeric material or foam or viscous liquid or resin. Normally, the matrix also acts as a damping medium for the blade. 
   Typically, the rupturable elements comprise capsules or other elements embedded within the matrix. Generally, these rupturable elements are uni-operational or reactive. Typically, uni-operational rupturable elements upon rupture provide their own outward pressure in co-operation with other rupturable elements. Generally, reactive rupturable elements release upon rupture reactants in order to react with other reactants from other reactive rupturable elements in order to provide the outward pressure. Alternatively, reactive rupturable elements react with the matrix in order to provide the outward pressure. Typically, the rupture elements incorporate a liquid or gas or generate a foam in order to provide the outward pressure. Possibly, the rupturable elements incorporate points or lines of weakness to facilitate rupture upon application of a percussive load. Advantageously, the rupturable elements provide a directional response upon rupture with regard to the outward pressure. Possibly, the rupturable elements rupture by discharge through perforations in the surface of the rupturable element. Generally, the rupturable elements are differentially distributed and/or sized and/or variously responsive at different locations within the hollow cavity. 
   Possibly, the blade incorporates means for blade venting to limit the extent of outward pressure presented within the hollow cavity. Possibly, blade venting is at a tip of the blade. Alternatively, blade venting is through perforation pimple release of a part of the blade surface. Possibly, blade venting provides an indication of percussive loads to the blade through indicator means. Possibly, such indicator means comprises viewing blade venting at a sight hole in which presence of releasable bias means indicates percussive loading of a fan blade. Possibly, the releasable bias means changes colour when subjected to percussive load to provide indication means upon application of that percussive load in use. 
   Also in accordance with the present invention there is provided a gas turbine engine incorporating a fan blade as described above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic section of a gas turbine engine; and 
       FIG. 2  is a schematic cross-section of a fan blade prior to a percussive impact; and, 
       FIG. 3  is a schematic cross-section of a fan blade after a percussive impact. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2  illustrating a schematic cross-section of a fan blade  20  in accordance with the present invention. This fan blade  20  comprises a hollow aerofoil  22  in which a cavity  23  is filled with a matrix  24 . Typically, the aerofoil  22  is made from titanium alloy panels, for example T 1   6 A 1   4 V alloy appropriately secured together. The matrix  24  is generally elastomeric and acts as a damping medium for the fan blade  20 . Within the matrix  24  a number of capsules or elements  25  are appropriately located and distributed. 
   The fan blade  20  as configured in  FIG. 2  is shown prior to a percussive impact. Thus, the fan blade  20  is consistent with the fan blade as installed within a gas turbine engine. 
   Referring to  FIG. 3  illustrating a schematic cross-section of the fan blade  20  subsequent to a percussive impact such as a bird strike. Thus, the blade  20  comprises an aerofoil  22  within which the cavity  23  is filled by the matrix  24  but in the after percussive impact configuration depicted in  FIG. 3  the capsules or elements  25  have expanded as a result of rupture or other activations due to that impact. In such circumstances, the elements  25  have releasably expanded due to the percussive impact in order to create an outward pressure in the direction of arrowheads  26 . This outward expansion within the matrix  24  ensures the removal of any voids or gaps in the cavity  23  as a result of the percussive impact. Furthermore, the fan blade  20  stiffness following percussive impact has been increased. In effect, the elements  25  expanding within the constriction of the cavity  23  and the matrix  24  exerts an increased internal pressure within the cavity  23  which causes greater blade  20  stiffness. The increased internal pressure within the cavity  23  also acts to prevent or reverse any buckling or collapse of cavity  23 . 
   In view of the reactive nature of the releasable expandable capsules or elements  25 , it will be appreciated that the aerofoil  22  may be made from lighter or a thinner cross-section of material than previously acceptable in order to provide appropriate resistance to bird strike level percussive impacts. Clearly, it is necessary to set and predetermine the appropriate level of percussive impact load to release the expansive bias within the cavity  23  in the form of increased internal pressure within that cavity  23 . As indicated above, generally, this level of impact load will be determined by expected bird strike scenarios. 
   As with previous hollow fan blades, it is necessary to provide vibration damping within the cavity of such blades. Furthermore, it will be appreciated that thinner cross-section or lighter materials used with fan blades in accordance with the present invention will render these fan blades more susceptible to vibration such that it is more important to provide appropriate damping. In such circumstances, the matrix  24  which fills the cavity  23  and within which the capsules or elements  25  are embedded will act as an appropriate vibration damping filler both during normal operation ( FIG. 2 ) and post percussive impact operation ( FIG. 3 ). 
   As can be seen in  FIG. 3 , generally the fan blade  20  will bulge about the point of percussive impact. However, the fan blade  20  will remain stiff and operational. Thus, the present fan blade  20  subsequent to a percussive impact will be able to continue to operate until appropriate replacement. Generally, the overall blade fan  20  thickness may be reduced which in turn leads to a lighter aerofoil, root attachment mechanism for the fan blade  20  and necessary containment system for the fan blade within the engine cowling/casing. 
   A trailing edge  22   b  of the fan blade  20  will generally have an improved integrity due to greater resistance to deformation. The filler in the form of matrix  24  and elements  25  in the trailing fan blade will also exert a reactive outward force. 
   It will be appreciated that as indicated above, the expandable capsules or elements  25  will react to percussive impacts. This reaction will generally be in terms of a rupture to the capsule or element  25  in order to release an expandable mechanism. Generally, the impact necessary for this rupture and expansion will be predetermined for each capsule or element  25 . 
   The fan blade  20  will incorporate appropriate release or vent mechanisms to prevent such excessive outward pressure within the cavity in the direction of arrowheads  26 . For example, the internal pressure created within the cavity  23  will vent as a fan blade  20  tip is broken. The fan blade  20  may be configured with a line of weakness to create such venting at the most appropriate location. It is most important that the fan blade  20  has a significant run on capacity after percussive impact in order to allow the gas turbine engine  10  incorporating the now damaged fan blade  20  to operate safely until the damaged fan blade  20  can be replaced or repaired. 
   As indicated above, the matrix  24  and expandable elements or capsules  25  in combination provide a releasable bias within the cavity  23 . The bias to create outward pressure in the direction of arrowheads  26  occurs when a percussive load is applied to the fan blade which ruptures the elements  25  in order to create expansion. As indicated previously, the matrix  24  is generally of an elastomeric nature in order to provide vibrational damping. In such circumstances, the matrix  24  will absorb some of the percussive impact but when that percussive impact is excessive, rupture and release of the elements  25  occurs. As an alternative to providing an elastomeric matrix, it will be understood that the matrix could be a foam or viscous liquid or resin with varying levels of deformation response. 
   The elements or capsules  25 , as indicated upon application of a percussive force, generate expansion. There are a number of mechanisms by which these elements  25  can create such expansion. The elements  25  could be uni-operational in that simple rupture of an element  25  releases a pressurised gas or other mechanism in order to create an expansion in volume over than of the original element  25  value. Alternatively, the elements  25  could incorporate reactants which upon specific release react together in order to create volumetric expansion and therefore increased pressure within the cavity  23  for a presented outward pressure in the direction of arrowheads  26 . 
   These reactants may be held within respective compartments of the elements  25  which mix on rupture or reactants from different elements  25  when released from their respective elements  25  acting to provide volumetric expansion or a reactant could be released from the elements  25  which react with the base matrix  24  in order to create expansion. Typically, the reactants within the elements  25  will be in liquid form but once released by rupture will create a gas or form a foam in order to create the outward pressure within the cavity  23 . Uni-operational elements  25  may simply constrain relatively high pressure gas which is released upon rupture. Examples of suitable gases for pressurised storage in uni-operational elements include inert gases such as argon, helium or nitrogen. 
   Reactants which may be stored within the elements  25  in order to create gas or foam for the outward pressure upon rupture are. Typically, these reactants take the form of an adhesive based upon an epoxy, polyurethane, silicon or similar mechanism matrix within which an appropriate foaming agent is provided. These foaming agents are also used with regard to detergents, forming closed cell foam products, fire extinguishers, concrete adhesives and soil stabilisers. Appropriate foaming agents will be chosen in accordance with particular requirements. With the present invention, fatigue failures in a fan blade can be controlled in order to prolong the lifespan of a fan blade component. The filler, including the reactant, releases a foaming adhesive which expands and so is able to fill large voids and cavities when activated under an impact load. 
   Reliable release of the outward pressure is a requirement when subjected to a predetermined percussive load. In order to achieve this reliability generally each element or capsule  25  will incorporate at least one line or position of weakness to ensure release when subjected to a predetermined impact load. In such circumstances, by appropriate determination of distribution and/or size and/or operational type of elements  25  it is possible to provide appropriate and proportional response within the fan blade  20  to impact loads. It will be understood that impacts at different parts of the fan blade  20  may require different responses and this as indicated can be reflected by element  25  distribution, size and type for best performance. In such circumstances, the distribution and/or size and/or operational type of elements  25  may be different in different parts or zones of the blade  20 . 
   In order to maximise performance by appropriate use of lines of weakness within the elements  25  it is possible to achieve directional response from the elements or capsules  25 . For example, by providing crossed lines of weakness, it will be appreciated that petal or bud rupture of the element  25  may be achieved which is of a directional nature. Alternatively, each element  25  may incorporate a perforated surface such that upon compression due to percussive load, reactants within the element  25  are forced through the perforations to create expansion either by gas release or reaction. These perforations may be asymmetrical about the elements  25  for directional control. It will be understood that if a directional response is required then each individual element  25  will require appropriate orientation within the cavity  23  and this will create manufacturing problems but may be achieved through magnetic alignment techniques and float bubble component manipulation processes. 
   A further alternative with regard to achieving release of the expansion within the elements  25  could be through providing a capsule within which a first reactant liquid is located along with a reactant pill floating within that liquid such that upon percussive impact the pill is broken to release a mixture of the reactants and expansion of the element. In such circumstances, the element itself may not rupture but simply expand due to the reaction between the reactants within the element. Such an approach may allow use of more aggressive reactants without danger of corrosive or detrimental action within the fan blade  20  or matrix  23 . 
   As indicated above, excessive outward pressure within the cavity  23  may cause ballooning of the fan blade  20 . In such circumstances, typically each fan blade will incorporate an appropriate venting or release mechanism for over expansion. This release mechanism will take the form of a hole or expansion cavity at an appropriate position within the blade  20 . Typically, such vent holes or expansion cavities will be located at the tip ends of the fan blades  20 . It will be understood that if such vent holes or expansion cavities can be visually inspected then these vent holes or expansion cavities act as sight holes to indicate occurrence of an impact above the predetermined percussive load and so indicate that the fan blade requires replacing. By incorporating appropriate dyes such sight inspection of vent holes or expansion cavities may be made easier. It will also be understood that pop-out sections or pimples could be provided which extend from the cavity  23  to the surface such that upon release of the elements  25  to cause expansion and therefore overpressure within the cavity  23  these pop-out elements are raised above the blade  20  surface. Similarly, it will be appreciated that vent holes may be plugged during normal operation but this plug displaced when elements  25  are released in order to create pressure within the cavity  23 . It is important that the overpressure created within the cavity  23  is not itself detrimental to fan blade  20  operation. For example, it will be appreciated if the over pressure within the cavity  23  were excessive then that overpressure itself could rupture the aerofoil  22  or at least create cracking significantly reducing the structural strength of the fan blade  20 . It is judicial use of overpressure within the cavity  23  in order to provide improved fan blade  20  stiffness after percussive impact which is of prime concern. It will be appreciated that insufficient over pressure will not provide the necessary improvement in stiffness necessitated by the diminution as a result of percussive impact whilst over pressure itself may create undue balloon distortion and rupture the cavity  23  itself. In order to avoid both of these scenarios, it is important that the distribution and/or size and/or type of elements  25  chosen for a particular fan blade  20  are determined for a proportional response. 
   Normally, the matrix  24  and elements  25  will simply be loaded in a liquid state into the cavity  23  and allowed to set in order to substantially fix element  25  location. As indicated above, techniques such as magnetic attraction with respect to elements  25  may be used in order to achieve some orientation and manipulation techniques with regard to the fan blade  20  may also be used to provide some control of element  25  distribution but nevertheless, with such a simple packing approach it may be difficult to specifically locate elements  25  as required. In such circumstances, bands or zones within the cavity  23  may be built up in staged steps from different combinations of matrix  24  and elements  25  in terms of proportion and types to achieve the desired response in use. It may be possible in some circumstances, to mould an insert for the cavity  23  externally of the fan blade  20 . In such circumstances, the insert may allow greater positioning of the elements  25 . It will be understood that once moulded and set the insert would simply be slipped into the cavity  23 . Generally, more elements  25  will be located towards the leading edge  22   a  of the fan blade  20  rather than the trailing edge  22   b  as it is the leading edge which is more likely to be exposed to percussive impacts. 
   Although the invention has been described with reference to a fan blade, it is equally applicable to other hollow blades. 
   Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.