GB2472053A

GB2472053A - Aircraft and engine deicing apparatus

Info

Publication number: GB2472053A
Application number: GB0912779A
Authority: GB
Inventors: Cosimo Buffone; David Sydney Knott
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2009-07-23
Filing date: 2009-07-23
Publication date: 2011-01-26
Also published as: GB0912779D0

Abstract

Deicing apparatus 30 for dislodging ice from a surface which is prone to ice accretion such as aircraft wings, aircraft engines and associated aerodynamic structures includes a plurality of pedestals 34 in contact with the surface 32 and arranged in a spaced two-dimensional array relative to said surface. Actuation means 38 are arranged to reversibly deform the surface 32 about said pedestals 34 between an at-rest condition and a deformed condition. The actuation means 38 is arranged to cause a variation in height of the surface between the pedestals in the two-dimensional array. The actuation means may include temperature dependent shape memory alloys in combination with an electrical heating system. The actuation means may comprise piezoelectric stacks (110, fig 7) acting on lever arms (120, Fig 7).

Description

Active De-icing System The present invention relates to a structure for dislodging ice and more particularly, although not exclusively, to a structure which can be selectively activated to dislodge a mass of ice accumulated thereon.

The accumulation of ice is widely acknowledged to be a problem for equipment which is subjected to temperatures below freezing. The problem is typically exacerbated for equipment which undergoes movement in freezing conditions such as for example aircraft and gas turbine engines therefore since motion through the air can increase the rate at which ice accumulates. Such conditions and problems associated therewith are not solely associated with aircraft and have also been found to affect, for example, wind turbines, telecommunications masts and antennas.

Particularly for moving equipment, accumulated ice will tend to become dislodged passively only when a significant mass has accrued. Such a mass of ice in motion poses a grave danger to individuals or else can damage equipment upon impact therewith. For gas-washed airfoil surfaces, a smooth profile is critical in achieving lift and the build up of ice on the surface itself can cause significant degradation in aerodynamic performance. It is known to mitigate such problems either by inhibiting formation of ice on a surface or else by dislodging ice in a manner which is less problematic and/or dangerous.

Known techniques for de-icing include either using chemicals to destabilize ice such that it is more readily dislodged by passing fluid, or else heating surfaces prone to ice build-up using hot gases, such as gases from a compressor or combustor, or else electric heating elements.

However both techniques are not without disadvantages. The energy required to effectively heat surfaces is unacceptable for a number of applications, especially in light of the current trend to reduce fuel burn and increase efficiency within the aerospace industry. Furthermore the continual application of chemicals consumes considerable volumes material, which is detrimental to the environment and costly to equipment operators.

In light of such disadvantages, further proposals for de-icing have involved the actuation of a surface in order to dislodge ice accumulated thereon in pieces which are sufficiently small to avoid the problems described above.

Known examples make use of actuators which apply impulse forces or vibrations to the iced surface in order to promote displacement of the ice. However the effectiveness and reliability of such systems leaves room for improvement. Furthermore, practical constraints such as available mechanical space, low power requirements, adverse operating environments and the need for long operational life impact on the successful adoption of such technologies.

International Publication Number Wa 95/34189 (international Application Number PCT/US95/07165) describes a system in which Shape Memory Alloy (SMA) materials are used to mechanically manipulate the iced surface. Sheets or wires of SMA material are used to deform the smooth surface to create ribs therein.

It is an aim of the present invention to provide an alternative and/or improved active de-icing system which avoids the occurrence of relatively large, and potentially dangerous, masses of dislodged ice.

According to one aspect of the present invention there is provided de-icing apparatus for dislodging ice from a surface which is prone to ice accretion, the de-icing apparatus comprising: a plurality of pedestals in contact with the surface and arranged in a spaced two-dimensional array relative to said surface; and actuation means for reversibly deforming the surface about said pedestals between an at rest condition and an actuated condition, the actuation means arranged to cause a variation in height of the surface between the pedestals in the two-dimensional array.

The array of pedestals according to the present invention may allow for deformation of the surface in two or more different orientations simultaneously. Such orientations are typically not parallel and, as such, may be oblique or orthogonal. Flexing of the surface in this manner may shear the ice accumulated thereon by changing the curvature of the surface in an improved manner. The surface to be de-iced is typically an outer surface of a body which provides a support structure for the surface.

In the at rest condition, the surface may have a flat or smoothly curving surface.

A layered structure may be provided in which the actuation means is sandwiched between a first layer and the support structure. The first layer may comprise the surface. The actuation means may be comprised within a second layer and may be embedded in either or both of the first layer and the support structure. The actuation means may be sandwiched between a first outer layer and a third inner layer which faces the support structure. The actuation means may be anchored or embedded in the first and third layers.

According to one embodiment, the pedestals are fixed in height and the actuation means serves to deform the surface about said pedestals in the actuated condition.

The pedestals may resist actuation of the surface in a passive manner.

The actuation means may comprise a lamina of actuation material in the form of a sheet, strips or wires which overlays the array of pedestals. The actuation means may comprise a shape memory material such as a shape memory metal or alloy. A grid or intersecting network of elongate actuation components may be provided. The intersecting network may pass between the pedestals in the array and may be shaped to correspond to arrangement of the pedestals in the array.

The array may comprise a plurality of rows and/or columns of pedestals. The array may comprise a regular pattern of pedestals.

In one embodiment, the pedestals are arranged to support the surface. The pedestals may be mounted on a body so as to support the surface relative to the body.

The body may comprise an airfoil body and may be arranged such that the surface comprises at least a portion of an outer fluid-washed surface of the body in use. The surface may pass over at least a leading edge of the airfoil.

The spacing of the pedestals in the array may be constant or else may vary in correspondence with a geometric feature or parameter of the surface. The density of the array may increase or decrease towards a leading edge of the body or else with changes in curvature of the surface.

The surface may comprise a first side or surface of a layer having a first side which is prone to ice accretion and a second side opposing the first side.

The actuation means may be actuated by heat and/or electricity so as to cause a geometrical change therein.

In one embodiment, the actuation means is arranged to adjust the height of one or more pedestals within the array. Each pedestal may comprise an actuation means. The pedestal actuators may selectively vary the height of the pedestal between at rest and actuated conditions. Rigid wall or spacer members may be located between adjacent pedestals in the array, which may resist the tensile force applied to the surface by the pedestal actuators. The actuation means may actuate all or a selected subset of the pedestals.

Mechanical amplification of the actuation means may be provided. The mechanical amplifier may comprise one or more lever arms. The lever arms may be mounted at an oblique angle between the actuator and the surface. The, or each, lever arm may be pivotably mounted to either or both of the actuator and/or surface. The lever arms may be arranged in opposing pairs, each arm of a pair depending from an opposing end of an actuator.

The actuation means may comprise a piezoelectric element. The actuation means may comprise a stack of aligned piezoelectric elements. The pedestals may be comprised within a pedestal-containing lamina. The depth of the lamina may be smaller than the length of the actuator which may be a piezoelectric stack. The actuation means may comprise a trapezoidal mechanism structure.

According to a further aspect of the invention there is provided a method of forming a de-icing structure for a surface as claimed in claim 19.

One or more working embodiments of the present invention are described in further detail below by way of example with reference to the accompanying drawings, of which: Figure 1 shows a half cross section of a gas turbine engine; Figure 2 shows a cross section through a structure according to the present invention; Figure 3 shows a plan view of the structure of figure 2; Figure 4 shows further detail of a cross section through a structure according to a first embodiment of the present invention; Figure 5 shows the structure of figure 4 in an actuated condition; Figure 6 shows a plan view of the structure according to the first embodiment; Figure 7 shows a cross section through the surface of a structure according to a second embodiment of the invention; Figure 8 shows further details of the actuation means of the embodiment of figure 7; Figure 9 shows the structure of figure 7 in an actuated condition; and, Figures lOa to lOf show various possible plan layouts of arrays of pedestals according to the present invention.

With reference to Figure 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, and intermediate pressure turbine 18, a low-pressure turbine 19 and a core engine exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines the intake 12, a bypass duct 22 and a bypass exhaust nozzle 23.

The gas turbine engine 10 works in a conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow into the intermediate pressure compressor 14 and a second air flow which passes through a bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the air flow directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 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 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 17, 18, 19 respectively drive the high and intermediate pressure compressors 15, 14 and the fan 13 by suitable interconnecting shafts.

The present invention may be applicable to any gas-washed surface of the gas turbine engine 10 which is prone to icing, such as for example the intake wall 12, fan blades 13 and/or other fixed or rotating components of the engine, such as vanes. The concept may also be applied to alternative types of fixed or moving structures which are prone to icing, including propellers, wind turbines, aircraft wings or masts for telecommunications or the like.

A schematic of the present invention used on an aerofoil surface is shown in figure 2. The aerofoil 24 comprises a body 26 which is prone to icing on a leading portion of the aerofoil outer wall 28 in use. At least the leading portion of the aerofoil is provided with a modified surface structure 30 comprising an outermost layer 32 supported by a plurality of spaced pedestal formations 34.

The pedestal formations 34 collectively support the outer layer 32 on the aerofoil body 24. In this embodiment, the layer 32 terminates part-way along the body 26 between a leading and a trailing edge. A smooth transition between the layer 32 and the remainder of the aerofoil body surface 28 is provided in order to maintain the desired smooth surface profile.

The arrangement of the pedestals 34 beneath outer layer 32 is shown in figure 3. As can be seen the pedestals are arranged in a two-dimensional array relative to the layer 32. It will be appreciated that the curved nature of the layer 32 results in the array of pedestals being three-dimensional within a global frame of reference.

However, relative to the curved layer 32 itself, the array may be considered two-dimensional. The pedestals are spaced about the area of the layer 32 such that they are upstanding relative there-to.

The pedestals 34 in this embodiment are arranged in a substantially regular array comprising a plurality of rows and columns. Rows are substantially perpendicular to columns and the spacing between each row and column is generally equal. However each adjacent row and column is offset from an adjacent row or column by half. Thus adjacent rows or columns are out of phase. Such an arrangement produces the pattern shown in figure 3, which may be useful for achieving deformation in the surface layer 32 as will be described below. However alternative embodiments may prescribe different arrays in which adjacent rows and columns may be offset by a different amount or else aligned such that each pedestal is arranged at a junction of a grid-type arrangement. In other embodiments, the pedestals may not be arranged in a regular array but may display varying spacing, for example according to the curvature of outer layer. Additionally or alternatively, the spacing between rows or columns of pedestals may vary as a function of distance from a defined feature such as a line or point. In one example, the spacing may vary as a function of distance from the leading or trailing edge of the aerofoil or else with distance from the root or tip.

In figures 4 to 6, there is shown a first embodiment of a system for deforming the layer 32 in order to dispel ice there-from. In figures 7 to 10 an alternative system for achieving deformation of the layer 32 is shown. The first embodiment generally relies on actuation of the surface layer(s) about fixed pedestals, whereas the second embodiment generally relies on the actuation of the pedestals to deform the surface layer. Whilst the two embodiments will be described separately below, it is to be noted that the first and second embodiments could be combined. That is to say the actuating pedestals of figures 7 to 10 could usefully be applied in conjunction with the actuated surface layer(s) of figures 4 to 6.

Accordingly the first and second embodiments are not mutually exclusive and any of the individual or combined features described in relation to one embodiment may be applied to other embodiments of the invention.

Turning now to figure 4, there is shown the body 26 of figure 2, which is aerofoil in section. The outer layer 32 is formed of a plurality of layers comprising an outer layer 36 an intermediate layer 38 and an inner layer 40.

The intermediate layer 38 comprises and actuation material in the form of a shape memory alloy (SMA), which is sandwiched between two polymer layers 36 and 40.

It has been found that a SMA material, such as a NiTi SMA, can be deposited as a fluidic coating on a desired surface. Heating of the fluidic coating, once applied, is used to produce the desired microstructure within the SMA material for subsequent use as an actuator for the purpose of de-icing. Heating of the NiTi coating can be achieved using infrared heaters or else by application of a voltage across the SMA layer using suitable electrical contacts at opposing ends or other spaced locations thereon.

Alternatively, resistive heating elements may be provided which contact the SMA and conduct heat thereto.

The inner 40 and outer 36 polymer layers are formed such that the inner layer 36 is thinner than the outer layer 40.

Turning now to figure 6, an exemplary layout of the actuating SMA layer 38 is shown. The actuation layer does not comprise a continuous surface but instead takes the form of a plurality of intersecting strips 42 which pass between the pedestals of the embodiment. In this regard, the actuation layer takes the form of a grid, mesh, lattice or other network structure which is positionable amongst the two-dimensional array of pedestals. In this embodiment, each pedestal is surrounded by the actuator network structure such that one or more strips separates each pedestal 34 from an adjacent pedestal in the array.

In the embodiment of figure 6, strips of actuation material are arranged in rows and columns which are substantially parallel with the rows and columns of the pedestal array. However an alternative layout of actuator strips is shown in phantom at 44 in figure 6, in which the strips 42 are oriented obliquely to the pedestal array rows and/or columns. Either arrangement may be preferred dependent on operational requirements.

Whilst the intersecting network of actuation strips is in many ways preferred, it is to be noted that alternative arrangements of actuation layer are possible to similar effect, such as a continuous or sheet layer of actuator material, or else a layer in which strips are arranged such that they extend in a parallel direction and provide a single column or row only. Such layers may be actuated in a manner similar to the network structure actuation layer described above to similar effect. The preferred choice of actuation layer will depend on operational requirements and may vary for different applications. In alternative embodiments, the actuation strips of SMA material may be The sandwich of SMA actuator 38 and polymer layers 36, is created away from the body 26, to which this de-icing structure is to be applied. The layered structure is then anchored along its free edges on the aerofoil or other surface to be de-iced. In figure 4, the opposing edges which are bonded to the body 26 are shown at 46 and 48.

In this embodiment, the aerofoil body has formed thereon several pedestals prior to attachment of the layered structure (36, 38, 40) . These pedestals can have different shapes such as cubical, conical, cylindrical, spherical, hemispherical, etc. In one embodiment, the pedestals may have a polyhedral shape in plan which corresponds to the orientation of the actuator strips. For example, as shown in figure 6, a cuboid or other rectangular parallelepiped may be used in conjunction with a perpendicular grid structure of actuator strips.

The SMA/polymer sandwich is laid on top of the pedestals 34 as shown in figure 4, such that, in an at rest condition, the SMA sandwich layer forms a smooth aerodynamic profile. Such a condition is achieved when the SMA actuator layer 38 is maintained at an ambient temperature. For icing conditions, the ambient temperature is typically at or below approximately 0°C, which causes accumulation of ice 50 over the body 26, and, in particular, over the de-icing structure 30.

When a voltage is applied to the SMA 38 heat is dissipated because of the Joule effect and its temperature is increased. The SMA is thus actuated by virtue of the microstructural martensitic to austenitic transformation which occurs upon heating. The SMA thus adopts an actuated condition in which its length is reduced when compared to the at-rest condition. The resulting tension in the SMA actuator layer 38 causes deformation of the SMA/polymer layered structure into an actuated condition as illustrated in Figure 5. The actuated surface structure 30 is pulled tightly over the rigid pedestals 34 which resist the tension so as to cause reversible deformation of surface structure there-over. This results in the smooth surface profile of the surface structure 30 becoming modified to form a plurality of peaks and troughs or other surface perturbations corresponding to the array of pedestals. The two dimensional array of pedestals causes perturbations in two or more orientations which serves to apply stress to the ice layer. Such stresses induce breakage in the ice sheet 50, resulting in de-bonding and eventual shedding of the resultant ice fragments 52 from the surface of outer layer 36.

The outermost polymer layer 36 is ticker than the inner polymer layer 40, which is closer to the aerofoil body 26. This difference in thickness produces relatively large deformations in the outer polymer layer 36, on which the ice accumulates, and thus improves the efficiency with which the ice is dislodged.

It is estimated that a 0.1% strain would be sufficient to cause ice breakage. This strain value is at least one order of magnitude smaller than that exhibited by the SMA and the strain levels that can be tolerated by the polymer layers 36 and 40. Accordingly the applied strain between the at-rest and actuated/deformed condition may be between 0.08 and 0.4% for a viable de-icing system and typically between 0.1 and 0.2%.

Whilst a continuous sheet SMA actuation layer may be sufficient for this purpose, the banded network SMA structure described above may allow for reduced heating power requirements. Such a banded or network actuator arrangement may allow the polymer to deform to a greater extent between the pedestals since the surface structure 30 can deform so as to create troughs there-between.

Accordingly the pedestals impede the SMA shrinkage to a lesser degree than for a continuous sheet actuation layer.

Using the above-described techniques, a relatively thin SMA coating can be applied, which may have a thickness less than approximately 100 microns and for which only a low power consumption would be required to heat up the material in order to undergo phase transformation.

Manufacturing cost for the system (depositing polymer and SMA coating) should also be lower than that for producing wire mesh embedded in polymers.

Turning now to the second embodiment as shown in figures 7 to 10, there is provided an actuation mechanism or system which differs from that of the first embodiment and which relies on active deformation of the pedestals in order to induce deformation of the surface to be de-iced.

As with the first embodiment, the strain required to break the ice layer is typically in the vicinity of 0.1% and so the The present proposed solution employs Piezoelectric actuators to deform the surface on which ice has formed and is accreting. The amount of strain required to break an ice layer is of the order of 0.1% and so the applied strain in the surface to be de-iced may be between 0.08 and 0.4% for a viable de-icing system and typically between 0.1 and 0.2% when actuated.

The basic arrangement of the constituent components of a de-icing structure 100 according to the second embodiment is shown in figure 7. The surface of the body 26 is shown at 102 and provides a base or supporting structure on which the surface structure 100 is provided. A bonding layer 104, which may comprise, for example, a polymer, is provided on body surface 102 and is positioned between the surface 102 and a lamina 106 containing pedestals 108. A bond is thus formed indirectly between the layer 106 and the body surface 102 via the bonding layer 104 such that the bonding layer 104 and lamina 106, including pedestals 108, are held in position on the body surface.

The pedestal lamina 106 supports an outer surface 107 which is spaced from the bond layer 104 by the pedestals 108. The outer surface 107 comprises a sheet material which may comprise a polymer or metal material dependent on the operational requirements thereof. Typically the surface structure comprises a pedestal-containing lamina sandwiched between inner 104 and outer 107 polymer layers.

Pedestals 108 comprise a piezoelectric stack 110 with mechanical amplification means as will be described below.

The piezoelectric stack 110 comprises a number of piezoelectric elements 112 which are oriented and connected in series. At each end 114, 116 of the piezoelectric stack 110 there is attached a lever mechanism 118 comprising a plurality of lever arms 120. Each lever arm 120 is connected at a first end to the piezoelectric stack 110 and at a second, opposing, end to a bonding or attachment formation 122. A first attachment formation 122A is bonded to surface layer 107 and a second attachment formation is bonded to inner layer 104. The layers 104 and 107 thus define the extremities of the lamina 106 in which the pedestals are embedded. The arms 120 are pivotally or hingedly connected between the piezoelectric stack 110 and the respective attachment formation 122 so as to function as a pin joint. The arms and associated attachment formation may be formed as an integrated component having suitable bending properties or else may be formed from separate connected parts.

The arms 120 are oriented obliquely to the inner 104 and outermost 107 surface. The arms 120 extending between opposing ends of the same stack 110 and attachment formation 122 are oriented obliquely in an opposing sense such that each of said arms sloes towards or away from the opposing arm.

It will be appreciated by the skilled person that piezoelectric elements are actuated by passage of current there-through. Accordingly the provision of a stack of elements 112, across which a voltage can be applied, allows a change in length to be achieved which is the sum of the changes in length of each individual element 112 in the stackll0. A power source 124 is shown schematically in figure 8 along with the electrical connections 126 for the stack 110.

Referring to figure 8, when the piezoelectric elements 112 are activated, they elongate or expand in a first direction, indicated by arrow A, and shrink or contract in the orthogonal direction, indicated by arrow B. The stack is oriented such that the direction of elongation is substantially parallel to the surface 107 in its at rest condition. This change in length is of the stack 110 is amplified by the mechanical lever arms 120 build around the piezo stack. In this regard the arms 120 pivot in response to the extension of the piezoelectric stack 110 and the angle C' formed between the arm 120 and the direction of elongation of the stack decreases. The ratio of the length of the stack in direction A to the depth of the pedestal in direction B results in the stack elongation becoming amplified upon transmission to the layers 104 and 107 via attachment formations 122. In this embodiment, the length of the stack 110 in an at-rest condition is greater than the depth of the pedestal. The trapezoidal mechanical structure around the stack also serves to keep together the piezoelectric elements.

Referring to figure 7, fixed elements in the form of spacers 128 are disposed between adjacent pedestals 108.

The spacers are relatively rigid in construction and of height equal to the depth of the pedestal-containing layer in its urideformed condition. The spacers 128 serve to resist deformation of the surface 107 by the pedestals in the vicinity of the spacer (i.e. at a location between pedestals) . Accordingly, when the pedestals 108 are actuated by application of a voltage across the piezoelectric stacks 110, the spacer 128 does not deform, or else deforms to a lesser degree than the pedestals, and accordingly undulations, deflections or perturbations in the outer surface 107 are achieved as shown in figure 9.

Either side of the spacer 128, the surface 107 is deflected inwards towards the body surface 102. This deflection serves to dislodge any ice accumulated thereon by application of stress to the ice in a manner similar to that described above in relation to figure 5.

The spacers also serve to create a tension in the surface 107 when deformed which serves to restore the surface to the at-rest condition when the force applied by the pedestals is removed by removal of the voltage across the piezoelectric actuator stack. The spacers 128 may be formed of or comprise of tougher plastics, metals or composites than the surface layer 107. The elastic properties of the surface 107 itself and the restorative force of the stack elements 112 upon removal of the voltage there-across, will also serve to restore the surface to its at rest condition.

This undulated shape may achieve local strains greater than 0.2% and will serve to break the ice sheet into individual ice pieces which undergo shedding under gravitational and aerodynamic loads in the event that the surface 107 is moving.

Different arrangements or arrays of pedestals 108 and spacers 128 are shown in figures lOa-f. As shown, spacers 128 may be interspersed between two or more pedestals in a two-dimensional array. Accordingly spacers may have a shape in plan which is similar to that of the pedestals 108 or else may be elongate in form so as to span a plurality of pedestals. The spacers may be aligned with rows and/or columns of the array of pedestals or else may be obliquely oriented relative thereto. In one embodiment, the spacers may comprise a lattice, grid or network structure similar in form to the arrangement of actuator strips shown in figure 6. As with the pedestals, the spacers may be any suitable geometric shape, such as cuboid, conical, cylindrical, spherical, hemispherical, etc. One benefit of using piezoelectric elements is that they act as capacitors, the activation energy for which is stored inside the element upon actuation and released upon removal of electric field. This offers a particularly efficient system compared with established icing mitigation techniques.

Another advantage of piezoelectric actuators is that an activation frequency in the region of 30kHz can readily be achieved, which provides for rapid actuation and effective ice breaking. The large bandwidth offered by piezoelectric elements may also serve to damp vibrations on an airfoil subject to flutter. Piezoelectric stacks of different dimensions, shape and number can be used depending on the wanted level of strain to be achieved. In addition, the spacing of the pedestals and the stiffness of the pedestal-containing lamina can be varied depending on the aerodynamic loads to which the airfoil is subject during operation. The stiffness of the pedestal lamina can also be improved by pre-stressing the lamina itself upon anchorage to the airfoil body.

The provision of an array of pedestals in conjunction with an actuation means embedded in a layered de-icing construction is proposed as a further definition of the invention.

In an alternative embodiment to that of figures 7 to 9, the intermediate spacers 128 may be omitted. Instead, adjacent pedestals may be arranged for actuation in opposing direction in order to achieve the desired surface variation in height or deformation. Accordingly, the piezoelectric stack of one pedestal may be arranged to increase the height of that pedestal, whereas the piezoelectric stack of an adjacent pedestal may be arranged to decrease its height. Thus an array of intermittently raising and lowering pedestals may be provided.

In an alternative embodiment, the bonding layer 104 may be omitted altogether and the pedestal-containing lamina 106 can be anchored to the airfoil body itself using known bonding or fixing techniques. Additionally or alternatively, the spacers 128 or pedestals 108 can be made hollow to enable a mechanical fixing to be applied through it between the outer surface 107 and the body support structure 102, thereby clamping all the elements to the support surface at the selected points.

A single lamina or else a plurality of pedestal-containing laminas may be anchored to the airfoil. This may allow redundancy and/or reduced costs of repair if a lamina fails.

The bonded piezoelectric lamina can also be removed from the underlining airfoil by applying heat.

A system of dots or a cross or other visual indicia may be applied to the outermost surface 107 such that a simple visual inspection of the operation can be undertaken. Correct actuation of the de-icing structure will result in distortion of the visual indicia on the outer surface which is visible to the naked eye.

Conversely, no distortion is a simple indicator of a failing de-icing structure.

A different embodiment of the present invention is to substitute the piezoelectric with springs of different stiffness. This will allow extracting energy from the airflow washing the airfoil; this energy will put the spring in self oscillation, if the right range of stiffness is chosen for the springs.

In any of the embodiments described above it is intended that the surface will be deformed once or a small number of times to fracture thereby dislodge the ice.

Accordingly it is the instantaneous strain placed upon the ice by the surface actuation which causes breakage in the ice. The process may be repeated to ensure ice is effectively removed. However such repetition is intended to occur over a time period which does not cause vibration of the surface. Accordingly the tension and resulting deformation of the surface is the mechanism by which ice is removed and is not significantly dependent on high frequencies of operation. The actuator according to the present invention may be operable at a frequency of anywhere between 0.1 Hz and a number of kHz. The required tensile force may accordingly be applied to the surface to achieve deformation over anywhere between a timescale of thousandths of a second and few seconds.

Claims

Claims: 1. De-icing apparatus for dislodging ice from a surface which is prone to ice accretion, the de-icing apparatus comprising: a support structure for supporting the surface; a plurality of pedestals depending from the support structure and in contact with the surface, the pedestals being arranged in a spaced two-dimensional array relative to said surface; and, actuation means for selectively deforming the surface about said pedestals between an at rest condition and an actuated condition, the actuation means arranged to vary the surface profile between the at rest and actuated conditions by causing a variation in height of the surface relative to the support structure between the pedestals in the two-dimensional array.
2. De-icing apparatus according to claim 1, wherein the pedestals in the two-dimensional array are arranged so as to cause deformation of the surface in two or more directions upon operation of the actuation means.
3. De-icing apparatus according to claim 1 or 2, wherein the actuation means causes an increase in curvature of a portion of the surface about the pedestals.
4. De-icing apparatus according to any preceding claim, wherein the surface comprises the outer surface of a first layer and the actuation means is comprised in a second layer arranged between first layer and the support structure.
5. De-icing apparatus according to claim 4, further comprising a third layer, wherein the actuation means is sandwiched between the first and third layers.
6. De-icing apparatus according to claim 5, wherein the first and/or third layers are formed of a polymer material.
7. De-icing apparatus according to any preceding claim, wherein the pedestals are mounted on the support structure so as to support the surface off the support structure at discrete locations.
8. De-icing apparatus according to any preceding claim, wherein the pedestals are arranged in a regular array comprising a plurality of rows and columns.
9. De-icing apparatus according to any preceding claim, wherein the support structure comprises at least a portion of an aerofoil body and the surface comprises a gas-washed surface of the body in use.
10. De-icing apparatus according to any preceding claim, wherein the pedestals are fixed in height and the actuation means comprises means for tensioning the surface over the pedestals.
11. De-icing apparatus according to claim 10, wherein the actuation means may comprises a lamina of actuation material in the form of a sheet, strips or wires aligned with the surface and arranged to overlay the array of pedestals.
12. De-icing apparatus according to claim 10 or 11, wherein the actuation means comprises a shape memory material which is actuated by heat so as to tension the surface.
13. De-icing apparatus according to any one of claims 10 to 12, wherein the actuation material is arranged in the form of an intersecting network arranged in plan to lie between the pedestals in the array
14. De-icing apparatus according to any one of claims 1 to 9, wherein the actuation means is arranged to actuate one or more pedestals in the array so as to adjust the height thereof relative to the support structure and thereby deform the surface between the at rest and actuated conditions.
15. De-icing apparatus according to claim 14, wherein each pedestal comprises actuation means in the form of a stack piezoelectric elements.
16. De-icing apparatus according to claim 14 or 15, wherein a mechanical amplification mechanism is connected in the force path between the actuation means and the surface.
17. De-icing apparatus according to claim 16, wherein the mechanical amplification mechanism comprises a plurality of obliquely arranged lever arms depending between the actuation means and the surface.
18. De-icing apparatus according to any one of claims 14 to 17, wherein a plurality of rigid spacer members are provided between the pedestals of the array, the spacer members arranged to contact the surface in the at rest condition and thereby resist deformation of the surface in the actuated condition so as to cause a change in gradient in the surface between the pedestals once actuated.
19. A method of forming a de-icing structure for a surface which is prone to ice accretion during use, the method comprising: forming a plurality of pedestals over a support structure in a spaced array, said pedestals arranged to depend outwardly from the support structure to a predetermined height; locating an outer surface layer over at least part of the support structure such that it contacts the plurality of pedestals on an inner side thereof, wherein the pedestals and/or surface layer are provided with actuation means for selectively altering the profile of the surface during use between an at rest condition and an actuated condition.
20. A method according to claim 19, wherein the actuation means comprises a lamina of shape memory material sandwiched between the outer layer and an inner layer so as to form a surface structure, the outer and inner layers being formed with the shape memory material there-between prior to location of the surface structure over the surface structure.