The present invention relates to heating elements for propeller blades, a method of forming heating elements, and a method of fastening a heating element to apparatus such as a propeller blade.

The need to prevent the build-up of ice on an aircraft in flight has been exercising aeronautical engineers for many years. It is particularly important to prevent ice building up on aircraft propellers. If allowed to form on a propeller, ice does not form uniformly causing the propeller to become unbalanced. This leads to vibrations as the propeller rotates, resulting in possible gear box failures and other dangers. Furthermore, the build-up of ice on a propeller causes loss of thrust, so the performance of the aircraft is affected adversely. Ice can be shed, causing damage to other parts of the aircraft.

Many techniques to avoid the build-up of ice have been tried over the years, including mechanical, electrical, thermal, and vibratory methods. None has proved entirely satisfactory. One disadvantage is that the extra equipment required for de-icing tends to increase the bulk of a propeller blade causing aerodynamic inefficiency. Other disadvantages are a tendency for the de-icing equipment to wear, fatigue and become damaged.

Also, conventional de-icing equipment is often difficult to repair on the aircraft whilst in service.

It is known to provide propeller blades with heating elements along their front edges where a heating element consists of a long wire which is laid out in a zig-zag configuration over the area to be heated. The wire is then covered with a rubber layer. Such a known heating element is easily damaged, as a single break in the wire is sufficient to make the heating element inoperable. Furthermore, the wire and rubber layer gives the front edge of the blade a raised profile causing reduced efficiency.

The present invention is defined in the claims to which reference should now be made. Preferred features are referred to in the sub claims.

The present invention in its first aspect provides a propeller blade having an electrically powered heating element, the heating element comprising a mat of fibres having surfaces which are electrically conductive. The fibres can be electrically conductive throughout, and are preferably carbon fibres. The fibres can be metal coated. The metal coating of the fibres is preferably of nickel.

The fibre mat has the advantage of being more damage tolerant compared with conventional zig-zagged wire heaters. Whereas a small hole in the region where wire is laid out leads to a break of the connection and total heating failure, a hole in a heating element according to the present invention merely results in a changed current path. This is a significant practical advantage in view of the relative likelihood of the front edges of

propeller blades being damaged. Furthermore, holes can be intentionally cut in a fibre mat so as to adapt the distribution of heat which the mat provides.

Preferred heating elements according to the present invention also have the advantage that they can be made significantly thinner than the wire and rubber layers known in the prior art. Thus, whereas the known solution requires blades to be formed with indentations into which the layers fit, the preferred heating element can be fitted directly on a blade without such indentations and still have low air resistance.

The mat of the heating element can be made up of more than one mat portion. The portions can be shaped to fit respective sides of a propeller blade adjacent the leading edge of the blade, or the mat or mat portions can be wrapped over the blade leading edge. Alternatively, the mat or mat portions can cover substantially all of the blade surfaces, this being especially suitable for anti-icing rather than de-icing operation. The mat portions can be connected in series or parallel, or could be continuous so as to constitute a single mat. Multiple mats or mat portions can be provided one over another, so as to form a composite. Furthermore, if a mat or mat portion becomes damaged, a repair can be made by simply fixing another mat or mat portion over the top of the first, so as to be electrically connected to the first.

The heating element can be adapted to produce an optimum heat

distribution pattern. The mat or mat portions of the heating element can be shaped so as to have variable resistivity along their length. To vary the heating pattern, dimensions such as length, width, and thickness can be varied, and gaps or holes provided in mats or mat portions.

The heating element can include a resilient cover such as a polyurethane layer.

The present invention in its second aspect relates to a heating element comprising a fibre mat in which at least the surfaces of the fibres are electrically conductive, the mat being backed by a perforated backing, the mat being fixed to the backing by film adhesive. The backing acts either to control the flow of adhesive to the mat and/or to absorb excess adhesive so as to provide a heating element with bonded layers in which the fibres of the mat have not become substantially separated by the adhesive. The backing is preferably glass fibre filter paper. Film adhesive is preferably a resin.

The heating element can be mounted on a propeller blade such that the perforated backing is towards the blade surface. The perforated backing can then electrically insulate the fibre mat from the blade.

The perforated backing preferably provides strength and stability whilst allowing the heating element to be flexible for fitting to the surface of apparatus to be heated. In particular, it is

suitable for fitting to an aircraft propeller blade.

The present invention in a third aspect is a method of producing a heating element by providing a fibre mat with a perforated backing, applying film adhesive to the surface of the fibre mat or backing, and curing to fuse the adhesive so as to bond the paper and mat. The curing step preferably results in a heating element which is still flexible. The resulting bonded element can then be applied to apparatus to be heated such as a propeller blade, and fixed thereto.

The present invention in its fourth aspect is a method of providing a heating element on an apparatus to be heated by applying a fibre mat with a perforated backing to the apparatus, at least a surface of the mat or a surface of backing being provided with a film adhesive, and curing to fuse the adhesive so as to bond the filter paper and fibre mat to each other and to the blade. This can be done as a single curing step. Alternatively, the curing can be done in two stages, first to bond the mat and paper, and secondly, to fix the resulting composite to the blade.

A preferred embodiment of the present invention will now be described by way of example, and with reference to the drawings in which:

Figure 1 is a diagrammatic oblique view of a heating element applied to a propeller blade;

Figure 2 is a diagrammatic oblique view of an alternative heating element; and

Figure 3 is a diagrammatic view illustrating the layered structure of the heating elements.

As shown in Figure 1, the heating element comprises two mats 1,2. The mats 1,2 are made of nickel coated carbon fibre. The mats 1,2 are fixed to the propeller blade 12 adjacent a leading edge 14. In alternative embodiments, multiple mats could be provided one over the other. The blade root of the propeller 4 includes two studs 5,6. The studs 5,6 can be connected directly to mats 1,2 as shown in Figure 1, or via connecting wires 7,8 as shown in Figure 2. One stud 5 is positive, whilst the other stud 6 is negative. To provide heat, power is supplied to the mats 1,2 via the studs 5,6 by way of bus bars 9,10 fastened to the mats 1,2. The two mats 1,2 are connected together near the tip of the propeller blade 12 via a further bus bar 16.

Multiple mats 1,2 can be laid one over the other so as to provide additional heating. Alternatively, they could be replaced by a single continuous layer or composite of multiple continuous layers.

The geometry of the mats 1,2 can be optimised so as to provide a preferred heating pattern. In particular, holes can be made in the mat for this purpose.

The heating element consists of nickel coated carbon fibre mats 1,2 backed by glass fibre filter paper 18. A fibre mat and filter paper backing are fastened together using a resin 20 such as a polyurethane resin. The resin 20 is preferably applied to the outwardly directed surface of the filter paper, then the mat and filter paper are keyed together by curing the resin 20. Basically, the perforations through the weave of the filter paper acts to soak up excess resin, avoiding excessive resin between the fibres of the fibre mat. Some resin soaks into the mat providing some protection and giving rise to a small reduction in resistivity as fibres move apart. The resulting composite is still flexible.

The layers of the preferred heating element before curing are shown in Figure 3. Upon curing, the resin layer 20 fuses, seeping through the filter paper 18 so as to bond the filter paper 18 to a mat 1,2.

The composite is fitted onto a propeller blade 12 preferably with the filter paper 18 toward the blade 12. The composite is further cured under applied pressure so as to fasten the heating element to the blade. The pressure can be supplied by covering the composite with a flexible membrane so as to define an enclosed space then causing a vacuum in that space, so that the membrane presses the composite.

An alternative approach is to apply the filter paper to the blade then a fibre mat, then resin layer, over the top. Another option

is to apply a resin layer to the blade then cover, first with the fibre mat, then the filter paper. In both cases, the order of applying the filter paper and fibre mat could be swapped around. Using any of these various approaches, a single curing step is sufficient to produce a blade with a heating element bonded to the blade and ready for use.

The present invention provides a simple yet robust heating element which can be produced and/or applied to apparatus to be heated in a simple and effective way.