The present invention relates to an arrangement for an aircraft to reduce the risk of icing on the aircraft's wings, horizontal stabilizer and vertical stabilizer.

Previously disclosed solutions to prevent icing on parts of an aircraft have included means for heating the aircraft parts concerned or for causing them to be coated with glycol. The intended surface has also been provided with a variable rubber coating, which, when filled, causes the ice attached thereto to fracture and break apart and to become detached from the aircraft in question.

The aforementioned previously disclosed means have not proved to be so effective, however, and have been costly and environmentally hazardous.

Icing on aircraft is an air safety factor. Ice on the wings, horizontal stabilizer and vertical stabilizer has an adverse effect on the aerodynamic characteristics. The consequence is an impairment of the performance, such as the aerodynamic lift and thrust. In more severe cases, the aircraft can perform movements that are difficult to control.

Icing represents a particularly high air safety risk to small aircraft (private aircraft). The impairment of the performance of the aircraft caused by icing, among other things on the leading edge of the wings, the horizontal stabilizer and vertical stabilizer, has resulted in crashes.

The risk of icing in practice prevents IFR flying (flying according to instrument flight rules) during the cold season of the year.

Meteorological conditions for icing.

A large proportion of the very small droplets of water which form clouds remains liquid even at minus

temperatures; these are known as supercooled water droplets.

These water droplets retain their liquid form for as long as they remain undisturbed and strike a cold body with a mass capable of absorbing the surplus heat in the supercooled water. If the water droplets strike the wing of an aircraft, for example, or some other object, they freeze immediately into ice and form a porous deposit of ice, known as rime.

Water droplets of the kind which occur in, for example, cumulonimbus clouds (cb) and in precipitation are significantly larger than those described above. When these strike the body of the aircraft, they first flow out along the surface and then freeze to form a hard, glass-clear ice deposit. This ice deposit forms first on the leading edge of the wings, the horizontal stabilizer and the vertical stabilizer, has a considerable effect on the aerodynamic characteristics and can render the aircraft completely uncontrollable. Water droplets (usually with a diameter of between 10 and 40 um) which form clouds remain liquid even at minus temperatures in the form of supercooled droplets. Their temperature can be low as-30°C. They retain their form in undisturbed surroundings. If they strike an object with the ability to take up the residual heat of the supercooled water, they will freeze immediately to form ice.

Water droplets in stratus-like clouds and in drizzling rain are very small and form a porous deposit (rime) in collision with an object. Water droplets in cumulus-like clouds and in rain are much larger. When these strike an object, they form a hard ice surface (ice deposit, horn ice).

The risk of icing is present at air temperatures between 0°C and-20°C.

The risk of icing is greatest at temperatures between-2°C and-8°C. Essentially, it is"slow"aircraft that are at risk of icing. Frictional heat is generated in the case of fast aircraft, for example jet aircraft, which

means that the shell temperature normally lies above 0°C.

Problems in conjunction with take-off and landing are the same, however, for the otherwise faster jet aircraft.

Ability of an aircraft to collect water droplets.

The airflow in front of an aircraft is deflected to one side by the projecting parts of the aircraft, for example the leading edge of the wings. The water droplets present in the air in clouds or precipitation exhibit a tendency to follow the flow lines of the deflected air, although since the droplets have a certain mass, their force of inertia will affect their movement and will attempt to cause them to move directly forwards. The resulting movement of the water droplets will follow lines which adopt a direction between the straight lines directed at the aircraft and the flow lines of the air.

Heating of the air by compression and friction.

Air which passes over the surface of an aircraft is the subject of so-called kinetic heating. This means that the temperature increases in an adiabatic fashion due to the increase in pressure at projecting parts of the aircraft, and that frictional heat is generated between the air and the surface of the aircraft body. These effects give highly variable heating of different parts of the aircraft body.

The principal object of the present invention is thus, in the first instance, to prevent icing on the aircraft by preventing the supercooled water, which from time to time strikes the aircraft, from coming into contact with the cold mass (metal surface) of the aircraft.

The aforementioned object is achieved by means of an arrangement in accordance with the present invention, which is characterized essentially in that there is applied, on the leading edge of the aforementioned parts, a thermally insulating, water-repellent and non-stick covering in the

form of a thin layer of a thermally insulating material, which is so arranged as to prevent supercooled water from coming into contact with the cold mass of the aforementioned parts at their respective leading edge and surrendering its excess heat at that point, but instead to remain in its liquid phase and to be deflected by the water-repellent insulating layer.

Previously disclosed technology and the invention in SE 509 588 C2 relate to a method and an arrangement for preventing the accumulation of snow or ice on an external surface of an element of a structure, in particular an element of a vehicle, and in the first instance a rail vehicle. The aforementioned method and arrangement involve cladding the surface with at least two layers, of which the outer layer is executed as a flat, rigid and smooth surface, while the inner layer consists of a material with a high thermally insulating capacity and high elasticity.

Use is thus made of a cladding in the form of two layers, and the aforementioned layers are intended to prevent the build-up of already formed snow or ice on the vehicle in question, etc.

Snow and ice are the solid form of water, i. e. the water has already lost its"ice-forming heat"of 80 cal/g.

The arrangement in accordance with the present invention is intended to prevent icing on aircraft.

A) Icing on aircraft normally occurs when a cold aircraft body enters a mass of air (e. g. a cloud) in which water droplets (water in its liquid form, its "aqueous phase") is present, and when the cold aircraft body is able take up the ice-forming heat of the water, at which point the water changes its phase to ice on the surface of the body which is capable of taking up this heat.

B) This patent application relates only to aircraft.

C) In accordance with the present invention, exposed parts must be thermally insulated with a thin, thermally insulating covering. If an arrangement in accordance with the previously disclosed patent were to be used to prevent icing, i. e. to change the aggrision phase, icing would occur since the mass of the outer layer is so great and its thermally insulating capacity is so low that icing would be bound to occur.

Significant differences thus exist between the present invention and what has been shown to have been previously disclosed through SE 509 588 C2, and the present invention can also not be regarded as being obvious to a person skilled in the art The idea of invention is thus to enable the supercooled water, which strikes the aircraft at those parts of the aircraft where de-icing normally causes major and serious problems, not to come into contact with the cold metallic parts of the aircraft, but instead to strike a special covering where the water cannot freeze, but is instead deflected by the water-repellent thermally insulating covering.

The invention is described below as a preferred illustrative embodiment, in conjunction with which reference is made to the accompanying drawings, in which: Fig. 1 shows a perspective view of an aircraft, which exhibits the arrangement in accordance with the present invention; Fig. 2 shows a sectioned view of the leading edge of a wing with the arrangement applied thereto; and

Fig. 3 shows the flow and movement of the air and the droplets of water in conjunction with an obstruction in their path.

An arrangement for an aircraft 2 to reduce the risk of icing on the wings 3, horizontal stabilizer 4 and vertical stabilizer 5 comprises a covering 1 of a specific kind. On the leading edge 3A, 4A, 5A of the aforementioned aircraft parts 3-5, a thermally insulating, water-repellent and non- stick covering 1 is formed by an essentially 1-2 mm thick T layer 6 of a thermally insulating material. The aforementioned covering 1 is so arranged as to prevent supercooled water 7 from coming into contact with the cold mass of the aforementioned parts 3-5 at their respective leading edge 3A-5A and instead to be deflected by the water- repellent thermally insulating covering 1.

Figure 3 shows the airflow lines as broken lines and the movement of the water droplets as solid lines in relation to an obstruction such as the leading edge of the wing of an aircraft.

It can be appreciated from the Figure that a water droplet which starts from the point A directly in front of the object will strike it at"A", whereas a droplet which starts from the point B will be deflected to"B". A droplet which starts from point C will be deflected severely to"C".

Droplets beyond point C will not strike the object at all.

Thus, a significant proportion of the droplets present in the air in front of the wing of the aircraft will not strike it.

The proportion of droplets which strike the aircraft will thus depend on the size of the droplets, the speed of the aircraft and the curvature of the projecting part of the aircraft.

The proportion of water droplets which can strike an aircraft increases with: 1. Increasing droplet size 2. Increasing speed of the aircraft, and

3. Increasing (more pronounced) curvature of projecting parts of the aircraft.

The above is of considerable significance, of course, for the degree of icing to which an aircraft is subjected when flying through clouds or precipitation with supercooled droplets of water. The extent of the surface of the aircraft which is exposed to significant icing also varies with the droplet size. If the droplets are small, icing will occur predominantly around the stagnation point "A", whereas the ice will be more extensive of the droplets are larger.

The aforementioned covering 1 is impregnated to make it water-repellent, in conjunction with which the impregnation essentially consists of silicon. The impregnation 8 is so arranged as to impart non-stick properties to the surface 1A of the covering 1.

The aforementioned thermally insulating material consists of a rubber material in the form of a cellular rubber material which exhibits closed cells. Chloroprene cellular rubber is a suitable cellular rubber material.

The thermally insulating material 1 consists essentially of a thin surface layer complete with a skin peeled from a body of cellular rubber produced by integral foaming.

The aforementioned thermally insulating material 1 exhibits a density of ca. 150 kg/m3 and is so arranged as to be capable of absorbing ca. 0.5% by volume of water.

The invention thus permits a simple but effectively functioning solution to be achieved, which is suitable for application to all kinds of aircraft with which problems of icing can be encountered.

The invention is naturally not restricted to the embodiment described above and illustrated in the accompanying drawings. Modifications are possible, especially in respect of the nature of the various component parts, or by the use of equivalent technology, without departing from the area of protection afforded to the invention, as defined in the Patent Claims.