GB2436861A

GB2436861A - Aerofoil

Info

Publication number: GB2436861A
Application number: GB0606679A
Authority: GB
Inventors: William Samuel Bath
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-04-04
Filing date: 2006-04-04
Publication date: 2007-10-10
Also published as: GB0606679D0

Abstract

The aerofoil comprises first and second surfaces 2, 4, the second being smooth at least part of the first comprising an undulating profile that may be produced by indentations 6 (which may be semi-spherical or tear drop shaped) or protrusions or a combination of the two. The aerofoil may comprise a wing, or the blade of a turbine or of a helicopter or may be used in toys.

Description

2436861

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AEROFOIL

The present invention relates to aerofoils, and in particular to the improvement in lift and performance of an aerofoil.

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It is generally known that as an aerofoil moves through the air, lift is produced by a pressure differential across the aerofoil created by air moving faster over the upper surface than the lower surface. However, air moving over the surface of the aerofoil also experiences drag. The layer 10 of air next to the surface of the aerofoil is know as the boundary layer and is almost stationary. Subsequent layers of air then move faster over this initial boundary layer. The drag experienced by air moving over the upper surface slows down the air stream, increasing the pressure on the upper surface of the wing. This reduces the pressure differential across 15 the wing and hence the lift.

Accordingly, there is provided an aerofoil comprising a first and a second surface, wherein at least a part of the first surface comprises an undulating profile and wherein the second surface comprises a 20 substantially smooth profile.

The undulating profile of the first surface may comprise peaks, troughs or both peaks and troughs.

25 The undulating profile reduces the drag across the surface of the aerofoil by reducing the area of aerofoil surface immediately in contact with the moving air stream. A boundary layer of air is in contact with the undulating first surface and is substantially stationary with respect to the aerofoil. The peaks and troughs may be arranged such that the boundary 30 layer of air fills the troughs of the undulating profile. The air stream moving over the first surface of the aerofoil moves over this boundary

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layer of air and has less contact with the aerofoil surface. This results in less drag.

The peaks and troughs are preferably formed by indentations in the first 5 surface. Alternatively, the peaks and troughs may be formed by protrusions out of the first surface or a suitable combination of protrusions and indentations. The indentations or protrusions may cover a part of the first surface or alternatively may cover substantially the whole of the first surface.

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Preferably, the second surface of the aerofoil includes substantially no indentations or protrusions.

The indentations may have a symmetrical cross-section. For example, the 15 indentations may be substantially semi-spherical. Alternatively, the indentations may have an asymmetrical cross-section such as the shape of half a teardrop or any other suitable shape. A first end of the half teardrop indentation may comprise a relatively steep drop and may be a trailing edge and a second end may comprise a gradual incline and may be 20 a leading edge with respect to an expected direction of flight. The two sides of the indentation may be substantially parallel.

The indentations may be distributed substantially symmetrically across the upper surface or, alternatively, they may be distributed asymmetrically. 25 For example, an aerofoil with no specific direction of flight may include symmetrical indentations in a symmetrical distribution. However, it may be preferably for an aerofoil with one specific direction of flight to have indentations with an asymmetrical shape in either a symmetrical or asymmetrical distribution.

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The first surface of the aerofoil may be a cambered surface. Alternatively, the first and second surfaces may be substantially flat.

The aerofoil may be any one of an aeroplane wing, a Frisbee, a rotary 5 blade such as in a turbine or a helicopter propeller or any other such aerofoil.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in 10 which:

Figure 1 schematically shows a cross section through an aeroplane wing according to one embodiment of the invention;

15 Figure 2 schematically shows the indentations of the aeroplane wing of Figure 1;

Figure 3 schematically shows the distribution of indentations across the upper surface of the aeroplane wing of Figure 1;

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Figure 4 schematically shows a cross section through an aeroplane wing only partially covered with indentations; and

Figure 5 schematically shows a Frisbee with indentations according to an 25 alternative embodiment of the invention.

Referring to Figure 1, which is not to scale, an aeroplane wing 1 is made from metal and comprises a cambered upper surface 2 and a substantially flat lower surface 4. Indentations 6 cover the upper surface 2 of the wing 30 from a leading edge 10 to a trailing edge 12. The lower surface 4 of the wing 1 is substantially smooth and has no indentations.

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The indentations 6 minimise an exposed part of the wing 8 on the upper surface 4 of the wing 1 and maximise the total surface area of the upper surface 4 of the wing 1. As the aircraft moves through the air, a 5 boundary layer of air will lie immediately next to the surface and so part of the boundary layer will lie in the indentations 6. The air stream following the line of the upper surface 2 of the wing 1 will therefore move over the air in the indentations 6 and a minimal amount of metal upper surface 8 instead of passing over a continuous metal upper surface 10 if the surface were to be smooth. This reduces the drag, therefore increasing the speed of the air stream and increasing the lift. Maximising the surface area of the upper surface 4 of the wing 1 further increases the speed of the air stream since the air stream travels further in the same length of time as air moving under the wing, also increasing lift.

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Referring to Figures 1 and 2, the indentations 6 have a shape similar to that of a half teardrop. In a direction from the leading edge 10 of the wing to the trailing edge 12 of the wing i.e. in the direction of expected intended flight, each indentation 6 has a gradual incline 16 up to the 20 upper surface 2 level followed by a sharp drop 14 from the upper surface 2 of the wing. For an aerofoil such as an aircraft wing that has only one direction of flight, an asymmetrical indentation such as the half teardrop of Figures 1 and 2 produces a more streamlined shape and further reduces drag in flight. The sides of each indentation are substantially parallel.

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Referring to Figure 3, the indentations 6 cover substantially the whole of the upper surface 2 of the aeroplane wing 1 to maximise lift. The indentations 6 may be arranged in a substantially symmetrical arrangement as shown. In other embodiments indentations may be 30 arranged in an asymmetrical arrangement.

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In an alternative embodiment, as shown in Figure 4, the indentations 6 cover only a part of the upper surface 2 of the wing 1. The leading edge 10 of the upper surface 2 of the wing is substantially smooth. Indentations 6 cover a portion of the upper surface of the wing towards the trailing 5 edge 12. It will be appreciated that different portions of the upper surface of the wing may be covered with indentations. For example, a part of the upper surface 2 of the wing towards the leading edge 10 may comprise indentations and a part of the upper surface 2 towards the trailing edge 12 may be substantially smooth.

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It will be appreciated that these indentations 6 can be applied to aircraft wings of all sizes and designs and that the size and shape of the indentations can be adapted accordingly. For example, the wing of a light aircraft may be covered with indentations with a diameter of a few

15 millimetres, between 2mm and 5mm for example, and there may be between around 4 and 7 indentations per cm2. The diameter of the indentations on the wing of a faster moving aircraft may need to be considerably smaller than this, whereas those on the wing of a toy aeroplane may be larger.

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Referring to Figure 5, in an alternative embodiment of the invention, a Frisbee 20 comprises an upper surface 22, which is substantially flat and covered with indentations 24. The indentations 24 are curved to follow the same curve as the circumference of the Frisbee to give a streamlined

25 shape. Providing indentations 24 on a Frisbee provides a greater length of air travel on the upper surface 22 of the Frisbee, which does not exist when both the upper and lower surfaces are flat and free from indentations.

30 There is not generally a particular direction of flight for a Frisbee and so the indentations on a Frisbee, or similar article, can be any other

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symmetrical shape such as circular or oval. Alternatively, in a further embodiment, the indentations are asymmetrical and are arranged to optimise the lift of a Frisbee designed specifically for a left-handed or right-handed thrower. For example, the indentations may be in the shape 5 of a half teardrop and may be arranged on the upper surface of the Frisbee so that, when the Frisbee is spun by a right-handed or left-handed thrower, a deeper portion of the teardrop is a leading edge and an inclined portion of the teardrop is a trailing edge. The sides of the teardrop may converge and possibly meet at a point, or alternatively may 10 be substantially parallel. The teardrop indentations may be curved to follow the circumference of the Frisbee and may be distributed either symmetrically or asymmetrically over the surface of the Frisbee.

It will be appreciated that the principle of including indentations on an 15 aerofoil may be applied to many other situations. For example, indentations may be included on the propellers of a turbine to increase suction or on the propeller blades of a helicopter. The shape, size and distribution of the indentations will be dependent on the size of the aerofoil, speed of travel and, in cases such as propeller blades, the angle 20 at which the aerofoil cuts through the air. Indentations may also be applied to a variety of toys such as model aeroplanes to increase the distance of flight. It will also be appreciated that the invention is not limited to metal aerofoil surfaces but may be applied to carbon fibre, plastic or any other suitable material.

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In alternative embodiments, indentations may be replaced by protrusions or any other suitable formation that provides the desired undulating profile such that the surface area is increased and actual contact of the wing with the moving air stream is minimised. In some embodiments a 30 combination of protrusions and indentations can be provided. The protrusions and/or indentations may be provided in a varying profile

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across an aerofoil such as an aeroplane wing, for example perpendicular to the expected direction of flight.

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Claims

1. An aerofoil comprising a first surface and a second surface, wherein at least a part of the first surface comprises an undulating profile

5 and wherein the second surface comprises a substantially smooth profile.

2. An aerofoil according to any preceding claim, wherein the undulating profile covers substantially the whole of the first surface.

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3. An aerofoil according to claim 1 or claim 2, wherein the undulating profile comprises peaks and troughs.

4. An aerofoil according to claim 3, wherein the peaks and troughs are formed by indentations in the first surface.

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5. An aerofoil according to claim 3 or claim 4, wherein the troughs have a symmetrical cross-section.

6. An aerofoil according to claim 5, wherein the troughs are 20 substantially semi-spherical.

7. An aerofoil according to claim 3 or claim 4, wherein the troughs have an asymmetrical cross-section.

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8. An aerofoil according to claim 7, wherein the peaks and troughs comprise a gradual leading edge and a relatively steep trailing edge with respect to the expected direction of flight.

9. An aerofoil according to claim 7 or claim 8, wherein the troughs 30 have the shape of half a teardrop.

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10. An aerofoil according to any one or more of claims 3 to 9, wherein the troughs are distributed substantially symmetrically across the first surface.

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11. An aerofoil according to any one or more of claims 3 to 9, wherein the troughs are distributed asymmetrically across the first surface.

12. An aerofoil according to any one or more of claim 3 to 11, wherein the sides of the troughs are substantially parallel.

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13. An aerofoil according to any preceding claim, wherein the first surface is a cambered surface and the second surface is substantially flat.

14. An aerofoil according to any one of claims 1 to 12, wherein the 15 first and second surfaces are substantially flat.

15. An aerofoil according to any preceding claim, wherein the first surface is an upper surface and the second surface is a lower surface.

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16. An aerofoil according to any preceding claim, wherein the aerofoil is any one of an aeroplane wing, a Frisbee and a rotary blade.

17. An aerofoil substantially as hereinbefore described with reference to any one or more of the accompanying drawings.

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