TECHNICAL FIELD

The present invention relates to a wing tip device and a method for controlling air flow at a wing tip.

BACKGROUND

During flight, aircraft’s wings create a rotating vortex of air shedding off the wing’s tips - the so-called‘tip vortex’. The energy lost as a result of the wing’s function inevitably creates wing-tip vortices extracting energy from the airframe of the aeroplane which significantly reduces the efficiency of the operation of the wing.

Most wing-tip devices of different designs are an extension of the wing in some form or another, and seek to inhibit the formation of the vortex and hence reducing those energy losses.

An alternative approach is to extract energy from the vortex. This has been previously considered in the context of a paddle type propeller at the trailing edge of the wing-tip harnessing energy as the rotating vortex causes the propeller to rotate and drive a generator to produce electricity. However, such propellers have not been adopted widely in the aircraft industry due to the increased maintenance costs of a device which inherently has multiple, moving component parts, any of which could break or seize up at any time.

There is a need for an improved energy harnessing wing-tip device.

STATEMENT OF INVENTION

Aspects of the invention are defined by the accompanying claims.

The inventor has realised that a wing tip device without moving parts can effectively and efficiently capture vortex energy and feed the energy back into the air-frame. In general terms, the present invention proposes a mechanism by which this is achieved by encouraging a vortex to develop within a shroud shaped wing tip device (hereinafter referred to as a shroud) surrounding each of the aeroplane’s wing-tips. The design of the shroud and the wing-tip is such that the rotating vortex within the shroud can be used to further reduce the pressure on the upper surface of the wing. The shroud can further be used, to provide an increased lifting force on an extension of the shroud itself.

The design of the shroud is such that it facilitates and enhances the vortex effect within the shroud. The means by which the shroud is attached to the wing provides a stationary propeller or turbine effect such that it has stationary blades on which the rotating air acts to provide forward thrust.

The overall design can be optionally further enhanced by the addition of winglets attached to the shroud extending vertically both above and below the shroud.

Unlike the majority of wing-tip devices, such as winglets, this device is not an extension of the wing but is instead an attachment. By providing the wing-tip device as an attachment, it can easily be retrofitted onto existing wings. This contrasts with winglets and other extensions of wings which must be manufactured integrally with the wing and which cannot be retrofitted onto existing wings. Additionally, by trapping the vortex in the shroud, any residual vortex outside of the shroud will be much less energetic than it would otherwise be. Thus the wing-tip device may also serve the purpose of a vortex inhibitor.

A shroud according to one embodiment of the invention is of a broadly cylindrical shape (without its main surface touching the wing-tip) which is spaced away from the wing-tip but encloses the wing-tip such that it wraps around the wing-tip in a manner designed to contain and manipulate the aerodynamic process so as to produce an energetic vortex within the shroud itself. Any known shape optimisation modelling technique may be used to determine the shape of the shroud. Shape optimisation may include, for example, generating a baseline shape and flying the shape in a simulated wind tunnel under different conditions (e.g. take-off, landing, climbing, cruising, weight increase/decrease) using well-developed computational fluid dynamics equations, and iteratively making incremental changes to the structure and shape details until the desired increase in vortex energy within the shroud is obtained by the computational fluid dynamics equations. It is also envisaged that shape optimisation may be performed by a convolution neural network trained on a suitable data set and configured to perform shape optimisation.

It is envisaged that the above described shape optimisation technique may result in the cross sectional shape of the shroud at right angles to its longitudinal axis resembling the shape of a spiral as the radius of the shroud’s cross sectional shape progressively decreases as it curls over from beneath the wing tip over to the top of the wing tip. That this is potentially the best optimised shape, is likely, is because the decreasing radius will have the effect of increasing the angular velocity of the air in the shroud as its upper surface curls over to end above the surface of the wing. By the laws of conservation of energy that increased angular velocity will cause a reduction of the air pressure at the throat of the venturi where the shroud is closest to the upper wing tip surface. This should further enhance the reduced pressure on the upper surface of the wing tip at that point, and hence provide an even greater pressure differential between the lower and upper surface of the wing at its tip - resulting in an increased lifting force.

Although the terms convoluted or logarithmic spiral are used above, it is envisaged that any similar shape which has an increasing curvature (i.e. decreasing radius) may produce the same effect and that the precise shape may result from a shape optimisation process such as that described above.

The inside surface of the shroud may further optionally comprise one or more flow vanes to manage the air flow of the vortex inside the shroud and to inhibit energy losses through turbulence which might otherwise develop in the air flow of the vortex. It is envisaged that flow vanes may be particularly advantageous in relation to air entering the shroud as the flow vanes may help to establish a smooth flow of air from the shroud entrance. It is also envisaged that flow vanes may assist in maintaining a streamlined flow inside of the shroud as the angle of attack of the wing changes in different phases of flight. Optionally the flow vanes may have an adjustable pitch so as to adaptively control air flow inside of the shroud during the above described changes in angel of attack of the wing, so as to optimise the effect of the shroud. The means of attachment of the shroud is a stationary, internal propeller or turbine, shaped so as to provide propulsive force to the airframe by extracting energy from the air vortex within the shroud. By attaching the shroud to the wing-tip with an internal structure instead of with the edges of the cylindrical shape of the shroud itself, a gap (hereinafter described as a venturi space) is provided between the upper surface of the wing and the top, curved part of the cylinder shape of the shroud.

The wing-tip profile (i.e. the shape defining the lower edge of the venturi space) is similarly designed using shape optimisation so that its shape matches and acts in combination with the shroud edge (i.e. the shape defining the upper edge of the venturi space) to produce the above described venturi space between the shroud where it curves over towards the upper surface of the wing and the wing itself. In some embodiments it is envisaged that the wing-tip profile may curve up inside the shroud so as to generate a further venturi which accelerates air flow into the side of the shroud.

As the vortex exits the shroud, it acts upon a flat plate formed from the lower part of the shroud, creating a lower pressure above the plate which provides a lift force on the plate. It is envisaged that the lift plate may have a slight degree of curvature to it about its longitudinal axis to optimise the lift effect. In particular, a vortex has lower pressure at its core. In some embodiments it may be beneficial to design the lift plate such that it in effect peels away a layer of air from the outer rotating flow (i.e. the curvature of the shroud where the lift plate starts is continued smoothly into the lift plate and then gently decreases into a flat surface). The effect of this is that the lower pressure in the core of the vortex impinges more directly on the lift plate, hence further enhancing the lift.

The synergistic effects provided by an embodiment of the invention are described below.

Creating a vortex in the Shroud

The front of the lower part of the shroud is beneath the leading edge of the wing. Thus the higher pressure from the air from the under wing surface is directed into the shroud to create a strong anti-clockwise rotating vortex in the shroud on the port wing tip as seen from the front of the aircraft. The vortex in the shroud around the starboard wing tip will rotate in the opposite, clockwise sense as see from the front of the aircraft.. Amplifying the vortex in the shroud

Whilst the shroud has been described above as being“broadly cylindrical” this is to provide a basis for describing the process by which it operates, and from which a baseline design can be generated in a suitable computational, shape optimisation process. The inventor has realised that further benefit could be obtained by amplifying the effect of the vortex in the shroud. This is achieved by decreasing the radius of the shroud along a portion of its length so as to provide a waist (i.e. a reduced cross section along part of its length). In particular, the velocity of the rotating air in the vortex along the waist portion increases when the radius decreases. A venturi along the shroud’s quasi cylindrical axis significantly accelerates the velocity of the vortex within the shroud at its throat, reducing the air pressure and further enhancing the lift effect of the venturi between the upper part of the shroud and the aeroplane’s tip.

Venturi effect along upper surface of the tip

The rotating air within the shroud then draws air into the shroud from the wing’s upper surface through the venturi space hence further reducing the already lower pressure on the upper surface of the wing compared to that on the lower wing surface.

Lift effect from increased pressure differential

The overall effect provides a vertical force from the increased air pressure differential between the lower (higher pressure) and upper (lower pressure accentuated by the shroud) surfaces of the wing at its tip.

A‘lift plate’

At its lower edge, the shroud extends horizontally, beneath and generally parallel to the plane of the wing at the tip. This part of the shroud structure also extends beyond the trailing edge of the wing in its plane surface, but below the plane of the wing. This extending, trailing part of the shroud is hereinafter described as a‘lift plate’. Along the chord of the wing (at a point determined by a suitable shape optimisation process as described above), the upper part of the shroud moves away from the wing surface to form a vertical plane hence leaving the horizontal plate (the lift plate) exposed. Thus, as the rotating vortex exits the shroud, it acts on the lift plate creating a lower pressure region above the lift plate. The lower pressure acting upon the surface of the lift plate provides a lifting force. It is thus envisaged that the lift plate may further increase the total lift generated by the shroud. It is further envisaged that the lift plate may comprise a curved, aerofoil-like cross sectional profile to enhance the lift it provides.

Attachment of the shroud - a thrust mounting

The inventor has realised that attaching the shroud to the wing with struts would inevitably have caused significant air drag and loss of efficiency as the struts would inevitably have been within the air vortex inside the shroud. The inventor has also realised that, whilst a rotating propeller or turbine creates thrust and a rotating wake of air behind it, it has numerous moving parts which, as described above, may break or jam. Instead the inventor realised that a stationary propeller or turbine (i.e. an propeller) in a strong rotating vortex may advantageously be provided to generate additional thrust. In particular, as one purpose of the shroud is to increase the rotational velocity of the vortex, a propeller was found to be unexpectedly more effective when used in the shroud than when used alone on a wing-tip without a shroud. The stationary propeller may have a secondary function of providing a connection between the wing and the shroud. Thus any mounting struts within the Shroud should be of an aerodynamic profile (propeller or turbine) to extract further energy from the vortex in the form of forward thrust. The mounting is hereinafter described as a‘thrust mounting’. Optionally, the pitch of the propeller blades can be adjusted to optimise the thrust. Where they are adjustable, they may be set to be flat faced to air flow to produce a sudden increase in both induced drag and form drag which may be beneficial in some phases of flight (such as approach and landing). It is envisaged that each of the blades may be attached at one end to an inner surface of the shroud and at their other end to a central, longitudinal rod or similar structure which passes down a longitudinal axis of the shroud to stabilise the thrust mounting structure. The additional drag generated by such a structure may be minimal given that it is in the centre of the vortex inside the shroud. Further, it is envisaged that the central, longitudinal rod may help to stabilise the vortex.

Winglets Irrespective of the efficiency and benefit of the shroud to enhance the efficiency of a wing, there will still be a likelihood that the whole of the wing, including the shroud itself will create a tip vortex outside of the shroud. Winglets are known to inhibit this effect, and could be used to reduce the effect of vortices outside of the shroud. It is envisaged that winglets may extend both above and below the shroud.

The above described features are summarised as follows:

• Wing-tip shroud: Use a wing-tip shroud to entrap and manage a high proportion of the energy from the wing-tip vortex.

• Venturi effect 1 : Design the shroud so that the circulating air inside the shroud creates a venturi effect between wingtip and the shroud itself. This further reduces the air pressure on the upper wing surface.

• Venturi effect 2: Shape the shroud so that the cross sectional area of the shroud reduces in the middle section, thus accelerating the circular flow which will amplify the venturi effect, thus reducing the pressure of the rotating air at its point of lowest cross sectional hence further reducing the air pressure on the upper wing surface towards the wing-tip.

• Lift plate: Extend the lower section of the shroud in a horizontal plane extending beyond and beneath the trailing edge of the wing. Any residual departing vortex bearing upon this plate will lower the pressure above the plate serving to provide lift onto the plate surface.

• Propulsive affect: Use the attachment points to connect the shroud to the wing as stationary propellers or turbines. By angling the blades appropriately, the rapidly rotating air passing through these will provide propulsion to the airframe.

• Winglets: Use the design of the shroud to create separate winglets (above and below) integrated into the shroud design.

The cumulative affect of the above features is to provide four different mechanisms to feed vortex energy back into the airframe, and to inhibit any residual vortex.

The above described features provide at least the following effects on lift and on air flow at the wing-tip: • A venturi mechanism develops between the rotating air within the shroud and the wing-tip which has the effect of amplifying the reduced pressure on the upper surface of the wing, hence increasing lift.

• Circulating air within the shroud bears upon the blades of the attachment structure - propeller, turbine or other such device providing propulsion.

• Reduced pressure created by the vortex emerging from the shroud will reduce the pressure above the‘lift plate’ and provide additional lift to the wing.

• The shroud winglets (above and below) will inhibit energy loss in any residual vortex extending outside the shroud itself.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 (a) shows a front elevation view of a wing tip device according to an embodiment of the invention, the view is taken to be looking into the wing tip device from the front of an air intake section.

Figure 1 (b) shows a plan view of the wing tip device of Figure 1 (a).

Figure 2 shows an elevation view of the wingtip device of Figure 1 , the view is taken from the side of the wing tip device, looking towards an aircraft fuselage.

Figure 3(a) shows a front elevation view of a wing tip device according to an embodiment of the invention.

Figure 3(b) shows a plan view of the wing tip device of Figure 3(a).

Figure 4(a) shows a front elevation view of a wing tip device according to an embodiment of the invention.

Figure 4(b) shows a plan view of the wing tip device of Figure 4(a).

DETAILED DESCRIPTION

Figure 1 (a) shows a front plan view of a wing tip device 100 (also referred to as a shroud) according to an embodiment of the invention. A corresponding wing tip device may be present on a wing on the opposite side of an aircraft. The longitudinal axis is defined as the axis along which the wing is moving forward in the air so that Figure 1 is said to be a view from along the longitudinal axis. The wing tip device 100 is mounted using a mounting device 101 onto the tip of a wing 102, the wing having an upper surface 103 and a lower surface 104. The mounting device comprises a plurality of aerodynamically shaped support structures 101 a-d, connected by a central support axis, which together comprise the blades of a stationary propeller. The support structures 101 a-d may all be positioned at a single position along the length of the wing tip device 100, or may each be spaced apart at different positions along the length, and/or at varied angular positions around a longitudinal axis of the wing tip device 100. The arrangement may optimised (e.g. using computational modelling) to ensure an optimum propulsive effect, attachment strength and even distribution of propulsive loads. There may be multiple sets of support structures 101 a-d so as to provide the functionality of a two or more propellers within the wing tip device 100. Whilst four support structures 101 a-d are shown in Figure 1 (a), the same propeller functionality may be achieved with two or three support structures, or with five or more. At least one of the support structures (i.e. stationary propeller blades in the case of Figure 1 (a) or, as will be described below, other mounting means such as support struts or an extended shroud shape) is secured to the wing 102 (not shown in Figure 1 (a)), and at least one of the of the support structures (i.e. stationary propeller blades or other mounting means as described below) is secured to an inner surface of the cylindrically shaped wing tip device 100 (i.e. shroud) so as to provide the mounting function of the mounting device 101. The blades may be fixed or their pitch may be adjustable with a suitable actuating mechanism.

The wing tip device 100 optionally comprises an upper winglet 105 and/or optionally a lower winglet 106. The gap defined between the wing tip device 100 and the upper surface 103 of the wing defines a venturi point 107 (also described as a venturi length as the gap may extend for a portion along the upper surface 103). The winglets serve the purpose of minimising vortex effects outside of the wing-tip device, such as those produced by the wing and wing tip device combined.

The wing tip device 100 is largely cylindrically shaped but is not connected to the wing 102 anywhere except for with the mounting device(s) 101 , thus defining a gap (i.e. a venturi length or point 107) along the upper surface 103 of the wing 102 and defining a gap (not labelled) along the lower surface 104 of the wing 102, where the wing tip device 100 curls around the lower surface 104 of the wing 102. The gap along the upper and lower surfaces of the wing may each be said to be an open side portion of the shroud. The shroud itself defines a longitudinal air channel which runs substantially parallel to the direction of movement of the wing. As the wing moves forward, air becomes channelled into the open front of the air channel of the wing tip device 100, and into the gaps along the upper and lower surfaces 103, 104 of the wing 102. The air moving into the wing tip device 100 from gaps along the upper surface 103 and lower surface 104 of the wing 102 gives the air inside the wing tip device 100 a rotational velocity so as to produce a vortex. The vortex acts on the propeller shape of the mounting device 101 so as to produce forward thrust. As described above, the pitch of the blades may fixed or adjustable where a greater control of thrust is required.

Figure 1 (b) shows an elevation view of the wing tip device 100 of Figure 1 (a). The wing is shown to have a direction of travel along the longitudinal axis so that the wing may be said to have a leading edge 108 and a trailing edge 109. The wing tip device 100 has a lift plate 1 10 extending rearwardly from the wing tip device 100. The lift plate in Figure 1 (b) is shown to extend beyond the trailing edge 109 of the wing itself when viewed from the elevation view of Figure 1 (b). The lift plate 1 10 may be the trailing part of the cylindrical shape wing tip device 100 of which approximately three quarters of the perimeter has been removed so as to open up the shape as shown in Figure 1 (b). Alternatively, it may be a separate component (such as a plate) secured to the lower side of the wing tip device 100. As the vortex air flow exits the wing tip device 100 it creates a region of lower pressure on the upper surface of the lift plate 1 10, thus generating lift. The stronger the vortex (i.e. the greater rotational velocity), the greater the increase in lift.

As shown in Figure 1 (b) the largely cylindrical shape of the wing tip device 100 has curved edges that taper down towards venturi point 107. The venture point 107 in this case is a venturi length which covers approximately two thirds of the centre part of the length of the wing). Its precise dimensions may be optimised by theoretical and experimental testing. It is envisaged that the length along which the venturi effect is generated may be more or less than two thirds of the length of the wing. In particular, it may be the entire length of the wing, or it may only be a small portion. The region of the upper surface 103 of the wing 102 along which the venturi effect is generated experiences a lower pressure due to the increased velocity of the air at that region. The lower pressure generates an increase in lift.

The mounting device 101 in Figure 1 (b) is shown to comprise two sets of support structures 101 (dotted lines), each set secured to the wing tip at positions approximately at the end of the venturi length. This would provide the functionality of two stationary propellers to generate forward thrust. Although not shown, optional additional multiplicities of blades may be present at other positions in the wing tip device 100.

The wing tip device 100 of Figures 1 (a) and 1 (b) does not have a tapered middle portion (i.e. a waist portion) to amplify the venturi effect. Despite this, it still provides enhanced lift and thrust as described above. The features of wing tip device 100 of Figures 1 (a) and 1 (b) may be used alone or in combination with any or all of the features of Figures 2 to 4. In particular, with the reduced tapered middle portion of Figures 3(a) and 3(b).

Figure 2 shows an elevation view of the wing tip device 100 of Figures 1 (a) and 1 (b), the view being taken from the side of the wing onto which the wing tip device 100 is attached. The leading edge 108 and trailing edge 109 are shown. The dashed lines show how the wing tip device 100 curls over to create a venturi effect along venturi point 107 along the upper surface 103 of wing 102.

The approximate sizes and shapes of the upper winglet 105 and lower winglet 106, and the lift plate 1 10 is also shown in Figure 2. It is envisaged that these may have any other suitable shape. The approximate shape of wing 102 is shown as a dotted line hidden behind the wing tip device 100.

The features of Figure 2 may be used alone or in combination with any or all of the features of Figures 1 (a), 1 (b), 3(a) ,3(b), 4(a), and 4(b).

Figure 3(a) shows a front plan view of a wing tip device 300 according to an embodiment of the invention, and Figure 3(b) shows an elevation view of the wing tip device 300 of Figure 3(a). The features of wing tip device 300 are the same as that of wing tip device 100 of Figures 1 (a) and 1 (b) with the following two exceptions.

First, the generally cylindrical shape of wing tip device 300 reduces in radius towards the centre of the length of venturi point 301 , before gradually increasing again in diameter towards the rear of the wing tip device 300. This creates a waist shape 302 (also described as a tapered portion) in the wing tip device 300. Thus the cylindrically shaped shroud may be said to have a central portion with a reduced radius.

Second, the mounting device 303 is positioned closer towards the point where the wing tip device 300 has the smallest radius i.e. at the waist 302. In other words, the blades of the stationary propeller may be said to be positioned in the central portion.

The effect of the first difference is that the air speed and thus the rotational velocity of the vortex inside the wing tip device 300 increases as the radius shrinks. Thus the venturi effect along venturi point 301 is greater.

The effect of the second difference is that the thrust generated by the stationary propeller functionality of the mounting device 303 is greater as the propeller is positioned where the air flow and rotational velocity of the vortex is fastest.

Whilst Figure 3(b) shows only one mounting device 303 at the waist 302, it is envisaged that two or more mounting devices may be positioned around or close to the position of the waist 302 in addition to any mounting devices positioned further towards the front and rear edges of venturi point 301.

Figure 3(a) also illustrates that, of the four shown support structures 303a-d, support structure 303c is what secures the wing tip device 300 to the wing. Where multiple sets of mounting devices are present, it is envisaged that each will have a similarly positioned support structure to help secure the wing tip device 300 to the wing.

The features of Figures 3(a) and 3(b) may be used alone or in combination with any or all of the features of Figures 1 (a) and 1 (b), 2, 4(a), and 4(b). Figure 4(a) shows a front elevation view of a wing tip device 400 according to an embodiment of the invention, and Figure 4(b) shows a plan view of the wing tip device 400 of Figure 4(a).

The features of wing tip device 400 are the same as that of wing tip device 100 of Figures 1 (a) and 1 (b) with the following three exceptions.

First, the cylindrical shape of wing tip device 400 is adjusted such that there is no curvature around the lower surface of the wing, as compared to wing tip device 100 of Figures 1 (a) and 1 (b). Instead, the wing tip device 400 ends approximately vertically downwards 401 at a right angle compared to the orientation of the wing. The effect of this may be that rotational velocity of the air flow is reduced as the air is not channelled into a vortex shape as effectively. Flowever, an advantage of this embodiment is that it is more easily manufactured. The means of attachment is not shown in Figure 4(a) but may be same as that of the other Figures. It may be necessary to move the positions of the stationary propeller

A second difference, is that venturi point/length 402 is longer and covers a greater portion of the length of the wing, as shown in Figure 4(b).

A third difference, is that the lift plate 403 is provided not as part of the wing tip device 400 but instead as an extension of the wing. It is envisaged that the lift plate may comprise a twist along its length to provide a means to adjust the angle of attack.

Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the art that many variations of the embodiment can be made within the scope of the attached claims.

For example, it is envisaged that a separate mounting device may be used instead of the stationary propeller, even though it may cause an increase in drag. In particular, the stationary propeller may be mounted in the shroud and a separate support strut may be used to connect the wing tip device to the wing tip.

The above described embodiments have been described in connection with a wing tip of a wing of an aircraft. Flowever, it is also envisaged that the wing tip device may be attached to any wing like structure where it is advantageous to harness vortex energy that would otherwise be lost. For example it is envisaged that the above described wing tip device may be used at the end of a helicopter rotor, or at the end of a blade of a wind turbine. The wing tip device may also be used in any field in which the fluid dynamics of lifting devices are a concern. For example, the above described techniques may also be applied to ship propellers or hydrofoil wings/blades.

In a further embodiment, which may be used in combination with any of the above described embodiments, the stationary propeller blades are not used to secure the shroud to the wing tip. Instead, separate support struts which are used. The separate support struts may either be connected between the central, longitudinal rod and the wing tip, or may alternatively or additionally be connected between other parts of the shroud and the wing tip, such as between the body of the shroud and the wing tip above and/or below the wing. In a further alternative to support struts, parts of the body of the shroud itself may be extended so as to connect and secure the wing tip device to the upper and/or lower surface the wing. It is envisaged that where the shroud body itself is connected and secured to the wingtip, enough of a gap along the upper and lower wing surfaces remain to ensure the above described venturi effects may be achieved. It is also envisaged in some embodiments that a stationary propeller thrust mounting, support struts and an extended, connected shroud body is used to secure the wing tip device to the wing tip. The shape of the support struts and/or extended shroud body may be generated using known computational techniques to ensure a desired aerodynamic performance is achieved.

Where additional support struts and/or an extended shroud body is used, there may be additional drag on the wing tip device and/or disruption of the stability of vortex in the shroud. In order to compensate for this, additional flow vanes may be positioned on an outer surface of the shroud. In particular, they may be positioned between where the shroud curls over towards the top of the wing and the outer part of the shroud itself towards the end of the curl over section. These flow vanes provide the advantage that air flow may be controlled towards and into the venturi which is created. Without the flow vanes, the additional drag and other disruptive effects of the additional support struts and/or extended shroud shape may reduce the efficiency of the wing tip device. The shape of the floe vanes may be generated using known computational techniques to ensure a desired aerodynamic performance is achieved. It is also envisaged that the shroud may be used without any stationary propeller at all. Rather, the shroud may simply provide additional lift through the above described venturi effects, without providing any additional thrust. This may be particularly advantageous where reducing weight is a priority and no additional thrust is required. By removing the weight of the propeller components, the total weight of the wingtip device is significantly reduced, while still providing the advantage of increased lift.

A further advantage of the above described embodiments is that it overcomes the need for constantly increasing wingspans. In particular, an ideal wing is one which has infinite wing span (i.e. a high aspect ratio). The higher the aspect ratio, the lower the tip vortex drag. Aircraft such as gliders and modern airliners have wings with a high aspect ratio for this reason. However, high aspect ratio wings inherently require larger hangers and more space at arrival gates. The above described embodiments provide a means to replace ever increasing wingspans with a compact device which does not require as much space, thus providing a commercial advantage by minimising hangar and arrival gate space required. In particular, it is envisaged that a fleet of airliners using the above described wing tip device may achieve the same operational efficiency as a fleet of airliners having large wingspans.

The inventor has also realised that, under normal circumstances, the air which enters the shroud from below the wing would be at a higher pressure than the air which enters the shroud from above the wing. Thus there is a risk that the air flow from below the wing will escape through the venturi gap to the upper surface if the air pressure inside the shroud is not low enough. The inventor has unexpectedly found that the shroud also generates a venturi between the lower surface of the wing tip and the shroud where air enters the shroud from the lower surface of the wing tip. Thus, air which is drawn into the shroud through this lower venturi gap is at a lower pressure and will join the vortex in the shroud, rather than escape the shroud over the upper surface of the wing. This effect is applicable to all the embodiments.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.