Patent Description:
<CIT> discloses a "winglet" device provided at a tip portion of a fixed-wing aircraft, and defining a surface which is a continuation of the aerodynamic lifting surface of the wing. The tip device has the function of reducing draft, by minimising the effects on the wing of caused by vortices developing at the trailing edge of the wing, due to the pressure differential between the air below the wing and the air above the wing. This, and related work, showed that a swept back and downward canted tip preferentially moves the vortex further outboard and reduces the intensity. However, no use is made of this purposely redirected airflow.

<CIT> similarly shows a "blended winglet", which is a wing-like device attachable to an airplane wing tip to achieve minimum induced drag for a given surface size. The wing tip includes a contoured surface which projects either above or below the wing plane and laterally from its tip.

European Patent Application Publication No. <CIT> discloses an aircraft, particularly a solar powered, high altitude, long endurance, unmanned aerial vehicle, which is equipped with a combination of canted down, raked back wing tips and trailing "tip tails" carried on booms from the tip regions of the mainplane.

United States Patent Application Publication No.<CIT> discloses a variable geometry wingtip comprised of two deformable yet relatively stiff airfoils that work in compression and tension against each other to change shape.

German Patent Application Publication No. <CIT> discloses an aircraft having left and right auxiliary wings movably and actively arranged at an outer end of left and right main wings.

United States Patent Application Publication No. <CIT> discloses a spiroid wing tip which includes a characteristic loop profile. The spiroid profile is lowered with respect to the wing chord plane, such that a portion of the spiroid wing tip extends below the wing chord plane.

The present invention proposes in general terms that a tip portion of an aerofoil is provided with an air-redirection formation including a surface which is positioned to generate uplift due to air vortices shed from the aerofoil.

More specifically, the invention proposes an air-redirection formation for a tip portion of an aerofoil according to claim <NUM>.

Here the term "horizontal" may be defined as level with the aerofoil (e.g. it may be defined as including multiple cords - imaginary straight lines joining the leading and trailing edges of the wing). Alternatively, in the case that the aerofoil is part of an aircraft, it may be defined based on the body of the aircraft. Note that conventionally wings are dihedral (angled up from the horizontal as defined by the aircraft body) or anhedral (angled downward from the horizontal as defined by the aircraft body), and indeed may have varying dihedral angle along the wing from wing root to wing tip. Instances of "horizontal" in this document are disclosures of both these possibilities (i.e. defining "horizontal" with reference to the aerofoil or with reference to the aircraft body). Major benefits of the proposed tip are to simultaneously reduce induced drag and modifying (typically increasing) net lift whilst unloading wing bending moment and/or the wing twisting moment under higher speed conditions (due to a neutralising moment applied by the device at the tip and associated modification of the lift profile), allowing an overall reduction in structural mass and possible expansion of the flight envelope.

The first portion deviates away from aerofoil (or "main wing") plane.

Thus, the first surface reduces drag by curtailing airflow over the tip and re-directing vortices generated by the aerofoil away from the aerofoil. In the base operating case ("nominal" flight conditions) the second surface may utilise the vortices shed from the aerofoil and directed by the first surface to provide lift. Thus, the weight of the formation is approximately offset by the additional lift, so that the stability and control benefits of the device are provided without a net penalty.

The second surface is spaced far enough away from main wing, via the first surface, such that it does not cause significant interference with the operation of the main wing or first surface. This separation may be greater than <NUM>% of the main wing MAC. The air-redirection formation directs and utilises rearwards circular/spiral airflow ("corkscrew vortices"), thereby allowing drag reduction and improved lift.

The second surface may incorporate an amount of 'sweep' to further maximise utilisation of the airflow and for traditional aerodynamic stability reasons.

Passive control of wing twist may be provided by the rear portion of the present air-redirection formation. During flight, efficient wings often tend to generate a pitching forwards moment. The rear portion of the air-redirection formation can counter this effect by providing a little less lift than its weight. At higher speeds when the wing tip begins to twist further, the air-redirection formation may decrease its angle of attack and, depending on the aerofoil section chosen, may prevent the wing from over-twisting. This would have the effect of expanding the safe flying speed. Active control (as described in the next paragraph) would further enhance this benefit and, in the case of aircraft in particular, would reduce the dependency on fine-tuned aerodynamics for all flight conditions.

Optionally, the rotational and/or translational position of the first and/or second surface relative to the aerofoil is controllable, thereby providing an additional flight control surface.

Various types of control are possible. Specifically, the control may comprise any one of more of the following:.

Control movements can be achieved using articulated hinges on or in the wing tip device or at the join to the aerofoil (or 'main wing' for an aircraft case). Alternatively, instead of a hinge, control movements can be achieved by deforming (i.e. controlling the shape of) a portion of the formation, such as the connection structure. This may be termed wing warping or wing morphing - that is, bending, twisting, expansion, contraction, thickening or any combinations of one or more of these, or other deformation of at least a portion of the formation - and can be used instead of a hinge, such that a rotation (curve or twist) is spread over a length of the portion of the formation rather than being concentrated at a hinge.

One or more of these control methods may be used to give overall attitude control for the aircraft, to optimise local airflow and efficiency along the wing, or to unload the structural forces through the wing structure.

The air-redirection formation is preferably provided as a device which is separate from, but connectable to, a tip portion of the aerofoil. The device may be formed as a single element having a surface which is a continuous curved sweep, or the device may be formed of multiple joined sections, e.g. straight sections which are joined to each other at angles. The curvature of any surface of the device may be selected to adjust the local angle of incidence of air onto the device.

The aerofoil may be any structure generating aerodynamic forces, including turbine blades, propellers and rotor discs, measurement devices, or wind energy generation devices. In a preferred case, the aerofoil may be a wing of an aircraft, such as a fixed-wing aircraft. One application is for high efficiency aircraft. Notable examples include competition gliders and solar-electric high altitude pseudo-satellites (HAPS). A HAPS would particularly benefit from potential higher aerodynamic efficiency due to better lift generation on the main wing with reduced drag, lower mass due to unloading the primary structure and higher airspeed limits specifically due to counter-acting the torsional forces at high speed, better high altitude stability (potentially increasing the operational ceiling) due to active counteraction of dutch roll and even the minor benefit of increased area for mounting photovoltaic panels. Alternatively, the aerofoil may be a portion of a kite, car, ship and any other vehicle. Indeed the present air-redirection formation may be provided for any aerofoil application for deliberate aerodynamic force generation and/or moderation.

Referring to the aerodynamic lifting surface of the aerofoil (the "main surface of the aerofoil") as lying in and defining a "horizontal plane", the first surface is provided as a horizontal or near-horizontal surface, and the first portion of the formation may project in a horizontal or near-horizontal direction from a tip portion of the main surface of the aerofoil.

Similarly, the second surface is horizontal or near-horizontal. In the case that the position of the second portion of the air-redirection device is controllable, this may change the relationship of the second surface to the horizontal plane.

The first and/or second portions of the air-redirection formation may extend generally parallel to an elongation axis of the aerofoil.

Alternatively, at least the first portion of the air-redirection formation may extend rearwardly from the main surface of the aerofoil in a direction opposite to the direction in which the aerofoil passes through the ambient air. In the case that that the aerofoil is a wing, the end of the first portion of the air-redirection device which is furthest from the body of the aircraft may be slightly inboard or slightly outboard of the tip of the wing. The second portion of the air-redirection surface is further from the body of the aircraft than the first portion. In the front-rear direction, it may be in line with a central axis of the wing, or may be positioned forwardly ("swept forward") or backwardly ("swept back") with respect to this axis. It is in the horizontal plane of the wing or below it.

The first and/or second surfaces may, in the case that the aerofoil is the wing of an aircraft, be level or canted in the dihedral or anhedral directions. The degree of canting may, in the case that the position of the second portion of the air-redirection device is controllable, depend upon that controlled position.

The first and second portions of the air-redirection formation are connected by a connection structure, which may be distinguishable from the first and second portions by having a smaller maximum thickness than the maximum thickness or chord length of the first and/or second portions of the air-redirection structure.

The first portion of the device may have a gradually reducing minimum thickness, though the direction of this minimum thickness may diverge from that of the main aerofoil chord direction (generally defined to be parallel to the freestream airflow in the lateral plane, though often with a pitch angle deviating from the lateral plane). The length in the chord direction will nominally reduce along the device with the exception of one group of implementations: if the wing tip device has an extreme sweep such that it is extending straight back, perpendicular to the main wing, then the length in the chord direction (approximately the freestream airflow direction) may actually increase, even though the local minimum thickness dimension of the device will still tend to reduce. Some (and possibly all) of the connecting section or the second portion of the device may have a greater length in the direction of the main aerofoil chord (approximately the freestream airflow direction) than the main aerofoil itself.

The connection structure is provided as at least two joined connection elements (which may be integral with each other, or separately but connected together).

Both the first and second connection elements project "downwardly" (e.g. as perceived by a person in the body of the aircraft) away from the respective first and second portions of the air-redirection formation. The first and second connection elements are connected to each other (via a third connection element) at ends of the first and second connection elements which are respectively distal from the first and second portions of the air-redirection formation.

Embodiments of the invention will now be described, for the sake of example only, with reference to the following drawings in which:.

Referring firstly to <FIG>, an aircraft is illustrated having two fixed wings <NUM>. The body of the aircraft (not shown) is in the central position <NUM> indicated in <FIG>. A respective tip portion of each wing <NUM>, furthest from the central body of the aircraft, is provided with an air-redirection formation <NUM> connected to the tip portion of the wing.

<FIG> is an enlarged view of a tip portion of one of the wings <NUM>. The wing has a central elongation direction indicated as <NUM>, which extends away from the body of the aircraft.

The underside of the wing <NUM> (as viewed in <FIG>) is an aerodynamic surface which defines or approximates a horizontal plane. For example, the lifting surface of the wing <NUM> may optionally be flat, or it may be curved but the horizontal plane may be defined by cords (imaginary straight lines joining the leading and trailing edges of the wing <NUM>). Note that a respective horizontal plane is defined with respect to each wing <NUM>, and the two horizontal planes are often not identical to each other due to dihedral or anhedral design. In practice, the plane described may also twist in the spanwise direction (a design feature known as `washout' in the aerospace industry). This twist is disregarded for the purposes of clear description below.

The air-redirection formation <NUM> provided at a tip portion of each wing <NUM> is typically formed as a device which is first constructed and then attached to the tip portion of the wing <NUM>. Note that in variations of the device the air-redirection formation <NUM> may be constructed as part of the fabrication process of the wing <NUM>, e.g. integral with the wing.

The device <NUM> includes a first portion <NUM> for directing vortices generated by the wing <NUM> away from the wing <NUM> which may reduce drag and may maintain higher overall lift on the main wing. A lower surface of the first portion <NUM> is a continuation of the aerodynamic lifting surface (under surface) of the wing <NUM>.

The device <NUM> further includes a second portion <NUM> that may produce lift using the vortices directed away from the wing <NUM> by the first portion <NUM>. More generally, in variants of the device it may generate local aerodynamic forces from the airflow, either positive (e.g. traditionally 'up' against the gravity vector) or negative (e.g. traditionally 'down' towards ground). The second portion <NUM> is spaced from the wing <NUM> by the first portion <NUM>.

The first and second portions <NUM>, <NUM> of the device <NUM> are connected by a connection structure <NUM>. The connection structure <NUM> maintains the first and second portions <NUM>, <NUM> of the device in a spaced-apart configuration. The implementation may blend <NUM>, <NUM> and <NUM> into a single continuous shape and structure in practice, and defining the boundaries between them is arbitrary for the sake of explanation.

Both the first and second portions <NUM>, <NUM> of the device <NUM>, or at least their respective lower surfaces, are substantially in the horizontal plane.

The connection structure <NUM> has a key aerodynamic effect, as well connecting the rear portion to the main wing. It preferentially moves the vortices, in the direction away from the wing <NUM> and towards the second portion <NUM>. It is canted downwardly, and also swept back. Canting downwardly means projecting in a direction which has a downward component (that is, in the direction away from the wing tip in the direction transverse to the horizontal plane of the wing, toward the lower surface of the wing from the top surface). Sweeping rearwardly means projecting in a direction which has a component in the rear direction (that is, a direction within the horizontal plane which is transverse to the central elongation direction <NUM>, and generally towards the rear of the aircraft).

As shown most clearly in <FIG> and <FIG>, the device <NUM> has its maximum chord (extension in the direction from the leading edge of the wing <NUM> to the trailing edge) near where it contacts the wing <NUM>. At successive positions in the direction <NUM>, the device <NUM> (and thus the combination of the wing <NUM> and the device <NUM>) has a narrowing (reducing) minimum distance between the leading and trailing edges (the local "thickness"), which generally corresponds to a reduction in the chord. Connection structure <NUM> may be the thinnest by minimum dimension (local thickness) but for implementations with more extreme sweep, this section may extend in the chordwise direction of wing <NUM>, meaning that the chord at this point, by strict interpretation, could be the longest even though the leading edge and trailing edge are closest to each other. The chord of the second portion <NUM> may be greater or less than the chord of the main wing <NUM>. In <FIG>, the direction A indicates the wing chord direction, and arrow D indicates the direction of the minimum local thickness at a particular local position.

The first portion <NUM> of the device <NUM> may have a fixed positional relationship with respect to the wing <NUM>. However, optionally, the second portion <NUM> of the device <NUM> may be provided with an actuator (not shown) which is controllable (typically based on signals generated within the body of the aircraft and transmitted to the actuator) to move relative to the wing <NUM>. Thus a control surface (e.g. lower surface) of the second portion <NUM> of the device <NUM> can act as a control surface, under control of the pilot, for steering the aircraft.

As indicated by the double-headed arrow in <FIG>, the position of the second portion <NUM> of the device <NUM> may, in one possibility, be movable about a yaw axis, e.g. by hinging or warping.

The device <NUM> is outboard the tip of the wing <NUM>, and extends principally away from the body of the aircraft, e.g. the outer tip of the second portion <NUM> is spaced from the central axis <NUM> of the wing <NUM> principally in the "vertical direction" (i.e. perpendicular to the horizontal direction). However, in variations of the device, the device <NUM> may project partially or wholly rearwardly from the wing <NUM> (i.e. in the direction in which air flows away from the wing <NUM>) or inboard of the outer edge of the wing.

Turning to <FIG>, an embodiment of the invention is shown. Reference numerals of elements corresponding to those of the device of <FIG>are <NUM> higher. In this embodiment too, each of the wings <NUM> of the aircraft, which extend outwards from the body <NUM> towards respective wing tips, is provided at a tip portion with a device <NUM>. This device may be constructed separately from the wing <NUM>, and attached to the wing <NUM> subsequently. Each of the wings <NUM> defines a respective horizontal plane, as described above.

The device <NUM> comprises a first portion <NUM> and a second portion <NUM>. Both lie substantially in the horizontal plane of the corresponding wing <NUM>. The first portion <NUM> includes a surface (e.g. lower surface) which acts as a continuation of the aerodynamic lifting surface of the wing <NUM>, and directs vortices generated by the wing <NUM> away from the wing <NUM> to reduce drag. The second portion <NUM> may produce lift using the vortices directed away from the wing <NUM> by the first portion <NUM>.

The first and second portions <NUM>, <NUM> are connected by a connection structure <NUM>, having a first connection element 123a and a second connection element 123b, joined together by a connection element 123c. The first connection element 123a is attached to the first portion <NUM> of the device <NUM>, but projects rearwardly (i.e. in the left-to-right direction in <FIG>; which is the general direction of airflow over the wing <NUM>) and downwardly from it. The second connection element 123b is attached to the second portion <NUM> of the device <NUM>, but projects forwardly and downwardly from it. The third connection element 123c which joins the first and second connection elements 123a, 123b is entirely below the first and second portions <NUM>, <NUM> of the device <NUM>.

Optionally, at least the second portion <NUM> of the device <NUM>, is movable relative to the first portion <NUM> and the wing <NUM>. This is done under the control of an actuator (not shown), typically controlled from within the aircraft body. Thus a control surface (e.g. lower surface) of the second portion <NUM> of the device <NUM> can act as a control surface, under control of the pilot or autopilot, for steering the aircraft.

For example, as illustrated by the arrow in <FIG>, the lower surface (control surface) of the second portion <NUM> of the device <NUM> may be movable in a roll axis, e.g. by hinging or warping. Alternatively or additionally, as illustrated by the arrow in <FIG>, the control surface of the second portion <NUM> of the device <NUM> may be movable about a pitch axis, e.g. by hinging or warping.

Many variations are possible within the scope of the claims, as will be clear to a skilled reader. For example, movement of the devices <NUM>, <NUM> may not be limited to movement of the second portions <NUM>, <NUM> of the devices <NUM>, <NUM>. Rather, the first portions <NUM>, <NUM> also may be movable relative to the wing <NUM>, <NUM>.

Claim 1:
An aerofoil (<NUM>) comprising an air-redirection formation (<NUM>) for a tip portion of the aerofoil (<NUM>), wherein the air-redirection formation includes:
a first portion (<NUM>), which includes a first surface which forms a continuation of the aerodynamic lifting surface of the aerofoil (<NUM>), the aerodynamic lifting surface of the aerofoil defining a horizontal plane; and
a second portion (<NUM>) connected to the first portion (<NUM>) by a connection structure (<NUM>), the second portion including a second surface spaced from the first portion (<NUM>) via the connection structure (<NUM>), and the second portion being in the horizontal plane of the aerofoil or below the horizontal plane of the aerofoil;
wherein, in use in nominal flight conditions, the first surface is substantially horizontal and is for directing vortices away from the aerofoil (<NUM>) and towards the second surface, and the second surface is substantially horizontal and is positioned to interact with the directed vortices, and
wherein the air-direction formation (<NUM>) is connected to the aerofoil (<NUM>); and
the connection structure (<NUM>) comprises at least a first connection element (123a) and a second connection element (123b), the first connection element (123a) and second connection element (123b) being joined by a third connection element (123c),
the first connection element (123a) being connected to the first portion (<NUM>) of the air-redirection formation (<NUM>), wherein the first connection element (123a) projects rearwardly and downwardly from the first portion (<NUM>); and
the second connection element (123b) being connected to the second portion (<NUM>) of the air-redirection formation (<NUM>), wherein the second connection element (123b) projects forwardly and downwardly from the second portion (<NUM>).