Patent Description:
Winglets are well-known and can take a number of forms. Examples of winglets, or wing tip devices incorporating winglets, are shown in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, and <CIT>. Generally speaking, winglets seek to reduce induced drag by increasing the effective span of the aircraft to which they are fitted.

The maximum aircraft span is effectively limited (for a given ICAO Annex <NUM> aerodrome code letter) by airport operating rules which govern various clearances required when manoeuvring around the airport (such as the span and/or ground clearance required for gate entry and safe taxiway usage). For some high-span aircraft, designers have considered moving away from using fixed wing tip devices, and have instead focussed on providing a moveable wing tip device which is moveable between a flight configuration (beyond the allowable gate limit span) and a ground configuration (in which ground configuration the wing tip device is moved away from the flight configuration such that the span of the aircraft is reduced to within the gate limit). Examples, of such moveable wing tip devices can be found in <CIT> or <CIT>.

Nonetheless, there is also still a desire to provide a fixed wing tip device that is suitable for use on a high-span aircraft. An example of a fixed wing tip device for use on a high-span aircraft is shown in <CIT>. In this arrangement, the upper wing-like element of the wing tip device meets a span limit when the aircraft is on the ground, and under <NUM>-g flight loading a lower wing-like element is arranged to offset some of the span decrease that occurs as the wing is aero-elastically deformed causing the upper wing-like element to move inboard. <CIT> discloses another arrangement having fixed upper and lower winglets, but one in which the lower winglet is arranged to increase the overall span under <NUM>-g flight loading.

It will be appreciated from the above that when facing size constraints imposed on span, the design of wing tip devices has recently tended to focus either on providing a moveable wing tip device, or on providing devices with both upper and lower winglets.

<CIT> (BOEING CO) relates to an aircraft having a pair of wings. A forward swept winglet is attached proximate to a wing tip of each wing. The forward swept winglet includes a leading edge and a trailing edge. The leading edge of each winglet extends from the wing at a predetermined forward sweep angle relative to a line perpendicular to a chord of the wing tip in a direction corresponding to a forward portion of the aircraft.

<CIT> (AIRBUS UK LTD) relates to a winglet having a root connected to a distal end of a wing; an upwardly angled section; a tip; and a transition between the upwardly angled section and the tip at which an outboard cant angle of the winglet decreases relative to a reference plane of the wing. In one embodiment the winglet is continuously curved and includes a point of inflection at which the span-wise curvature of the winglet changes sign. In another embodiment the upwardly angled section is planar. The winglet generates a tip vortex when in flight. The tip vortex is displaced outboard from the winglet due to the decrease in outboard cant angle at the transition.

<CIT>) relates to an aircraft wing tip construction for utilizing the vortex airflow around the wing tip to produce a forward acting thrust on the wing comprising, airfoil means supported by the wing substantially normal to the wing span and extending above and below the tip of the wing with the surface of the airfoil means twisted to provide an angle of attack with respect to the vortex airflow such that the lift force on the fin has a component producing a forward acting thrust on the wing.

<CIT> (BOEING CO) relates to an airfoil/winglet combination comprising an airfoil having an airfoil root portion and an airfoil tip portion spaced apart from the airfoil root portion, the airfoil tip portion having a wash-out twist relative to the airfoil root portion; and a winglet extending outwardly from the airfoil tip portion of the airfoil, the winglet being swept forward relative to the airfoil.

When designing wing tip devices, the shape of the wing tip device is often considered based on the so-called 'jig-shape' (i.e. the shape under no-load conditions). Many disclosures of wing tip devices tend to be shown in this jig-shape. For example, <CIT> discloses the shape of the winglets in their 'no-load' state.

In reality, the shape of a wing tip device tends to be governed by the operating conditions of the aircraft on which it is being used. For example, the shape and orientation of a wing tip device tends to differ from the jig shape when the wing is under a <NUM>-g flight condition (under which the wing and winglet are aero-elastically deflected) or when the wing is under a static load condition when the aircraft is stationary at the airport (under which the wing tends to be elastically deflected by its own structural weight and fuel-load weight). Designing to the jig shape is beneficial as it matches the shape of the wing tip device as manufactured. However, in reality, the shape and/or orientation of the wing tip device may not exactly match the jig shape once it is installed on the aircraft wing.

Aspects of the present invention seek to provide an improved wing tip device, especially when facing constraints on span.

According to a first aspect of the invention, there is provided an aircraft according to claim <NUM>.

The present invention recognises that a beneficial winglet design can be obtained by considering the span of the winglet when the wing is under the worst-case static loading conditions. More specifically, by ensuring the winglet tip is at the maximum spanwise extent when the wing is under worst-case static loading, the aircraft should always be complying with airport compatibility gate limits, whilst optimising the position of the winglet tip. Furthermore, by having the wing-like region canted inboard in the no-load condition (such that the tip of the winglet is located inboard of the maximum spanwise extent of the winglet) a winglet with a relatively long unrolled length may be obtained (discussed in more detail below).

The worst-case static loading will be readily understood by the skilled person. It is the highest static loading the aircraft wing would be expected to encounter during normal use (for example when the aircraft is stationary on the ground, and fully fuelled).

Reference herein to the 'maximum spanwise extent of the winglet' will be understood to mean the most outwardly-located structure of the winglet under the corresponding specified load condition. The part of the winglet that is located at that maximum spanwise extent may change in dependence of the load condition of the wing (for example the maximum spanwise extent may be at the winglet tip when the aircraft wing is under the worst-case static load, but may be located lower down the winglet (e.g. distal from the tip) when the aircraft wing is under the no-load condition.

It will be appreciated that the winglet is a <NUM>-dimensional structure. The tip of the winglet is considered to be located at the maximum spanwise extent of the winglet when the lower (outer-most) surface of the tip is at the maximum spanwise extent (even though the upper (inner-most) surface, and leading edge, at the tip will necessarily be slightly inboard of that due to the thickness of the structure). The tip of the winglet will be readily identifiable, and may include a tip cap.

It will be appreciated that the span of the aircraft may be different depending on the load condition that the wing is under. The span of the maximum spanwise extent of the winglet when the aircraft wing is under worst-case static loading, is a parameter set by an airport compatibility limit (for example relating to clearance restrictions for buildings, signs, other aircraft). The compatibility limit is a gate limit. The span of the maximum spanwise extent of the winglet when the aircraft wing is under no-load may be less than the compatibility limit.

The winglet is a fixed winglet. Reference to the wing-like region being canted inboard under the no-load condition, will be understood to refer to a 'passive' change (i.e. a change in cant due to the different load conditions) rather than any actuated, or moveable components per se on the winglet.

The wing-like region may be curved. In embodiments in which the wing-like region is curved, the wing-like region may be an extension of the transition region. The wing-like region may be less curved than the transition region. The wing-like region comprises a planar portion. The planar portion extends away from the tip. The planar portion may include the winglet tip. When the aircraft wing is under worst-case static loading, the planar portion extends vertically downward from the winglet tip, such that it lies along the maximum spanwise extent of the winglet. In the no-load condition the planar portion is canted inboard beyond the vertical. Providing a planar portion extending along the span limit (in the worst-case static loading conditions) has been found to facilitate a relatively long un-rolled length of winglet because it 'pushes' the transition region outboard which, when the root and tip locations are fixed, increases the length between these end points and hence the unrolled length of the winglet (see <FIG>, and especially 6a and 6b described below).

In some embodiments, all of the wing-like region may be substantially planar. The transition region is preferably curved. The substantially planar portion may extend tangentially from the distal end of the curved transition region. The substantially planar portion need not necessarily be exactly planar. For example, the substantially planar portion may be part of a conic section that has a sufficiently high radius that it can be considered as substantially planar.

During flight, the aircraft wing is subjected to loading, and tends to undergo aero-elastic deformation. When the aircraft wing is under the <NUM>-g flight conditions, the wing-like region may be canted further inboard, relative to when the aircraft wing is under the no-load condition, such that the tip of the winglet is located yet further inboard of the maximum span of the winglet.

The aircraft is preferably a passenger aircraft. The passenger aircraft preferably comprises a passenger cabin comprising a plurality of rows and columns of seat units for accommodating a multiplicity of passengers. The aircraft may have a capacity of at least <NUM>, more preferably at least <NUM> passengers, and more preferably more than <NUM> passengers. The aircraft is preferably a powered aircraft. The aircraft preferably comprises an engine for propelling the aircraft. The aircraft may comprise wing-mounted, and preferably underwing, engines.

According to a second aspect of the invention, there is provided a method of manufacturing a winglet according to claim <NUM>. By designing for the worst-case static load condition, and then designing the required jig-shape (i.e. for the no-load condition), the winglet should always be complying with airport compatibility gate limits in use, but may have an optimised shape to reach that span. The step of designing the jig-shape (step (ii)) is preferably subsequent to step (i).

The step of designing for the worst-case static load condition comprises the step of orientating a planar portion vertically downwards from the winglet tip along the maximum spanwise extent of the winglet.

The step of designing the jig-shape comprises the step of canting the wing-like region inboard such that the tip of the winglet is located inboard of the maximum spanwise extent of the winglet.

It will be appreciated that, unless otherwise specified, the shape of the winglet (or parts thereof) referred to herein, refers to the shape in a frontal projection (i.e. on to a y-z plane). The shape of the winglet may be defined by the ¼ chord line running along the winglet. The cant may be measured relative to the vertical (i.e. the y-axis). The y axis is preferably in an absolute reference frame.

<FIG> shows a frontal view (i.e. in the y-z plane) of a winglet <NUM> at the end of a wing <NUM> of an aircraft <NUM> according to a first embodiment of the invention (the aircraft being shown schematically in <FIG>). The winglet comprises a root <NUM>, a tip <NUM>, a curved transition region <NUM>, and an upwardly extending, substantially planar, wing-like region <NUM> extending from the distal end <NUM>' of the transition region <NUM> to the tip <NUM>. The leading edge of the winglet is shown as a dashed line.

In <FIG>, the wing <NUM> is shown with the aircraft stationary on the ground and with a full fuel load (i.e. the worst-case static load). Under this load condition, the planar portion <NUM> extends vertically downwards from the tip <NUM>. As indicated by the vertical dashed line <NUM>, the maximum spanwise extent of the winglet <NUM> is therefore defined by the winglet tip <NUM> and the vertical planar portion <NUM> extending downwardly therefrom (marked between two X's in <FIG>).

It is desirable to maximise the effective length of the wing within the confines of any airport restrictions on wing span. Accordingly, the tip <NUM> and the vertical portion <NUM> are also at the maximum span (Smax) set by the airport gate compatibility limit (e.g. see wing spans in FAA groups I to IV or ICAO codes A to F).

In <FIG>, the wing <NUM> and winglet <NUM> are shown in a no-load condition (i.e. in their jig shape). In the jig shape, the planar portion <NUM> in canted inwardly such that the tip <NUM> is moved inboard (relative to the worst-case static load condition). Accordingly, the maximum spanwise extent of the winglet <NUM> is then shifted lower down the winglet <NUM> to the junction <NUM>' between the transition region <NUM> and the planar portion <NUM> (marked with an X in <FIG>). The magnitude of that span is also slightly reduced (see <FIG>).

In <FIG>, the wing <NUM> and winglet <NUM> are shown in a <NUM>-g flight condition. In the flight condition, the wing <NUM> is flexed upwards under aero-elastic loading and the planar portion <NUM> in canted further inward such that the tip <NUM> is moved further inboard (relative to the worst-case static load condition and the no-load condition). The maximum spanwise extent of the winglet is shifted onto the transition region <NUM> (marked with an X in <FIG>). The magnitude of that span is also further reduced.

<FIG> overlays the images of the winglet in the three load conditions of <FIG>, which illustrates the above-mentioned changes in the magnitude of the span and the location of the point of maximum span on the winglet.

As is evident from <FIG>, the winglet tip <NUM> and planar portion <NUM> are at the maximum spanwise extent when the wing is under worst-case static loading, but are canted inboard (sometimes referred to as being 'over-canted') in the jig shape. This arrangement is beneficial. Firstly, it ensures the aircraft should always be complying with airport compatibility gate limits as it is sized for the worst-case load scenario. Secondly, by providing the tip and planar portion extending along the span limit (in the worst-case static loading conditions) the total un-rolled length of winglet is relatively long because it 'pushes' the transition region outboard which, when the root and tip locations are fixed, increases the length between these end points, and hence increases the unrolled length of the winglet.

These benefits can be seen in <FIG> and <FIG> which compare the winglet <NUM> of the first embodiment of the invention with a previously suggested winglet <NUM> (shown on the left-most side of <FIG>/<FIG>). Referring first to <FIG> the winglets <NUM>, <NUM> are shown overlaid with one another when attached to a common wing <NUM>. <FIG> shows the wing under a worst-case static load.

It can be seen from <FIG> that the transition region <NUM> on the winglet of the first embodiment of the invention is necessarily further outboard (than the region <NUM> on the previously suggested winglet <NUM>) in order to blend into the vertical planar portion <NUM>. Since the root <NUM> and tip <NUM>, <NUM> locations are largely the same for both winglets <NUM>, <NUM> the unrolled length of the winglet <NUM> of the first embodiment is longer than the unrolled length of the previously suggested winglet <NUM>. This provides a winglet with a longer effective length, and with a more open transition region, both of which give rise to improved aerodynamic performance (primarily in terms of a drag reduction).

<FIG> shows the wing under a <NUM>-g flight load. It can be seen that in this load condition, the previously suggested winglet <NUM> has a planar portion <NUM> that is essentially vertical, whereas on the winglet of the first embodiment of the invention, the tip <NUM> of the winglet is moved inboard such that the planar portion <NUM> is over-canted. The maximum spanwise extent of the winglet <NUM> is moved lower down the winglet to the transition region. Thus, the tips <NUM>, <NUM> of the two different winglets are essentially coincident at this load condition, yet the total span of the aircraft is greater with the winglet of the first embodiment of the invention.

Whilst the present invention has been described and illustrated with reference to the first embodiment, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations. By way of example, the winglet may also be used as part of a wing tip device having a downwardly extending winglet 201b. Such an embodiment is shown in <FIG>, which illustrates such a wing tip device <NUM> in the three load conditions (worst-case static load (lower-most image), no-load condition (middle image), and <NUM>-g flight load (upper-most image). The upwardly extending winglet 201a is essentially the same as the first embodiment, but the downwardly extending winglet 201b seeks to offset some of the span reduction experienced when the aircraft is in <NUM>-g flight.

Claim 1:
An aircraft (<NUM>) comprising a wing (<NUM>) and a winglet (<NUM>) at the end of the wing, the winglet comprising: a root (<NUM>); a tip (<NUM>); a transition region (<NUM>) extending away from the root; and a wing-like region (<NUM>) extending from the distal end of the transition region to the tip, wherein the wing-like region comprises a planar portion (<NUM>) extending into the winglet tip,
and characterised in that
when the aircraft wing (<NUM>) is under the worst-case static loading, the worst-case static loading being the highest static loading the aircraft wing would be expected to encounter during normal use, when the aircraft is stationary on the ground and with a full fuel load: the tip of the winglet (<NUM>) is located at the maximum spanwise extent of the winglet; the planar portion extends vertically downward from the winglet tip such that it lies along the maximum spanwise extent of the winglet; and the winglet (<NUM>) extends substantially vertically along an airport gate compatibility limit span (Smax),
but when the aircraft wing (<NUM>) is under the no-load condition, the wing (<NUM>) and winglet (<NUM>) being in their jig shape in that no-load condition, the wing-like region (<NUM>) is canted inboard such that the tip of the winglet is located inboard of the maximum spanwise extent of the winglet (<NUM>) and the planar portion (<NUM>) is canted inboard beyond the vertical.