Passively actuated fluid airfoils

Abstract

The present invention provides a fluid wing and a method for operating the fluid wing, wherein the fluid wing has a main fixed part and a tip part movably mounted at the tip end of the main fixed part. The main fixed part has an upper surface and a lower surface, and the tip part has an upper surface and a lower surface. The fluid wing can be operated to be in: a) a first configuration, in which the upper surface of the tip part is tilted downward relative to the upper surface of the main fixed part; and b) a second configuration, in which the tip part is rotated upward relative to the first configuration, so that the upper surface of the tip part and the upper surface of the main fixed part are substantially continuous surfaces. The movement of the tip part relative to the main fixed part is passively actuated exclusively by the movement of the wing relative to the surrounding fluid.

Description

Technical Field

[0001] The present invention relates to a fluid airfoil having a movable portion. Background Art

[0002] The aerodynamic efficiency of an aircraft is related to the effective wingspan, because the deflection of the wing due to the inherent structural flexibility of the wing and the effects of aerodynamic loads will reduce the wingspan in the flight shape. It is usually not possible to compensate by simply increasing the fixed wingspan of the wing, especially in commercial aviation, because fixed aircraft categories are matched to terminal gates, runways, and taxiway widths, which makes geometric expansion impossible or will incur higher airport costs, affecting the economics and maneuverability of the aircraft.

[0003] Various attempts have been made to overcome this limitation, but each has limitations:

[0004] Folding wingtips – system complexity and weight

[0005] Increase wing stiffness – trade structural weight for wingspan

[0006] Downward pointing winglets - benefit from wing deflection, but the possible benefit level is limited due to geometry constraints.

[0007] A disadvantage of many downwing devices is that while the downwing device benefits from the flexibility of the wing, the connection of the downwing device to the wing is primarily a rigid connection, so the downwing device is a compromise between aircraft category / gate restrictions, minimum ground clearance height restrictions, and the geometry of the downwing device that is then deflected for the cruise portion of flight.

[0008] Even in the case where a mechanically folding wing does not have the disadvantages of a downwing, ie the attached winglet arrangement can be positioned for an optimal position by mechanical folding, a mechanically folding wing still has disadvantages in terms of weight, complexity and reliability of the folding mechanism.

[0009] It should also be noted that an increase in span due to the attraction of aerodynamic loads at a greater distance from the wing root will also lead to new structural challenges, and means to mitigate the load staging due to span will also benefit any solution to the problem of span reduction in the flying shape.

[0010] The problem of maintaining effective wingspan is not unique to airfoils on commercial aircraft, and the problem also exists in airfoils of all fixed wing aircraft types as well as in hydrofoils used for boats. Summary of the invention

[0011] A first aspect of the present invention provides a fluid airfoil having a main fixed portion and a tip portion movably mounted at a tip end of the main fixed portion, wherein the main fixed portion has an upper surface and a lower surface, and the tip portion has an upper surface and a lower surface, and the fluid airfoil is operable to be in:

[0012] a) a first configuration in which an upper surface of the tip portion is inclined downwardly relative to an upper surface of the main fixing portion; and

[0013] b) a second configuration in which the tip portion is rotated upward relative to the first configuration so that an upper surface of the tip portion and an upper surface of the main fixing portion are a substantially continuous surface,

[0014] Therein, movement of the tip portion relative to the main fixed portion is passively actuated exclusively by movement of the airfoil relative to the surrounding fluid.

[0015] Another aspect of the invention provides a method of operating a fluid airfoil having a main fixed portion and a tip portion, the tip portion being movably mounted at a tip end of the main fixed portion, wherein the main fixed portion has an upper surface and a lower surface, and the tip portion has an upper surface and a lower surface, and the method comprises placing the fluid airfoil in:

[0016] a) a first configuration in which an upper surface of the tip portion is inclined downwardly relative to an upper surface of the main fixing portion; and

[0017] b) a second configuration in which the tip portion is rotated upward relative to the first configuration so that an upper surface of the tip portion and an upper surface of the main fixing portion are a substantially continuous surface,

[0018] Therein, movement of the tip portion relative to the main fixed portion is passively actuated exclusively by movement of the airfoil relative to the surrounding fluid.

[0019] An advantage of the present invention is that the downward orientation of the tip portion in the first configuration can provide a reduced wingspan to meet aircraft category / gate restrictions. The rotation of the tip portion relative to the main fixed portion to assume the second configuration means that the fluid wing can have an optimal shape with the most beneficial wingspan in operation, such as a cruising flight shape when the fluid wing is an aircraft wing. The rotation is entirely caused passively by the movement of the fluid wing relative to the surrounding fluid. The rotation can be around a flexible or spring element used to attach the tip portion to the main fixed portion, as opposed to conventional rigid attachments or complex actively actuated folding mechanisms. The tip portion can be considered a "floating" wing portion.

[0020] The fluid airfoil can also be operated to:

[0021] c) A third configuration in which an upper surface of the tip portion is inclined upward relative to an upper surface of the main fixing portion.

[0022] Upward rotation to the third configuration may be used to provide a passive load relief function.

[0023] The tip portion may be movably mounted to the main fixed portion by a hinge or a flexible element. Alternatively, the entire tip portion may be constructed as a flexible element that droops under its own weight when aerodynamic loads are low (stationary or slow gliding) (a first configuration) and deflects upward when aerodynamic loads increase (a second configuration).

[0024] The fluid airfoil may have a span that increases between the first configuration and the second configuration.

[0025] The tip portion may be biased toward the first configuration. The tip portion may be biased toward the first configuration by a variable spring. The variable spring may be configured to apply a spring force that varies as the tip portion is rotated upward relative to the first configuration.

[0026] The tip portion may be configured to rotate about a pitch axis as the tip portion rotates upward relative to the first configuration. The rotation about the pitch axis may be used to provide a beneficial change in the angle of attack (α) of the tip portion as the tip portion rotates upward relative to the first configuration. The rate of change of the angle of attack of the tip portion may be nonlinearly related to the angle of rotation of the tip portion relative to the first configuration. The change in the angle of attack may be positive or negative, or may undergo a transition from positive to negative or from negative to positive as the tip portion rotates upward relative to the first configuration. The hinge or other flexure element may be configured to simultaneously change the airfoil pitch of the tip portion as the tip portion deflects in the vertical plane.

[0027] The tip portion may be arranged to have an angle of attack (positive α) that decreases as the tip portion is rotated upward relative to the first configuration.

[0028] The first configuration may correspond to a lowermost position of the tip portion. A stop may be provided to prevent excessive rotation of the tip portion. The lowermost position of the tip portion may correspond to a minimum height ground clearance.

[0029] The tip portion may have a mass, and the weight of the tip portion may bias the tip portion toward the first configuration.

[0030] The tip portion may be configured to remain in the tip portion's lowermost position during relative forward motion of the airfoil through the fluid at a speed below a predetermined forward speed. The predetermined speed may be, for example, a normal taxi speed for aircraft applications.

[0031] The tip portion may have a mass and a spring, and the weight of the tip portion together with the spring may bias the tip portion to remain in a lowermost position of the tip portion below a predetermined advancement speed.

[0032] The tip portion may include a first lift surface having an upper surface and a lower surface of the tip portion, and the tip portion may further include one or more second lift surfaces.

[0033] The second lifting surface may be fixed or rotatable relative to the first lifting surface. The second lifting surface may be coupled to the first lifting surface or to the main fixed portion of the fluid airfoil. The second lifting surface may be coupled by a hinge or flexible element similar to that used to couple the tip portion to the main fixed portion.

[0034] The fluid airfoil may be a wing airfoil, preferably a wing of a fixed wing aircraft, or a hydrofoil. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

[0036] Figure 1 A plan view of a fixed-wing aircraft is shown;

[0037] Figure 2 Shows Figure 1 a rearwardly viewed view of a wing tip region of an aircraft shown in , wherein the wing tip portion is tilted downwardly in a first configuration;

[0038] Figure 3 Shows something like Figure 2 , wherein aerodynamic forces acting on the wing tip portion are insufficient to lift the wing tip portion from the first configuration;

[0039] Figure 4 Shows something like Figure 3 , wherein the aerodynamic forces acting on the wing tip portion shown are sufficient to lift the wing tip portion to the second configuration;

[0040] Figure 5 Shows something like Figure 4 , wherein the aerodynamic forces acting on the wing tip portion shown are sufficient to lift the wing tip portion from the second configuration (solid line) to the third configuration (dashed line);

[0041] Figure 6 Shows Figure 3 A plan view of a wing tip portion in a first configuration and a cross-sectional view taken along AA showing the angle of attack of the tip portion;

[0042] Figure 7a plan view showing a wing tip portion between a first configuration and a second configuration and a cross-sectional view taken along BB showing the angle of attack of the tip portion;

[0043] Figure 8 Shows Figure 4 A plan view of the wing tip portion in a second configuration and a cross-sectional view taken along CC showing the angle of attack of the tip portion;

[0044] Fig. 9 Shows Figure 5 A plan view of the wing tip portion in a third configuration and a cross-sectional view taken along DD showing the angle of attack of the tip portion;

[0045] Fig.10 shows a schematic plan view of a wing tip section wherein the hinge axis is offset relative to the flight path;

[0046] FIG. 11 shows another example of a wing tip portion having a first lifting surface and a second lifting surface, Fig.11a The wing tip portion is shown tilted downward in a first configuration, Fig.11b The wing tip portion is shown rotated upward to a second configuration;

[0047] FIG. 12 shows yet another example of a wing tip portion having a first lifting surface and a second lifting surface, Fig.12a The wing tip portion is shown tilted downward in a first configuration, Figure 12b The wing tip portion is shown rotated upward to a second configuration;

[0048] FIG. 13 shows yet another example of a wing tip portion having a first lifting surface and a second lifting surface, Fig.13a showing a first lifting surface inclined downwardly in a first configuration, Fig.13b The wing tip portion is shown rotated upward to a second configuration; and

[0049] Fig.14 A schematic diagram looking backwards of a vessel comprising a hydrofoil with a movable tip portion is shown. DETAILED DESCRIPTION

[0050] Figure 1A plan view of an aircraft 1 is shown, the aircraft 1 having a fuselage 2, wings 3, engines 4 mounted below the wings, a nose end 5 and a tail end 6, wherein the tail end 6 includes a horizontal stabilizing surface 7 and a vertical stabilizing surface 8. The aircraft 1 is a typical jet passenger aircraft, but the present invention is applicable to a wide variety of fixed-wing aircraft types, including commercial fixed-wing aircraft, military fixed-wing aircraft, passenger fixed-wing aircraft, cargo fixed-wing aircraft, jet fixed-wing aircraft, propeller fixed-wing aircraft, general aviation fixed-wing aircraft, etc., wherein any number of engines are attached to the wings or fuselage.

[0051] Each wing 3 has a root end adjacent to the fuselage 2 and a tip end opposite the root end. The tip end of the starboard wing 3 of the aircraft 1 is generally indicated by a box 9 shown in dotted lines. The port wing 3 of the aircraft 1 is an approximately identical mirror image of the starboard wing.

[0052] Figure 2 A view of the starboard wing 3 viewed rearwards (i.e. viewed from the nose end 5 along the centre line C of the aircraft 1) is shown in detail, the starboard wing 3 having a tip end 10 and a tip section 11 which is movably mounted at the tip end 10 of the main fixed part of the wing 3. The main fixed part has an upper surface 12 and a lower surface 13. The tip section has an upper surface 14 and a lower surface 15. The tip section is movably mounted to the tip end 10 of the main fixed part of the wing 3 by a hinge 16. The hinge enables the tip section 11 to rotate relative to the main fixed part of the wing 3 in a vertical plane which is substantially perpendicular to the centre line C of the aircraft 1. The axis of rotation may be considered to be substantially in the direction of the spire, wherein the spire of the wing 3 is a line extending between the leading edge and the trailing edge of the wing.

[0053] Hinge 16 is a spring hinge that biases tip portion 11 in a downwardly hanging orientation, such as Figure 2 As shown, in this downward hanging orientation, the upper surface 14 of the tip portion 11 is generally downwardly inclined relative to the upper surface 12 of the main fixed portion of the wing 3. The spring hinge 16 can be considered similar to a "living hinge". The spring hinge can be constructed with one or more flexible (semi-rigid) elements. Various different structures of the spring hinge 16 will be obvious to those skilled in the art.

[0054] The operation of the movable tip portion 11 will now be described. Figure 3 In the embodiment, the lifting force L acting on the tip portion 11 due to the forward movement of the wing 3 relative to the surrounding air is insufficient to overcome the biasing effect of the spring hinge 16, which acts together with the mass of the tip portion 11 to force the tip portion 11 to the lowermost position corresponding to the first configuration of the tip portion.

[0055] When the aircraft 1 is stationary or taxiing (typically at speeds below about 20 knots), the first configuration of the tip section 11 provides a reduced overall span of the wing 3 so that the aircraft 1 can comply with various span constraints. The length of the tip section and the downward angle of the tip section 11 in this lowermost position are selected to ensure that the tip end 17 of the tip section meets the ground clearance constraints of the aircraft 1. By combining the weight of the wingtip section 11 with the biasing action of the spring hinge 16, the tip section 11 remains in the first configuration and has the appearance of a conventional descending wing when the aircraft 1 is stationary or taxiing.

[0056] Figure 4 The wing tip portion 11 is shown in a second configuration in which the tip portion 11 is oriented relative to Figure 3 The first configuration shown in FIG. 1 is rotated upward so that the upper surface 14 of the tip portion 11 and the upper surface 12 of the main fixed portion of the wing 3 are substantially continuous surfaces without forming any significant angle between these surfaces. When the aircraft 1 exceeds normal taxi speed and moves in the take-off run, the lifting force L on the tip portion 11 creates a moment that will exceed the weight of the tip portion 11 and the spring load of the spring hinge 16, and this moment allows the tip portion 11 to rotate upward. In this second configuration, the span of the wing 3 is increased to a maximum value, and this configuration is maintained for most of the climb, cruise, descent and general flight of the aircraft 1.

[0057] Then, if Figure 5 As shown, a third configuration of the wing tip portion 11 is shown. If during a flight phase, there are loads on the tip portion 11 that exceed a predetermined load level, inherent passive load relief can also be provided by allowing the tip portion 11 to deflect to the cruise-optimized second configuration. Figure 5 As shown, the upper surface 14 of the tip portion is inclined upwardly relative to the upper surface 12 of the main fixed portion of the wing 3. This passive load reduction can be used to provide gust load reduction during flight. By being able to provide this third configuration, due to this passive load reduction, the structural weight of both the tip portion 11 and the remaining portion of the wing 3 to which the tip portion is attached can be reduced due to the reduction in the maximum aerodynamic load limit of the wing 3. In addition, this load reduction can also be used to improve the passenger ride quality when the aircraft 1 is a passenger aircraft. When the temporary gust load subsides, the spring bias of the spring hinge 16 provides a restoring moment M to return the tip portion 11 to Figure 4 The second configuration shown in .

[0058] One way in which the spring hinge 16 or other flexible element for rotatably mounting the tip portion 11 to the main fixed portion of the wing 3 may be constructed is by making a variable spring part of the design of the spring hinge 16 or flexible element. The variable spring may be configured to change in size as the tip portion 11 rotates relative to the main fixed portion of the wing 3. Figure 3 The first configuration shown in FIG. 1 is rotated upward to apply a variable spring force. The tip portion 11 is rotated upward to apply a variable spring force. Figure 4 The second configuration shown in FIG. Figure 3 The predetermined load level at which the third configuration shown in FIG. 1 is located can be controlled by adjusting the variable spring force.

[0059] Although the spring hinge 16 can be configured to provide simple rotation in the vertical plane, the spring hinge 16 is most advantageously configured as an offset hinge (see Fig.10 ) to allow the tip portion 11 to also rotate about the pitch axis as the tip portion rotates upward in the vertical plane. Rotation about the pitch axis causes the angle of attack (angle of attack, α) of the tip portion 11 to change as the tip portion 11 rotates upward. As an alternative to an offset hinge, an unbalanced flexure element may be used. Reference will now be made to Figures 6 to 9 This optimized hinge geometry is discussed in detail.

[0060] Figure 6 A plan view from above the outboard end of the wing 3 is shown with the tip section 11 in a first configuration. The direction of the oncoming free stream S is indicated (opposite to the direction of flight) and the angle of attack of the airfoil section of the tip section 11 is shown through section AA of the tip section. As shown, in the first configuration, the tip section 11 has a significant nose-up positive angle of attack +veα.

[0061] The spring load applied by the spring hinge 16 and the weight of the tip section 11 should be considered for normal taxiing speeds so that low levels of lift do not move the tip section beyond the aircraft category span limitations even at this high positive angle of attack.

[0062] Figure 7 1 shows the change in the angle of attack as the tip portion 11 rotates upward between the first configuration and the second configuration. Figure 7 As shown in section BB in , the airfoil section of the tip portion 11 is still in nose-up pitch, but the angle of attack +veα is less than that in section AA for the first configuration.

[0063] When the aircraft moves in the takeoff roll, because the forward aircraft speed V exceeds the normal taxiing speed, the combination of the tip section speed and α should cause the lift generated by the tip section 11 to exceed the combination of the tip section weight and the spring load of the spring hinge 16 and begin to rotate the tip section 11 in the second configuration for flight. Variations and optimizations of the spring load of the hinge 16 and α of the tip section 11 can be considered to optimize the lift coefficient C1 of the wing 3 to obtain the best runway field length.

[0064] Figure 8 The outboard end of the wing 3 is shown in a second configuration in which the speed of the free stream S is of a magnitude sufficient to cause the tip portion 11 to adopt the second configuration.

[0065] In this second configuration for the cruise phase of the aircraft's flight, the tip section 11 can be deflected slightly nose-up torsional to have an optimal α / twist, giving the tip section 11 and the wing 3 an optimal performance as a whole. As shown in section CC, the tip section 11 now adopts a slightly nose-up pitch angle of attack +veα.

[0066] Fig. 9 The outboard end of the wing 3 is shown in a third configuration in which the free stream S creates a temporary aerodynamic load sufficient to overcome a preset load level at which the hinge 16 enables the tip portion 11 to rotate upward from the second configuration to the third configuration. Fig. 9 This third configuration is shown, in which the angle of attack of the tip portion 11 is as shown in section DD. Figure 8 The tip portion 11 is rotated downward to a lower angle of attack α than the second configuration shown in FIG. The lower angle of attack (angle of attack) α may be an intermediate value or may be a small negative value. By reducing the angle of attack, the upward lifting force generated by the tip portion 11 is reduced in the third configuration compared to the second configuration, which helps to reduce the gust load applied to the main fixed portion of the wing 3 and reduce the load acting on the tip portion 11 to a predetermined load level, at which the tip portion 11 can return to Figure 8 The upward movement of the tip portion 11 due to temporary aerodynamic loads such as gusts of wind together with the offset hinge 16 causes a negative incidence effect to reduce the lift on the tip portion 11, thereby transferring the lift to the inside of the wing 3 and thus reducing the load.

[0067] The third configuration may be employed when the loads on the tip portion exceed a predetermined level, for example due to gust loads or extreme maneuvers. The spring load in the vertical axis exceeding the hinge 16 will move the tip portion 11 beyond the optimum span length, thereby reducing the bending moment on the entire wing 3, and also due to the vertical movement of the components, reducing the effective angle of attack in the second mode and thus reducing the lift and bending moments. In addition, continued rotation of the hinge 16 (with an offset hinge axis) or other unbalanced flexure elements will also cause a further downward twisting nose moment, thereby reducing the α of the tip portion 11 and thus further reducing the lift force L. All three mechanisms reduce the lift on the tip portion and the moment arm, thereby alleviating some of the loads.

[0068] The spring hinge 16 or other flexible element may be tailored so that the angle of attack α of the tip portion 11 changes as the tip portion 11 deflects in the vertical direction, depending on the speed of the aircraft 1 and the aerodynamic forces on the tip portion 11 .

[0069] Passively actuated wingtip portions may provide higher performance for a comparable type of aircraft through the enhanced aerodynamics of the increased span and the inherent load reductions incorporated into the passive actuation of the system without the complexity of additional systems to actuate the wingtip folds.

[0070] Fig.10 The offset hinge 16 is schematically shown. As can be seen, the axis of rotation of the hinge 16 is offset by an angle θ relative to the free stream S (opposite to the direction of aircraft flight) when viewed in plan. The offset hinge 16 causes a change in the angle of attack of the tip portion 11 from positive to negative as the tip portion 11 rotates upward about the hinge.

[0071] FIG. 11 shows another example in which a tip portion 110 includes a first lifting surface 20 substantially identical to the tip portion 11 described previously, with the addition of a second lifting surface 21. The same reference numerals as in the previous example are used to denote the same components. However, the first lifting surface 20 is Fig.11a In the first configuration of the downwind wing shown in FIG, the second lifting surface 21 protrudes outward from the vicinity of the hinge 16 so that the extent of the second lifting surface in the wing span direction is substantially the same as the extent of the first lifting surface 20. The second lifting surface 21 is fixed in a manner to rotate with the first lifting surface 20. The second lifting surface 21 not only assists the tip portion 110 from Fig.11a The first configuration shown in FIG. 1 is rotated to Fig.11b The second configuration shown in FIG. 1 and wing load redistribution is achieved when in the second configuration. As in the previous example, the tip portion 110 can also be configured to be further rotated to a third configuration to provide load relief.

[0072] FIG12 shows a further example of a tip portion 210 which shares many common features with the example shown in FIG11 and in which like reference numerals are used to denote like parts. The example shown in FIG12 differs only in that the second lifting surface 31 is connected to the first lifting surface 30 by a spring hinge 32 or other flexure element constructed similarly to the spring hinge 16 which couples the first lifting surface 30 to the main fixed portion of the wing 3. Fig.12a The tip portion 210 is shown in a first configuration, Figure 12b The tip portion 210 is shown in the second configuration. As can be seen, the second lifting surface 31 is configured to rotate from the first configuration toward the second configuration in a direction inboard of the aircraft fuselage 2. As with the previous example, the tip portion 110 may also be configured to rotate further to a third configuration to provide load relief.

[0073] In another embodiment not shown, the second lifting surface 31 is coupled to the tip end 10 of the main fixed part of the wing 3 by means of a spring hinge 32 instead of being coupled to the first lifting surface 30 .

[0074] FIG. 13 shows another example in which a tip portion 310 includes a first lifting surface 40 substantially identical to the tip portion 11 described previously, with the addition of a second lifting surface 41. The same reference numerals as in the previous example are used to denote the same components. However, the first lifting surface 40 is Fig.13a In the downdraft wing in the first configuration shown in FIG, the second lifting surface 41 projects outwardly from the vicinity of the hinge 16 so that the extent of the second lifting surface in the spanwise direction of the wing is substantially the same as the extent of the first lifting surface 40. The second lifting surface 41 is fixed to the outboard end 10 of the main fixed portion of the wing 3. The second lifting surface 41 is fixed so that it does not substantially rotate relative to the main fixed portion of the wing 3. Therefore, the second lifting surface 41 is attached on the opposite side of the hinge 16 relative to the first lifting surface 40. The first lifting surface 40 is configured to project outwardly from the tip portion 11 in the same manner as previously described for the tip portion 11. Fig.13a The first configuration shown in FIG. 1 is rotated to Fig.13b As with the previous examples, the first lift surface 40 may also be configured to be further rotated to a third configuration to provide load relief.

[0075] It should be noted that in the above described embodiments, the operation of the spring hinge 16 is mechanically independent of the operation of the spring hinge 32 and the rotation about the hinges 16, 32 is entirely passive and controlled by the aerodynamic loads acting on the lift surface.

[0076] Although in the embodiments described above the fluid airfoil is described in the context of an aircraft wing, it will be appreciated that the fluid airfoil may alternatively be any other airfoil, and may alternatively be a hydrofoil for a racing boat or boat. Fig.14 Schematically shown is a boat 400 having a hydrofoil having a main fixed part 401 and a tip part 402 which is movably mounted at the tip end of the main fixed part 401 by means of a spring hinge 403 or other flexure element. Fig.14 , the tip portion 402 is shown in a first configuration in solid lines and the tip portion 402 is shown in a second configuration in dashed lines. The operation of the hydrofoil of the vessel 400 is similar to that described above with reference to Figures 2 to 5 The operations described are essentially the same.

[0077] Where the word "or" appears, the word should be interpreted as meaning "and / or" such that the items involved are not necessarily mutually exclusive and can be used in any appropriate combination.

[0078] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A fluid wing, the fluid wing having a main fixed portion and a tip portion, the tip portion being movably mounted at the tip end of the main fixed portion by a flexible element or a hinge, wherein: The main fixed portion has an upper surface and a lower surface, and the tip portion has an upper surface and a lower surface, and the fluid airfoil is operable to be in: a) a first configuration in which an upper surface of the tip portion is inclined downwardly relative to an upper surface of the main fixing portion; and b) a second configuration in which the tip portion is rotated upward relative to the first configuration so that an upper surface of the tip portion and an upper surface of the main fixing portion are substantially continuous surfaces without forming any significant angle between the upper surface of the tip portion and the upper surface of the main fixing portion, wherein movement of the tip portion relative to the main fixed portion from the first configuration to the second configuration is passively actuated exclusively by movement of the fluid airfoil relative to a surrounding fluid, and wherein the tip portion is biased in the first configuration by the flexible element or hinge during relative forward movement of the fluid airfoil through a fluid at a speed below a predetermined forward speed.

2. The fluid wing according to claim 1, wherein: The fluid airfoil is operable to: c) A third configuration in which an upper surface of the tip portion is inclined upward relative to an upper surface of the main fixing portion.

3. The fluid wing according to claim 1 or 2, wherein: The fluid airfoil has a span, and the span increases between the first configuration and the second configuration.

4. The fluid wing according to claim 1 or 2, wherein: The flexible element or hinge comprises a variable spring.

5. The fluid wing according to claim 4, wherein: The variable spring is configured to apply a spring force that varies as the tip portion is rotated upward relative to the first configuration.

6. The fluid wing according to claim 1 or 2, wherein: The tip portion is configured to rotate about a pitch axis as the tip portion is rotated upward relative to the first configuration, wherein the flexure element is configured to be unbalanced or the hinge has an offset axis.

7. The fluid airfoil according to claim 6, wherein: The tip portion is arranged to have an angle of attack that decreases as the tip portion rotates upward relative to the first configuration.

8. The fluid wing according to claim 1 or 2, wherein: The first configuration corresponds to a lowermost position of the tip portion.

9. The fluid wing according to claim 1 or 2, wherein: The tip portion has a mass, and the weight of the tip portion biases the tip portion toward the first configuration.

10. The fluid wing according to claim 1 or 2, wherein: The tip portion includes a first lift surface having an upper surface and a lower surface of the tip portion, and the tip portion also includes one or more second lift surfaces.

11. The fluid airfoil according to claim 10, wherein: The second lift surface is fixed or rotatable relative to the first lift surface.

12. The fluid wing according to claim 1 or 2, wherein: The fluid airfoil is a wing airfoil and the surrounding fluid is air.

13. The fluid airfoil according to claim 12, wherein: The wing airfoil is the wing of a fixed-wing aircraft.

14. A method of operating a fluid airfoil having a main fixed portion and a tip portion, the tip portion being movably mounted at the tip end of the main fixed portion by a flexible element or a hinge, wherein: The main fixed portion has an upper surface and a lower surface, and the tip portion has an upper surface and a lower surface, and the method includes placing the fluid airfoil in: a) a first configuration in which an upper surface of the tip portion is inclined downwardly relative to an upper surface of the main fixing portion, and b) a second configuration in which the tip portion is rotated upward relative to the first configuration so that an upper surface of the tip portion and an upper surface of the main fixing portion are substantially continuous surfaces without forming any significant angle between the upper surface of the tip portion and the upper surface of the main fixing portion, wherein movement of the tip portion relative to the main fixed portion from the first configuration to the second configuration is passively actuated exclusively by movement of the fluid airfoil relative to a surrounding fluid, and wherein the flexible element or hinge biases the tip portion in the first configuration during relative forward movement of the fluid airfoil through a fluid at a speed below a predetermined forward velocity.

15. The method of claim 14, further comprising placing the fluid airfoil in: c) A third configuration in which an upper surface of the tip portion is inclined upward relative to an upper surface of the main fixing portion.

16. The method according to claim 14 or 15, wherein: The flexible element or hinge includes a variable spring, and the method further includes varying a spring force applied by the variable spring to the tip portion as the tip portion is rotated upward relative to the first configuration.

17. The method of claim 14 or 15, further comprising rotating the tip portion about a pitch axis as the tip portion rotates upward relative to the first configuration, wherein: The flexible element is configured to be unbalanced or the hinge has an offset axis.

18. The method of claim 17, further comprising decreasing the angle of attack of the tip portion as the tip portion rotates upward relative to the first configuration.

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