WO2021148338A1

WO2021148338A1 - Load control for an aircraft wing

Info

Publication number: WO2021148338A1
Application number: PCT/EP2021/050890
Authority: WO
Inventors: Francesco Gambioli; Philipp BEHRUZI; Michele SCIACCA; Emmanuel MORLANNE
Original assignee: Airbus Operations Limited; Airbus Operations (S.A.S.)
Priority date: 2020-01-22
Filing date: 2021-01-18
Publication date: 2021-07-29
Also published as: GB2591252A; GB202000918D0

Abstract

An apparatus attached near the tip end of a cantilevered aircraft wing for controlling the loads on the wing. The apparatus comprises a first body having a mass rotatable about a first axis by an actuator device; and a second body adjacent to the first body and having a mass rotatable about a second axis by an actuator device. The first and second bodies each have a centre of mass offset from their respective axes and are configured to counter rotate about their respective axes.

Description

LOAD CONTROL FOR AN AIRCRAFT WING

FIELD OF THE INVENTION

[0001] The present invention relates to an apparatus and a method for controlling load on an aircraft wing.

BACKGROUND OF THE INVENTION

[0002] Aircraft control systems currently use the controlled motion of fast-acting movable control surfaces (e.g. ailerons and spoilers) to redistribute aerodynamic loads, and so reduce wing bending at the root. These systems are commonly known as load alleviation function (LAF) systems and can act to affect steady state loads (initial conditions) and dynamic loads (e.g. that develop during a gust encounter).

[0003] The design and operation of the control surfaces is therefore dictated, at least partially, by the load alleviation function requirements.

SUMMARY OF THE INVENTION

[0004] A first aspect of the invention provides an apparatus attached near the tip end of a cantilevered aircraft wing for controlling load on the wing, the apparatus comprising: a first body having a mass rotatable about a first axis by an actuator device; and a second body adjacent to the first body and having a mass rotatable about a second axis by an actuator device, wherein the first body and the second body are configured to counter rotate about their respective axes, and wherein the first and second bodies each have a centre of mass offset from their respective axes.

[0005] A further aspect of the invention provides a method of controlling load on a cantilevered aircraft wing, comprising: simultaneously counter rotating a first body and a second body about respective axes in order to generate an asymmetric inertial load, wherein the first body and second body are adjacent and attached near the tip end of the aircraft wing.

[0006] A further aspect of the invention provides a method of retrofitting a wing tip device to a tip end of an aircraft wing, including installing an apparatus according to the first aspect. [0007] Reference to 'near the tip end of the wing' should be interpreted as closer to the tip end of the wing than the root end of the wing. For example, the first and second bodies may be located outboard of the mid-point between the wing root and wing tip, preferably located outboard of the 80% span location from the wing root, and more preferably located outboard of the 90% span location from the wing root. The first and second bodies may be located within a wing tip device attached to the outboard extent (or tip) of the wing. The further outboard on the wingspan the bodies are located, the greater effect they may have on the behaviour of the wing root bending loads.

[0008] The centre of mass of the bodies is offset from their respective axes and asymmetric about the axes such that an asymmetric inertial force is developed. Counter rotation of the respective bodies therefore causes an asymmetric inertial force to be generated that can be used to excite (increase) loads on the structure or to alleviate (reduce) loads on the structure. In the case of load excitation, the system could be used to perform ground and/or in-flight tests on the aircraft wing. Alternatively, the system could be used to limit the response of the wing to loads, for example dynamic loads such as gusts or landing impact, and thereby limit the impact of these loads on the aircraft structure. A lighter wing can thereby be constructed, or wing strengthening may be avoided or reduced, due to the reduced loads experienced by the wing.

[0009] The system is particularly advantageous when large winglets/wing tips are retrofitted to existing aircraft wings, which can increase the effect of gust loads on the aircraft. The system could therefore be used to reduce the loads level, so as to limit (or eliminate) the load exceedance beyond a given target level, and therefore allow large and/or retrofitted wingtip devices to be fitted without modifications to the existing wing structure.

[0010] The system may help to limit the vibration and load transmission from the engine/pylon into the wing-box, and to mitigate/reduce the effects of mounting external storage to the wing.

[0011] The rotating masses may be complimentary or as an alternative to the use of flight control surfaces, which primarily develop aerodynamic forces to act on the aircraft wing. In this way, the fatigue life of the control surfaces can be increased, as they are no longer needed to perform at least a portion of the load alleviation function. The size of the control surfaces, which may partially be dictated by a requirement to perform a load alleviation function, may also be decreased. The size of actuators used to move the control surfaces, which may partially be dictated by a requirement to perform a load alleviation function, may also be decreased. The use of the control surfaces, or at least the degree of deflection needed, may also be decreased. The rotating masses may be used instead of or in addition to the flight control surfaces in a flight test regime, which may expand the number of vibration modes or other excitation that the aircraft wing may be subjected to beyond use of the control surfaces alone. The rotating masses may be used in an aircraft wing ground test regime, to excite the wing structure.

[0012] The axes of the rotating masses may extend in the span-wise direction and be substantially parallel to each other, such that the asymmetric inertial force that is generated acts in the vertical direction.

[0013] The inertial force may vary sinusoidally with time.

[0014] The first axis may spaced from the second axis. The first axis may be co-axial with the second axis.

[0015] The inertial force generated by the rotating first body may be the same as the inertial force generated by rotating the second body. The masses of the first and second bodies may be the same. The location of the centre of mass of each of the first and second bodies to their respective axes may be the same distance.

[0016] The first and second axes may be arranged symmetrically with respect to a flexural axis of the aircraft wing. The flexural axis is the line about which the wing bends without twisting and vice versa. With this arrangement, the resultant force of the rotating bodies will damp or excite the wing bending substantially without any wing twist.

[0017] The first and second axes may be arranged asymmetrically with respect to a flexural axis of the aircraft wing. With this arrangement, the resultant force of the rotating bodies will additionally damp or excite the wing twist as it bends, for example to develop a nose-down or nose-up pitch response from wing root to wing tip. [0018] The first body and second body may be configured to rotate at the same frequency. The first body and second body may be configured to rotate at different frequencies.

[0019] The first and second bodies may be configured to rotate with the same moment of inertia and/or amplitude and/or mass and/or acceleration and/or angular velocity. The first and second bodies may be configured to rotate with different moments of inertia and/or amplitude and/or mass and/or acceleration and/or angular velocity.

[0020] The first and second bodies may be configured to rotate in phase. The first and second bodies may be configured to rotate out of phase.

[0021] The first and second axes may be arranged substantially parallel to a flexural axis of the wing.

[0022] A common actuator may rotate the first and second bodies. The apparatus may include two actuators: a first actuator device configured to rotate the first body about the first axis and a second actuator device configured to rotate the second body about the second axis.

[0023] The first and second bodies may be configured to rotate no more than half a revolution (180 degrees) about their respective axes. The first and second bodies may be configured to rotate at least half a revolution (180 degrees). The first and second bodies may be configured to rotate at least one full revolution (360 degrees). The first and second bodies may be configured to rotate at least two full revolutions (720 degrees).

[0024] The apparatus may further comprise a sensor configured to detect an input affecting load on the aircraft wing. The apparatus may be configured to at least alleviate load on the aircraft wing by rotating first and second bodies in response to the detected input.

[0025] The load may be an aerodynamic load on the aircraft wing. The load may be a ground load acting on the aircraft wing. The load may be an unbalanced load generated by a dynamic part coupled to the aircraft wing. The load may be a predicted load that is predicted to be experienced by the aircraft wing, e.g. from a forward looking pressure sensor. The load may be a determined from a part of an aircraft to which the wing is attached, e.g. a fuselage accelerometer. The load may be measured directly on the aircraft wing.

[0026] The load may be a dynamic load. The aircraft wing may have a dynamic response to the dynamic load. The first and second bodies may be configured to rotate about their respective axes to alter the dynamic response to the aircraft wing.

[0027] The apparatus may be configured to excite the aircraft wing to vibrate at one or more modes of vibration by rotating the first and second bodies.

[0028] The apparatus may be enclosed within an aerodynamic profile of the aircraft wing. The apparatus may be enclosed within a fairing. The apparatus may be enclosed within an aerodynamic profile of a wingtip device attached to the tip end of the aircraft wing.

[0029] The first and second bodies may be configured to be held in a rest position when the bodies are not rotating. Before and/or after rotating the first and second bodies, the first and second bodies may be held in the rest position. The rest position may be substantially the rotational angle with the highest potential energy due to gravity.

[0030] The method may further comprise detecting an input affecting load on the aircraft wing, and wherein the first and second bodies are rotated to at least alleviate load on the aircraft wing in response to the detected input.

[0031] The method may further comprise rotating the first and second bodies to excite the aircraft wing to vibrate at one or more modes of vibration. The rotating masses may therefore be used to increase the bending load on the aircraft wing. This may be advantageous when performing ground or in-flight testing.

[0032] The first and second bodies may be rotated in flight.

[0033] The first and second bodies may be rotated below 10 Hz. The first and second bodies may be rotated below 4 Hz. The first and second bodies may be rotated between 0.5 Hz and 2 Hz.

BRIEF DESCRIPTION OF THE DRAWINGS [0034] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0035] Figure 1 illustrates a plan view of an aircraft;

[0036] Figure 2 illustrates schematically a wing structure;

[0037] Figure 3 illustrates schematically an apparatus to control loads on the aircraft wing;

[0038] Figures 4A & 4B illustrate schematically an example of the apparatus attached to an outboard end rib according to a first example;

[0039] Figure 5 illustrates the inertial force generated by the apparatus according to a first example;

[0040] Figure 6 illustrates the rest position of the apparatus when not in operation;

[0041] Figure 7 illustrates the apparatus offset from the wing flexural axis according to a second example;

[0042] Figure 8A illustrates the apparatus enclosed by a fairing;

[0043] Figure 8B illustrates the apparatus enclosed within a wing tip device;

[0044] Figures 9 & 10 illustrate the apparatus with a movable centre of mass relative to their respective axes;

[0045] Figures 11A-C illustrate the apparatus with coaxial axes.

[0046] Figure 12 illustrate the apparatus with bodies spaced along the spanwise direction of the wing.

DETAIFED DESCRIPTION OF EMBODIMENT(S) [0047] Figure 1 shows an aircraft 1 with port and starboard fixed wings 2, 3, a fuselage 4 with a nose end 5 and a tail end 6. Adjacent to the tail 6 are horizontal and vertical stabilisers 7, 8 attached to the empennage portion of the aircraft fuselage 4. The aircraft 1 has under-wing mounted engines 9. Each wing 2,3 and the horizontal and vertical stabiliser surfaces 7, 8 have a variety of flight control surfaces including spoilers, flaps, ailerons, elevators, rudder, etc., although depending on the aircraft configuration a variety of different flight control surfaces including those not listed here may be adopted.

[0048] The aircraft 1 is a typical jet passenger transonic transport aircraft but the invention is applicable to a wide variety of fixed wing aircraft types, including commercial, military, passenger, cargo, jet, propeller, general aviation, etc. with any number of engines attached to the wings or fuselage.

[0049] Each wing has a cantilevered structure with a length extending in a span-wise direction from a root to a tip, the root being joined to an aircraft fuselage 4, and the tip attaching to a wingtip 10. The wings 2, 3 are similar in construction so only the starboard wing 3 will be described in detail with reference to Figure 2.

[0050] The aircraft 1 includes a centre wing box 9 within the body of the fuselage 4. The centre wing box 9 is joined to an inboard rib 14a which forms the root of the wing 3. The wing 3 includes a series of evenly spaced ribs 14 between the inboard rib 14a and an outboard rib 14b at the starboard wing tip. In the example shown in Figure 2, the wing 3 only has 5 ribs, but it will be apparent that the wing may have any number of ribs 14.

[0051] An apparatus for controlling loads on the aircraft wing 3 is coupled to the end rib 14b, such that the apparatus is near to the tip end of the wing 3 of the aircraft 1. The apparatus 20 is shown schematically in Figure 3, and includes a pair of bodies 21a, 21b configured to rotate about respective axes 22a, 22b. The centre of mass of the bodies 21a, 21b is offset from the respective axes 22a, 22b, and asymmetric about the axes 22a, 22b, such that an asymmetric inertial force is developed. [0052] Each of the bodies 21a, 21b is coupled to a respective actuator 23a, 23b by an axial arm 24a, 24b extending from the actuators 22a, 22b along the respective axes 22a, 22b, and a lever arm 25a, 25b extending perpendicular to the axes 22a, 22b. The first axis 22a and the second axis 22b are substantially parallel to each other, although a small amount of relative misalignment between the axes 22a, 22b may be permitted. The actuators 23a, 23b are connected to a controller 26 that is itself connected to a sensor configured to monitor forces acting on the aircraft 1 or about to act on the aircraft 1 (or a part of the aircraft 1, such as the wing 3).

[0053] The controller may be a standalone unit, or form part of the flight computer of the aircraft 1. The sensor may be part of the existing aircraft flight monitoring system, or may be a separate device not linked to the existing flight monitoring system. The sensor 27 may be any suitable sensor known in the art, such as an accelerometer, a strain gauge, a weather detector, or any sensor suitable for detecting loads acting on the aircraft 1 or about to act on the aircraft 1.

[0054] The controller 26 will typically receive a signal from the sensor 27 representative of a load such as an aerodynamic load, or a ground load acting on the aircraft, or an unbalanced load generated by a part of the aircraft. The controller 26 will then analyse the signal and determine an appropriate response.

[0055] In operation, the aircraft 1 will often encounter gust loads, or other dynamic loads, that excite the wing or other cantilever structures on the aircraft. The dynamic load will excite the wing and cause the wing to bend (in a direction generally normal to the wing spanwise and chordwise directions) at a harmonic frequency, or natural frequency. [0056] The dynamic load is recorded by the sensor 27 and a signal sent to the controller 26. The controller 26 will then send a command to the actuators 23a, 23b so that the actuators 23a, 23b rotate the bodies 21a, 21b and generate an asymmetric inertial force. By rotating the bodies 21a, 21b about the axes 22a, 22b, the inertial forces generated by the rotating bodies 21a, 21b can counter the dynamic loads acting on the structure. Typically the rotational frequency of the bodies 21a, 21b will be a harmonic frequency of the wing 3. The rotational frequency of the bodies 21a, 21b may be the first natural frequency of the wing 3 or a higher order natural frequency. The wing 3 may have a plurality of vibration modes. The inertial force varies in amplitude to match a dynamic load experienced by the aircraft, and will generally be a sinusoidal varying force with time.

[0057] The bodies 21a, 21b are rotated in opposite directions, in-phase and at the same rotational frequency, such that a fixed plane of symmetry 28 exists between the two rotating bodies 21a, 21b, as shown in Figures 4a & 4b. The plane of symmetry is normal to the chord-wise direction. The in-phase relationship between the two rotating bodies 21a, 21b may be achieved using gearing/gearbox (not shown).

[0058] As shown in Figure 5, rotation of the bodies 21a, 21b generates a centripetal force 30a, 30b directed from the centre of mass of the body towards the axis of rotation 22a, 22b. The axes 22a, 22b are substantially parallel to the span-wise direction of the wing 3 such that by counter rotating the two bodies 21a, 21b in-phase and at the same frequency, the chord- wise components 31a, 31b of the centripetal force 30a, 30b cancel each other out. In contrast, the vertical components 32a, 32b of the centripetal force 30a, 30b combine to generate a resultant vertical force. This resultant vertical force can be used to counter any dynamic load experienced by the aircraft structure, so as to provide a load alleviation function. The term "Vertical" is used in accordance with the normal orientation of the aircraft 1, i.e. substantially perpendicular to the chord-wise and span-wise directions of the wing 3. [0059] The axes 22a, 22b are substantially parallel to the span-wise direction of the wing 3, and so the span-wise components of the centripetal force are negligible (or at least small) compared to the chord-wise components 31a, 31b and vertical components 32a, 32b.

[0060] Typically, the load will be detected in real-time as it is experienced by the aircraft. In some situations, the sensor may predict a load that the aircraft is expected to experience. For example, the sensor 27 may be an alpha vane (angle of attack sensor) that detects a gust about to produce a load on the aircraft. In this case, the controller is able to anticipate (i.e. predict) the gust load and produce a counter-load that coincides with the development of the gust load. The system is thereby not just reactionary with a lag but active to reduce the effect of the gust load to an optimum.

[0061] In an alternative example, the apparatus may not respond to a dynamic load but may instead be used to generate a dynamic load, for instance when undertaking a flight test or a ground test.

[0062] In this case, the operation is substantially similar as in the case of load alleviation except that the rotational frequency of the bodies 21a, 21b is not chosen to counter a dynamic load, but is instead chosen to excite a desired load on the aircraft structure. The bodies 21a, 21b may be rotated at one or more natural frequencies of the aircraft structure to excite the structure and generate a bending load. The response of the aircraft structure may be observed over a range of frequencies, for instance the frequency may be increased between a lower rotational frequency and an upper rotational frequency.

[0063] The wing 3 may be excited to observe how the structure behaves at different frequencies, and to see the natural frequencies and mode shapes of the dynamic of the wing. For instance, the system may analyse up to 10 or more mode shapes. [0064] In the all of the examples, the bodies 21a, 21b may not necessarily continually rotate, and in those cases will be fixed in a rest position when not rotating. In order that the bodies 21a, 21b can be operated with minimal effort, and therefore rapidly respond to any signal from the controller 26 and/or reduce actuator wear, the rest position of the bodies 21a, 21b is arranged so that the bodies are substantially inclined vertically upwards so that they have substantially the highest potential energy due to gravity, as shown in Figure 6. The activation of the bodies 21a, 21b is therefore assisted by gravity. In this context, 'substantially' refers to a rotational angle near the top of the rotational travel of the bodies 21a, 21b, such as within 30 degrees of the top (i.e. within 30 degrees of vertical), and preferably with 10 degrees of the top.

[0065] In another example shown in Figure 7, the chordwise distance between each of the axes 22a, 22b and the flexural axis of the wing 3 may be different, i.e. the mid-point between the first axis 22a and the second axis 22b is offset from the flexural axis of the aircraft wing. In this case, the resultant force generated by the rotating bodies 21a, 21b will also cause some relative twisting of the wing 3 relative to the inboard end of the wing 3. This can be used to produce nose-down and nose-up pitching movements of the wing.

[0066] Typically, the apparatus will be enclosed within the skin of the aircraft 1 so that it is not exposed to external airflow around the aircraft 1, such that the aerodynamic effect of the rotating bodies 21a, 21b on the aircraft 1 is minimal.

[0067] In some examples, as shown in Figures 8a & 8b, the lever arm 25a, 25b may have a diameter greater than the thickness of the wing 3. In this case, or even if the diameter is not greater than the thickness of the wing 3, a fairing may be fitted around the apparatus 20 to shield the rotating bodies 21a, 21b from any external airflow. In an alternative example, the apparatus may be fitted within a wing tip device 10.

[0068] Even in examples in which the bodies 21a, 21b are exposed to any external airflow, the aerodynamic forces generated by the movement of the bodies 21a, 21b will be small compared to the inertial forces generated by the movement of the bodies 21a, 21b. [0069] Typically, the inertial force generated by the rotating bodies 21a, 21b will be controlled by controlling their rotational frequency. Alternatively, the radial extent of the bodies 21a, 21b from their respective axes 22a, 22b may be altered. For example, Figures 9a & 9b show a linear actuator 38 coupled to the body 21 so that the radial distance of the body 21 from the axis 22 can be altered during, or prior to, operation. In Figures 10a & 10b, a rotary actuator 39 is positioned at the intersection of the axial arm 24 and lever arm 25 so that the lever arm 25 can be articulated and thereby alter the radial distance of the body 21 from the axis 22. The radial distance from the axis 22 may be increased or decreased.

[0070] In a further example, shown in Figures 11 a- 11c, the axis 22a of the first body 21a and the axis 22b of the second body 21b may be co-axial. In this case, if the span- wise locations of the bodies 21a, 21b are the same, each of the bodies 21a, 21b will be restricted to rotating substantially 180 degrees about the axes 22a, 22b, i.e. a half revolution. In this case, the rotational direction of the bodies 21a, 21b will each reverse at the end of the half revolution.

[0071] Alternatively, the axis 22a of the first body 21a and the axis 22b of the second body 21b may be co-axial, but the bodies 21a, 21b spaced along the span- wise direction of the wing 3 so as to allow each body 21a, 21b to complete one or more full revolutions.

[0072] It will be clear to the skilled person that the examples described above may be adjusted in various ways. For example, the apparatus may include more than two bodies, for instance, three, four or five bodies with masses that are arranged to generate a desired inertial force. An aircraft 1, or aircraft wing 3, maybe have multiple sets of rotating bodies. These sets may be aligned or oriented relative to each other, to generate the required effect.

[0073] The axes 22a, 22b may not be parallel to the wing span- wise axis, but instead parallel to the chord- wise or vertical axis of the wing, or at an angle to any of these axes.

[0074] Each of the masses may be able to rotate in either direction, as desired, for example clockwise and anti-clockwise. The bodies may be rotated out-of-phase, and/or at different speeds, in order to tailor the generated inertial load and its vector. [0075] The controller and/or sensor may be part of the apparatus located on the wing, or may be positioned on a different part of the aircraft such as the fuselage.

[0076] The bodies are driven by an actuator, such as a rotary actuator. The actuator may be an electric motor. The bodies may be driven by a common actuator using a gearing mechanism. The bodies may be driven off an actuator not dedicated for rotating these bodies, for example they may be driven off a slat actuator used for deploying and retracting leading edge wing slats. The slats are not typically operable during cruise flight when the loads alleviation function is operating. This helps reduce the additional mass of the system.

[0077] The additional mass of the system may be offset by reducing the size, and therefore mass, of flight control surface actuators which may otherwise be sized larger to provide the required LAF. The additional mass of the system may also be offset by reducing the wing structural weight.

[0078] Where the word 'or' appears this is to be construed to mean 'and/or' such that items referred to are not necessarily mutually exclusive and may be used in any appropriate combination.

[0079] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. An apparatus attached near the tip end of a cantilevered aircraft wing for controlling load on the wing, the apparatus comprising: a first body having a mass rotatable about a first axis by an actuator device; and a second body adjacent to the first body and having a mass rotatable about a second axis by an actuator device, wherein the first body and the second body are configured to counter rotate about their respective axes, and wherein the first and second bodies each have a centre of mass offset from their respective axes.

2. An apparatus according to claim 1, wherein the first axis is coaxial with the second axis.

3. An apparatus according to claim 1, wherein the first axis is spaced from the second axis.

4. An apparatus according to any preceding claim, wherein the first and second axes are arranged symmetrically with respect to a flexural axis of the aircraft wing.

5. An apparatus according to any of claims 1 to 3, wherein the first and second axes are arranged asymmetrically with respect to a flexural axis of the aircraft wing.

6. An apparatus according to any preceding claim, wherein the first body and the second body are configured to rotate at the same frequency, and/or with the same moment of inertia, and/or with the same amplitude, and/or with the same mass, and/or acceleration, and/or angular velocity, and/or at the same phase.

7. An apparatus according to any preceding claim, wherein the first and second axes are arranged substantially parallel to the flexural axis of the aircraft wing.

8. An apparatus according to any preceding claim, further comprising a first actuator device configured to rotate the first body about the first axis and a second actuator device configured to rotate the second body about the second axis.

9. An apparatus according to any preceding claim, wherein the first body and/or the second body are configured to rotate at least one half revolution about their respective axes, or at least one full revolution about their respective axes.

10. An apparatus according to any preceding claim, further comprising a sensor configured to detect an input affecting load on the aircraft wing, and wherein the apparatus is configured to at least alleviate load on the aircraft wing by rotating the first and second bodies in response to the detected input.

11. An apparatus according to claim 10, wherein the load is one or more of: an aerodynamic load on the aircraft wing; or a ground load acting on the aircraft wing, or an unbalanced load generated by a dynamic part coupled to the aircraft wing.

12. An apparatus according to claim 10 or 11, wherein the load is a dynamic load and the aircraft wing has a dynamic response to the dynamic load, and wherein the first and second bodies are configured to rotate about their respective axes to alter the dynamic response of the aircraft wing.

13. An apparatus according to any of claims 1 to 9, wherein the apparatus is configured to excite the aircraft wing to vibrate at one or more modes of vibration by rotating the first and second bodies.

14. An apparatus according to any preceding claim, wherein the apparatus is enclosed either within an aerodynamic profile of the aircraft wing, or within an aerodynamic profile of a wing tip device attached to the wing, or within a fairing.

15. An apparatus according to any preceding claim, wherein the first and second bodies are configured to be held in a rest position when the bodies are not rotating, wherein the rest position is substantially the rotational angle with the highest potential energy due to gravity.

16. A method of controlling load on a cantilevered aircraft wing, comprising: simultaneously counter rotating a first body and a second body about respective axes in order to generate an asymmetric inertial load, wherein the first body and second body are adjacent and attached near the tip end of the aircraft wing.

17. A method according to claim 16, wherein before and/or after rotating the first and second bodies, the first and second bodies are held in a rest position, wherein the rest position is substantially the rotational angle with the highest potential energy due to gravity.

18. A method according to any one of claims 16 to 17, further comprising detecting an input affecting a load on the aircraft wing, and wherein the first

and second bodies are rotated to at least alleviate load on the aircraft wing in response to the detected input.

19. A method according to any one of claims 16 to 17, wherein the first and second bodies are rotated to excite the aircraft wing structure to vibrate at one or more modes of vibration.

20. A method according to any one of claims 16 to 19, wherein the first body and second body are rotated in flight.

21. A method according to any one of claims 16 to 20, wherein the first and second bodies are rotated between 0.5 Hz and 2 Hz.

22. A method of retrofitting a wing tip device to a tip end of an aircraft wing, including installing an apparatus according to any of claims 1 to 15 near the tip end of the wing.