The present invention concerns aerodynamic devices, such as slats, ailerons, elevators, spoilers, flaps, Krueger flaps and rudders. More particularly, but not exclusively, this invention concerns aerodynamic devices for being rotatably mounted to an aerodynamic structure, such as a wing, horizontal tailplane or vertical tailplane, of an aircraft, the aerodynamic device having a spanwise length, a chordwise width, a leading edge section along a leading edge of the device, for being rotatably mounted to the aerodynamic structure of the aircraft, and a trailing edge section along a trailing edge of the device, for providing a required aerodynamic profile. The invention also concerns an aircraft comprising such an aerodynamic device.
Typical aircraft wings for passenger aircraft are provided with a number of slats. These slats are rotatably mounted along the leading edge of the wing. When stowed, they are positioned adjacent to the wing leading edge and in line with the wing. When deployed, they translate forwards away from the leading edge of the wing and rotate downwards away from the wing. The slats effectively increase the area of the wing and provide additional lift to the wing.
FIG. 1a shows a spanwise end view of a typical wing assembly 100. The wing assembly 100 comprises wing primary structure 110, a leading edge device 120 (located at the leading edge of the wing primary structure 110) and a trailing edge device 130 (located at the trailing edge of the wing primary structure 110). The leading edge device 120 is a slat. The trailing edge device 130 is a flap. The wing primary structure 110 has a chordwise minor axis 111, extending through the centre of the structure 110. The minor axis 111 is the axis about which the out-of-plane spanwise bending stiffness is at a minimum and the axis about which out-of-plane spanwise bending takes place.
FIG. 1b shows a schematic perspective view of the slat 120 of FIG. 1a. The slat 120 comprises a leading edge D-nose section 122 at the leading edge of the structure and a wedge shaped trailing edge section 123. The axes show the spanwise direction 124 extending along the length of the slat, the chordwise direction 125 extending along the width of the slat and the out-of-plane direction 126 extending upwards away from the slat 120. Equivalent directions (spanwise, chordwise, out-of-plane) also apply for a trailing edge device 130 or any other kind of leading edge device.
In the stowed configuration of FIG. 1a, the minor bending axes of the slat 120 (and flap 130) are substantially the same as the minor bending axis 111 of the wing primary structure 110. Hence, when stowed, the entirety of the slat 120 (and flap 130) are close to the minor bending axis 111 of the wing primary structure 110.
If the slat (and flap 130) are mounted to the wing primary structure 110 by more than two supports (e.g. slat tracks), the slat (and flap 130) are coupled to the wing primary structure 110 deformations and are forced to sympathetically bend with the wing primary structure 110 about an axis parallel to the wing primary structure 110 minor axis 111. When the wing primary structure 110 bends up and down (out-of-plane spanwise bending) and the coupled slat 120 (and coupled flap 130) are forced to sympathetically bend with it, the slat (and flap) experience out-of-plane spanwise bending 137 (up and down about the minor axis 111—see FIG. 2c) and the stress level in the device is relatively low. The slat (and flap) also experience this out-of-plane spanwise bending due to out-of-plane aerodynamic load (lift).
FIG. 2a shows a spanwise end view of the wing assembly 100 of FIG. 1a with slat 120 and flap 130 deployed. Here it can be seen that the wing primary structure 110 has the same minor axis (about which the out-of-plane spanwise bending stiffness is at a minimum) 111, about which it bends. The leading edge device 120 and the trailing edge device 130 are shown with forced bending axes 121, 131 marked on them. These axes 121, 131 run through the centroid of the devices and are parallel to the minor axis 111 of the wing primary structure 110. If coupled to the wing primary structure 110, the leading edge device 120 and trailing edge device 130 are forced to bend about these axes 121, 131 respectively during out-of-plane spanwise bending of the wing primary structure 110.
The stress level during wing bending is determined by the distance from the bending axis and the stiffness of the material. As can be seen from the stress representations 140, 150 of the leading edge device 120 and trailing edge device 130, respectively, the maximum stress on the devices 120, 130 is increased due to the increased distance of the extremities of the devices from the forced bending axes 121, 131.
Also, as the minor bending axes of the slat 120 (and flap 130) are rotated, when the wing bends up and down and the coupled slat 120 (and coupled flap 130) are forced to sympathetically bend with it, the slat 120 (and flap 130) experience out-of-plane spanwise bending 127 (up and down about the slat/flap minor axis 127a—see FIG. 2c) and also in-plane bending 128 (forwards and backwards about the slat/flap major axis 128a—see FIG. 2d). The slat (and flap) also experience this in-plane bending due to chordwise aerodynamic load (drag).
It should be noted that a slat (and flap) typically also experiences out-of-plane chordwise bending 129, about a mid-principal axis 129a (perpendicular to both major axis 128a and minor axis 127a), as shown in FIG. 2f. Bending about this axis 129a is induced by an out-of-plane aerodynamic load (lift) and is the method by which the slat (or flap) trailing edge section 123 carries the aerodynamic load forward to the leading edge section 122.
FIG. 2e shows a schematic plan view of the slat 120 of FIG. 1b, showing the in-plane bending. Here, it can be seen that the slat 120 (including a leading edge section 122 and a trailing edge section 123) bends about major axis 128a (the axis about which in-plane bending occurs), perpendicular to the minor axis 127a (the axis about which out-of-plane spanwise bending occurs).
The devices are much stiffer in in-plane bending and so this puts high loads on the mounting points to the wing primary structure 110 and generates large stresses. As well as generating large loads and stresses, this can also lead to undesirable deflection and/or distortion of the slat 120 itself.
Hence, for a typical aircraft, a slat (or other aerodynamic device) is only mounted to the wing at two support points (e.g. slat tracks) so as to allow the slat to be decoupled from wing bending. If only using two support points for the slat, this means that the slat has to be stiff and strong enough to support the aerodynamic load between the two supports. Making a stiffer and stronger slat incurs a weight penalty (as the slat needs to be heavier in order to be stiffer and stronger). This puts an effective limit on the length of the slat. Hence, in order to provide additional lift over the required length of the wing, an aircraft wing is typically provided with a number of slats along its length. This results in a high part count for the wing, resulting in cost and complexity in design and manufacture.
FIG. 2b shows an aft view of a typical wing assembly in an upwardly bent position. Here, the wing assembly 100 comprises four deployed slats 120a, 120b, 120c, 120d, each attached to the wing primary structure 110 by two slat tracks; inboard track 120e and outboard track 120f. This Figure also shows the wing assembly undeformed position 112 (in dashed lines) and the upwardly bent position due to out-of-plane spanwise bending 113.
Instead of, or in addition to, limiting the length of the slat, another way of dealing with the issue is to limit the deployment angle of the slat. However, this is also undesirable as it means the slat may not be effective, or as effective, over the whole flight envelope.
Another solution is to structurally reinforce the slat. However, again, this also has disadvantages as the reinforcement adds weight and complexity.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aerodynamic device.