Patent Publication Number: US-2023159158-A1

Title: Aircraft landing gear bay door

Description:
RELATED APPLICATION 
     This application incorporates by reference and claims priority to French patent application FR 2112528, filed Nov. 25, 2021. 
     FIELD 
     The present disclosure relates to an aircraft landing gear bay door. The present invention concerns aircraft landing gear bay doors. 
     BACKGROUND 
     There are many different landing gear bay door arrangements, often comprising doors used to open and close the landing gear bay to allow a landing gear to deploy from the bay. These doors are often generally planar (see U.S. Pat. No. 8,708,272, for example) and may be designed to have an external side that is contoured to match the surrounding contour of the aircraft where it is located. This is done to maintain the aerodynamic profile of the part of the aircraft where the door is located, when the door is closed. The aerodynamic profile will typically be shaped so as to reduce the drag of the aircraft when the landing gear bay doors are closed. 
     However, such landing gear bay doors are not designed or adapted for noise reduction purposes. In particular, there is no consideration of noise reduction when the doors are open and the landing gear is deployed, for example on landing approach. Here, it is important to reduce the noise footprint of the aircraft. This is different to wishing to reduce the drag, as that is not a concern when the aircraft is landing and slowing down anyway. 
     The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft landing gear bay door. 
     The project leading to this application has received funding from the European Union&#39;s Horizon 2020 research and innovation programme under grant agreement No 769350. 
     SUMMARY 
     The invention described here concerns an aircraft landing gear bay door comprising an attachment mechanism suitable for pivotally mounting the door to an aircraft such that the door can move between a closed position, in which the door at least partially closes an aircraft landing gear bay to stow a landing gear therein, and an open position, in which the door at least partially opens the landing gear bay to allow the landing gear to deploy, and a first side arranged to face inwardly in relation to the aircraft landing gear bay when the door is closed and face towards one side of a main leg of the landing gear when the door is open and the landing gear is deployed, a second opposite side arrange to face outwardly in relation to the landing gear bay when the door is closed and thus provide an aerodynamic outer surface of the aircraft, and face away from the main leg of the landing gear when the door is open and the landing gear is deployed. 
     The invention also concerns an aircraft landing gear arrangement, an aircraft, methods of operating an aircraft and a method of reducing noise. 
     The invention provides, according to a first aspect, an aircraft landing gear bay door comprising an attachment mechanism suitable for pivotally mounting the door to an aircraft such that the door can move between a closed position, in which the door at least partially closes an aircraft landing gear bay to stow a landing gear therein, and an open position, in which the door at least partially opens the landing gear bay to allow the landing gear to deploy, and a first side arranged to face inwardly in relation to the aircraft landing gear bay when the door is closed and face towards one side of a main leg of the landing gear when the door is open and the landing gear is deployed, a second opposite side arrange to face outwardly in relation to the landing gear bay when the door is closed and thus provide an aerodynamic outer surface of the aircraft, and face away from the main leg of the landing gear when the door is open and the landing gear is deployed, wherein the shape formed from the first and second sides provides an aerofoil profile of the door. 
     Such a door with the aerofoil profile acts to slow down the airflow flowing past the inner side. This reduces the noise of the airflow flowing past the landing gear. Hence, it means that the aircraft using such a door will have a lower noise footprint, especially when the landing gear is deployed and the aircraft is coming into land. 
     Throughout the specification, the terms fore/aft, forward/backward, behind/in front, height, side etc. should be construed in relation to the conventional terms for an aircraft during normal flight. For example, upstream corresponds to a forward/fore region and downstream corresponds to a backward/aft region. Hence, if the first side directly faces the one side of a main leg of the landing gear, it will be directly facing laterally (starboard or port-wards) and so at 90 degrees to the aircraft longitudinal axis. Hence, the door itself will be roughly aligned with the direction of the aircraft longitudinal axis, when open. Height corresponds to the dimension in the upward/vertical (z) direction, in relation to a level orientation of the aircraft with respect to the ground. 
     The attachment mechanism may attach the door to the aircraft directly. For example, the door may be pivotally mounted adjacent to a landing gear bay of an aircraft. However, the attachment mechanism may also attach the door to a deployable landing gear leg so that the door can pivot with respect to the aircraft as the leg is deployed. 
     The door may be roughly aligned with (i.e., substantially in) the direction of the aircraft longitudinal axis, when open. A pivotal axis of the door relative to the aircraft may be substantially aligned with the direction of the aircraft longitudinal axis. For example, a plane containing the door (i.e., a plane enclosing the door thickness/smallest dimension) is orientated in a fore-aft direction of the aircraft, when open. For example, it may be that the door is substantially parallel to a longitudinal axis of the aircraft. 
     The landing gear door may be one of several landing gear doors working in combination to close and open the landing gear bay. 
     The first side may be contained within the landing gear bay when the door is closed. For example, it may be that only a second opposite side of the door may provide an aerodynamic surface of the aircraft when the door is closed. 
     Thus in certain embodiments, the second side of the door, at least when the door is deployed, may be contoured to deviate from the shape that would be defined, with the door in its closed position, by a continuance of the surrounding exterior (largely planar) contour of the aircraft (e.g. the fuselage or the landing gear fairing). 
     The first side may provide one half of an aerofoil profile and wherein the second side provides a corresponding other half of an aerofoil profile. In embodiments, it may be that the inner side of the door has a profile that matches one half of an aerofoil profile. For example, a lower half of an aerofoil profile. The aerofoil profile may run along a longitudinal direction of the door, in use. The outer side of the door has a profile that matches one half of an aerofoil profile. For example, both inner and outer sides of the door may provide respective halves of an aerofoil profile. For example, the half provided by the outer side may be the opposite half to the inner side of the door, for example an upper half of an aerofoil profile. The aerofoil profile may run along a longitudinal direction of the door, in use. The leading edge of the complete aerofoil profile may be located at an upstream edge of the door, when open. 
     A leading edge of the aerofoil profile may be located at an upstream edge of the door, when open. 
     The aerofoil profile may have a height corresponding to at least 80%, or at least 90%, of the height of the door. Here, height refers to the vertical dimension when the door is open. 
     The aerofoil profile may have a camber. The camber may move inwards or outwards from leading edge to trailing edge of the aerofoil profile. The leading edge may be nearer or further away from the landing gear leg, than the trailing edge in the lateral dimension. For example, the trailing edge may partly curve around the landing gear leg, in use, wherein the trailing edge of the aerofoil profile is straight. For example, in embodiments, it may be that the aerofoil profile has an abrupt trailing edge of a certain thickness, rather than tapering to a point. Alternatively, the trailing edge of the aerofoil profile may be tapered. 
     A region of the aerofoil profile may be provided with a bulged portion, bulging towards the second side of the door. Such a curved bulged portion acts to significantly slow down airflow and hence, reduce noise. The bulged portion may be located towards a rear/trailing end of the aerofoil profile. For example, in embodiments, it may be more than halfway along the length of the aerofoil profile from the leading edge. The first side of the aerofoil profile may be provided with the bulged portion. For example, the first side at the bulged portion may be offset past the rest of the second side, wherein the second side of the aerofoil profile may also be provided with the bulged portion. 
     The shape of the door may be such that along a longitudinal section of the door when open, the thickness of the door is greater than a first thickness for at least a fifth of its length in the longitudinal direction and is less than a second (lower) thickness for at least a fifth of its length in the longitudinal direction. It may be that the first thickness is at least 25% greater than, or 50% greater than, for example twice as great as or more than twice as great as, the second thickness. The thickness of the door may be defined as the separation between the first side and the second side at the particular location along the longitudinal section. The longitudinal section may be taken at a mid-way point along the height of the door, when open. 
     The first thickness may be located upstream of the second thickness. In other words, the first (greater) thickness occurs nearer a leading edge of the door, than the second thickness. The thickness of the door may be greater than the first thickness for at least half of its length in the longitudinal direction and is less than the second thickness for at least a fifth of its length in the longitudinal direction. The thickness of the door may be greater than the first thickness for at least three fifths of its length in the longitudinal direction and is less than the second thickness for at least a fifth of its length in the longitudinal direction. 
     The door may comprise a moveable component, moveable from a first position when the door is closed to a second position when the door is open and wherein, in the second position, the moveable component moves to form the aerofoil profile. This movement may be automatic, for example due to abutment of the moveable component against a door closure frame, or other part of the aircraft in contact when the door is closed. The moveable component may be a memory shape component and wherein the memory shape component is biased to be in the second portion and is urged to the first position as the door closes. For example, due to abutment of the memory shape component against a door closure frame, or other part of the aircraft it contacts when the door is closed. The memory shape component may, for example, comprise a memory shape alloy or a memory shape composite material. 
     According to a second aspect of the invention there is also provided an aircraft landing gear arrangement including the aircraft landing gear bay door as described above and a landing gear including a main landing gear leg, the main landing gear leg being pivotally mountable in relation to an aircraft landing gear bay between a stowed and a deployed position, and comprising a number of landing gear wheels arranged to roll in a fore/aft direction when in the deployed position. 
     When the door is in the open position and the landing gear is in the deployed position, the aerofoil profile may at least be partially aligned with the landing gear leg in the fore/aft direction. For example, it may be that the landing gear leg and the aerofoil profile are at the same longitudinal position along the aircraft, in use. The landing gear leg may be aligned with a rear/trailing end of the aerofoil profile. For example, it may be aligned with the aerofoil profile more than halfway along the length of the aerofoil profile from the leading edge. 
     When the door is in the open position and the landing gear is in the deployed position, the bulged portion of the aerofoil profile may be at least partially aligned with the landing gear leg in the fore/aft direction. 
     When the door is in the open position and the landing gear is in the deployed position, the aerofoil profile may be offset from the landing gear leg in the lateral direction by an offset distance. In other words, the landing gear leg and the aerofoil profile are separated in the dimension across the aircraft, in use. The offset distance may be at least 20 mm, for example at least 30 mm, or at least 50 mm, or at least 100 mm. They may be offset by less than 5 times the width of the landing gear leg. They may be offset by less than 200 mm. The offset distance may, at least in part, be dictated by the flow velocity in the region and/or by the clearance requirements between the landing gear and the door. 
     According to a third aspect of the invention there is also provided an aircraft comprising the aircraft landing gear bay door or the aircraft landing gear arrangement as described above. 
     According to a fourth aspect of the invention there is also provided a method of operating an aircraft, including the step of using the aircraft landing gear bay door, the aircraft landing gear arrangement or aircraft as described above. 
     According to a fifth aspect of the invention there is also provided a method of operating an aircraft comprising the steps of opening a landing gear bay door, deploying a landing gear, including a main leg, and an aerofoil profile to the door causing the velocity of the air flow in between the side of the main leg of the landing gear and the landing gear bay door to be reduced. 
     According to a sixth aspect of the invention there is also provided a method of reducing noise generated by a landing gear on landing approach, comprising the steps of opening a landing gear bay door, deploying a landing gear, including a main leg, and an aerofoil profile to the door causing the velocity of the air flow in between the side of the main leg of the landing gear and the landing gear bay door to be reduced. 
     It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG.  1    shows a side view of a landing gear arrangement according to a first embodiment of the invention; 
         FIG.  2    shows a schematic plan view of the landing gear arrangement of  FIG.  1   ; 
         FIG.  3    shows a schematic plan view of a landing gear arrangement according to second embodiment of the invention; 
         FIG.  4    shows a schematic plan view of a landing gear arrangement according to third embodiment of the invention; 
         FIG.  5    shows a schematic plan view of a landing gear arrangement according to fourth embodiment of the invention; 
         FIG.  6    shows a schematic plan view of a landing gear arrangement according to fifth embodiment of the invention; 
         FIG.  7    shows a schematic plan view of a landing gear arrangement according to sixth embodiment of the invention; 
         FIG.  8    shows a schematic plan view of a landing gear arrangement according to seventh embodiment of the invention; 
         FIG.  9    shows a schematic plan view of a landing gear arrangement according to eight embodiment of the invention; 
         FIG.  10    shows a schematic plan view of a landing gear arrangement according to ninth embodiment of the invention; 
         FIG.  11    shows a schematic plan view of a landing gear arrangement according to tenth embodiment of the invention; 
         FIG.  12    shows a schematic plan view of a landing gear arrangement according to eleventh embodiment of the invention; 
         FIG.  13    shows a schematic plan view of a landing gear arrangement according to twelfth embodiment of the invention; 
         FIG.  14    shows a schematic plan view of a landing gear arrangement according to thirteenth embodiment of the invention; 
         FIG.  15   a    shows a schematic upside down side view of a stowed and closed landing gear arrangement according to fourteenth embodiment of the invention; 
         FIG.  15   b    shows a schematic plan view of the landing gear arrangement of  FIG.  15   a   , in an open and deployed configuration; 
         FIG.  15   c    shows a schematic plan view of the landing gear arrangement of  FIG.  15   a   , in a second open and deployed configuration; and 
         FIG.  16    shows a schematic front view of an aircraft with a nose landing gear arrangement and two main landing gear arrangements, the landing gear arrangements being suitable for being in accordance with any of the embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a side view of a landing gear arrangement  100  according to a first embodiment of the invention.  FIG.  2    shows a schematic plan view of the landing gear arrangement of  FIG.  1   . 
     The arrangement  100  comprises a landing gear main leg  110 , which, in use, is pivotally connected to an aircraft by a pivot mechanism  113  at the top of the leg. At the bottom of the leg is an axle, which mounts two landing gear wheels, only one (labelled  131 ) seen in  FIG.  1   . The wheels roll in a fore/aft (or longitudinal) direction. A side stay (not seen) is pivotally mounted on the leg  110  and also to the aircraft, in use. 
     The landing gear main leg  110 , side stay and axle/wheels are entirely conventional and comprise various other elements/features, such as oleo struts, support arms/braces, electrical installations, brake assemblies, actuators etc. which will not be described here. 
     The arrangement  100  also includes a landing gear bay door  140 . The door  140  has an outer side  142  (facing away from the leg  110 ) and an inner side  141  (facing the leg  110 ). Slightly below the top edge  143  on the inner side  141  is an attachment mechanism to mount the door  140  to the leg  110 . 
     The door  140  has a front edge  145  and a rear edge  146 . The rear edge is the full door height but the front edge  145  is slightly shorter. It is the longest dimension, i.e., at the rear edge, that defines the door height. The front edge  145  is shorter as the bottom edge  144  of the door  140  is shaped to curve upwards towards the front edge  145 . 
     The arrangement  100  is shown in relation to an oncoming airflow  1001 . 
     As can be seen in  FIG.  2   , a unique feature of the door  140  is that the door  140  has a cross-sectional shape, in plan view, of that of an aerofoil. In other words, the front edge  145  has the shape of a rounded leading edge of an aerofoil and the shape tapers towards a trailing edge at the rear edge  146 . 
     The aerofoil shape provided has a planar outer side  142 . Hence, when the landing gear bay door  140  is closed in use, the planar outer side  142  forms an outside contour of the aircraft. It, in fact, provides a contour that matches/follows that of the surrounding generally planar contour of the aircraft. The aerofoil shape also has a straight edge at the rear edge  146  to provide a finite thickness at that edge  146 . 
     The aerofoil shape provided has a curved concave inner side  141 . Hence, when the landing gear bay door  40  is open airflow between the inner side  141  and the landing gear leg  110  is caused to slow down and thus reduce the noise generated. 
     The aerofoil profile shape is provided on the door along its entire height/length (i.e., from the top  143  to the bottom  144 ). 
       FIG.  3    shows a schematic plan view of a landing gear arrangement  100  according to a second embodiment of the invention. The second embodiment is similar to the first embodiment and only the differences will be described. A side view of the second embodiment is in fact the same as the side view of the first embodiment (i.e.,  FIG.  1   ). 
     Here, an aerofoil shape is still provided in the plan view, but it is a slightly different aerofoil shape. Firstly, the outer side  142  is not planar and instead provides a convex profile. Hence, when the landing gear bay door  140  is closed in use, the convex outer side  142  forms an outside contour of the aircraft. It can also be seen that the rear edge  146  is angled downwards to provide a more concave profile to the inner side  141 . In particular, the landing gear leg is located in line (perpendicular to the airflow  1001 ) with a location near to the maximum concave point  147  of the inner side  141 . 
       FIG.  4    shows a schematic plan view of a landing gear arrangement  100  according to third embodiment of the invention. The third embodiment is similar to the second embodiment and only the differences will be described. A side view of the third embodiment is in fact the same as the side view of the second embodiment (i.e.,  FIG.  1   ). 
     Here, the landing gear leg is offset from the aerofoil door  140  by an offset distance (labelled as  148 ) of 50 mm. 
       FIG.  5    shows a schematic plan view of a landing gear arrangement according to fourth embodiment of the invention. The fourth embodiment is similar to the third embodiment and only the differences will be described. A side view of the fourth embodiment is in fact the same as the side view of the third embodiment (i.e.,  FIG.  1   ). 
     Here, the landing gear leg is offset from the aerofoil door  140  by an offset distance (labelled as  148 ) of 100 mm. 
       FIG.  6    shows a schematic plan view of a landing gear arrangement  100  according to fifth embodiment of the invention. The fifth embodiment is similar to the second embodiment and only the differences will be described. A side view of the fifth embodiment is in fact the same as the side view of the second embodiment (i.e.,  FIG.  1   ). 
     Here, the aerofoil shape includes a “rear bulge”  149  on the upper (outer) side  142 . Hence, the outer side  142  of the door has a bulged portion, where the contour is further outwards of the rest of the outer contour of the door. This rear bulge  149  matches and corresponds to the contour of the concave shape of the inner side  141 . 
       FIG.  7    shows a schematic plan view of a landing gear arrangement  100  according to sixth embodiment of the invention. The sixth embodiment is similar to the fifth embodiment and only the differences will be described. A side view of the sixth embodiment is in fact the same as the side view of the fifth embodiment (i.e.,  FIG.  1   ). 
     Here, the leading region of the outer side  142  is further outwards, corresponding to the level of the rear bulge of the fifth embodiment. This gives a much thicker forward region of the door  140 . Hence, the outer side  142  is much more planar compared to that of the fifth embodiment. However, it still enables a concave shape in the inner side  141 . 
       FIG.  8    shows a schematic plan view of a landing gear arrangement according to seventh embodiment of the invention. The seventh embodiment is similar to the sixth embodiment and only the differences will be described. A side view of the seventh embodiment is in fact the same as the side view of the sixth embodiment (i.e.,  FIG.  1   ). 
     Here, the trailing edge  146  is located much further inwards (towards the leg  110 ). Hence, the concave shape of the inner side  141  can be retained with a substantially planar outer side  142  and with a thinner door  140  thickness. 
       FIG.  9    shows a schematic plan view of a landing gear arrangement according to eight embodiment of the invention. The eighth embodiment is similar to the fifth embodiment and only the differences will be described. A side view of the eighth embodiment is in fact the same as the side view of the fifth embodiment (i.e.,  FIG.  1   ). 
     Here, the bulge  149  is much reduced meaning that the outer side  142  of the door  140  extends less far outwards. Consequently, the concaveness of the inner side  141  is reduced. 
       FIG.  10    shows a schematic plan view of a landing gear arrangement  100  according to ninth embodiment of the invention. The ninth embodiment is similar to the eighth embodiment and only the differences will be described. A side view of the ninth embodiment is in fact the same as the side view of the eighth embodiment (i.e.,  FIG.  1   ). 
     Here, the forward portion of the inner side  141  extends further inwards and hence increases the thickness of the door  140 . This means that for the same outer side  142  contour (i.e., still substantially planar), the concaveness of the inner side  141  is increased. 
       FIG.  11    shows a schematic plan view of a landing gear arrangement according to tenth embodiment of the invention. The tenth embodiment is similar to the eighth embodiment and only the differences will be described. A side view of the tenth embodiment is in fact the same as the side view of the eighth embodiment (i.e.,  FIG.  1   ). 
     Here, the trailing edge  146  takes on a slightly different shape with the tail being inflected so as to not extend inwards as far (if it followed the “bulge” contour). Instead, the trailing edge  146  portion is substantially straight and extends back in line with the airflow  1001 . 
       FIG.  12    shows a schematic plan view of a landing gear arrangement according to eleventh embodiment of the invention. The eleventh embodiment is similar to the tenth embodiment and only the differences will be described. A side view of the eleventh embodiment is in fact the same as the side view of the tenth embodiment (i.e.,  FIG.  1   ). 
     Here, the trailing edge  146  is thicker than in the twelfth embodiment. 
       FIG.  13    shows a schematic plan view of a landing gear arrangement according to twelfth embodiment of the invention. The twelfth embodiment is similar to the eleventh embodiment and only the differences will be described. A side view of the twelfth embodiment is in fact the same as the side view of the eleventh embodiment (i.e.,  FIG.  1   ). 
     Here, the inner side  141  extends further inwards in front of the bulged portion  149  (giving a thicker leading portion of the door  140 ). 
       FIG.  14    shows a schematic plan view of a landing gear arrangement according to thirteenth embodiment of the invention. The thirteenth embodiment is similar to the twelfth embodiment and only the differences will be described. A side view of the thirteenth embodiment is in fact the same as the side view of the twelfth embodiment shown in  FIG.  1   . 
     Here, the outer side  142  extends further outwards at the bulge  149  so as to reduce the planar-ness of the outer surface whilst still providing a large concaveness of the inner side  141 . 
       FIG.  15   a    shows a schematic upside downside view of a stowed and closed landing gear arrangement  100  according to fourteenth embodiment of the invention. In other words, this is a view looking along the leg  110  when the leg is stowed in an aircraft landing gear bay (defined by walls  150  of the aircraft surrounding the bay). 
     The fourteenth embodiment is similar to the first embodiment in that a shaped door  140  (e.g., any suitable aerofoil shape) is provided. However, the door shape when the door is stowed (as it is in  FIG.  15   a   ) is planar (on both sides  141 ,  142 ). The door  140  only takes up a curved shape when it is opened away from the walls  150 . In other words, the door  140  is biased to take up the curved shape, but is forced to take up the planar shape of  FIG.  15   a    by the walls  150 . 
     This could be achieved by having the door being pre-stressed and/or being made out of a memory shape alloy. 
       FIG.  15   b    shows a schematic plan view of the landing gear arrangement  100  of  FIG.  15   a   , in an open and deployed configuration. Here, the door  140  has started to move away from the walls (not shown) and the trailing edge of the door has moved to curve downwards, providing a concave inner side  141  and convex outer side  142 . 
       FIG.  15   c    shows a schematic plan view of the landing gear arrangement of  FIG.  15   a   , in a second open and deployed configuration. As the door has moved further away from the walls, the leading edge has also curved downwards, providing an even more concave inner side  141  and even more convex outer side  142 . 
     In  FIGS.  15   b  and  15   c   , the resulting airflow can be seen. Airflow is directed by the trailing edge of the door smoothly behind the landing gear leg  110 . 
       FIG.  16    shows a schematic front view of an aircraft  1000  with a nose landing gear arrangement  300  and two main landing gear arrangements  100 ,  200 , the landing gear arrangements being suitable for being in accordance with any of the embodiments of the invention. 
     Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
     The landing gear leg may be offset from the aerofoil door by different offset distance, being greater or less than the figures stated herein, and the distance need not be exactly the same for the length of the leg. 
     The door inner side  141  may not be concavely curved and instead may have any suitable curved profile. 
     The door outer side  142  may have any suitable profile. The outline of the door may be differently shaped for a different aircraft in view of the shape of the opening of the landing gear bay required to allow the deployment therethrough of the landing gear, which may be differently configured depending on the design of the aircraft concerned. For example, not all landing gear legs are provided with a side stay. 
     The door rear edge  146  may taper to a point instead of having a straight end. 
     The door may be made of any suitable material. It may be 3D printed. 
     The door  140  may be attached directly to an aircraft (not via the leg  110 ). For example, at the door top edge  143  may be an attachment mechanism to pivotally mount the door  140  to an aircraft, in use. 
     It will be appreciated that the landing gear leg  110  may have any suitable number of wheels, for example being greater than two. 
     Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. 
     Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments. 
     It should be noted that throughout this specification, “or” should be interpreted as “and/or”, unless stated otherwise. 
     Although the invention has been described above mainly in the context of a fixed-wing aircraft application, it may also be advantageously applied to various other applications, including but not limited to applications on vehicles such as helicopters, drones, trains, automobiles and spacecraft.