Abstract:
A reusable bumper ( 10 ) for a rotorcraft ( 1 ) suitable for protecting a rear end ( 3 ) of said rotorcraft ( 1 ). The bumper ( 10 ) comprises a resilient outer shell ( 20 ) forming a first chamber ( 11 ) having a resilient inner shell ( 30 ) placed therein and forming a second chamber ( 12 ), said outer shell ( 20 ) having at least one main orifice ( 21 ), said inner shell ( 30 ) being provided with management means ( 50 ) for managing the pressure that exists inside said second chamber ( 12 ) and suitable for enabling said pressure to increase up to a predetermined threshold in order to inform the pilot that the bumper ( 10 ) has made contact with the ground (S), and then to enable said pressure to drop in order to absorb energy resulting from said contact.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of FR 09 04148 dated Sep. 2, 2009, the disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a rotorcraft bumper for protecting a structural element of an aircraft against an impact with the ground, and to a rotorcraft provided with such a bumper. 
         [0003]    More precisely, the present invention provides a bumper for protecting the rear end of a rotorcraft, in particular the anti-torque tail rotor of a helicopter, and it therefore lies in the restricted technical field of protecting structural elements of a rotorcraft. 
       BACKGROUND OF THE INVENTION 
       [0004]    Regulations for certifying a helicopter require the anti-torque tail rotor to be protected so as to avoid said tail rotor impacting against the ground when the helicopter takes a nose-up position close to the ground. 
         [0005]    For example, when landing in autorotation, the pilot makes the helicopter take a nose-up position close to the ground, a procedure known as “flaring”. Consequently, there is a risk of the tail rotor coming into contact with the ground, and that could lead to a catastrophic situation. 
         [0006]    Furthermore, when landing at high speed, the pilot causes the helicopter to take a nose-up position in order to brake it. 
         [0007]    Thus, aircraft, and in particular helicopters, advantageously include protection against impact with the ground in the event of the aircraft taking a strongly nose-up position. 
         [0008]    For example, a crushable and replaceable tail bumper is known that is arranged in a structural element. For example, the Applicant&#39;s helicopter known under the trademark Gazelle® is provided with such a bumper inside the keel of its faired tail rotor, i.e. inside the bottom structural element of said faired tail rotor that faces the ground when the helicopter is standing thereon. 
         [0009]    Such a bumper nevertheless needs to be changed or repaired after each occasion it comes into contact with the ground. The repairs are sometimes performed by the owner of the rotorcraft and thus under conditions that are not as good as they might be. 
         [0010]    Alternatively, other rotorcraft are provided with a resilient skid having a single blade. 
         [0011]    A first end of the skid is provided with a curved portion, while its second end is fastened by two distinct fastener means to the structural element for protection. Document FR 2 554 210 describes a skid of that type arranged under the tail boom of a helicopter. 
         [0012]    The skid is thus cantilevered-out, which explains why the term “cantilever” is sometimes also used to designate such a skid. 
         [0013]    Although effective, it should be observed that the cantilever of such a skid gives rise to high levels of force at the fastener means between the skid and the structural element. The structural element therefore needs to be dimensioned accordingly so as to be capable of withstanding the forces generated by the skid on coming into contact with the ground, with this being achieved by using local reinforcement, where such reinforcement is harmful to the overall weight of the rotorcraft and also constitutes extra expense. 
         [0014]    Furthermore, the skid sometimes leads to a disagreeable bounce effect that tends to push the structural element away from the ground. 
         [0015]    In addition, it has been observed that such a skid has sometimes given rise to incidents on certain aircraft. 
         [0016]    Starting from a given nose-up angle, the angle between the skid and the ground as formed by the curved portion can cause the skid to become jammed and consequently leads to the structural element being dented where it is fastened to the skid. Furthermore, certain occasions of losing control of a rotorcraft have been caused by the skid catching on the ground (net, vegetation, etc. . . . ). 
         [0017]    Furthermore, certain touchdowns can lead to the structural integrity of the tail boom being put at risk. As a result significant maintenance action should be expected. In order to remedy such incidents, various solutions may be provided, such as, for example: 
         [0018]    accepting plastic deformation of the skid during an impact; or indeed 
         [0019]    hinging the skid to the structural element for protection, and arranging an oleo-strut between the skid and the structural element. 
         [0020]    Finally, skids are difficult to develop. Thus, skids need to be sufficiently stiff for the pilot to be aware that contact has occurred between the skid and the ground, while also being sufficiently flexible to absorb the energy that results from such contact without generating excessive forces in the structure. 
         [0021]    Other damper devices have also been envisaged in the following additional documents. 
         [0022]    For example, additional document FR 362 365 relates to a car suspension and cannot seriously be considered for protecting a helicopter tail rotor. Document FR 362 365 describes a buffer provided with an elastomer having a recess formed therein to constitute an air chamber, with orifices enabling the air chamber to communicate with an outside medium. 
         [0023]    Similarly, additional document FR 2 873 641 relates to a device that is applicable to car doors, the device having a deformable shell extending between two panels of a door. 
         [0024]    Additional document U.S. Pat. No.  5 , 445 , 430  relates to an armrest having a shell that is closed in part by a membrane that is suitable for rupturing in the event of an impact. 
         [0025]    Additional document WO 2008/054401 describes an anti-crash device having an airbag that is inflatable by two distinct sources of gas and that is provided with venting valves. 
         [0026]    Finally, additional document U.S. Pat. No.  4 , 399 , 963  relates to a skid wheel for light aircraft that is inserted in a body made of an elastic material. 
         [0027]    It can be understood that those additional documents relate to subjects that are far removed from the restricted technical field of the invention. 
       SUMMARY OF THE INVENTION 
       [0028]    An object of the present invention is thus to propose a reusable rotorcraft bumper for protecting the rear end of the rotorcraft and enabling the above-mentioned limitations to be overcome. It should be observed that the term “rear end” is used to designate the tail boom or indeed the tail rotor of the rotorcraft, for example. 
         [0029]    A particular object of the bumper is to reduce the bounce effect and the risks of jamming that have been observed with devices that use a skid, to avoid generating high levels of force at the interface of the device with the structural element, and to avoid requiring repair in order to be reused. 
         [0030]    According to the invention, a reusable rotorcraft bumper suitable for protecting a rotorcraft rear end is remarkable in that it comprises a resilient outer shell forming a first chamber having a resilient inner shell placed therein and forming a second chamber, the outer shell having at least one main orifice, the inner shell being provided with management means for managing the pressure that exists inside the second chamber and suitable for enabling the pressure to increase up to a predetermined threshold in order to inform the pilot that the bumper has made contact with the ground, and then to enable the pressure to drop in order to absorb energy resulting from the contact of the bumper with the ground. 
         [0031]    Thus, when the rear end of the rotorcraft fitted with the bumper strikes the ground at a low impact speed, e.g. less than 0.5 meters per second (m/s), the outer shell deforms, with the air contained in said outer shell escaping through the throttling main orifice(s). This deformation thus gives rise to damping, thereby avoiding sudden contact of the rear end with the ground. Because the speed of impact is low, this damping suffices to absorb the energy that results therefrom. 
         [0032]    In parallel, the deformation of the outer shell causes the inner shell to be compressed. It can be understood that the volume of the outer shell becomes smaller on engaging the ground and as a result the volume of the inner shell is reduced in proportion. So long as the pressure of the air situated in the second chamber defined by the inner shell remains below a predetermined threshold, the management means allow the pressure that exists in the second chamber to rise. 
         [0033]    During the increase of pressure in the inner shell, the movement towards the ground of the rear end of the rotorcraft is progressively limited. As a result, the pilot can physically feel that the bumper has come into contact with the ground. 
         [0034]    It should be observed that this effect also occurs with skid devices presenting a relatively high level of stiffness. This is greatly appreciated by pilots, insofar as it enables them to know the position of the rotorcraft relative to the ground. 
         [0035]    However, unlike skid devices, the bumper of the invention reduces the bounce effect due to restoration of the energy that was absorbed while making contact with the ground. 
         [0036]    More precisely, at the end of making contact with the ground, the outer shell tends to return to its original shape during this redeployment stage, with the outer shell sucking in air from outside the bumper through its main orifice. This makes the suction progressive so the bounce effect is reduced. 
         [0037]    It can be understood that the bumper is reusable insofar as it returns to its original shape without significant damage. Furthermore, it should be observed that the bumper does not exert high levels of force at the points where it is fastened to the rear end. Since the contact force arises as a result of pressurizing the chambers, the transmission of this force to the structure of the rotorcraft is distributed over the full area of the interface. 
         [0038]    The bumper thus presents the advantages of a skid without suffering from its drawbacks. 
         [0039]    For an impact with the ground at a high speed, e.g. greater than 0.5 m/s, during an initial stage of making contact, the air contained in the first chamber escapes from the inner shell via its at least one main orifice, while the pressure of the air contained in the second chamber defined by the inner shell increases. 
         [0040]    When this air pressure situated in the second chamber defined by the inner shell reaches a predetermined threshold, a second stage begins. The management means then allow the air contained in the second chamber to escape from the second chamber to outside the bumper. This threshold enables the pressure to be limited and thus enables the force that is generated by the impact to be limited. 
         [0041]    As a result, the inner shell is deformed to a large extent and contributes actively to absorbing energy. 
         [0042]    The tail bumper of the invention is thus sufficiently stiff during a first stage for the pilot physically to feel that the bumper has made contact with the ground, and then sufficiently flexible during a second stage to absorb the energy that results from the bumper making contact with the ground at a relatively high speed. 
         [0043]    The invention also includes one or more of the following additional characteristics. 
         [0044]    In order to enhance redeployment, said outer shell is made of an elastomer material suitable for conserving its shape under the effect of air pressure in flight or on the ground, and that is intrinsically suitable for returning to its original shape after making contact with the ground. 
         [0045]    In order to ensure that the outer shell does not risk being deformed by the aerodynamic forces to which the outer shell is subjected in flight, it is possible to envisage mixing its elastomer with stiffener materials, e.g. glass fibers. Thus, the outer shell is given a desired shape on being molded, a shape that is preferably aerodynamic and suitable for co-operating with the shape of the rear end. After being flattened on making contact with the ground, the outer shell itself tends to return to its original given shape. 
         [0046]    Likewise, the inner shell is optionally also made of an elastomer material suitable for conserving its shape under the effect of air pressure in flight or on the ground, and that is intrinsically suitable for returning to its original shape after making contact with the ground. 
         [0047]    Advantageously, the management means comprise a pressure-relief valve that is rated at said predetermined threshold. 
         [0048]    Under such circumstances, below said predetermined threshold, the pressure-relief valve remains closed, holding captive the air that is inside the second chamber. Conversely, above said predetermined threshold, the pressure-relief valve opens and releases the air that previously filled the second chamber. 
         [0049]    Furthermore, in a first embodiment, the outer shell may be fastened directly to the rear end for protection. 
         [0050]    In contrast, in a preferred second embodiment, the bumper has a rigid outer plate to which the outline of said outer shell is bonded by conventional methods, e.g. by adhesive or by vulcanization. 
         [0051]    Similarly, a top wall of the inner shell, presenting an inner outline in the form of a lip, is bonded to an inner plate. The pressure-relief valve is then fastened to the inner plate to close at least one slot through said inner plate so long as the pressure that exists inside the second chamber defined by the inner shell remains below a predetermined threshold. 
         [0052]    The inner shell is then fastened to the outer shell so that the inner shell is arranged inside the outer shell, the pressure-relief valve passing through a main opening in the outer plate so as to open out to the outside of the bumper, i.e. to the outside of the first and second chambers. 
         [0053]    The inner and/or outer plates are rigid, being based on a metal or a composite material. 
         [0054]    Thus, the bumper is a completely self-contained element. It suffices to fasten the outer plate to the rear end using conventional means such as screws, rivets, etc., in order to put the bumper into place. It can also be removed easily and without any difficulty. 
         [0055]    In addition, a contact wall of the outer shell is suitable for facing the ground when the bumper is fastened to the rear end, and a bottom wall of the inner shell is in contact with the contact wall, and possibly secured to the contact wall. Any contact between the outer shell and the ground thus immediately has consequences for the inner shell. 
         [0056]    Optionally, the inner shell comprises a bottom wall and a top wall connected together by a side wall, and the bumper is provided with a spring surrounding the side wall of the inner shell. 
         [0057]    The spring participates actively in redeploying the outer shell and consequently in redeploying the inner shell. In addition, the spring then makes it possible firstly to control the deformation of the inner shell in a privileged direction by holding the inner shell within the spring. 
         [0058]    Consequently, the side wall of the inner shell may advantageously be in the form of a bellows that is folded concertina-like. 
         [0059]    In addition, in a first variant, the management means are provided with means for filling the second chamber, e.g. a pump. 
         [0060]    Thus, after contact has been made with the ground, the management means prevent air from escaping from the second chamber, with the pressure-relief valve closing as soon as the pressure of said air drops below the predetermined threshold. The filler means then fill said second chamber so that it returns to its original shape. 
         [0061]    In a preferred second variant, the management means includes at least one secondary orifice connecting the second chamber to the outside of the bumper, the secondary orifice being formed for example through a closure plate of the pressure-relief valve or through a region of the inner plate that faces the opening in the outer plate. 
         [0062]    In the second embodiment, this secondary orifice thus co-operates with the opening in the outer plate. 
         [0063]    Under such circumstances, said at least one secondary orifice is dimensioned by calculation or by testing so as to avoid preventing the pressure from increasing in the second chamber during the first stage. 
         [0064]    At the end of the second stage, the inner shell tends to return to its original shape, as a result of the characteristics of the material from which it is made and possibly with the assistance of a spring. During this redeployment stage, air penetrates naturally into the second chamber through said at least one secondary orifice. 
         [0065]    In addition to the bumper, the invention also provides a rotorcraft having a lift-providing rotary wing located between a nose and a rear end of the rotorcraft. The rotorcraft is then remarkable in that a bottom face of its rear end, that faces the ground when the rotorcraft is standing on the ground, is provided with a bumper as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0066]    The invention and its advantages appear in greater detail from the following description of embodiments given by way of illustration and with reference to the accompanying figures, in which: 
           [0067]      FIG. 1  is a side view of a rotorcraft provided with a bumper of one embodiment of the invention; 
           [0068]      FIG. 2  is a longitudinal section through a bumper in a preferred second embodiment; 
           [0069]      FIG. 3  is a cross-section through a bumper in the preferred second embodiment; and 
           [0070]      FIG. 4  is a plan view of a bumper in the preferred second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0071]    Elements present in more than one of the figures are given the same references in each of them. 
         [0072]    Three mutually orthogonal directions X, Y, and Z are shown in  FIGS. 1 and 2 . 
         [0073]    The direction X is referred to as the “longitudinal” direction insofar as it relates to the longitudinal dimension of the bumper extending in said longitudinal direction X. 
         [0074]    Another direction Y is referred to as the “transverse” direction insofar as it relates to a transverse dimension of the bumper extending said transverse direction. 
         [0075]    Finally, a third direction Z is referred to as an “elevation” direction, and it corresponds to the vertical dimensions of the structures described. 
         [0076]      FIG. 1  shows a rotorcraft extending longitudinally rear to front from a rear end  3  to a nose  2 . The rotorcraft  1  has a rotary wing  8 , i.e. a lift and propulsion rotor located between the rear end  3  and the nose  2 . 
         [0077]    The rear end  3  is provided with a tail boom  4  that is secured to the fuselage of the rotorcraft  1 , the tail boom  4  carrying a ducted tail rotor  5 , sometimes known under the trademark Fenestron®. 
         [0078]    In one embodiment, a tail bumper  10  of the invention is fastened to the bottom face  7  of the bottom fairing  6  of the faired rotor that faces the ground S when the rotorcraft is standing on the ground. 
         [0079]      FIG. 2  is a longitudinal section through the bumper  10 . 
         [0080]    The bumper  10  has an inner shell  30  made of an elastomer-based elastic material suitable for returning to its original shape after being deformed as a result of coming into contact with the ground S. 
         [0081]    It should be observed that the person skilled in the art knows how to make such an elastic material, which is similar to the material used for making tires, for example. 
         [0082]    The inner shell  30  has a bottom wall  31  from which there rises a side wall  33  in the form of a bellows, i.e. it is folded concertina-like. 
         [0083]    A bottom end of the side wall  33  is thus secured to the bottom wall  31 . Furthermore, a top end of the side wall  33  is extended by a top wall  32 , specifically by a lip that is fastened to the top end of the side wall  33 . 
         [0084]    Consequently, the inner shell  30  constitutes a container of elastic material defining an empty space therein, referred to as a second chamber  12 . 
         [0085]    The second chamber  12  is partially closed by an inner plate  34  secured to the top wall  32  of the inner shell  30  by adhesive or by vulcanization, for example. 
         [0086]    As explained below, the inner plate  34  has slots, and these slots can be closed by management means  50  for managing the pressure that exists in the second chamber  12 . 
         [0087]    Finally, the bumper  10  has a spring  40  that surrounds the inner shell  30 , with the turns of the spring  40  co-operating with the folds in the concertina-folded side wall  33 . 
         [0088]    In addition to an inner shell  30 , the bumper  10  has an outer shell  20  made of an elastomer-based elastic material that is suitable firstly for conserving its shape under the effect of air pressure, and secondly for returning to its original shape after being deformed by making contact with the ground S. It should be observed that the person skilled in the art know how to make such an elastic material, which is similar to the material used for making tires, for example. 
         [0089]    While the outer shell  20  is being molded, said outer shell  20  is advantageously given an aerodynamic shape for co-operating with the aerodynamic shape of the rear end  3  that is fitted with the bumper  10 . 
         [0090]    In a first embodiment, the outer shell  20  is secured to the bottom face  7  of the rear end that is to receive the bumper  10 . 
         [0091]    In the preferred second embodiment shown diagrammatically in  FIGS. 2 to 4 , the bumper  10  is completely independent of the rear end  3 . 
         [0092]    Under such circumstances, the inner plate  34  is fastened by conventional means such as screw fasteners, adhesive, or indeed rivets, to the underside of an outer plate  24  suitable for being fastened to the bottom face  7  of the rear end  3 . 
         [0093]    The outer periphery  22  of the outer shell  20  is secured, by adhesive or vulcanization, to the outer plate  24 , taking care to arrange the inner shell  30  inside the outer shell  20 . 
         [0094]    Thus, the outer shell constitutes a kind of resilient container that is closed by the outer plate  24 . The outer shell  20  then defines a first chamber  1  containing the inner shell  30 . 
         [0095]    Furthermore, the outer shell  20  is provided with a contact wall  23  facing the ground S and suitable for coming into contact with the ground during landing or takeoff, it being optionally possible to secure the bottom wall  31  of the inner shell  30  to said contact wall, e.g. by adhesive or even by bolts secured to the bottom wall  31  and co-operating with fastener orifices in the outer shell  20 . 
         [0096]    It should be observed that the purpose of the management means  50  is to release the air contained in the second chamber  12  to the outside of the bumper  10 . Consequently, the outer plate  24  is provided with a main opening  24 ′ enabling the management means to open to the outside EXT of the bumper  10 , specifically inside the bottom fairing  6 . 
         [0097]    Similarly, the bottom face  7  has a secondary orifice suitable for co-operating with said main opening  24 ′. The management means open to the outside EXT of the bumper  10 , and more specifically to the inside of the bottom fairing  6 . 
         [0098]    Finally, the outer shell  20  may be provided with at least one main orifice  21  connecting the first chamber  11  to the outside of the bumper  10 , such as two main orifices at the front of the outer shell  20  and one main orifice at the rear of said outer shell  20 , in the example shown. 
         [0099]    With reference to  FIGS. 3 and 4 , the inner plate  34  has two slots  34 ′ and  34 ″ opening out into the second chamber  12 . Under such circumstances, the inner plate  34  closes the second chamber defined by the inner shell  30  only partially. 
         [0100]    Furthermore, the inner shell  30  is fitted with management means  50  for managing the pressure that exists within the second chamber  12 . 
         [0101]    In a purely mechanical variant, the management means  50  is provided with a pressure-release valve having a base  62  secured to the inner plate  34  and a closure plate  61  that closes the slots  34 ′ and  34 ″ in the plate  34  when the pressure within the second chamber  12  is below a predetermined threshold. 
         [0102]    In addition, the management means  50  is provided with at least one secondary orifice  70 , specifically with two main orifices formed in the closure plate  61  of the pressure-release valve  60  in  FIGS. 3 and 4 . 
         [0103]    Nevertheless, it can be understood that the secondary orifices need essentially to open to the outside EXT of the bumper  10 . Under such circumstances, these secondary orifices may be arranged in the inner plate  34 , for example. 
         [0104]    Consequently, when the rear end  3  of the rotorcraft  1  approaches the ground at slow speed, the bumper  10  may be caused to impact against the ground S. 
         [0105]    As a result, the outer shell  20  deforms and expels the air it contains in its first chamber  11  through its main orifices  21 . This gives rise to a first absorption of the energy that results from the impact. 
         [0106]    In parallel, the inner shell  30  deforms. The top wall  32  of this inner shell  30  is secured to the rear end  3  via the inner plate  34  and the outer plate  24 , with the bottom wall  31  of the inner shell  30  approaching its top wall  32 . 
         [0107]    The deformation of the inner shell  30  is more controlled because of the presence of the optional spring  40 . 
         [0108]    The air present in the second chamber is expelled towards the inside of the bottom fairing  6  through the secondary orifices  70 . This results in second absorption of energy by the inner shell  30 . 
         [0109]    Nevertheless, these secondary orifices  70  present an area that does not enable all of the air to be expelled. Consequently, the pressure of the air contained in the second chamber  12  increases. This enables the pilot of the rotorcraft to have a better feeling of contact being made with the ground. 
         [0110]    Since the speed of impact is small, the first and second absorptions of energy suffice to damp the energy that results from the impact. 
         [0111]    After this impact, the contact wall  23  of the bumper is no longer in contact with the ground S. The inner and outer shells  20  and  30  then tend to return to their original shapes, possibly being assisted by the spring  40  and by the stiffness of their walls. 
         [0112]    While it is redeploying, the outer shell  20  sucks in outside air through its main orifices  21  in order to fill the first chamber  11 . Similarly, the inner shell  30  sucks in outside air through its secondary orifices  70  in order to fill the second chamber  12 . 
         [0113]    When the bumper  10  strikes the ground S at a higher speed, e.g. greater than 0.5 m/s, the outer shell  20  deforms and expels the air it contains in its first chamber  11  via its main orifices  21 . This provides first absorption of the energy that results from the impact. 
         [0114]    In parallel, the inner shell  30  deforms, the bottom wall  31  of the inner shell  30  approaching its top wall  32 . The air present in the second chamber is expelled towards the inside of the bottom fairing  6  through the secondary orifices  70 . This results in second absorption of energy by the inner shell  30 . 
         [0115]    Nevertheless, these secondary orifices  70  present an area that does not enable them to evaporate all of the air. 
         [0116]    Consequently, during a first stage, the pressure of the air contained in the second chamber  12  increases. Contact with the ground is then felt by the pilot of the rotorcraft. 
         [0117]    When said pressure reaches a predetermined threshold, a second stage begins. The pressure-release valve  60  opens to enable the air contained in the second chamber  12  to be exhausted via the slots  34 ′,  34 ″. This results in third absorption of energy of sufficient magnitude. 
         [0118]    After the impact, the contact wall  23  of the bumper is no longer in contact with the ground S. The inner and outer shells  20  and  30  tend to return to their original shape, possibly being assisted by the spring  40 . 
         [0119]    While it is redeploying, the outer shell  20  sucks in outside air through its main orifices  21  in order to fill the first chamber  11 . 
         [0120]    In parallel, the pressure that exists in the second chamber drops below said predetermined threshold. The pressure-release valve closes and closes the slots  34 ′,  34 ″ in the inner plate  34 . The inner shell  30  then sucks in outside air through its secondary orifices  70  in order to fill the second chamber  12 . 
         [0121]    Finally, it should be observed that provision may be made for the outer shell  20  to carry a proximity sensor suitable for informing the pilot that the bumper is at a distance that is less than some given minimum distance from the ground S, e.g. by triggering an audible or visible alarm. 
         [0122]    Naturally, the present invention is capable of being subjected to numerous variations as to its implementation. Although several embodiments are described, it will readily be understood that it is not conceivable to identify exhaustively all possible embodiments. It is naturally possible to replace any of the means described by equivalent means without going beyond the ambit of the present invention. 
         [0123]    For example, the bumper  10  is arranged on the bottom fairing  6  of the faired rotor  5  at the rear end. Nevertheless, it is entirely possible for the bumper to be placed on a bottom face of the tail boom  4 .