Abstract:
Arrangement for mounting a propulsive system on an aircraft airframe  12  by means of an external strut  4  which connects the engine  1  with the airframe  12,  said strut  4  comprising a plurality of spars  8  intruding into the airframe  12  and means  7, 9, 11  for attaching said spars  8  to the internal airframe structure  10,  the cited means  7, 9, 11  being dampers, shock absorbers or active actuators with characteristics chosen so that the said means  7, 9, 11  are particularly suited for the efficient reduction of attachment and internal dynamic loads arising mainly but not only from hard landing events and aircraft flight manoeuvres, and for damping sustained vibrations of the propulsive system-airframe  12  assembly, the said arrangement allowing the easy removal of the strut  4  from the rest of the said airframe  12  and the interchangeability of the said strut  4  between different aircrafts, being also the cited arrangement tolerant to damage or complete failure of at least one of the said spars  8.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention discloses an arrangement for mounting an aircraft engine support structure on the airframe of said aircraft. 
       BACKGROUND OF THE INVENTION 
       [0002]    Modern aircraft propulsive systems tend to have increasingly large by-pass ratios in order to achieve the high energetic efficiencies desirable for their operation. The said aircraft propulsive systems may be of the cowled type (like turbofans) or uncowled (including systems having propellers and those variously referred to as unducted fan engines (UDF), open rotor engines, prop-fans or high speed turbo-prop engines). 
         [0003]    All the modern high by-pass engines used in the aircraft industry have a characteristically large diameter which imposes geometric constraints to their installation on the airframe, particularly when the propulsive system is mounted on the fuselage as a certain clearance must be maintained between the airframe and the engine cowl or open rotor blade tips, depending on whether the propulsive system is cowled or uncowled. In the case of fuselage mounted engines, aerodynamic considerations favour a concept of installation consisting on a single slender strut or pylon extending between the airframe and the engine. Examples of this engine mounting arrangement can be seen in patents U.S. Pat. No. 2,863,620 and U.S. Pat. No. 4,953,812, presenting specific design problems particularly in respect of the large static and dynamic loads that arise at the attachment of the strut to the airframe. 
         [0004]    There exist solutions in the state of the art which have the problem of presenting high structural discontinuities or apertures in the airframe, like it is described for example in document U.S. Pat. No. 5,065,959, which discloses a configuration of branch or yoke that does not allow the dismounting of the engine support structure or that needs big cuts in the airframe for allowing the dismounting of the structure. The possibility of dismounting the engine support structure is highly desirable for allowing the easy change of said support structure in the case of damage produced for example by engine FIRE or by the accidental impact of objects onto the support structure. 
         [0005]    The present invention comes to solve the above-mentioned drawbacks. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention describes a structural arrangement which provides means for mounting an engine to an aircraft which reduce the internal loads and the magnitude of the dynamic response of the airframe due to continuous engine and rotor vibration, aircraft manoeuvres and dynamic load cases. The said structural arrangement comprises at least one strut, or pylon, between the engine and the airframe. The said strut includes a plurality of spars, at least three in a fail safe configuration, which are attached to the internal structure of the airframe by means of a plurality of damping elements. 
         [0007]    The present invention provides also means to attach an engine to an airframe in such a way that the mounting strut can be easily removed from the airframe and at the same time reduces the dynamic response of the assembly and the static loads at the attachments due to the favourable geometry of the arrangement. 
         [0008]    Other features and advantages of the present invention will be disclosed in the following detailed description of an illustrative embodiment of its object in relation to the attached figures. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows a perspective schematic view of a rear fuselage mounted open rotor propulsive system illustrating the mounting arrangement according to the present invention. 
           [0010]      FIG. 2  shows a detailed side sectional view of the structural mounting arrangement on the aircraft airframe according to the present invention. 
           [0011]      FIG. 3  shows a detailed sectional front view of the attachment of one of the strut spars to the internal airframe structure according to the mounting arrangement of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The installation of modern high by-pass engines, cowled or uncowled, on aircraft requires consideration of the large loads introduced in the airframe due to the dynamic response of the engine-pylon-airframe assembly to normal and emergency flight conditions. Consideration must also be given to the level of sustained vibration in the cabin during normal flight and also in such conditions as those produced in and after the event of losing one or more rotor blades in flight. The problems caused by the dynamic response can be particularly severe in the case of high by-pass engines due to the large centrifugal forces generated by the unbalanced rotation of the relatively heavy rotor blades at high speed. The rotor unbalance may be caused by damages in the blades by the impact of small debris during the normal operation of the aircraft or, in the extreme case, by the loss of one or more rotor blades. Moreover, the large diameter of the fan or open rotor of modern high by-pass engines requires the use of long struts, or pylons, for their mounting in the airframe which due to their own flexibility and the flexibility of the attachment result in the engine mounting assembly having resonant frequencies similar to those of the airframe. The effect of this similarity of natural frequencies is a possible and undesirable amplification of the dynamic loads due to normal flight manoeuvres, aircraft response to atmospheric turbulence or to dynamic landing loads, among other dynamic flight conditions. 
         [0013]    The physical reason for the large magnitude of the static loads at the interface is that the bending moment produced by the mainly vertical forces at the engine centre of gravity must be balanced by a tensile force on one side of the strut and a corresponding compressive force at the opposite side. The magnitude of the said force couple is equal to the magnitude of the vertical force at the engine centre of gravity times the distance between the engine and the strut attachment to the fuselage divided by the effective distance between the load bearing elements on the upper and lower sides of the strut, or strut depth. In a typical application, considering a propulsive system weight of 5000 Kg, a distance between the engine C.G. and the side of the airframe of 2500 mm and a strut depth of 500 mm, the magnitude of the force arising at each side of the strut at the attachment point for a vertical acceleration of the airframe of 9 gs, this being one of the load cases corresponding to a dynamic landing which analysis is required by the aircraft certification authority, results after calculation in 2205000 Newton at each side of the strut. 
         [0014]    The obtained loads are substantial and require careful design of the strut attachment and supporting fuselage internal structure in order to satisfy the static strength requirements with minimal weight and while satisfying any other design requirements, particularly the possibility of removing the strut from the airframe and the tolerance to large structural damage to the strut or its attachment to the airframe. 
         [0015]    The calculation above does not consider dynamic effects, which in the case of heavy engines with large diameter rotors and mounted on long struts pose a particular problem due to the potentially large response to transient excitations such as those produced by dynamic landings, flight manoeuvres and discrete atmospheric turbulence, which may increase the statically calculated loads by a large factor. 
         [0016]    Also of concern in the presented engine installation concept is the response of the airframe and propulsive system assembly to sustained vibration produced by the inertial or aerodynamic imbalance of the rotating elements of the propulsive system. 
         [0017]    On consideration of the transient response, it can be shown by simple analysis that for a typical open rotor engine mounted on a rear fuselage as described above, where the strut attachment to the airframe is assumed to be a continuous structure and the airframe and strut are designed using good aircraft design practise, the first natural frequencies of the vertical oscillation of the engine-pylon assembly are in the range from 10 Hz to 20 Hz. Existing transport aircraft carrying 100 to 200 passengers have natural frequencies of their first airframe vibration modes in the range from 2 Hz to 15 Hz. Therefore it can be seen that if certain modes of the airframe are excited which corresponding natural frequencies are similar to the frequency of vertical oscillation of the engine-pylon assembly, a large dynamic response of the propulsive system can be expected resulting in large attachment loads. As an illustrative measure of this effect it can be said that a well known result of the theory of the dynamics of systems of one degree of freedom (DoF) is that if the frequency of excitation is √2*fn or fn/√2, being fn the natural frequency of a lightly damped 1 dof system, the amplitude of the response is 2 times the amplitude of the excitation. Considering the above mentioned ranges of airframe and engine-pylon natural frequencies it is clear that dynamic amplification effects may increase very substantially the loads calculated above for the airframe-strut interface for a dynamic landing case in which calculation dynamic response effects were not included. 
         [0018]    Of high importance for the overall merit of the design of the airframe is its dynamic response, and that of the propulsive system installation, to sustained vibration induced by the rotating parts of the propulsive system. If the level of vibration is not very small it will be felt by the passengers and will cause discomfort. Moderate levels of sustained vibration may cause failure of aircraft systems and material fatigue in the metallic parts. 
         [0019]    At high levels of vibrations such as those produced after the loss of one or more blades of the engine fan or open rotor, the transient dynamic loads may produce immediate structural failure. High levels of sustained vibration may produce structural failure by fatigue and/or failures of the aircraft control systems and/or even impair the pilots ability to read the instruments in the cockpit and operate the aircraft. 
         [0020]    Low to moderate levels of vibration of the propulsive system may have their origin in small damages to the fan or open rotor blades caused by the input of small debris or other accidental damage during the operational life of the aircraft. The said blade damages may cause vibration due to the unbalanced centrifugal forces if there is a loss of blade mass or due to unbalanced aerodynamic forces caused by shape distortion of one or more blades. 
         [0021]    Therefore, the present invention consists of a structural assembly configuration of the supporting structure  4  of a high bypass ratio engine  1  or an open rotor  2  engine on the fuselage  12  of an aircraft. The supporting structure  4  of the engine  1  comprises a closed structure part  6  outside the fuselage  12  and two or more essentially straight stringers  8  penetrating the fuselage  12  approximately perpendicular to its outer surface, connecting with the internal structure  10  of said fuselage  12  through several viscous dampers, elastic supports or actuators. Each of the stringers  8  penetrating the fuselage  12  is supported on the inner structure  10  of the fuselage  12  at two points  11 ,  9  arranged essentially on the longitudinal axis of said stringer  8 , these points  11 ,  9  on the same stringer  8  being spaced in the mentioned stringer  8  such that they form an effective support with a large lever arm to react due to the mechanical moments produced by gravitational and inertial forces acting on the centre of gravity of the engine  1 . One or more connecting members  7  additionally connect the closed supporting structure  6  of the engine  1  to the fuselage  12  such that they transmit the forces in the longitudinal direction of the airplane. 
         [0022]    According to the configuration of the present invention, the connecting members  7 ,  9  and  11  are thus clustered in members  9  connecting the end of the stringers  8  of the closed supporting structure  6  of the engine  1  with the internal structure  10  of the fuselage  12 ; which members  11  connect the mentioned stringers  8  with the closed supporting structure  6  of the engine  1  at points of the mentioned stringers  8  closest to the engine  1 ; and which members  7  connect the closed supporting structure  6  of the engine  1  to the fuselage  12  such that they transmit the forces in the longitudinal direction of the airplane. Connecting members  9  and  11  transmit forces essentially perpendicular to the longitudinal axis of the stringers  8 , at least one of the two groups of connecting members  9  and  11  further transmitting forces essentially focused along the longitudinal axis of said stringers  8 . The described arrangement of the connecting members  7 ,  9  and  11  according to the present invention is effective in restricting all the degrees of freedom of the engine  1  in relation to the airplane. The connecting members  7 ,  9  and  11  can be adjusted such that in normal conditions only a sufficient number of them transmit forces, such that the connection of the closed supporting structure  6  of the engine  1  to the fuselage  12  is statically defined, the rest of the connecting members transmitting forces only in the event of structural failure or breakage of other connecting members. It is also possible that several of the connecting members  7 ,  9  and  11  redundantly transmit forces in normal conditions, which requires that the geometry of the mentioned connecting members connecting the stringers  8  to the internal structure  10  of the fuselage  12  can be adjusted. 
         [0023]    The connecting members  7 ,  9  and  11  can be different types of those commonly used in aeronautics, formed by elastomers, wire mesh, viscoelastic type elements, electrohydraulic elements or any other type of element that allows transmitting the forces required between the stringers  8  and the internal structure  10  of the fuselage  12 , these members  7 ,  9  and  11  having the desired rigidity and dampening. 
         [0024]    The connecting members  7 ,  9  and  11  can also be active insofar as their rigidity, dampening or elongation can be changed by an automatic control system regulating these mentioned features to obtain the desired dynamic response in every flight or landing condition. 
         [0025]    The use of at least three stringers  8  in the supporting structure  4  of the engine  1  makes the assembly concept redundant from a structural point of view, which is a certification requirement for airplanes, particularly in this case in which the supporting structure  4  of the engine  1 , comprising the closed structure  6  and the stringers  8 , and or the inner structure  10  and/or the connecting members  9 ,  11  can be damaged by fragments of the engine  1  in the event the latter should explode. 
         [0026]    One of the main added advantages of the configuration object of the present invention is furthermore the fact that it can be easily disassembled from outside the airplane without needing to have large structural gaps or openings in the fuselage  12 . The possibility of being able to disassemble the supporting structure  4  of the engine  1  is highly desired so as to be able to easily change said supporting structure  4  in the event of damage caused, for example, by a fire in the engine  1  or by the accidental impact of objects on the mentioned supporting structure  4 . 
         [0027]    The configuration object of the present invention further has a geometry that allows spacing out the supporting points  9 ,  11  of the stringers  8 , as can be seen in  FIG. 3 , much more than in other known configurations without compromising the disassembly capacity, the rigidity of the supporting structure  4  of the engine  1  and preventing concentrations of stress in the stringers  8  since there are no changes in the direction of the forces inside the stringers  8 , while further allowing the reduction of the inner loads in the fuselage  12  transmitted by the supporting structure  4  of the engine  1 . 
         [0028]    Any modifications comprised within the scope defined by the following claims can be introduced in the preferred embodiment just described.