STUDY OF RUNNING ENGINES INERTIAL AND GYROSCOPIC PROPERTIES INFLUENCE ON THE DYNAMIC SYSTEM ENGINE – PYLON – WING STRUCTURAL CAPABILITIES

A modern large-sized aircraft dynamic properties analysis, determined by the specificity of its layout scheme, demonstrates that the engines on under the wing elastic pylons lightly damped oscillations cause a number of undesirable phenomena, including intense accumulation of fatigue damage of the pylon-to-the-wing attachment, in fact in the area of engine installation in the pylon and the wing. The results of theoretical and experimental research show that with some engine attachment to the pylon structural modification it becomes possible to use the engines inertial and gyroscopic properties to absorb these oscillations. In this case, the motor tones damping coefficients increase by an order of magnitude or even more, so the gyroscopic coupling of elastic vibration tones is realized. With the rational choice of the additional parameters of elastic and dissipative bonds in the engine attachments it is possible to affect the aircraft wing and engines aero elastic vibrations effectively, which has a significant effect on the aircraft elements structural capabilities. A mathematical model of aero elasticity (MMAE) with respect to the kinetic moment of the engine rotors and specially designed units for attaching the engines to the pylons was developed in order to study the influence and the selection of rational elastic-dissipative parameters of the pylons-under-the-wing aircraft engine mounts. The method of predetermined basic forms is used for the aircraft with running engines on the pylons MMAE synthesis. The given forms are considered as the aircraft basic structure forms natural vibrations in the void. This work treats the engine nacelle and the rotor as absolutely rigid bodies, the elasticity of the rotor to the nacelle attachment is neglected. The pylon is modeled by an elastic beam, and the elastic and dissipative properties of the pylon-to-the-wing and the engine-to-the-pylon attachments are correspondingly by elastic-dissipative bonds. Schematic diagrams of the engine to the pylon attachments are proposed. The results of the study devoted to the influence of the proposed attachment points modifications on the load and integral strength characteristics of the main structural elements of the engine – pylon – wing dynamic system on the example of an An-124 aircraft are presented. The practical implementation of the proposed solutions aimed to reduce the level of fatigue damage to structural elements of the aircraft feasibility is proved.


INTRODUCTION
One of the main features of a modern aircraft layout is the location of engines on elastic pylons under the wing. With obvious design and operational advantages of such a layout, an actual problem arises: engines on elastic pylons lightly damped oscillations. This phenomenon is conditioned by the fact that the oscillations energy dissipation in the dynamic engine -pylon -wing system is only performed by internal and structural damping. Engines on elastic pylons oscillations lead to the elements of the wing and pylon structure fatigue damage accumulation and to the aero elastic dynamic instability of the "pylon" flutter type occurrence as well as to the worsen the crew and equipment operation performance.
At the same time, the results of theoretical and experimental studies [1][2][3][4][5][6][7][8][9] show that certain engine to pylons attachment points design modifications, aimed at some engine and pylon relative displacement provision, can significantly increase the dissipative properties of the engine -pylon -wing dynamic system. In this case, special elastic elements and damping devices are introduced into the structure, and the running engine can be used as an inertial and gyroscopic oscillations dampener [10][11][12][13][14][15]. Research shows that with rational (tuning) values of the engine to the pylon attachment elastic dissipative parameters, it is possible to influence the aero elastic oscillations of the engine -py-lon -wing dynamic system effectively. At the same time, the damping coefficients of motor tones increase by an order of magnitude or more.
This paper studies some design schemes of engine to pylon attachment points and the influence of the proposed modifications on the aero elastic oscillations in the engine -pylon -wing dynamic system is analyzed. Mathematical model of an aircraft with operative elastic pylons engines, which is developed by the authors, is used for analytical research [1]. The results of the study devoted to the influence of the proposed attachment points modifications on the load and integral strength characteristics of the main structural elements of the engine -pylon -wing dynamic system on the example of an An-124 aircraft are presented. The practical implementation of the proposed solutions aimed to reduce the level of fatigue damage to structural elements of the aircraft feasibility is proved.

MATHEMATICAL MODEL OF THE AIRCRAFT AERO ELASTICITY WITH THE ENGINES RUNNING ON ELASTIC PYLONS
The development of a mathematical model of aero elasticity (MMAE) is an important stage in the study of the engine attachment points to elastic pylons under-the-wing structural modifications influence on the aircraft structural elements strength characteristics. The method of predetermined basic forms is used for the aircraft with running engines on the pylons MMAE synthesis [1,[16][17][18][19][20]. In accordance with this method, the deformations of structures in perturbed motion take the form of expansion in a series of known coordinate vector functions (forms). The given forms are considered to be the forms of the aircraft basic structure natural oscillations in the void. This work treats the engine nacelle and the rotor as absolutely rigid bodies, the elasticity of the rotor to the nacelle attachment is neglected. The pylon is modeled by an elastic beam, and the elastic and dissipative properties of the pylon-to-the-wing and the engine-to-the-pylon attachments are correspondingly by elasticdissipative bonds. The engine has a kinetic moment the influence of the engine thrust dynamic components is neglected. Thus, in [3] it is shown that the influence of dynamic thrust components on the natural frequencies and forms of oscillations, as well as on the flutter characteristics of the aircraft, is insignificant and can be ignored. When calculating aerodynamic characteristics, the engine is schematized as a set of vertical and horizontal thin bearing surfaces modeled by a family of attached and free vortices.
We consider an elastic plane that performs a steady horizontal flight at the speed of  V in its initial state. Having the set-up inertia-mass engine parameters, the pylon geometric characteristics, the rotor kinetic moment , the engine-to-the-pylon attachment points elastic-dissipative parameters, using the Lagrange equations, it is possible to form a perturbed motion equation of an elastic aircraft with the engine running on the pylon as shown in [1]. In the matrix form the mathematical model of an aircraft aero elasticity (MMAE) has the following form:   Let's assume that all basic structure parameters are known, and the matrix of internal and structural damping coefficients is set. Let the coordinate system be connected to the engine (Fig. 1). The following designations in Figure 1 and Figure 2 are used: 1-beam model of the wing console; 2 -beam model of the pylon; 3 -engine; 4 -additionally introduced elastic elements of the engine attachment points to the pylon design; 5 -additionally introduced dissipative elements of the engine attachment points to the pylon design (for example, hydraulic dampers). The engine generally has six degrees of freedom of relative displacements, which are set by the vector  d q . At the same time are -the coordinates describing the engine linear displacement due to its attachment points to the pylon malleability; 6 5 4 , , are -the coordinates of relative angular deviation; -is the partial coefficients of stiffness and damping of the corresponding elastic and dissipative elements.
The research has shown that the maximum effect of damping the motor tones vibrations with minimal modifications of the original engine to the pylon attachment scheme is achieved by ensuring

RESEARCH RESULTS OF THE RUNNING ENGINES INFLUENCE ON THE AIRCRAFT STRUCTURAL CAPABILITIES
The aircraft with modified engine attachment points aero elastic and structural capabilities mathematical modeling process consisted of two stages. At the first stage, the rational (set-up) elasticdissipative parameters of the engine attachment points to the pylons for a given suspension variant were determined ( fig. 1 or fig. 2). The mathematical model of aero elasticity created in accordance with the method of specified basic forms was used. Dynamic reaction of the large-sized plane of the An-124 type to external disturbance and control influences was investigated in particular. The analysis of the obtained data shows that the engine with modified attachment points has a significant impact on the intensity and repeatability of dynamic loads acting on the pylon and wing of the aircraft. Thus, Figure 3 shows the normalized oscillogram of the bending moment increments in the root section of the external engine pylon   The rational choice of the engine suspension elastic-dissipative parameters allows you to reduce significantly the amplitude of its oscillations, primarily in the area of torsion and engine elastic tones. As an example, Figure 5  It is obvious that decrease in level and repeatability of the dynamic loads which effects the engine -pylon -wing system leads to the reduction of fatigue damageability of the design elements and to the increase of their resource. An important quantitative characteristic of the aircraft structure loading capacity is the spectral density of the bending moment in the calculated cross sections.  The obtained spectral characteristics of changes in the integral force factors in the cross sections of the aircraft structure elements were used to quantify the resource in accordance with the hypothesis of spectral summation of fatigue damage. In particular, some impact assessment results of the proposed improvements of the engine-to-the-pylon attachment points, strength characteristics of the large An-124 type aircraft main structural wing elements are shown in Figure 7 in the form of the graphs showing the resource increase coefficient changes for the lower N T and upper V T panels at the external engine attachment point. The solid line shows the released engine dependence relatively the vertical axis (diagram fig. 1), the dashed line shows the released engine dependence relatively the horizontal axis (diagram fig. 2). The dashed line corresponds to the l value of the production An-124 aircraft. All the mentioned above data are obtained with taking into account the engine rotor kinetic moment influence.