A Finite Element Approach to Modal Parameter Estimation of Vertical Tail Fin

Abstract

The vertical stabilizers or fin of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip and provide direction stability. Vertical tails designed for aircrafts suffer from reduced stability and control effectiveness at high dynamic pressures due to aeroelastic effects. Adequate tail performance requires a detailed study on dynamic performance before investigating aeroelastic effects. Experimental modal analysis is a very extensive, time consuming and expensive process while finite element analysis is quick, accurate, fast and economical. Hence, in the present work it is purposed to carry out a detailed finite element simulation of vertical tail fin for a UAV to evaluate modal parameters such as natural frequencies and its corresponding modal shapes. CAD modeling of the vertical tail fin was done in UG NX using spline and swept commands. The vertical fin is sandwiched with polyurethene foam of density 70kg/m 3 bonded with thin Aluminum sheet of 1mm thickness as face sheets. Similarly, biwoven e-glass fiber was considered as face sheets along with PU foam as core. The meshing of model was carried out using a 10 noded solid tetrahedral element. Boundary conditions were simulated and dynamic analysis was performed to evaluate natural frequencies and corresponding modal shapes. Keywords: Modal Analysis, Natural Frequencies, Mode Shapes, Vertical Tail, FEM I. Introduction For modern UAV's, the ability to fly and maneuver at high angles of attack and at high loading conditions gives tactical advantages. For UAV's the maneuverability at very high angles of attack is achieved through a combination of the wing root leading edge extensions (LEXs) and the placement of vertical tails. A pair of large vortices generated by highly swept wing root LEXs contribute to enhanced vortex lift by developing high suction areas over the wings. The UAVs are designed to utilize unique characteristics of these vortex structures which are prime contributors to the aerodynamics of an aircraft during high angle of attack flight. The vertical tails of the UAV's are placed in closer proximity to the LEX vortex flow field to take full advantage of the concentrated energy contained in the LEX vortices to provide the directional stability and control necessary for high angle of attack maneuverability. Large buffet loads and associated fatigue damage have been observed on the tails of such aircrafts during certain flight conditions, including rapid maneuvering at subsonic Mach numbers and dynamic pressures considerably less than the maximum allowable values (1, 2).The interactions of a flexible structure with the aerodynamic forces acting on it are severe enough to influence the structural and aerodynamic design. The dynamic and aeroelastic analysis of an aircraft with main reference to its lifting and control surfaces is an essential aspect for finalization of the design cycle and is also required for obtaining flight clearance and certification of the aircraft. Correlation of the dynamic characteristics of the aircraft obtained from analysis with the Ground Vibration Test results of the aircraft was studied (3). The airframes of high performance aircraft, have suffered from an aeroelastic tail buffet problem for many years. This problem is inherent to vertical flows used to generate lift at high angles of attack as they tend to break down causing severe empennage dynamic loading and premature fatigue failures. Better understanding of the empennage buffeting problem is required for development of reliable fatigue usage monitoring systems and for the fleet management of aircraft. The challenges associated with computational simulation of empennage buffet vary from prediction of the nonlinear separated vortical flows about complex configurations to the coupled interaction between the flow and the dynamic response of the tail structure (4).Vibration analysis of an aircraft component can characterize the structural dynamics, determine the fundamental frequencies and define the complete modal data of the component. From this data, engineers can objectively evaluate their concerns about the impact of vibrations on adjacent aircraft components. These concerns have very real consequences since excessive vibrations can lead to premature component fatigue and failure (5, 6). Vertical tail fin is a critical component that must meet very high quality standards. Vertical tails designed for aircrafts suffer from reduced stability and control effectiveness at high dynamic pressures due to aeroelastic effects. Therefore, adequate tail performance requires a detailed study of dynamic performance before investigating aeroelastic effects (7, 8). Also, modal analysis is not only useful in its own right, but it also provides the basis for a number of further dynamics analyses. To this end, modal test plays an important role in the certification process of any new or