Study of orifice characteristics of undercarriage

Abstract

The goal of this research is to develop a simulation technique, to estimate the total time required for complete percolation of oil from upper chamber to lower chamber through orifice and vice versa for nitrogen with the help of CFD technique. To create a damper (multi-phase) model to predict accurately the time required for the air / nitrogen percolating to upper chamber from lower chamber during flight after extension of undercarriage. This study pertains to oleo pneumatic damper used in aircraft undercarriage applications. newlineThe aircraft undercarriages are designed to perform the same way in all possible landing conditions like normal or hard landing, during taxiing on a normal runway or rough runway (unprepared or unevenness in surface) resulting in unsatisfactory oscillations leading discomfort to passenger and crew, with maximum or minimum aircraft payload, experienced or new pilot, or combination of some or all factors, etc. The major function of the undercarriage is to absorb the vertical loads (kinetic energy) of the airplane at the time of touch down, with no / minimum (slow) rebound and dissipate to the aircraft structure (and occupants) to the tolerable levels. Its secondary objective is to provide comfort to humans in the aircraft during taxiing. A shock strut internal (damper chamber) contains orifice and a variable controlled metering pin cross-section (varying cross sectional area) is capable of reducing fuselage vibrations effectively (fuselage is very long, slender and is highly sensitive to even small external disturbances or vibrations). newlineExtensive research has been carried out by the aircraft OEMs to develop an understanding of the shock strut behaviors like internal pressure, telescoping velocity, stroke, time, etc., on a single stage oleo pneumatic shock strut to determine the characteristics of the orifice at the time of touch down using testing. The relationship between internal strut pressure, overall loads developed by the struts and time for compression were available in public domain. Literature survey shows that not much information is available on the study carried out on the nitrogen percolation to lower chamber of undercarriage during flight, whereas lot of research on water percolation in soil and plant stems has been studied by various research scholars. newlineix newlineThe simple oleo pneumatic (shock absorber) model will be built to characterize the percolation phenomena in the strut, for a better understanding of quantity of nitrogen is chocked in the lower chamber during aircraft touch down, the parameters to be studied are flow of oil and air / nitrogen through orifice along with time. The model will include oil height, Orifice parameters (orifice thickness, orifice diameter, number of orifice, inlet chamfer, and various fluid). The time would be obtained by solving the CFD models. newlineThis problem to be studied is a single stage oleo pneumatic shock strut without a separator having oil flowing from top chamber to bottom chamber during flight and vice versa after the extension of undercarriage before landing which has fixed boundaries, multi-physics problem (having oil (incompressible fluid) and gas medium (compressible fluid)) using Volume of fluid technique, which is very effective in determining the free surface phenomenon. In this work, an implicit unsteady and k-and#949; turbulence model was developed at the ambient conditions. Investigations and analytical model development were performed for air percolating to upper chamber through the orifices. Experiments were conducted in laboratory for different configuration orifice. The simple representative model was developed using two bottles (represents chambers) and the results are validated / compared with test result. The time required for the air percolation from lower chamber to upper chamber with respect to various conditions and its influence with respect to time was studied. Comparison of analytical model and experiments were presented and the overall match between the simulation and experimental result was in a good agreement. The methodology can be adopted in real life to reduce the development time and cost of the system by designing the orifice and the undercarriage shock strut internals to meet the damping characters requirements. newline