Landing Gear Concept and Dynamic Landing Loads of the Unmanned Space Re-entry Vehicle USV3

Abstract

This paper presents the methodology to perform a conceptual structural design of the landing gears and the dynamic landing loads for the Unmanned Space re-entry Vehicle USV3, currently under development at CIRA. The landing gear preliminary design is usually carried-out by responding to requirements of maximum overall dimensions, maximum weight and maximum attainable load factor. The oleo-pneumatic mechanism is conceived with the aim of maximizing the oleo-pneumatic efficiency, since it is critically depending on the maximum vertical load factor and on the strut length, whereas the structure is generally sized by combining the design vertical loads with the maximum expected horizontal loads. The horizontal drag component simulating the force required to accelerate the tires and wheels up to the landing speed (spin-up) and the forward acting horizontal load resulting from rapid reduction of the spin-up drag loads (spring-back) must be evaluated. The dynamic model of the landing gear system, fully nonlinear and taking into account the structural flexibility has been conceived. In particular, the dynamic model is made of subsystems based on statistical evaluations and/or semi-empirical equations; these subsystems should be easy manageable, making the whole system simple to analyze for different values of the project parameters. The model is used for material selection and preliminary sizing of components through dynamic analyses. At the same time, maximum expected horizontal loads are evaluated in an iterative manner, the entire process leads to the conceptual design of the landing gear. A method to carry-out the dynamic landing simulations by integration of the vehicle equations with those of each undercarriage (pseudo-elastic, fully nonlinear dynamic equations) has been defined and applied to the USV3 vehicle. The coupling between the vehicle and the landing gears has been performed by using a kinematic approach, i.e. the aircraft is considered rigid. The developed methodology is entirely encoded in the MATLAB/Simulink environment, and linked to excel sheets for the input and output data management. Dynamic landing simulations on two and three points have been performed for evaluating the maximum responses of spin-up and spring back loads in a rational manner. A sensitivity analysis involving landing horizontal and vertical speed, and tire inflation pressure completes the whole work. A comparison of resulting horizontal loads with those obtained by using the Appendix C and D of STANAG 4671 (USAR) shows the convenience to use the rational approach presented herein. In fact the attained lower structural loads may lead to potential weight and cost reduction from the earliest phases of design.