In this paper an automatic procedure aimed at preliminary designing an oleo-pneumatic landing gear strut for a light unmanned aircraft is presented.The whole work is motivated by the necessity to design the landing gear of the unmanned aircraft X-MALE, currently in development at the Italian Aerospace Research Center (CIRA SCpA), according to the Airworthiness Requirements NATO STANAG 4671 (USAR).The landing gear preliminary design is usually carried-out by responding to requirements of maximum overall dimensions, maximum weight and maximum attainable load factor. While the oleo-pneumatic mechanism depends essentially on the maximum vertical load factor and on the strut length, thus on the oleo-pneumatic efficiency, 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 tyres 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) have to be calculated.Differently from other Airworthiness Requirements for piloted aircraft, USAR.479, “Level Landing” imposes that, in the absence of specific experimental tests, the determination of spin-up and spring-back maximum loads must be carried-out by means of more rational analysis.The procedure is made of the following steps: first static sizing of the strut structure under maximum expected vertical loads, selection of oleo-pneumatic chamber architecture and choice of minimum operating pressure and pressure ratios, run of dynamic landing for orifice sizing and calculation of maximum spin-up and spring-back loads, re-sizing of the strut structure under combined vertical and calculated horizontal loads.The horizontal loads due to a dynamic landing depend on the stiffness and inertia distribution along the landing gear strut. Since these distributions are modified by the re-sizing following the dynamic landing analysis, the final solution (in terms of piston and cylinder wall thicknesses and orifice diameter) is found in an iterative manner.Calculations performed for the X-MALE landing gear conceptual design show that the convergence to a final solution respecting all given requirements is reached in few iterations. The method presented herein can be used also to simulate and set-up the dynamic drop test of an existing oleo-pneumatic landing gear.