Methodology for the Validation of Loads in Rational Turning Analysis
A methodology for the validation of the turning loads is needed when the aircraft has a non- conventional landing gear configuration. This is the case for the A400M aircraft in which the main landing gear (LH and RH) consists of three independent articulated-type struts positioned along the aircraft longitudinal axis. As the legs are positioned along the aircraft in the longitudinal axis, during the turning manoeuvres the lateral loads reacted by the main landing gears are distributed between each of the three legs. The complexity of the landing gear is increased as all the struts have a twin wheel arrangement, fitted to the main fitting by a trailing arm assembly. Aircraft regulation for turning load calculation is not entering in how the tyres mechanics affects the lateral load sharing in a multi-leg landing gear. In fact, the paragraph CS25.495 is sharing the lateral loads between the legs proportional to the vertical load of the leg. For the calculation of this vertical load, it is considered the aircraft in a static condition with the shock absorbers and tyres in their static position. This approach is far away from the physics of the manoeuvre. First, no effect of the steering angle is impacting the load sharing as occurs in the reality. In a real situation, the steering angle is affecting the side slip angle of the tyres contributing directly to the lateral load sharing. Second, the vertical load is depending on the lateral load factor applied to the centre of gravity of the aircraft when this one is manoeuvring on ground. Knowing that there is a huge difference between what is required by the rule and what is the real physics of the turning manoeuvre, it was decided to build rational models that approach real dynamic of the aircraft complying with the load factors required by the rules. This way it will be minimised the risks of load exceedance during the real operation of the aircraft. The impact of this decision was affecting the validation of models as more parameters enter in the simulations. Due to that, it was necessary to develop tests to obtain the lateral load versus side slip curves associated to the tyres. Moreover, it was performed flight tests with a fully instrumented aircraft to obtain the validation of the loads over the landing gears during real aircraft turning manoeuvres. The flight tests were performed with different aircraft configuration in order to capture the effects of different aircraft parameters: aircraft weight, centre of gravity position, steering angle, aircraft longitudinal speed, braking and thrust. For high speed turnings it was required to reach different levels of lateral load at the centre of the gravity till the maximum possible.