Experimental Structural Validation of a Landing Gear System for High-Speed Helicopter Applications

The design and technological demonstration of a Landing Gear architecture were addressed for Airbus fast rotorcraft end application within the Clean Sky 2 Racer project. Numerical activities including advanced modelling approaches were carried out to substantiate the feasibility of structural concepts in compliance with industrial standards and CS-29 applicable airworthiness requirements. In order to demonstrate the goodness of design strategies, a true-scale prototype was manufactured and tested for demonstrating its capability to withstand static loads representative of the limit and ultimate cases expected in service. The paper will focus on the qualification of the Nose and Main Landing Gear systems. Sizing process was validated and verified by test whose results allowed for validating/calibrating the FE model. In such a way, the design database could count on a reliable tool available for analysing the effect of any further load condition change.


Introduction
The Landing Gear (LG) system is one of the critical aircraft subsystems requiring a thorough interdisciplinary development process.The targets of a minimum weight figure, reduced volume envelope, high structural strength and optimal performance are the most recurring challenges to be faced early in the aircraft design cycle.Several engineering ways are being developed over the yearsnumerical prediction tools, new lightweight materials and advanced manufacturing processes -allowing for a more confident accomplishment of qualification standards and therefore a shorter lead time for the final integration onto the aircraft.A LG aims to dissipate landing loads thereby reduce the impact entity transmitted to the aircraft attachments (suspension function); it guarantees the ride comfort during taxing as well as the ground manoeuvres completion as braking and steering (control function).All the design requirements are assured by the Airworthiness Regulations to meet operational, structural and safety requirements.The technical literature outlines a clear scenario of different modelling, analysis and testing approaches for landing gear systems.A validated Main Landing Gear (MLG) model with respect static and dynamic test data is presented in [1].The mathematical methods for determining the LG dynamic characteristics as demand for the aircraft (A/C) architecture definition are discussed in [2].The fatigue testing with cyclic loading to replicate the LG flight mission profile (drift-to-right/left, braking, rolling, towing, etc…) with an accent on the residual stress formations are explained in [3].The paper [4] emphasizes instead the applicability of numerical models to predict structural behaviour considering artificially introduced flaws.Current research plans are raising awareness of developing innovative concepts for LG systems in order to achieve improved weight and cost, which are the major stakes for A/C manufacturers and airlines.In the European scenario, the core mission of Clean Sky 2 framework falls in leading breakthrough technologies to improve the A/C environmental aspects providing so a solid contribute to the aeronautical competitiveness and mobility, [5].The ALLEGRA project has been developed in response to assess low noise technologies applied to both NLG and MLG architectures, [6].Within ITEMB project [7][8], carbon fiber materials have been used to design a midsize aircraft MLG bay bulkhead for Airbus next generation aircraft.ANGELA consortium is aimed to design the whole LG system of the Airbus Helicopters Racer flight prototype ([9-10]) with the objective to reach a high TRL (Technology Readiness Level, NASA [11]), i.e. 6 to demonstrate the functionality in relevant operative environment.The present paper provides an overview on the structural strenght qualification of LG system comprising a focus on the correlation between Finite Element (FE) models and test results; the activity saw in particular a cooperation between Magnaghi Aeronautica S.p.A. and AVIATest Lab.

LG system design description
The ANGELA project aims at developing, manufacturing, testing and qualifying innovative LGs to be integrated into the Racer High Speed Rotorcraft, developed in the framework of Clean Sky 2 research program.The LGs system involves a tricycle wheeled type and oleo-pneumatic shock absorber (S/A) for dissipation of the energy during landing.The NLG is based on an articulated architecture including two wheels and tires, linked to a pivoting trailing arm; the main fitting is then attached by a pair of pintle pins at two attachment brackets of helicopter (H/C) bay, Figure 1(a).A drag brace actuator (DBA), which can extend/retract the leg and lock it in the up/down position, supports the NLG leg frontally.MLG is a direct cantilever type with one wheel and brake, supported laterally by dedicated hydraulic side-brace actuator (SBA), Figure 1

Numerical models description
The design activity has been supported by the development of FE models to investigate the expected stress state during the static test, in particular by focusing on the most critical element.An overview of numerical models representative of LGs design architecture is outlined in Figure 2. Material references of main structural parts -reported in parts considering the actual nonlinear contact between the interfacing surfaces (i.e.lugs, holes, bearings, bushings, etc.) according MSC Nastran ® references, [13].The static load conditions are evaluated to investigate the in-service (ground manoeuvrings, landing phases as summarized in the Table 2) ground forces as well as reactions to H/C side.Once the worst cases have been determined, the stress analyses allow for evaluating the relevant safety margins of the structural components.

NLG Load Conditions MLG Load Conditions
Towing Swivelled (FWD/AFT direction)*, ref. [14] Braked Roll, ref. [14] Towing Swivelled +45°/-45° (FWD/AFT direction), ref. [14] Reverse Braking, ref. [ -limit loads without detrimental or permanent deformation; -ultimate loads for at least 3 seconds without failures or buckling.Because of the relevance of the loads to be applied, detailed numerical simulations were performed to: -reasonably lead the experimental tasks in order to control any possible dangerous deviation with respect the expected structural deformation; -accurately select the most relevant area of the prototypes for checking strain and displacements; -substantiate the test rigs stiffness and the load transfer chain.In Figure 3     In Table 3 are indicated for a single critical load condition: • the locations selected for strain gages (S/Gs) applications; • strain data obtained from static test; • strain predicted by FEM.
The following Figure 5 and 6 represent the numerical strain maps for the significant NLG and MLG components.
Table 3. Limit static test case, S/Gs max values.

Discussion and conclusions
In the scenario of Racer Clean Sky 2 project, authors investigated a high TRL LGs architecture tailored for a high-speed rotorcraft.Numerical predictions were carried out to substantiate the system feasibility in compliance with the airworthiness requirements and customer specification.Full-scale prototypes of both NLG and MLG were finally manufactured and tested for demonstrating in an efficient test campaign the strength capability of the conceived structural concepts considering the loads expected in service.Test outcomes with following dimensional checks and non-destructive inspections (NDI) showed a structural behaviour compliant with design requirements for both limit and ultimate loads.Additionally, good correlation levels compared to the theoretical calculations were appreciated in terms of static deflections and elastic deformations as well.Strains obtained by FE analysis higher than test data were however in a deviation range still acceptable for qualification.

Funding
(b).Both the S/As are nitrogen-oil based consisting of one stage: a separator piston divides the oil and nitrogen phases.

Figure 1 .
Figure 1.Design details of Racer LG system.
and 4 the assembled LGs + test rig are represented.Single lumped forces were introduced in all relevant directions (Z: vertical, X: drag; Y: side) by means of hydraulic jacks positioned at the at the wheel location.

Figure 5 .Figure 6 .Figure 7 .Figure 8 .
Figure 6.MLG FE strain results, 3pt Spin Up load condition.The two example cases reported herein (NLG Level Landing with Drag and MLG Spin Up) involve both load components acting predominantly in the xz plane (drag and vertical load combined).With reference to the NLG, the ground forces induce bending moments equally oriented resulting in a stretching action of the Main Fitting rear side, Figure5(a).The drag brace actuator reacts essentially as a strut rod element, which pushes downward the interface lug with a following tension of Main Fitting front side too, Figure5(b).The deformation maps in Figure7explain this load path.The Lever Arm would freely swivel around its pivot except for the S/A strut stiffness which induces a flexural deflection (pulling its bottom face and squeezing the upper side close to the S/A lower attachment), Figure5 (c)-(d).The behaviour of the MLG can be led to a cantilever beam based scheme.The Main Fitting deforms mostly in the global xz plane with an elongating of the front arm (Figure6(a)) and a compression of the rear arm (Figure6(b)).The eccentric position of loads application point involves a S/A rotation with a subsequent torsional deformation of the torque links, Figure6(c)-(d).The static deformation is given in Figure8.The side brace is quite unloaded as the loads in the y direction are of negligible entity.The simulation results are consistent with the experimental stress distributions: the limit deformation values indicated above are compliant with the material characteristics.The maximum values of strain tensor do not exceed the plasticity allowable values of 7050 aluminium alloy (used for both Main Fittings) and Titanium Ti-6Al-4V (of NLG Lever Arm and MLG Torque Links).The test demonstrated the structural stability and integrity at the respective ultimate load conditions too.The correlation discrepancies are within an acceptable range indicating that the engineering results are accurate and reliable.
This research was funded by Clean Sky 2 Joint Undertaking under the European Union's Horizon 2020 research and innovation program under grant agreement No. CS2-GAM-FRC-2014-2015 and the following extensions.

Table 1
[12] obtained from the Metallic Materials Properties Development and Standardization (MMPDS) Handbook,[12].The FE models involves mainly 3D mesh x z y

Table 2 .
Limit load conditions (ground/landing) planned for the static test.

Static test on full-scale prototypes
[14]ic tests were carried out at AVIATest Lab to demonstrate the LGs capability (as per EASA airworthiness requirements CS 29.305 part,[14]) of withstanding: */Drift to Left Normal Landing*, (from drop test) Take-Off, ref. [14] 3pt Level Landing with drag Normal Landing*, (from drop test) Spin Up*, Spring-Back, Tail Down*, (from drop test) (*) load case considered also as ultimate condition (1.5 x limit load) Max Side Load*, (from drop test) Drift to Right/Drift to Left*, (from drop test)44.