Vibration Test campaign performed on a Landing Gear System

The landing gear system is one of the most critical systems of the aircraft. The need to design a landing gear with high performance, longer life and with a significant reduction in terms of weight, production and maintenance costs represents a real challenge for a sustainable future that Europe is heralding. This paper presents the results of the experimental campaign of vibration tests performed on the main and side landing gear system, in both extended and retracted configurations, to be installed on the AIRBUS Group/Helicopter RACER compound helicopter demonstrator and it is part of the Project ANGELA within the European Research Program Clean Sky 2 Fast-Rotorcraft. Furthermore, in this paper will be described the tailoring of the standard. The hybrid nature of the RACER, not envisaged in any of the categories of the standard, required a tailoring phase to test the landing gear systems in a conservative condition. Thus, it will be shown how this phase led to defining a test sequence, a test setup and the vibration loads. The test campaign, conducted with RTCA DO 160-G tailoring, is part of a wider experimental activity aimed at the development, production and qualification of processes and materials that will allow the landing gear system to achieve the “Permit to Flight”.


Introduction
The purpose of this paper is to illustrate the results of the experimental campaign of vibration tests performed on the entire Racer landing system, to be installed on the AIRBUS Group/Helicopter RACER compound helicopter demonstrator.The vibration test campaign is part of a wider experimental activity aimed to achieve at least TRL 6 (1), required for a transition from research levels to the industrial scale.In particular, in this work the purpose of the fixture design and the tailoring phase of the standard used during the vibration test campaign will be described.From this tailoring phase arise the test sequence, the test loads, as well as the test setup that will be shown.Finally, the results obtained during the test campaign will be illustrated.Significant dynamic loads generally affect the whole landing gear system during flight, primarily due to the aerodynamic excitation and to the presence of rotating parts.In order to identify an efficient testing strategy, it was necessary to understand which loads were the most representative and how to apply them in the laboratory to be as close to reality as possible.Due to the hybrid nature of the aircraft, the first point led to tailoring of the standard, while the second point, due to the complexity of the Equipment Under Test (EUT), to a careful design phase of the fixtures to ensure that the loads derived from the tailoring phase were introduced in the most suitable manner onto the test articles.

Equipment under test
The tests conducted in the CIRA's Space Qualification Laboratory (2) involved the entire Racer landing system, composed of the left Main Landing Gears (MLG) shown in Figure 3, its actuator (Side Brace Actuators, SBA) in both retracted (see Figure 1) and extended (see Figure 2) configuration, the Nose Landing Gear (NLG) shown in Figure 6 and its actuator (Drag Brace Actuator, DBA) in both retracted (see Figure 4) and extended (see Figure 5) configuration.

RTCA DO-160F Tailoring
Racer is a high-speed helicopter demonstrator currently being developed by Airbus Helicopters as part of the Clean Sky 2 research program.The aircraft configuration combines fixed wings for energy efficient lift, propellers for energy-efficient propulsion and a main rotor that provides energy-efficient vertical takeoff and landing flight capabilities.
Due to the hybrid nature of the aircraft, the vibration test required a dedicated analysis of RTCA DO-160-F (3) in order to identify the most representative configuration of the real system and to test the equipment in a conservative condition in terms of mechanical loads and test setup.The category identified was the U2 -Unknown Helicopter Frequencies.Each EUT was then tested with Sine on Random vibration loads.Test levels were calculated according to the aircraft sources excitation frequencies values (4), by following the RTCA DO-160-F (Table 8.2.A and Table 8.2B).The results contained in these tables are directly relate to some information classified as confidential and for this reason they are not be shown in this paper.• Repeat performance test level for a minimum of 10 minutes; • Repeat sine-sweep at 0.5 g from 10 Hz to 2000 Hz at 1 octave/minute.
The Sine-on-Random test is a combined test where several sine-sweep tones are superimposed to the random background signal.The result of the RTCA-DO160F tailoring phase is summarized in Table 2 where the vibrational loads to which it was subjected are indicated for each set of EUT.The data are classified as confidential and for this reason the reported sweeping sine tone amplitude values have been normalized to the maximum value.Instead, the random values appear to be as per standard (4) and therefore can be reported.According to RTCA DO 160-G, the sinusoidal frequencies during sine-on-random tests has varied at a logarithmic sweep rate not exceeding 1 octave/minute from 0.8•fn to 1.2•fn, where fn are the sinusoidal frequencies of the test spectrum (4).

Test setup
Once the test profile has been identified, it is necessary that the vibration loads are applied to each set of EUT, specified in Table 1, in the most suitable manner.The aerodynamic loads and those due to the rotating parts induce excitations to the landing gear systems through the attachment points of the system with the aircraft.The control accelerometers were therefore positioned at these points.The multipoint control strategy requires the use of the average of the control accelerometers (4).
To make the constraint conditions as realistic as possible, the attachment points on the fixture were equipped, as in real operating conditions, with movable bearings and bushings.Figure 8 shows the attachment points of the main landing gear fixture.

A B C
Figure 8 -MLG fixture -attachment points For each EUT and for each actuator configuration the dynamic characterization of the mechanical fixture was executed with resonance search tests.The purpose of this phase is to verify that in the test range the fixture does not behave by amplifying the response provided by the shaker (5) (6).To achieve this goal, the fixtures were subjected to sine at 0.5g in the test frequency range (10-2000 Hz ( 4)) by monitoring the responses of the EUT attachment points to the fixtures themselves.
After verifying that the fixtures were suitable, due to the mass and dimensions shown in Table 1, each subsystem was tested individually on the vibration system coupled with slip table (X and Z axes excitation) or with head expander (Y axis excitation) by means of a mechanical fixture.
Moreover, in order to simulate the operative conditions each actuator was tested in both retracted and extended configurations.Since each set of EUT showed similar behaviors and results, for reasons of clarity and brevity, in this paper only the procedures and results obtained by testing the Main Landing Gear along the Z axis will be shown.For each set of EUT, the vibration tests were conducted using Model 356A16 PCB triaxial accelerometers installed near the attachment points of the EUT with the fixture, characterized previously, as control accelerometers.Measurement triaxial accelerometers (PCB Model 356A01) on the EUT near the attachment points were used.Furthermore, while the accelerations of the wheels were recorded for the landing gears, for the actuators measuring accelerometers were positioned at the midpoint of the length of the actuators themselves.As an example, Figure 10 shows the MLG setup during tests along the Z axis.

Test results
According to the test sequence reported previously, for each test article a sine sweep be-fore and after the sine on random sequences was performed for each axis (4).The sine sweeps were performed in order to detect any dynamic response variation or structural fault, highlighted by changing in the frequency of the peaks or amplitude response modification, that could arise from the applied test loads (6).
From Figure 11 to Figure 14 the resonance searches performed before and after the entire sequence of vibrations along the Z axis are shown.Those graphs evidenced that the operational vibration loads applicated caused no dynamic response variation, in turn related to any macroscopic structural failure such as damage failure, oil leaks or loss of system performance related to its ability to extend and retract without any problems.In fact, the over-lapping of the two curves evidenced no significant peaks variation, both in terms of amplitude and frequency.Furthermore, at the end of the test sequence, each set of EUT, in each configuration, were visually inspected and no evidence of oil leaks or structural failure were found.Moreover, successive functional tests carried out by MAGNAGHI, proved the correct functioning of the landing gear system and its capability to extend and retract without any problems.The experimental results were used as reference values to evaluate the accuracy of the developed numerical models.The excitation in X and Y direction showed similar results.

Conclusion
The current study highlighted the most significant aspects of a vibration test campaign conducted as part of the Clean Sky 2 ANGELA project, aimed at complying with vibration requirements contained in the landing gear system technical specification supplied by Airbus Helicopters.The present campaign is part of the whole qualification campaign foreseen on the landing gear system in order to achieve the Permit to Flight and TRL 6 level (1), required for a transition from research levels to the industrial scale.The outcomes of the experimental tests demonstrate that the operational vibration loads imposed did not alter the dynamic response in a way that led to any macrostructural failure.The test campaign was successfully completed and the landing gear system flight articles were delivered to Airbus Helicopters and installed on the RACER compound helicopter demonstrator.

Figure 7 -
Figure 7 -Racer zone subdivisionBased on previous analyses, each set of EUT was assigned, as shown in Figure7, to the following zones:• NLG and DBA go under the cockpit and therefore belongs to area A;• MLG and SBA go under the wing and therefore belongs to zone B.• The duration of each test, as well as the random vibration levels was the same for each set of EUT.• As foreseen in RTCA DO-160-F, the test vibration sequence includes the following steps:• Sine-sweep at 0.5 g from 10 Hz to 2000 Hz at 1 octave/minute; • Sine-on-random performance test level for a minimum of 10 minutes; • Sine-on-random endurance test level for a minimum of 20 minutes;

Table 1 -
Equipment Under Test -Length and Mass

Table 2 -
Test levels