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Design of dynamic test specimens and fixtures for
large structural components of civil aircraft
To cite this article: Yuanzhou Lyu and Ruowei Li 2024
J. Phys.: Conf. Ser.
2764 012001
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2023 2nd International Conference on Aerospace and Control Engineering
IOP Conf. Series: Journal of Physics: Conf. Series 2764 (2024) 012001
IOP Publishing
doi:10.1088/1742-6596/2764/1/012001
1
Design of dynamic test specimens and fixtures for large
structural components of civil aircraft
Yuanzhou Lyu1,2, Ruowei Li1
1Shanghai Aircraft Design and Research Institute, Shanghai, China
2Corresponding author’s email: lvyuanzhou@comac.cc
Abstract: In the course of designing and certifying civil aircraft, a significant number of
dynamic tests are necessary. If the test specimen is too large in volume or weight, it will place
higher demands on the loading capacity of the test equipment. The main focus in the design
process of dynamic test specimens is to ensure consistency in the dynamic characteristics
compared to the original structure, specifically with regard to natural frequency and mode shapes.
Factors such as stiffness, damping, and boundary conditions are taken into consideration. In
order to facilitate test implementation and reduce manufacturing costs, a combined approach
involving finite element analysis and experimental verification is employed to provide guidance
for the design of dynamic test specimens and fixtures for civil aircraft.
1. Introduction
To comply with airworthiness regulations and ensure optimal aircraft performance, civil aircraft often
require numerous dynamic tests during the design and certification process [1]. Examples of such tests
include vibration fatigue [2] and acoustic fatigue tests. When conducting dynamic tests, the loading
capacity of the test equipment is more demanding if the test specimen's volume or weight is too large.
Thus, it is essential to design dynamic test specimens and fixtures that are reasonably sized.
When designing dynamic test specimens, maintaining the test specimen's dynamic characteristics in
line with the original structure is crucial, particularly the natural frequencies and mode shapes. The
primary factors influencing the structure's dynamic characteristics include stiffness, damping, and
boundary conditions [3]. As a result, focusing on these factors becomes imperative during the design
process of structural dynamic test specimens and fixtures.
This study investigates the design of dynamic test specimens and fixtures for a specific type of civil
aircraft's engine pylon box section acoustic fatigue test specimen and fixture, serving as a case example.
The objective of this research is to facilitate dynamic testing for large structural components of civil
aircraft, as well as reduce the manufacturing costs associated with test specimens. To achieve this, a
combination of finite element analysis and experimental validation approaches are employed to study
the design methods for aircraft dynamic test specimens and fixtures.
2. The structure of the engine pylon
The engine pylon of civil aircraft typically comprises the upper fairing, center box section, trailing edge,
lower fairing, and bypass inlet flow integration (BIFI) structure [4][5]. The center box section, which
incorporates a combination of conventional beams, wall panels, and frames, acts as the primary load-
bearing structure of the engine pylon. The area where the side wall panel of the engine pylon box section
is located undergoes evaluation for acoustic fatigue.
2023 2nd International Conference on Aerospace and Control Engineering
IOP Conf. Series: Journal of Physics: Conf. Series 2764 (2024) 012001
IOP Publishing
doi:10.1088/1742-6596/2764/1/012001
2
Figure 1 illustrates the structure of the engine pylon box section and the area designated for acoustic
fatigue evaluation.
Figure 1. Engine pylon box section structure and acoustic fatigue assessment region.
3. Design of dynamic test specimen
Since the pylon box section is excessively large, conducting fatigue testing becomes challenging due to
high manufacturing costs. Therefore, it needs to be trimmed. To mitigate the influence on the dynamic
characteristics of the engine pylon box section and considering its structural features, the specimen is
composed of the local single-sided side panel, front and rear bulk frames, upper panel, and bottom beam
belly panel at the assessment position [6][7] (Figure 2 illustrates this).
4. Design of dynamic test fixtures
The pylon box section is connected to the wing through the upper link rod, middle joint, side link rod,
and lower link rod. To accurately replicate the installation method of the engine pylon box section on
the aircraft, a fixture is designed with fixed support at the front and rear ends, and it is installed on the
ground by using anchor bolts [8]. Figure 3 illustrates the fixture and the installation arrangement of the
specimen.
Figure 2. Structure of specimen. Figure 3. Structure of fixtures.
5. Dynamic analysis
5.1. Pylon section model
According to the dynamic modeling method of large civil aircraft structures [9][10], HyperMesh is used
for pre-processing the finite element model. The wall panel section of the engine pylon box section
model is simulated with a reference grid size of 5 mm. The thickened parts are simulated by using solid
2023 2nd International Conference on Aerospace and Control Engineering
IOP Conf. Series: Journal of Physics: Conf. Series 2764 (2024) 012001
IOP Publishing
doi:10.1088/1742-6596/2764/1/012001
3
grids. CWELD elements are used to simulate the fasteners. The link structure, consisting of the upper
link rod, lateral link rod, diagonal bracing rod, engine thrust rod, front mounting joint, and engine
connecting parts, is simulated by using CROD elements. SHELL elements are used to simulate the
remaining structures.
The power unit is simulated with CMASS elements to model the actual weight and inertia of the
engine. RBE2 elements are used to connect it to the front mounting joint, rear mounting joint, and engine
thrust rod. The pylon is connected to the wing via the upper link rod, middle joint, side link rod, and
lower link rod, allowing rotational degrees of freedom around the release link connection position while
constraining other degrees of freedom.
5.2. Specimen model
The specimen model has the same grid size and form as the pylon box section model. The specimen is
fixed to the ground through a fixture. The anchor bolt positions are subjected to rigid support constraints.
5.3. Comparing the analysis results
Figure 4 shows the first-order modal shapes of the side wall panels of the pylon box section obtained
through modal analysis, with a frequency of 300.3 Hz.
Figure 5 shows the first-order modal shape of the side wall panel of the specimen, with a frequency
of 290.0 Hz.
The comparison reveals that there is only a difference of 10.3 Hz in the first-order modal frequencies
of the side wall panels of the two, resulting in an error of approximately 3.3%.
Figure 4. The first mode of the pylon box
section side wall panel.
Figure 5. The first mode of the test piece side
wall panel.
6. Experimental verification
To validate the findings of the finite element analysis [11], frequency response tests are performed on the
pylon in the installed state. Figure 6 shows the amplitude and phase curves obtained from the three tests,
indicating the presence of the first-order modal shape of the side wall panel of the pylon box section at
approximately 255 Hz.
Modal testing is concurrently conducted on the experimental specimen to further analyze its
characteristics. The model used for modal testing is depicted in Figure 7. The obtained modal testing
results demonstrate that the frequency of the first-order mode is measured at 239.9 Hz, as illustrated in
Figure 8.
The test results indicate that the first-order modal frequencies of both the pylon and the specimen are
lower by approximately 50 Hz when compared to the analysis results. Additionally, the difference
between the first-order modal frequencies of the pylon and the specimen is noted to be 15 Hz, resulting
in an error rate of approximately 5.8%.
2023 2nd International Conference on Aerospace and Control Engineering
IOP Conf. Series: Journal of Physics: Conf. Series 2764 (2024) 012001
IOP Publishing
doi:10.1088/1742-6596/2764/1/012001
4
Figure 6. The result of the suspension frequency response test.
Figure 7. Mode experiment model of the
specimen.
Figure 8. The first-order mode of the
specimen side wall panel.
2023 2nd International Conference on Aerospace and Control Engineering
IOP Conf. Series: Journal of Physics: Conf. Series 2764 (2024) 012001
IOP Publishing
doi:10.1088/1742-6596/2764/1/012001
5
7. Conclusions
This study focuses on the design of specimens and fixtures for dynamic testing of large structural
components of civil aircraft. The following conclusions have been drawn:
1) The first-order modal frequency in the actual test is slightly lower compared to the analysis results.
This difference may be attributed to the lack of complete rigidity in the actual connection.
2) The pylon and the specimen exhibit a consistent difference in the first-order modal frequency, as
observed by comparing the results from finite element analysis and modal testing. These analysis results
can provide insights into the relative dynamic characteristics.
3) The error in the first-order modal frequency between the suspension and the test component is
kept below 10%, indicating the potential for ensuring dynamic characteristic consistency between the
two through localized truncation of the large structure and fixture design. This finding can serve as a
guideline for designing dynamic test components and fixtures for large structural components.
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