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Numerical study of the strength characteristics of the V-shaped tail of an unmanned aerial vehicle based on composite materials

https://doi.org/10.38013/2542-0542-2020-3-54-61

Abstract

In this research, we investigate the strength characteristics of the V-shaped tail of an unmanned aerial vehicle made of polymer composite materials based on an epoxy matrix filled with fiberglass and carbon fibre. The study was carried out by numerical modelling of the stress-strain state of the system in the Ansys Mechanical software and the Composite Prep-Post module allowing a model of layered structures of polymer composite materials to be set up. The values of stresses and strains under static loading conditions were determined. The numerical calculation was verified by comparing its results with the values obtained during a full-scale exper-iment. It is shown that the values obtained by numerical calculation differ from those obtained in the full-scale experiment by 10–15 %. An assumption is made that this discrepancy can be associated with macro-structural inhomogeneities of polymer composite materials appearing as a result of using the autoclave-free moulding method, which are not considered by numerical modelling based on idealized micro-models. In order to increase the accuracy of numerical modelling in the first approximation, it is proposed to introduce a correction factor when calculating the amount of deformation in problems on the rigidity of design structures.

For citation:


Lazorin A.E., Degtyarev A.A., Polikarpov A.A. Numerical study of the strength characteristics of the V-shaped tail of an unmanned aerial vehicle based on composite materials. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(3):54-61. https://doi.org/10.38013/2542-0542-2020-3-54-61

Introduction

It is known that increasing fineness ratio results in better performance of the tail unit [1], but at the same time increases the effect of structural elastic strains, the values of which depend on aerodynamic loads. This effect may drastically affect modern high-speed aircraft and unmanned aerial vehicles (UAV), especially those featuring thin swept lifting surfaces. In comparison to a straight tail unit with the incidence angle changing only due to torsion, the incidence angle of a sweptback tail unit also changes as a result of bending [2]. Thus, tail bend­ing during flight will enhance the UAV stability. This, it turn, will lead to an increase in the amount of elevator displacement for efficient trimming, and therefore, to a decrease in the UAV flight speed and range.

In order to maintain the design perfor­mance of the UAV, the tail unit strain value needs to be decreased during flight, but the tail unit weight shall remain unchanged. Also, it is necessary to elaborate an approach to rational design of structural load-bearing elements of the UAV based on the tail unit made of poly­mer composite materials (PCM) regarding the requirements to minimum weight and sufficient stiffness, at the same time maintaining high aerodynamic characteristics.

The most common materials used in air­craft engineering are fibrous composite materials consisting of a polymer epoxy or polyester matrix and high-modulus fibres based on organic, carbon or boric individual filaments. In compa­rison to metals, the main advantages of polymer composite materials are higher strength and stiff­ness characteristics along with lighter weight, as well as the possibility to control diverse material properties, depending on the type of the struc­ture to be developed. As a rule, multi-layer PCM are best suited to thin-wall shells where stresses in fibre layers are considerably higher than inter­layer stresses. The possibility to combine vari­ous lay-up patterns and to alter reinforcement directions allows to obtain materials best suited for the relevant applications.

Problem statement

This study was intended to investigate strength and bending stiffness of the V-shaped swept tail unit, and the symmetrical profile of small relative thickness in static loading conditions [3].

The object of this study is the V-shaped tail unit of the UAV, the general view of which is shown in Figure 1.

Fig. 1. General view of UAV V-shaped tail unit

One of the key problems in the UAV design is to reduce the weight and to increase the stiff­ness of the structure. It is difficult to find a solu­tion to this problem, because the UAV airframe is exposed to high aerodynamic loads during flight and its production cost shall be low.

In order to ensure accommodation of flight loads determined through the aerodynamic analy­sis, at the design stage we selected a structural layout (SL) consisting of two spars with partially load-bearing skin. The general view of the tail panel SL is shown in Figure 2 with the upper skin panel omitted for clarity.

The front rectangular spar is used to take up loads caused by aerodynamic forces during flight. In terms of design, the front spar com­prises two caps made of fiberglass roving and two fiberglass walls with the core made of foam plastic Rohacell® 110 WF. Structurally, it closes the tail unit nose, forming a closed-type torsion box, which is resistant to bending strains and considerably increases the general structural stiffness. The rear spar is similar to the front one in design. It is intended to take up loads caused by aerodynamic forces as a result of elevator displacement and serves as a mounting surface for the elevators. Both spars are also used for accommodating the docking pins and load transmission through these pins from the UAV tail unit to the fuselage. To interconnect the top and bottom tail unit skins, there are thin aluminium ribs on the end faces.

The skin is a three-layer sandwich panel comprising two fiberglass layers and an interme­diate layer of filling material made of foam plastic Airex® С70.75 or non-woven polyether material Soric® LRC 2. There is no layer of filling material at the spar mounts, as well as in the area where the upper and lower skins overlap near the front edge of the tail unit in order to prevent panel rupture occuring in the filling material due to flight loads. The general view of the skin sandwich panel struc­ture is shown in Figure 3.

Fig. 2. General view of the tail panel SL

Fig. 3. General view of skin sandwich panel structure

Solution description

To define strength characteristics of the tail unit panel more precisely, we conducted a strength analysis using the finite element (FE) method in the environment of the Ansys Mechanical soft­ware package along with the Static Structural static strength analysis module and the special Composite Prep-Post module intended to design composite structures.

Figure 4 shows a general view of the tail unit’s FE model for a combined strength analysis (for parts made of PCM and isotropic materials) with a generated grid (the upper skin is not shown for clarity); Figure 5 shows an enlarged view of the model part; total number of the grid nodes is 1,953,601. This large amount of grid nodes is needed to build a high-quality 3D model of parts made of PCM using the Composite Prep- Post module in order to successfully implement contact interactions. Dynamic effects under static loading were not taken into account, all the ma­terials were assumed to be linear-elastic, the anisotropy of mechanical properties of parts made of PCM was taken into account by setting the relevant values of physical and mechanical properties determined in a test lab within a sys­tem of three coordinate axes.

Fig. 4. General view of the model for strength analysis

Fig. 5. Enlarged view of the model section for strength analysis

The loading pattern is shown in Figure 6. Attachment is similar to parts attachment used in the UAV structure - the fastening pins are at­tached with the fixed support, and displacement of the end face of the root rib “into the fuselage” is limited by compression only support. The panel is exposed to the design load of 444.3 N determined through numerical modelling in the Ansys CFX software for a given flight mode.

Fig. 6. General view of the loading pattern, where A - compression only support, B - distributed load from CFX, C - fixed support

A stress-strain model obtained by the ana­lysis clearly demonstrates the fields of stress dis­tribution over the tail unit skin (Fig. 7) and the spars (Fig. 8), as well as deformation of the tail unit structure under load (Fig. 9).

Fig. 7. Stress distribution in tail unit skin

Fig. 8. Stress distribution in tail unit ribs

Fig. 9. Tail unit structural deformation under load

As a result of numerical modelling, we have determined maximum stress and strain values for the tail unit, which are equal to 186.03 MPa and 35.6 mm, respectively. The calculated stress value for this design is not critical as the factor of safety regarding load is 1.78. Yellow and red colors in the represented stress distribution are not clearly visible and depend on the peculiarities of FE model generation.

In the course of full-scale tests to verify the strength analysis, a design loading pattern was reconstructed. The panel was rigidly attached to the fastening pins thrusting against the root rib and was loaded at five points by loads with the weights equal to values M1-M5 determined by numerical modelling of the selected flight mode in the Ansys CFX software as pressure coefficients Cp for each selected cross-section. Deformation under load was measured at the end section point corresponding to the maximum displacement point that was de­termined by numerical modelling. The loading pattern formed during full-scale tests is shown in Figure 10. The general view of the stress-strain model of the tail unit showing its bending beha­viour under load in comparison with the reference model is shown in Figure 11.

Fig. 10. Loading pattern

Fig. 11. General view of tail unit stress-strain model

The experiment result is the stress-strain value equal to 42 mm. This figure exceeds the de­sign value by 17.9 %. This considerable difference in stress-strain values obtained through numerical modelling and full-scale test is probably caused by inefficient stability of PCM mechanical proper­ties obtained in the course of manufacturing of the tail unit. For numerical computations, we used moduli of elasticity in tension and bending deter­mined based on the analysis of PCM samples in a test lab. However, according to the experiment, mechanical properties of PCM in series-produced parts may differ from those determined based on the test samples.

In order to increase the stiffness of the tail unit with its weight unchanged, the following changes were introduced at the design stage: cross-section of the spar caps in the end section was reduced, wall thickness within the entire spread was increased, PCM fiberglass filling ma­terial for spar caps and walls was replaced with carbon fibre, Figure 12.

Fig. 12. Diagram showing changes in tail unit design

As a result of the repeated numerical mo­delling, we determined the stress-strain value equal to 20.9 mm (Fig. 13). This figure is less than the initial one by 40 %. Within the scope of the study objective, this stress-strain value based on numerical modelling of the selected flight mode in the Ansys CFX software is considered tolerable.

Fig. 13. Tail unit structural deformation under load

Based on the loading pattern, we repeated the full-scale test similar to the initial one but with a new design of the tail unit. The test shows that the maximum stress-strain value is 23 mm. This figure exceeds the design value by 9 % only. This deviation may be associated with the imperfection of the loading pattern and with parameter measu­rement accuracy during the experiment.

Conclusions and recommendations

Computations in the Ansys Mechanical soft­ware and material replacement with the design weight unchanged allow to increase the stiffness of the tail unit and to reduce the stress-strain value under load by 45 % as compared with the initial variant. The flight test experiment con­firms that the trim angle in the pitch channel is reduced from 4 to 2 degrees in the cruise flight mode. Performance of the UAV with a new tail unit configuration meets the requirements for the amount of elevator displacement for effi­cient trimming. This, in turn, allows to reach the design values of the maximum flight speed and range.

Comparison of the results of numerical modelling in the Ansys Mechanical software with the experimental strain values shows that the analysis does not consider all critical fac­tors. Numerical values of mechanical charac­teristics of PCM under discussion may vary for the design and physical models of the tail unit. This variation in values is presumably as­sociated with the reproducibility of structures with homogeneous properties during the PCM parts production process based on the manual fibre impregnation method and autoclave-free moulding in a matrix under vacuum. In this case, the matrix-to-filling material ratio may vary within a certain range depending on the uniformity of binder distribution in each layer of the pack being formed when making a PCM package.

For design calculations within the structural analysis of PCM-based products made using the above-mentioned method, it is proposed to intro­duce a correction factor of around 1.1-1.5 when calculating the design stress-strain value in order to compensate the inhomogeneity of mechani­cal properties of formed multi-layer PCM on the macrolevel.

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About the Authors

A. E. Lazorin
Central Scientific Research Institute of Chemistry and Mechanics named after Mendeleev (TsNIIKhM)
Russian Federation

Lazorin Alexander Evgenievich – Leading Design Engineer, Design Department of the Special Design Bureau. Research interests: design of aircrafts based on composite materials, CAE.

Moscow



A. A. Degtyarev
Central Scientific Research Institute of Chemistry and Mechanics named after Mendeleev (TsNIIKhM)
Russian Federation

Degtyarev Alexander Alexandrovich – Cand. Sci. (Phys.-Math.), Head of the Special Design Bureau, Deputy General Director. Research interests: dynamics of complex technical systems.

Moscow



A. A. Polikarpov
Central Scientific Research Institute of Chemistry and Mechanics named after Mendeleev (TsNIIKhM)
Russian Federation

Polikarpov Alexey Andreevich – Leading Design Engineer, Design Department of the Special Design Bureau. Research interests: aerodynamics, automatic flight control, aircraft system design.

Moscow



Review

For citation:


Lazorin A.E., Degtyarev A.A., Polikarpov A.A. Numerical study of the strength characteristics of the V-shaped tail of an unmanned aerial vehicle based on composite materials. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(3):54-61. https://doi.org/10.38013/2542-0542-2020-3-54-61

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ISSN 2542-0542 (Print)