Computer Aided Drafting, Design and Manufacturing Volume 26, Number 4, December 2016, Page 30

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1 Computer Aided Drafting, Design and Manufacturing Volume 26, Number 4, December 2016, Page 30 CADDM Aircraft wing box rapid modeling based on skeleton model Zhang Chao, Xi Ping School of Mechanical Engineering and Automation, Beihang University, Beijing , China. Abstract: In the process of traditional aircraft structure design, repetitive manual modeling of aircraft wing box parts is time consuming. Meanwhile, it s difficult to update associated models when product design changes. Aiming at solving these problems, a method for rapid modeling and updating associated models was put forward. Based on skeleton model, combining with the technology of template re-use, it makes aircraft wing box rapid modeling and correct updating possible. The detailed implementation process of the method was given, and a certain aircraft wing box was taken as an instance to verify the feasibility and effectiveness of this method. Key words: skeleton model; parametric template; template re-use; rapid modeling 1 Introduction With the development of CAD technology, model based definition (MBD) technology is rapidly developed and applied [1-2]. Aircraft design process is becoming a full three-dimensional process. Being the only data source in the process of product design, three dimensional model is the inevitable developing trend of CAD technology. Therefore, it is quite important to create and maintain three-dimensional model accurately and quickly. Aircraft wing structure is an extremely complex system. It has a few part types but a lot of parts in number. Meanwhile, there is high association among the parts which interact with each other. And their shapes change frequently according to the design changes [3]. Aircraft wing box structure has the following characteristics. First of all, there are a lot of parts in the same type, which have similar reference elements and similar structure but with different size. For example, the wing rib parts have different size, but there structure and main reference elements are similar. Secondly, a large number of geometric and non-geometric association relationships exist among aircraft wing box parts, which will lead the part s model to be affected by other models. In the traditional aircraft structure design process, on the one hand, designers usually create parts models one by one manually to achieve the establishment of the aircraft wing box model. On the other hand, because there is no effective association among aircraft wing parts, the change information among models generally delivered by manual notice such as making a call or sending a letter of notice. In the process of product design changes, structural designers mostly modify the associated model by replacing the elements, modifying the model parameters manually and other interactive means. Hence, this design method has the following shortcomings. Firstly, repetitively manual creating a lot of similar parts models results in time-consuming and longer aircraft structure design cycle. Secondly, when a part model is changed, the associated models can only be modified manually. Because of the highly association between aircraft parts, it is often unable to update the associated models or updating is incomplete. Furthermore, the modifying operation is cumbersome and inefficient. Focusing on the problems above, and concentrating on the preliminary stage of aircraft wing box detailed design, a rapid modeling method based on skeleton model which could update associated models was put forward. The aircraft wing box model could be created rapidly by reusing templates encapsulated by parametric models while the association relationship among downstream parts models will be established with the reference elements published from skeleton model. When elements in skeleton model are replaced or modified, the relevant parts and the whole assembly product would automatic update rapidly and accurately through the association. Corresponding author: Xi Ping, Female, Ph.D., Professor, xiping@buaa.edu.cn.

2 Zhang Chao et al., Aircraft wing box rapid modeling based on skeleton model 31 2 Skeleton model of aircraft wing box Skeleton model is the foundation and carrier for wing box rapid modeling and accurate updating. It defines the overall framework of the wing box. Elements in skeleton model mainly include datum points, datum lines, datum planes and datum surfaces. These elements are the geometric reference for modeling of wing parts. They define the basic spatial orientation of parts and assembly relation among parts, but not involving the specific parts size and details [4]. Skeleton model of aircraft wing box is derived from aircraft overall shape skeleton model by decomposition. It is located on the top of product structure tree as a datum part for product design. Skeleton model of aircraft wing box mainly includes the reference elements of wing skins, wing spars, wing ribs and wing stringers. The elements of skeleton model should be categorized. As shown in Fig.1, a certain type aircraft wing box skeleton model with datum points, lines, planes and surfaces was classified in the corresponding geometrical sets under the structure tree of skeleton model. Fig. 1 Skeleton model of aircraft wing box. When skeleton model elements are needed by aircraft wing parts, an element publication mechanism was essential to publish reference elements. The element publication mechanism could copy the elements which are to be referenced for modeling to the target part s geometrical reference set from the skeleton model with Paste with a link function. A geometrical reference set was used specially to preserve these published geometric reference elements. With these published reference, designers could create parametric model, and encapsulate it as parametric modeling template. Then, they can reuse the template to create a new similar model rapidly at different position on the skeleton model. Meanwhile, because of that all geometric reference elements for modeling come from skeleton model and different downstream parts would reference the same geometric reference, the association relationships among downstream wing parts models are established through the elements of skeleton model. The reference elements of skeleton model are not only the basis of parametric modeling template, but also the carrier of achieving association among parts. 3 Parametric modeling template Parametric modeling template is the key to realize aircraft wing box rapid modeling. Template is a kind of knowledge reuse technology [5]. Aircraft wing box consists of a lot of parts but with a few part types, mainly including wing spars, wing ribs and wing stringers. The parts in the same type generally have similar structure and similar reference elements but different size. In the process of aircraft structure design and modeling, the quality of wing parts model varies due to the different levels of designers and the different methods of modeling. And the repetitive manual modeling of similar parts is not only time consuming, but also laborious. Template can encapsulate design intent and modeling information such as modeling process, geometric reference elements, parameters, efficient modeling methods and experience [6]. In this way, we can get parametric modeling template with high quality; and then reuse these templates by changing the input data of template; finally, we can get new similar model automatically without repeating modeling process manually. 3.1 Template implementation process CATIA is a common software in aerospace field. Based on the CATIA, the implementation process of template consists of three parts as shown in Fig.2. Firstly, we should analyze the structural features of all type of aircraft wing parts, extract the geometric reference datum and the modeling parameters, and create the parametric model. Secondly, encapsulate the parametric model as template. At the same time, publish the geometric reference and the parameters for modeling. Finally, reuse the obtained template. Input the corresponding geometric reference and parameters to generate a new part model. Fig. 2 Process of template implementation.

3 32 Computer Aided Drafting, Design and Manufacturing (CADDM), Vol.26, No.4, Dec (1) Create parametric model. The process of creating parametric model include five steps as shown in Fig.3. Taking creating parametric model of wing rib as an example, the process is as the following Fig.4. The parametric model is built by geometric reference and model parameters, and the size and position of the model can be updated by changing them. geometry reference and parameters should be set to generate a new corresponding part model. In order to ensure the correct establishment of the association relation, the reference elements of template must be the published elements in the skeleton model. The model parameters can be input directly or fetched from other parts according to the parameter relation among the parts. (2) Encapsulate parametric model as template. After the parametric model is created, the parametric model should be encapsulated as template by the creating knowledge template tool provided by CATIA. The following Fig.5 shows the implementation process of the model s encapsulation. The model geometric reference and parameters are used as two input interface for template reuse. When the template is reusing, they should be set to generate a new model. (3) Reuse template. The process of reusing parametric modeling template is as the following Fig.6. Firstly, we should open the skeleton model. Secondly, we should select the parametric modeling template document for reusing. Finally, the model Fig. 3 Process of creating parametric model. (a) (b) (c) (d) Fig. 4 Process of creating wing rib parametric model. (a) Create empty part. (b) Build reference. (c) Build parameters. (d) Build wing rib model.

4 Zhang Chao et al., Aircraft wing box rapid modeling based on skeleton model 33 model and set the parameters to generate new model in the corresponding location on the skeleton model, without repeating the manual modeling. Meanwhile, the work of parts assembly is also omitted, and thus saving a lot of time. As far as we can see, the re-use of parametric modeling templates can realize the automatically repetitive modeling, which can effectively guarantee the quality of the model and improve the efficiency of modeling, so as to realize the rapid modeling of the aircraft wing box. Fig. 5 Process of encapsulation. Fig. 7 The fast modeling of the Rib models. Fig. 6 Process of template reusing. As shown in Fig.7, the wing rib parametric modeling template is used for several times. We can get similar wing rib models at different assembly position with different size through selecting the corresponding published reference elements in the skeleton model and setting parameters every time. In the modeling process of wing box, the parametric models should be built for the wing spar, wing rib and wing stringer respectively, (encapsulating them as templates and then reusing these template). The designer just needs to select the appropriate reference elements from the skeleton 3.2 Features of aircraft wing parts From the above declaration of parametric modeling template, we can know that, the parametric model is the basis of parametric modeling template. At the same time, the geometric reference and parameters are the basis of creating parametric model. Reasonable definition of the parameters and rational selection of geometric reference is the key to make templates with good reusability. Hence, analyzing the geometric reference and modeling parameters of wing parts are the prerequisite for wing box rapid modeling. The aircraft wing is an important part of the aircraft, its main role is to provide lift for the aircraft and meet performance requirements when the aircraft flies in various flight conditions. The main parts of the wing

5 34 Computer Aided Drafting, Design and Manufacturing (CADDM), Vol.26, No.4, Dec box are as follows: wing stringers along the chordwise direction, wing spars and wing ribs along the spanwise direction. (1) Wing spar. As the most important longitudinal component, spars bear most even all the bending moment and shear force. The spar is attached to the upper and the lower wing surfaces of the wing. The geometric reference for modeling includes the spar positioning plane, the spar starting plane, the spar termination plane, the upper and the lower wing skin surfaces. The geometric reference elements are as the following Fig.8. The spar positioning plane is used to determine the position of the spar in the wing assembly model. The starting and termination plane of spar are used to determine the beginning and end of the spar along the spanwise direction. experience of designers, the parameters of parameterized model are the flange thickness, the upper flange width and the upper flange thickness. Fig. 9 Geometric reference of upper side of wing stringer. The geometric reference for the lower side wing stringer includes the lower wing skin surface, the starting plane of the stringer, the termination plane of the stringer and the positioning plane of the stringer as shown in Fig.10. The parameters of parameterized model are the flange thickness, the upper flange width and the upper flange thickness. Fig. 8 Geometric reference of wing spar. Wing spar is composed of the flange and the web. Its cross-section shape mainly includes C-shape, I-shape and other shapes. According to the characteristics of spar structure and modeling experience, the model parameters are web thickness, upper flange width, lower flange width, upper flange thickness and lower flange thickness. (2) Wing stringer. The wing stringer is the longitudinal part that serves to support the aircraft wing skin. The wing stringers are attached to the upper or lower wing surface. Hence, wing stringers are divided into the upper side wing stringers and the lower side wing stringers. Wing stringer s structure is the flange with different cross-section shape, the common types of wing stringer are C-shape, Z-shape, T-shape and so on. As shown in Fig.9, the geometric reference for the upper side wing stringer include the upper wing skin surface, the starting plane of the stringer, the terminating plane of the stringer and the positioning plane of the stringer. Based on the structural characteristics of wing stringer and modeling Fig. 10 Geometric reference of lower side of wing stringer. (3) Wing rib. In the process of aircraft structure design, wing ribs are divided into two types, common ribs and stiffened ribs. The common ribs serve primarily to maintain the wing skin shape and to impart aerodynamic loads from the wing skin and the wing stringer to the spar. And the stiffened ribs serve to withstand concentrated loads. The shape of the rib is constrained by the upper and lower wing skin surfaces. Therefore, the geometric reference datum of the wing ribs include the upper and the lower wing skin surfaces, wing rib starting plane, wing rib termination plane, wing rib positioning plane, and stringer positioning plane as shown in Fig.11. According to the structural characteristics of wing ribs and the modeling experience, the parameters include the web thickness, the upper flange thickness,

6 Zhang Chao et al., Aircraft wing box rapid modeling based on skeleton model the upper flange width, the lower flange thickness, the lower flange width, the depth of the stringer notch and the thickness of the stringer notch as shown in Fig.11. The stringer notch of wing rib is the structure for assembling with wing stringer. Fig. 11 Geometric reference of wing ribs. 4 Design example In order to verify the efficiency and the feasibility of this method, a certain aircraft wing box model is selected as an instance. Before rapid establishing wing box model, the parametric modeling templates of wing spar, wing stringer and wing rib are created firstly. 35 created automatically and quickly. Finally, we realize the aircraft wing box rapid modeling. The model created by the parametric modeling template can be modified according to the design change. The way of modifying model includes the modification of the geometric reference and the adjustment of parameters of the model. When the model information is changed, the affected model would be automatically checked and updated according to association relationship. Taking the skeleton model reference elements change as an example, as shown in Fig.13, when the wing rib positioning plane of skeleton model was changed by creating a new rib positioning plane and replacing the original one, the aircraft wing box model automatically checks the downstream parts models which are associated with this rib positioning plane, and then automatically create a new correct box model. After the skeleton model was changed, both rib 01 model and rib 02 model were updated at the new correct position automatically. (a) (b) Fig. 12 Process of aircraft wing box rapid modeling. As shown in Fig.12, the complete implementation process of wing box rapid modeling consists of two parts. ① Open the skeleton model; ② Reuse the parametric templates of wing rib, wing spar and wing stringer, and then parts models of aircraft wing can be (c) Fig. 13 Automatic updating after the skeleton model changing. (a) Before the skeleton model changing. (b) Replacing the wing rib positioning plane with new plane. (c) After the skeleton model changing.

7 36 Computer Aided Drafting, Design and Manufacturing (CADDM), Vol.26, No.4, Dec The rapid modeling of the wing box based on the skeleton model can avoid the repetitive modeling of the designer. It just needs to analyze the structural features of the aircraft wing parts and create the corresponding parameterized modeling templates, then the wing box model could be rapid established under the re-use of templates. At the same time, because of the wing parts models are linked with each other through the geometric reference of skeleton model, the design changes can be quickly and precisely updated. 5 Conclusion (1) This paper aims at solving the problem that aircraft wing box parts repetitive manual modeling and inaccurate associated updating. Based on skeleton model, a wing box rapid modeling method was proposed. (2) The aircraft wing box skeleton model and parametric modeling template was introduced. And the three parts of implementation process which are creating parametric model, encapsulating the model as template and reusing template were illustrated in detail. The features of aircraft wing parts were analyzed. (3) A certain aircraft wing box was given as an instance to complete implementation process of wing box rapid modeling to verify the feasibility and effectiveness of the method. (4) This method is also suitable to model other products which have a large number of similar parts and the parts are highly associated with each other. Aeronautical Manufacturing Technology. 2016, (5): (in Chinese). [2] Zhang B Y, Xi P. Development of 3D marking technology [J]. Journal of Engineering Graphics. 2011, 32(4): (in Chinese). [3] Hu B F. Research and implementation of fast design and finite element modeling technology for aircraft wing structure [D]. Beijing: Beijing University of Aeronautics and Astronautics, 2010 (in Chinese). [4] Fu L. Research on correlation design method based on multilevel framework model [J]. Forging Equipment and Manufacturing Technology. 2013, (4): (in Chinese). [5] Shi M, Zhu C G, Huang S. Product modeling method based on knowledge template [J]. Computer-Aided Engineering, 2011, 2(2): (in Chinese). [6] Li T U, Zhang S Y, Chang-Ming L U. Design reuse method of complex product based on knowledge template [J]. Computer Integrated Manufacturing Systems, 2009, 15(6): (in Chinese). Zhang Chao is currently a master candidate in the school of mechanical Engineering and Automation, Beihang University. His research interests include rapid design and manufacturing technology and variant design technology. Xi Ping is a professor and Ph.D. advisor at Beihang University. Her current main research interests include aircraft preliminary design and associated technologies, CAD/CAM. References [1] Liu R, Duan G J. MBD technology development and application in the field of aviation manufacturing [J].