The development of sport aviation, has had considerable development since the early 70s when they were introduced to the market the first light and ultralight aircraft. These devices are characterized by their light weight and ability to operate from short and unprepared runways. In Mexico there is a significant amount of these planes in private hands and employees mostly with sports / recreational purposes; however, in this field dependence of the industrialized countries, mainly the United States, causes the costs, both acquisition and maintenance are high.
The different uses than can be given to a light aircraft, besides recreation, can be varied, such as monitoring, training, photography, advertising and search missions. In this cases a light aircraft can replace larger aircraft, with significant savings in operating expenses. For an aircraft of this type can perform such missions advantage over current equipment is required to be two-seater, enclosed cabin, cruising speed of around 80-100 kts, easy construction and maintenance, and a low fuel consumption. To achieve these design specifications, the use of composite materials is proposed, essentially a sandwich structure of fiberglass polyurethane foam sheet.
The required technology to manufacture an aircraft of this type has been developed in our organization for several years, through experimentation by persons engaged in handling fiberglass and other composite materials. However, most of his work has been done empirically, so it is necessary to define the scope of the technology developed, not only to develop an aircraft, but to transfer the technology to other composite materials applications, and enter this diverse and relatively new world of engineering, so many possibilities open to those willing to delve into its features. For this continuous research and development in cooperation with schools of higher education, the productive sector and researchers together to raise the course of development in this area is needed.
DEVELOPMENT PROGRAM OBJECTIVES
•Build a prototype at 1: 4 scale of an aircraft with the characteristics mentioned in the justification of the project.
•To characterize composite materials with which it is provided, with emphasis on the control of manufacturing processes.
•Develop analytical methods and manufacturing required for this type of structures, as well as check the capabilities of the current resources.
•Involve young undergraduates in mechanical engineering in the development of a research project, promoting and taking to practice their knowledge.
•Once built, a flight testing to validate structural models has to be performed, also all considerations during the design process have to be assumed, as well as verify the properties of materials in a functional model.
The goals achieved to this date in the project are:
•Part of the conceptual development of the aircraft was performed, achieving a satisfying design, able to meet the requirements of the missions.
•Building models and molds has been accomplished, which is the last stage of the preparation of prototype manufacture itself.
•Material tests were performed, whereby the physical and mechanical properties were obtained, a process in which students of mechanical engineering career from the Technological Institute of Durango were involved .
•The structural analysis method used was perfected, and the finite element model was completed, which allow you to select a configuration of material, number of layers close to the optimum.
•A method for organizing and managing large amounts of information required for the modeling of the system development.
The present work has as its ultimate goal to acquire more knowledge on the different aspects of composite materials, such as design (at all stages), manufacturing, properties, etc. For many application in different fields of Mexican industry. Technology here developed can be replace or innovate in different areas, this is one of the great dreams of the authors of this work; however is necessary the complete understanding of the materials to make this possible.
This project is the way to prove the knowledges acquired so far in the area of the compounds, altogether with another subject long forgotten in our country: aircraft construction. The first part of this project (development stage) is building a two-seater prototype light aircraft scale that can meet a wide range of missions and be economical to purchase and operate. Computational models, plans, real models, materials testing, creating molds were made, it remains to finish the final prototype, test and validate our calculations. We have learned a lot during the project, and hope this is the basis for development of composite materials industry as well as the first stage of the construction of an aircraft design and construction purely national
A flow chart is presented in Figure 1 with the stages of development and construction of prototype scale. The completed project phases are marked with a “check mark” in green. An important aspect of the development process is that, after completion of the three-dimensional modeling three parallel lines are still working, the corresponding manufacturing, analysis and testing of materials.
This represents a major advantage of using composite materials, as once defined geometry can start working on the models and molds, although no charges, thickness or number of layers of material required in each element are defined aircraft. Structural analysis define these parameters based on the testing of materials, which is performed while the finite element model was built. This methodology saves time and resources, in addition to allowing the crossing of information between different processes. Once these steps are necessary to laminate the parts of the plane by the method of vacuum bag molding, to finally assemble all the pieces. The final step is to start flying prototype to implement a program of test flights where to test flight characteristics and verify that adhere to the estimated. It also will test the structure to validate the finite element models and make a decision about whether the decisions made during the simulation process were correct.
The first step of this project was to define the type of aircraft that was required for both recreational, sporting and business activities of our country. The selected requirements were:
•Seater aircraft, this allows use in flight instruction and observation, search and rescue.
•Single engine. It involves less initial investment and maintenance, if like design simplicity.
• Ideal for high-wing monoplane observation activities, since the wing does not interfere with visibility down. Besides the high wing configuration is much more stable than others, which is a plus considering that one of the missions is the training of new pilots.
•Made of composite materials: a main frame sandwich fiberglass over styrofoam, carbon fiber reinforcement in the areas of attachment of the major structural components, powerplant and workload.
•Their planes flaperons have full scope which allows excellent STOL relatively low angles approximation, allowing good visibility landings necessary short runways.
With these features in mind was continued aerodynamic design. aerodinámico.
To meet these requirements the selected airfoil for supporting floors is the Eppler 420; its main characteristic is having high curvature, which allows you to achieve high lift coefficients. In the early stages of design considered using a slat Eppler 421 profile, features similar to 420 but with a greater thickness; however since we want to keep the simplicity of the design it was decided to ignore, which impacts the minimum approach speed but provides manufacturing and reduces the final cost model. Figure 2.
Furthermore, according to experiences excessively low speed approach is necessary, as this involves a very pronounced attack angle, which complicates the pilot’s visibility and reduces the accuracy of the landing, which negates to some extent the advantage of the low speed. The composite construction allows the use of such profiles, which are otherwise very difficult to construct. An additional advantage of these profiles is that both the upper surface and the lower surface with concave surfaces, thereby increasing the structural efficiency. The selected profile is symmetrical to the flaps in Figures polar curves (Figure 3, corrected for elongation corresponding to 7.5) with different angles of flap deflection obtained for Reynolds numbers corresponding to operating speeds expected occur obtain in prototype 1: 4 scale radio controlled whereby the performance of an aircraft shall be ensured.
A general dimensions available aircraft modeling proceeds thereof, which was done in three dimensions. This allows a better understanding of the different parts of the plane, and consider aspects such as ergonomics, configuration and arrangement of internal components such as fuel tanks, instruments, joysticks, etc. Besides the three-dimensional model is the source for obtaining the plans and templates for building the actual model, and provide the geometric model that served as the basis for generating the finite element element. Figure 4 shows some views of three-dimensional model are presented.
STRUCTURAL ANALYSIS USING THE FINITE ELEMENT METHOD
The proposed structure combines the advantages of the mechanical properties of composite materials, such as high specific strength and relatively easy to adapt to curves such as fanciful involves the geometry proposed for the plane strength. For the construction of the composite panels is proposed to employ fiber sandwich / epoxy and reinforce critical areas with carbon fiber. For joints of controls with control surfaces rods ball joints (push-pull) will be used, except the rudder, where control is exercised by cables. Finite element model was used to analyze the structural response to loads that supported the model in flight. (Figure 5) The process consisted of discretizing the finite element model. To assign each physical and mechanical properties of the parties they represent.
One of the challenges at this point was to get the correct orientation of the axes of each element, since this depends the correct application of the mechanical properties of the materials, as to be considered orthotropic, its mechanical properties vary with orientation. In Figure 6 oriented elements is bounded by arrows, and the difference observed between untargeted. The next step is to restrict and apply loads to the model. The results are analyzed using contour graphs, as in Figure 7, which shows the stress distribution of Von-Mises around the aircraft. This allows applying a failure criterion and reach the structure optimization.
One of the most important aspects of this work is to characterize the physical and mechanical properties of materials used in building the model properties. The materials used, despite being commercial, are highly sensitive to the manufacturing process that is subject to when speaking of mechanical properties.
That’s why testing is due to the different materials available. Specimens for this test (figure 8) were made using the same procedure used for aircraft laminates, vacuum bag molding. This process is to laminate the layers of fiberglass and epoxy resin on the mold, after application of a separating film to prevent adhesion, and then covering the laminate with a plastic bag, which is removed in vacuo to obtain an impregnation uniform resin in the fiber and to make laminate follow the geometric pattern of the mold.
Once the specimens finished laminates were cut to recommended size, organized and proceeded to stress testing (Figure 9). The results of such tests are processed and grouped in curves as shown in Figure 10, with the experimental data points and the correlation as shown as curves obtained.
The data obtained from these tests, such as elastic moduli, Poisson, maximum allowable effort in rolling, turning points in the stress-strain curve are of vital importance to have a better understanding of materials and complement the finite element model and can estimate the structural behavior with much greater precision than using only generic data published for these materials.
During the project it was necessary to create a database that would handle the large amount of information required. An example is given below. The fuselage consists of several components, and each component is subjected to various stresses, which are assigned specific mechanical properties to absorb the loads they are having. This is done by varying the type, amount and orientation of the layers constituting the laminate. Figure 11 shows a schematic of the fuselage, where the different components of the airframe, grouped according to the loads exerted is shown. B The table shows the data needed to characterize the physical properties of each component. Also needed is another set of properties, the mechanical, the material defining each layer, which involves a large amount of information, which would otherwise be very difficult to handle. All these data are introduced into the finite element model.
The technique used to build the prototype application, the use of molds with the geometry of the plane. To build on these first necessary to construct a model identical to the prototype geometry. Due to the curved surfaces of the aircraft, ma modeling is more complicated than traditional models; to not have a CNC machine templates model cross sections (Figure 12) were generated using a printer.
Estas plantillas contenían la información de la forma de las secciones y también guías para la alineación; fueron impresas en papel y se transfirieron a un laminado de madera para darles la rigidez adecuada, tras lo cual sirvieron como guía para cortar secciones transversales de los componentes en espuma de poliestireno de alta densidad. Estas secciones se pegaron una tras otra para formar la geometría de los diferentes componentes del avión (figura 13). Debido a que la espuma de poliestireno es muy suave, se recubrió el modelo con una fina capa de fibra de vidrio para proceder a darle un acabado fino, del cual depende el acabado de los moldes y por ende, de las piezas terminadas. Una vez terminados los componentes se ensamblaron para verificar el reglaje de las superficies, se hicieron los ajustes requeridos y se procedió a dar el acabado definitivo (figura 14).
The construction of the molds is made by covering a layer model with no structural fiber glass-epoxy. The first layer was of “tooling” abrasion resistant which further contributes to a better finish of the finished parts resin. In Figures 15, 16 and 17 the mold of the main structural parts of the prototype are presented.
•The finite element model allowed to quantify the magnitude of the stresses in the laminates of different structural components considering the orthotropic material properties, which was the basis for defining the combination of laminated materials.
•The physical and mechanical properties obtained by experiment show that the manufacturing process is consistent, which allows relatively accurate predictions of the structural behavior of the prototype.
•The construction method model and mold is suitable to the resources available in the state, and it is possible its extension to the actual aircraft, and for other composite structures.
•The possibility of developing parallel activities is a clear advantage of the proposed method of manufacture, and allowed more efficient resource management.
•The database generated materials will serve as a tool for future design projects in which these materials are involved.
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