Chapter II: CAD modeling
Before moving onto aerodynamic modeling, we should complete the 3D model of the Century. In this chapter we will talk about the existing model that we have, then we will detail the missing parts and the concept of work we’ve chose to each part.
2.2 Existing model
The modeling process of Century airplane has already begun in Avionav, the existing model is shown in the figure 11.
The material used for this type of aircraft is aluminium because it offers very good deformation capabilities and good weldability.
Plus, it has a high resistance to corrosion. In order to develop their product, Avionav had changed some of the constituent of their aircraft which are the canopy and its stand. The shape had been developed and the material as well, we will detail these changes in the next section.
2.3 Reverse engineering
In the scientific literature, the reverse engineering have a lot of definition switch the field of application:
– The process of retrieving the new geometry of a fabricated part by digitizing it and modifying its existing CAD model  and applications in mechanics are diverse :
– Inspect and compare the geometry of the real object with the CAD model 
– Measure the behaviour of manufacturing tools (control of wear, for example)
– Capture factory or production line geometry to use CAD software for piping design, boiler room, ventilation, air conditioning; Capturing the deformations of an object, in a load case of a beam for example; in order to compare the piece of information with the predictions of the finite element analysis
– To capture the shapes of an existing part whose CAD model does not exist so that a piece of replacement is manufactured by rapid prototyping for example.
Reverse Engineering RE is used for many applications such as business, virtual reality, gaming, to intrust how competitor products are manufactured and how they work RE is decomposed in 3 general steps:
1- Data acquisition: the physical object is numerically divided into two or three dimensional data using 3D scanning technologies like CMMs, laser scanners, structured light digitizers, or Industrial CT Scanning (computed tomography). , The measured data are represented as a point cloud, lacks topological information and design intent.
2- Data processing: data can be segmented, for example, to extract data from canonical surfaces (plane, cylinder, and sphere) or complex surfaces by converting the point cloud to a triangular-faced mesh.
3- The reconstruction of the cad model: the extracted surfaces can for example be ‘thickened’ and if necessary reworked to rebuild a 3d solid.
In this section, we will present the steps we had used in order to model the bonnet and the winglet of the airplane.
The bonnet of the plane is composed of two parts: a top one (figure 12) and a lower one. That are assembled together and added to the airplane in order to protect the engine.
The modeling of these parts is very difficult using the direct modeling method, that’s why we decided to use the reverse engineering: scan to CAD using CATI V5 tools.
We had as scan the smartphone Lenovo Phab 2 Pro (figure 13) that includes tango which is a new technology from Google that enables augmented reality (AR) gaming and utilities. This technology offers a lot of particularities that make this device an innovative one.
2.5 SCAN to CAD
The scan to CAD is the development of a 3D model from a cloud of points witch is the scan of an object. This operation takes place in 5 steps:
1st step: importing the cloud of point using the «digitized sheep editor »
Figure 14: Clouds points importing
Figure 15: Cloud of points
2nd step: Cleaning of the cloud of points: deleting of useful points: the points from noise
Figure 16: Deleting of point
Figure 17: Deleting of point
3rd step: Filter
Figure 18: Flirtation
4th step: meshing
Figure 19: Meshing
Figure 20: Mesh
5th step: creation of the plan section
In this step we should choice carefully the position and the direction of the plan sections. In effect, the number of curves influences the surface quality.
Figure 21: Plane Section Tools
Figure 22: Plane Section
6th step: creation of curves
There is 3 types of curves: curves on scan, curves on mesh and 3d curves.
The curves on scan are the intersection of the plane section with the cloud of points. And the curves on mesh are the intersection of the plane section and the mesh. However, the 3d curves are independent of both scan and mesh.
Figure 23: Curves on scan
7th step: surfaces creation
In this step, we create the surface from the mesh.
We can use 4 methods:
1- Surface network: From curves network composed of guide curve and contours. Its easy to get but it have bad surface quality.
Figure 24: Curves network
Figure 25: Surface network
2- Multi-Suctions: From profile curves and curves of guide. It takes time and requires a huge number of curves. But, it can get an acceptable surfaces quality.
Figure 26: Multi-section surface
3- Power fit and filling: from a contour or intersection of curves. It requires a huge number of intersections and get a bad surfaces quality.
Figure 27: Power fit
4- Automatically Surface: The easiest method. But it requires an acceptable scan and mesh quality. And we get a mediocre quality.
Figure 28: Automatic Surface generator
Figure 29: Automatic surface
So, to get the best surface quality by the easiest way we develop this methodology:
1- Use the automatic surface generator to get the initial surface
2- Determine areas to correct.
3- Use multi-scan with 3D curves
4- Analyse the surface using analyse tools from free style workshop
5- Repeat the 2nd, 3rd and 4th point until getting the best surface quality that respect the more possible details and curves continuity.
Finally, we get this result:
Figure 30: Final Surface
Figure 31: Final result analyses
We use this methodology to for modeling the top cover. And we get this result (next figure).
Figure 32: Top cover
In this chapter, we used reverse engineering technology: SCAN to CAD, to complete the 3D model of the storm. This model well be used in the next chapter to find the geometry and the aerodynamic coefficients for the Simulink model. But, before that, we should have a theoretical study of the different aerodynamic models to choice our mathematical model.