Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach

# Fuselage

During preliminary design, our use of the AirCONICS approach has also allowed us to develop the lifting surface geometries to a reasonable level of fidelity suitable for first-pass analysis and

Figure 17.3 Example configuration studies.

further detailing. Thus far in the design process, however, relatively little thought has been given to the precise shape of any fuselage elements. Typically, the two main functions of a conventional fuselage are to

• • provide a low-drag “package” for payload (cameras, cargo, other sensors, etc.) and systems
• (engines, avionics, batteries, fuel tanks, etc.), and
• • structurally connect masses and sources of load.

In order to construct the “package,” it is useful to have both a structural construction approach in mind and dimensionally correct geometry of the main items that have to be fitted into the fuselage. To lower costs, the construction method may well impose some restrictions on the range of possible fuselage shapes. One way of providing the package geometry is to construct an accurate 3D model of each element to be housed; examples for engines are shown in Figure 17.4. Clearly, such models require a significant amount of effort to construct.[1]

An alternative that requires less effort is to create a simple solid object that is an approximate representation of the object onto which the CAD tool can drape “decals,” which follow the contours of the solid. This can generate very realistic geometry from photographs without a great deal of effort. Such objects can also be given accurate mass properties to aid center of gravity (CoG) calculations. An example is given in Figure 17.5, which is a camera model that we constructed for a UAV project.[2]

At the concept/preliminary design stage, the level of effort required for such high-fidelity component models is possibly not appropriate (if they are not already available), and hence a third alternative is to directly use sketches or photographs in the fuselage model. Furthermore, photo-realistic or complex 3D models at the concept stage can slow down rebuild times potentially, making the model unwieldy and frustrating to use. Figure 17.6 shows 2D drawings of the 3D models given in Figure 17.4. These are a very lightweight representations of the geometry but are good enough to undertake the construction of fuselage geometry concepts.

Figure 17.4 Example 3D models of Rotax aircraft engine and RCV UAV engine. Courtesy of Chris Bill and RCV Engines Ltd.

Figure 17.5 Example of images used to create realistic looking 3D Solidworks geometry model.

Figure 17.6 2D side elevations of Rotax aircraft engine and RCV UAV engine. Courtesy of Chris Bill and RCV Engines Ltd.

Figure 17.7 Scaling dimension added to drawing (mm). Courtesy of Chris Bill.

If one does have a 3D model of the object that one wants to put in a fuselage, it is very easy to create a 2D drawing from this. However, as we will be planning to put a 2D representation directly into our Solidworks fuselage 3D model, it is important to add in an accurate reference dimension so that the image can be scaled to match the fuselage, see Figure 17.7. This drawing needs to be saved as a picture format (we use *.jpg). It is also important to crop the image as close to the geometry as possible to prevent the empty borders obscuring other parts of the fuselage model.

Figure 17.8 “Spaceframe” aircraft structure.

The designer can now start to think about how to “package” these objects into a low-drag fuselage shape. Although we cannot calculate the drag at this stage, we can use some good aerodynamic design principles to try and minimize drag by minimizing the frontal and wetted areas as well as having a generally “smooth” profile.

At this stage, the importance of having parametric geometry becomes all too apparent. We are essentially “hand sketching” a shape around the main fuselage objects. In order to develop a successful low-drag aircraft, we will need to use good engineering practice of analysis and iteration to find the best solution. Detailed CFD studies of the shape will ultimately allow it to be “fine-tuned”; hence the need for a flexible parametric geometry at this stage. In order to do this, we need to sketch a series of closed “bulkhead” profiles that encompass our fuselage objects. At this stage we also need to understand what fundamental materials and processes we are going to use to construct our fuselage. This has a constraining effect on the sorts of shapes that we can produce. A space frame structure (Figure 17.8) cannot be used to create a smooth double-curvature fuselage shape. Therefore we cannot use circular or elliptical “bulkhead” profiles if we intend to use such a structural approach.

For the low-cost UAVs associated with student projects, double- curvature fuselages are difficult to achieve. Double curvature generally implies complex, costly manufacturing processes such as composite molding (for nonmetallic) or stretch-forming/press-forming (for metallic materials) or even large nylon SLS 3D-printed parts that are relatively expensive.

• [1] For some brought-in components, the manufacturers will make available suitable CAD models to customers.
• [2] Of course, this can be achieved only if there are orthogonal photographs available. Instructions on how to createrealistic looking approximate models are given under “Decals” in the Solidworks help files.

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