Imagination blends with reality when a vision has to become a concrete project. The design, which can be defined in wider terms as a process aimed for deciding how something will work and look like, carries out that function.
Indeed, at Pie Aeronefs SA, one of the most strategic phases of the all-electric UR-1 race aircraft development is represented by the mechanical design engineering work led by our engineer Oliver Ensslin. Giving a shape to an idea and making it concrete are not simple tasks especially when you work on a cutting-edge project such as ours. Those phases into industrial design are closely related to each other as every step in a direction must include every aspect of the other.
In mechanical design engineering, projects need several stages of development and require the respect of strict rules because every little change is part of a general frame consisting of a complicated equilibrium between variables and limitations.
But this process is also highly creative and that is why we, at Pie Aeronefs SA, want to give you the opportunity to have a glimpse behind the scenes of our mechanical design engineering work.
The first thoughts about the design process of UR-1 parts were related to what functions the parts must accomplish. It means essentially what are these parts made for. An aircraft being a complexe system, we often have to optimize and combine multiple functions in a single part. In addition, the design of a part can be influenced by multiple aspects, such as elements and components that are placed near it.
Among all the variables and limitations to take into account, we also have to think about load paths: which means the forces that go through the part. For example, while the design of an element was underway, the mechanical design engineering team was not only focused in finding the most suitable material, but also in finding out the place where the forces and the moments will go.
It’s important to highlight that the weight and the loads are always the main drivers, when you design a part.
There are lots of aspects in the design procedures to take into account. A strategic role is carried out from the discussion between teams that have different tasks. The mechanical design engineering team has to know what the parts manufacturers, e.g. our experts in composite materials, can do regarding their know-how and equipment. At the same time, the design is also influenced by the recommendations provided by our stress analysts.
In addition, during the design process, there are many specific questions that demand accurate answers: will the designed part withstand the environment (corrosion)? What if someone handles it wrong? How can it fail and what could be the consequences? What happens when it reaches its envelope in terms of tolerances? Etc.
Obviously, the complexity of a part conditions the amount of questions as well as the duration of work needed for the mechanical design engineering team. A simple pin might take less than a morning to be designed, be checked by the stress analyst and then be drawn for the manufacturing. On the other hand, our wing, which is our most complicated part so far, requires several weeks of designing work. When a solution does not work, we go back a few steps and start over.
The communication between the teams is fundamental because, as our engineer Oliver Ensslin says: “Engineering is not something one can do alone”.
Choosing a material
Lots of different materials have a place in our aircraft. They all have various properties but the main characteristics we are looking for are the weight per a given volume (density), the load it’s possible to sustain on a given cross section (strength) and how much it will be deformed under a force (stiffness).
Generally, carbon fiber has a very good mix of those characteristics and we use it widely in the wing. But it is not easy to manufacture. Because it is laminated out of fiber, it is preferable to make quite flat parts.
To design parts with a complicated and precise geometry, we do prefer Aluminium. It is easy to mill out of a block but it’s important to consider that the milling head has to reach all the surfaces.
Steel and Titanium have their place too. We mainly use them for small, very resistant parts like bolts and pins. Lately, 3-D printed plastics have gotten much better over the last decades. Through this manufacturing process, it is now possible to create very complicated geometries in a cheap way. But it can not take much load because of the material properties. This aspect circumscribes the opportunities to use it.
Concerning our battery enclosures, we selected a very special material to withstand the heat in case of a fire. After many tests with different materials, we came up with a ceramic that encloses the carbon fiber cloth.
To make the UR-1 electric aircraft design, our mechanical design engineering team uses a specific software called CAD (Computer Aided Design). It is the main tool for a designer and helps the team to see how things can fit together. It allows us to move parts around, try to adopt different solutions and, at last, see how things would work.
The team is also able to send the geometry made to the stress analysts so they can run different analyses of loads withstanding on a specialized software such as ANSYS. After that, the team can export the geometry to our aircraft technicians in order to mill the molds, for example. The CAD is also useful to make drawings for the workshop.
Another key software we use is SVN (Subversion), which is useful to store and organise our data. We have developed a simple and fast process to track drawing versions and releases.
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