The benefits of electric motors are obvious in systems like trains or static industrial machinery, where no internal energy storage is needed and the electric power is delivered externally. However, in applications where the electric power needs to be stored on-board (like cars or planes) the electric propulsion system has one big drawback: the energy storage. More specifically the energy density.
Gasoline for example has an energy density of 36MJ/L while Lithium-ion batteries have around 2MJ/L and hydrogen (compressed at 70MP) has 5.6MJ/L.
There are many experimental projects exploring solar planes, some of them really ambitious and generating good results. All of these planes are restricted to slow soaring planes with huge wing surfaces so they have both enough area for more power and less power requirements.
There is a lot of development and research on technologies regarding electric flight and future alternatives (batteries are improving really fast), but out of the soaring solar planes, there are not much projects looking for innovative configurations for electric airplanes that could be built in the present future. Furthermore, most of the designs are based on conventional internal combustion engines planes that are adapted to electrical propulsion; some projects simply replace the engine and use exactly the same structure.
The electric propulsion system is completely different from an internal combustion engine or jet propulsion. These differences should be used right in the ideation of the design so they become an advantage rather than an issue. Among all the projects found, the few ones more related to this motivation were the Flight of the Century and the E-fan projects.
The top level objective was to introduce the electric plane into the aerobatics and competition sector, propose an attractive image and motivate further acceptance and new ideas on the whole range of electric vehicles.
Of course that was a broad and subjective objective that can be approached in many ways. The approach and the specific objective of this project is to do a conceptual design of a technological demonstrator as a high performance aerobatic aircraft.
Out of these abstract objectives, I extracted some general but more specific requirements in order to start a research on the desired characteristics:
Then it came a more intense research on each of these general requirements. Specifically on their state of the art and their allowable limits. With the research done and trying to balance all of the requirements, the more specific top level requirements were:
The aircraft should achieve the same speed and maneuvers as any internal combustion aerobatic aircraft.
It aimed at the aerobatics and competition sector as potential marketing use, trying to make electric vehicles more attractive. Similar to how Formula 1 technology and advertising applies to normal cars.
Now came the fun part. With the above requirements in mind I tried to explore many fitting configurations, pointing out their pros and cons in terms of aerodynamic, structure, manufacture, operation, etc.
Some of the ideas were really interesting but were developed as separate projects because they did not fit for the objectives of this particular project.
Then I took the most promising concept and started developing and pushing it in different directions to see where the limits could be. The concept that I came up tried to group all the elements really tight around the line of flight. Propellers, motors, batteries and pilot were aligned to improve aerodynamics and maneuverability.
Starting from the last drawing as a spark, I did several iterations mixing hand drawing and software for accurate dimensions and sizing. I used Blender as a more creative and free-form modeling tool and Catia for parametric surfaces and for the final CAD drawings.
In this small aircraft I took the height of the seated pilot as the maximum. The space needed behind the pilot to provide a smooth flow into the motor area seemed a good solution to place the heavy and bulky batteries. Then, the cabin surfaces should wrap around the pilot and the batteries.
The fairing of the motors was longer than necessary, increasing the aerodynamic drag both inside and outside. The first idea was to locate the reduced fairing at the rear to suck as much boundary layer as possible (an aerodynamic improvement). However, this geometry presented several structural and weight balance complications.
Following the iterations, the motor group is shifted forward. This allows for an easier structure and weight balance. Several ideas for the tail are explored. Finally, the ground clearance at takeoff is the driver of the decision.
With the overall configuration looking like the picture above, I started to explore the structure and the materials to be used. Despite the common use of carbon fiber in the industry, the idea of an aluminum structure could reduce manufacture cost.
With the internal structure in aluminum, the external surfaces had no need to carry structural loads. They could be done in hand lay-up carbon or glass fiber and cured at room temperature and pressure, using cheap foam as a mold.
Throughout the whole evolution process, I constantly had to assess the geometry regarding weight balance and performance both on ground and airborne.
The final aircraft was to have the same power as internal combustion engines aerobatic aircrafts and a really agile performance. It used the advantages of the electric propulsion like the relative small size of the electric motors and the possibility to place the batteries anywhere in the aircraft.
This was an ambitious project that I enjoyed a lot. It allowed me to develop a concept from the idea to a consistent product.
This was the original thesis document with the research, evolution of the concept and detail drawings, calculus, data, etc. Some details have evolved since the publication of the thesis in 2014.