Electric propulsion has gained popularity over the recent years because it has no environmental impact during its operation. However, if the vehicle has no external energy supply (like a train connected to a power line), the key point is the energy source. Compared to fossil fuels, all the known alternatives have a huge disadvantage: they can store way less energy for the same space and weight.
This is known as energy density and specific energy and are the amount of energy stored per liter or kilogram respectively. Gasoline for example has an energy density of 46MJ/Kg while Lithium-ion batteries have around 0.8MJ/Kg. More than 40 times higher!
This huge difference is critical in aircrafts where weight and air resistance have to be minimum.
In order to overcome this issue, an external energy source like the sun has been extensively explored using solar panels in aircrafts. Some of these projects, like Solar Impulse were really ambitious and generated good results. All of these projects are centered on maximizing the autonomy of the aircraft, thus restricted to slow soaring planes with huge wing surfaces so they have both enough area for solar panels and less power requirements.
Aside from soaring solar planes, there is a lack of projects looking into innovative configurations and geometries for electric airplanes. Most of the designs are based on conventional internal combustion engines planes that are adapted to electrical propulsion; in some cases only the engine is replaced, keeping the original structure.
The electric propulsion system is completely different from an internal combustion or jet engine. Such differences could be used right in the ideation of the design so they become an advantage rather than an issue. Among all the projects found in the state of the art research, the few ones more related to this motivation were the Flight of the Century and the E-fan projects shown below.
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. The goal of the thesis was 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 balancing 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, aiming at making electric vehicles more attractive. Similar to how Formula 1 technology and advertising applies to normal cars.
After that came the fun part. With all the above requirements in mind I explored many fitting configurations, pointing out their pros and cons in terms of aerodynamics, structure, manufacture, operation and maneuverability.
Some of the ideas were really interesting but were developed as separate projects because they did not fit the objectives of this particular project.
Out of the brainstormed ideas, I selected 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 with 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 was shifted forward. This allowed for an easier structure and weight balance. Several ideas for the tail were explored. Finally, the ground clearance at takeoff was 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 is 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.