Trasgo
Preliminary design of a light, electric and aerobatic aircraft
Final thesis Degree on aerospace engineering Polytechnic school
Alejandro GarcĂa Soto 10/06/2014
Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Universidad Europea de Madrid
Title Trasgo: Preliminary design of a light, electric and aerobatic aircraft
Author Alejandro García Soto
Supervisor Raúl Carlos Llamas Sandín
Degree or course Aerospace engineering
Date 20/06/2014
Alejandro García Soto
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Abstract This project tries to find and propose a new configuration or geometry for electric planes that allow them to enter the aerobatic and exhibition sector. Electric propulsion is considered from the beginning in order to achieve advantages that would not be possible other way. A new powerful and agile figure is to land in the electric aviation community.
Resumen Este proyecto trata de proponer una nueva configuración o geometría dentro de los aviones eléctricos que les permita introducirse en el campo de la aerobática y las exhibiciones aéreas. El sistema de propulsión eléctrico se tiene en cuenta desde el primer momento para conseguir ventajas que no se podrían alcanzar de otro modo. Una imagen ágil y potente está a punto de aterrizar en el panorama de la aviación eléctrica.
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Universidad Europea de Madrid
Acknowledgements Almost in everything we want to do, develop or create we can find some already existing roads that can get us closer, avoiding misleading routes and fatal mistakes. These roads were paved by many people before us. Their ideas, work and experiences are the shoulders of the giants we climb on to discover higher routes. None of this is relevant, however, without people around that make you feel like you can make it. It does not really matter if it's true because trying hard is the only way to find out. I want to show my deep gratitude to them. To my parents that want to keep me safe and comfortable, are struggling and confused with every decision I make, but always support me in the end. To my professors, the ones that teach with their own voice and encourage their students to build their own. In particular to my supervisor RaĂşl who always pushed me to pursue my ideas and showed me another perspective of engineering. To my friends that always show a different point of view, support me when I need it and help me with my living. To Carmen, for everything.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
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Universidad Europea de Madrid
Index Abstract/Resumen
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Acknowledgements
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Index
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Motivation: the reasons behind this project
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Objectives: From general to specific
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Top level requirements
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Exploration: Brainstorming and analysis of ideas
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Evolution: The development of the concept
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Consolidation: Final check and geometry details
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Alejandro GarcĂa Soto
Views of Trasgo
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Aerodynamic surfaces
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Weight balance
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Conclusions
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References
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
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Universidad Europea de Madrid
Motivation: The reasons behind this project
Motivation
The reasons behind this project Electricity is not a human invention. However, the knowledge on how to manipulate it has been, and still is, one of the most life-changing discoveries of mankind. Since the early days people have always feared lightings, felt electrostatic charges and wandered about electrical fishes. It was in the 17th century when deeper investigation started and the word electricus was coined by William Gilbert. Two centuries later, people started to manipulate and control the electromagnetic phenomena and the first electrical motors began to spin. Since then, electric vehicles have always been around, increasing the attention whenever petroleum crisis rise and fading when oil companies push the automotive market. Nowadays hybrid and electric are attractive words in car advertisement. Electric propulsion has been used since functional electric motors were introduced (trains and trams are a great example). Electric motors are small and very efficient compared with internal combustion engines, and using electricity makes them independent of the energy source as long as this source can be transformed into electric potential. This independence is a huge advantage because separating the energy source from the motor makes it easier to find the most efficient decision on each side. The most suitable motor can be used and the energy source can be changed as much as desired. Furthermore, if clean sources like solar or wind energies are used, the total environmental impact of the propulsion system is reduced to the manufacture of the components.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
The benefits of electric motors are far from discussion 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 onboard (like cars or planes) the electric propulsion system has one big drawback: the energy storage. More specifically the energy density. The energy density is the energy provided per unit of mass or unit of volume. It is critical for any vehicle because of the inertia and space available, but especially for flying vehicles as it affects directly the power consumption and the performance in the air (mass) and the aerodynamics (volume). Gasoline for example has a specific energy of 46MJ/kg and 36MJ/L while Lithium-ion batteries have less than 1MJ/kg and around 2MJ/L and hydrogen (compressed at 70MP) has 142MJ/kg and 5.6MJ/L. One of the most promising energy sources for electric flying vehicles is the solar energy. Planes normally have some relatively large surfaces (wings) to place the solar panels and can go above the clouds to receive more direct sun light. However, the same problem of the energy density persists: too much area is needed to provide low power. 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 is not much projects looking for alternative configurations for electric airplanes that could be built now. Furthermore, most of the designs are based on conventional internal combustion engines planes that are adapted to electrical propulsion; some projects go further and simply replace the engine and use exactly the same structure.
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www.solarimpulse.com
Universidad Europea de Madrid
Motivation: The reasons behind this project
The electric propulsion system is completely different from internal combustion engine and jet propulsion. These differences should be used right in the ideation of the design so they become an advantage rather than an issue, and allow the electric planes to evolve in its own direction. Following the conventional designs made for other propulsion systems will only make planes on the wake of others. Among all the projects found, the ones more closely related to the spirit of Trasgo are the Flight of the Century and the E-fan projects.
http://www.flightofthecentury.com/
Airbus technology demonstrator E-Fan
The flight of the century project, led by Chip Yates, is aimed at breaking records in electrical flight. They use the Long-ESA plane designed by Burt Rutan, a modified long-EZ adapted to carry batteries and a motor. The E-fan project by innovation works from Airbus Group is researching on the use of an efficient nacelle to improve the efficiency of the electric motors. This plane was designed from scratch to be electric.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
With the reasoning stated so far, the questions that made this project start were: What can I do to try to change the oil-based economy and polluting transportation network? This is an aerospace degree and I think electric systems are the best way to combine both design and sustainable energy source freedom ‌ Let's design an electric airplane! So I want to push electric vehicles. How can I do it? How do I see the electric vehicles now? When I think of electric vehicles right now I think of solar planes and electric cars. When I think of leading edges on electric cars I think of Tesla Roaster electric supercar beating a Mustang (I don´t like the mass product itself because is a rich people toy, but I like the change it makes on the perception of electric cars). When I think of the leading edge of electric planes I think of RC-planes, or the solar impulse flying nonstop. UAVs and solar planes being able to fly non-stop are both directions with huge opportunities and advantages. But I want to go in a different direction with this project: I want to make an impression in order to push new alternative ideas to flourish. I want to see an electric superplane on the airshows and competitions that shifts the perception on the electric planes and opens up the possibility for the whole range of aviation to become electric in the near future with actual technology, not in a potential future with super-batteries.
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Universidad Europea de Madrid
Objectives: From general to specific
Objectives
From general to specific
By introducing the electric plane into the aerobatics and competition sector this project wants to make an impression, propose an attractive image and motivate further acceptance and new ideas on the whole range of electric vehicles.
Of course that is 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 demonstrator to add a new game for the electric plane: high performance aerobatics. Out of these abstract objectives, some general but more specific requirements are derived in order to start a research on the desired characteristics of the Trasgo plane. What characteristics does Trasgo need to have? •Electric propulsion •Autonomy for one aerobatic show/competition •Good Aerobatic performance •Realistic manufacture
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Once some general requirements of the design are stated, research and analysis is done on specific subjects in order to try to find more specific requirements and limitations. Autonomy Autonomy is of great importance in many uses of an aircraft like transportation or surveillance, but not in the recreational, sport or exhibition fields. A single pilot will be really tired after 2 hours (car drivers need to stop after 2 hours), aerial exhibitions flights are not longer than 30 minutes and normal weekend and private pilots don't even fly for more than one hour (if they go for a long trip, they plan some stops on the way). For this reasons, autonomy is not going to be the driver of this project as long as a minimum of 30 minutes to do some aerial exhibition is covered. Aerobatic performance The first point was whether design the airplane for two pilots thinking on the use as a trainer or keep it to one and save weight and complexities. Because this first step is to design a demonstrator and a prototype needs to be built before further development, the simplicity of one pilot is preferred. The design of a trainer version with two pilots is left out of the scope of this first iteration. Taking a look at existing aerobatic airplanes and their weight to power ratio and assuming the total weight of this airplane is going to be the same order of magnitude, an estimate of 200kW is defined as a first shot for the airplane to have a decent weight to power ratio. Several pilots were asked for both objective and subjective aspects of aerobatic flight. The most relevant information was provided by the pilots in the www.aerobaticsforum.com community being the entry post for this research one of the
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Objectives: From general to specific
most active and viewed. Sadly, the forum has been closed some weeks ago because of a lack of funds. The outcome provided by the pilots was that a decent roll rate and a sharp stall are the most important flight characteristics. They prefer direct controls through cables, and not fly-by-wire systems. Realistic manufacture This is not going to be a mass production aircraft. As a technology demonstrator, it will need to be proven by a prototype. A single prototype that will be redesigned afterwards. For this reason, no economies of scale or batch production systems should be used. It should be as easy and cheap as possible to build just one aircraft. In order to be built in a reasonable period of time no further innovation on any systems included should be necessary either, as they could delay considerably (or forever) the production of the airplane. Several companies were asked to provide information on commercially running electrical propulsion systems, more specifically on motors and batteries. The technology that seemed to be more suitable in terms of weight efficiency and also had been proven and developed for some years now is the propulsion systems used to self-propel gliders, either as a kit to be installed on an existing glider or the ones used in the design of new motor gliders. One motor and battery system is chosen. However, it is important to keep in mind that no deep market analysis have been done, and the products selected are just for geometrical reference. Products already available in the market were chosen despite of potential or future better alternatives for which there was no data available.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
They put one big problem on the table that was not expected. The bottle neck for the electric vehicles was the batteries because of their energy density which makes vehicles struggle with a tight tradeoff between energy storage and weight. But one more reason was not taken into account: the constant power delivery. There is no current battery able to provide more than 50kW constantly in a safe way; so they said there was no possibility to provide one 200Kw motor for this project. The products selected were the propulsion system called Front Electric Sustainer (FES) by LzDesign. Despite this motor-battery system provides only 20kW power. LzDesign, as many other companies in this sector, make the propulsion systems on request and can adapt to the needs of the customers. They stated that a motor-battery system providing
http://www.front-electricsustainer.com/Manuals/FESM130%20MOTOR%20manual%2 0v1.0.pdf
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Objectives: From general to specific
http://www.front-electricsustainer.com/Manuals/FES%20 BATTERY%20PACK%20GEN2% 20manual%20v1.12.pdf
50kW of continuous power was possible to manufacture on demand. This value is the one used in the evaluation of the performance. For the geometrical reference the 20kW system is used. The expected power for the airplane is 200kW, so the solution came along:
Four separate and independent battery-motor systems providing 50kW each.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
With the research made so far, some more specific requirements can be stated: Top level requirements: •One pilot •200kW of electric power distributed in four independent systems •Low moment of inertia •30 minutes of autonomy •Ease to manufacture one single prototype
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Exploration: Brainstorming and analysis of ideas
Exploration
Brainstorming and analysis of ideas In order not to restrict from the beginning the design possibilities to 4 propeller configurations, different ways of joining the motors into one propeller were explored.
The first approach was to feed an external ring from the inside with the motors. This design would allow for changes in the diameters both on the external ring and on the motors to play with. These differences in diameter could be used as a gear to adjust the rotation speed. This design is the one that would be used in the configurations using one single propeller that will be explored later.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Motor 3 Motor 2
Stators
Motor 1 Rotors
Another idea would be to put each motor on top of the other, sticking together the rotor of one motor to the stator of the next motor and so on. The rotations of each motor will sum up. This design has many problems like the diameter it would reach with four motors or the need for brushes between stages to transfer the power.
Other alternative would be to connect all the motors to a common gear driving the propeller. This would allow different allocations of the motors but in this case the motors would be perpendicular to the axis of the propeller, requiring most probably bigger fuselage diameters to fit in.
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Exploration: Brainstorming and analysis of ideas
Stators
Rotors
One last option would be to join all rotors together in the longitudinal dimension and have the stators fed from separate batteries. This would require, in order to avoid brushes and bigger complications, that the rotors where permanent magnets. This requirement, depending on the size of the motors, may push the safety of the system too much.
Despite this exploration was done in order not to restrict the design possibilities, the four propeller solution is thought to be more simple and efficient in terms of mechanism, manufacture, assembly and maintenance.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
The first configuration idea was to analyze the common geometry for the aerobatic aircraft already in the market: motor in front, one main wing and empennage in the back. Afterwards the designs disperse trying to figure out how to arrange the electric systems and make an advantage of them.
Common configuration, easy to manufacture and operate
Batteries could fit here
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Exploration: Brainstorming and analysis of ideas
Propulsion spread wide along the wing for propulsive efficiency
Increased inertia
Motors spin in opposite directions to recover rotational energy
Distribution of the propulsion is mantained but the inertia is reduced
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Motor in the center of gravity, reduced moments
Good rolling rate
Long landing gear and curved lateral shape for propeller clearance
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Exploration: Brainstorming and analysis of ideas
Opening of the diamond shape Good pitch rate
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Tail control increased by the motor wake
Structure will push controls or avionics elsewhere
Boundary layer suction
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Tail downwards to protect propeller
Universidad Europea de Madrid
Exploration: Brainstorming and analysis of ideas
Increased inertia and instability if motor fails
Complicated structure
Reduced inertia
Distributed propulsion
Boundary layer suction
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Center of gravity will go too much forward
No active lateral control
Boundary layer suction
Special control surfaces to compensate the lack of vertical stabilizer
Spread propulsion
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Universidad Europea de Madrid
Exploration: Brainstorming and analysis of ideas
No active lateral control
Use of two wings as a fairing to increase motor efficiency. Span will be reduced and so the inertia
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Boundary layer suction
Tail control increased by motor stream
Motors aligned with the line of flight
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Universidad Europea de Madrid
Exploration: Brainstorming and analysis of ideas
Everything merging as much as possible towards the center in order to reduce the inertia.
Tail control increased by motor stream
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Fairing to increase efficiency and reduce diameter
Fairings may be used as structure for the tail
Vane to suck boundary layer from the fairing
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Exploration: Brainstorming and analysis of ideas
Air inlet
Motors
More diameter, less rpm
Less diameter, more rpm
Propulsion going inside the structure Structure is taken completely outside Air inlet
Structure allows the air to flow through
Alejandro GarcĂa Soto
Space not useful in normal configurations is the duct for the exhaust
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Air inlet
Air outlet
Clean outline
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Evolution: The development of the concept
Evolution
The development of the concept Starting from the last drawing as a spark, several approximations were done mixing hand drawing and software for accurate dimensions and sizing (both blender and Catia). Blender was used as a more creative and free modeling tool (mainly at the beginning) and Catia was used for parametric surfaces and for the final CAD drawings. The space needed behind the pilot in order to provide a smooth flow into the motor renders the obvious solution for placing the heavy and bulky batteries. The cabin lines flow surrounding both the pilot and the batteries. Batteries arranged iteratively
Center line of the plane is defined
Pilot and motors are placed aligned with the center line
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
The first 3D model of Trasgo looked like this:
Starting with the model in Catia. The arrangement of the batteries keeps changing.
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Evolution: The development of the concept
It was noticed that the fairing of the motors was far longer than necessary, increasing the resistance both inside and outside. The first intention was to locate the reduced fairing at the rear to suck as much boundary layer as possible. This geometry presents several structural and weight balance complications.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Following the iteration, the motor group is shifted forward. This allows for an easier structure and weight balance. Several arrangements for the tail are explored. Finally, the ground clearance at takeoff is the driver of the decision at this point.
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Evolution: The development of the concept
So far, all forces from the tail were assumed to go through the cylinder. However, a continuous beam through the whole structure seemed more efficient. Both positions up and under the cylinder were assessed, but the upper one seemed simpler and the beam could go straight to the tails.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
With the overall configuration looking like the picture above, more exploration is done on the structure and the materials to be used. Despite the common use of carbon fiber in the industry, the idea of aluminium structure comes along in order to reduce manufacture cost. Using CFRP would need to rely on serial production to pay of the tooling costs. Because this project is to be one single prototype at the time, it may be more profitable to avoid expensive tooling and processes. The first approach is to do the whole airplane in aluminium, using tubes for the structure and developable surfaces for the external shape. Using developable surfaces would have an impact on the aerodynamics of the airplane, but was thought to be compensated by cost and ease to manufacture. Making a cheap prototype to prove the concept and leave the aerodynamic refinement for following iterations of the prototype made sense.
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Evolution: The development of the concept
Transparent plastic, could be made two curbature surface Cylinder
Cylinder
Cone
Cone
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Up to this point, the first structural idea to assemble the wing is explored. Sheet metal removable covers
Batteries
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Evolution: The development of the concept
Due to the internal structure in aluminium, the external surfaces had no need to carry structural stresses. They could be done in hand lay-up carbon or glass fiber and cured at room temperature and pressure, using cheap foam as mold. This way, the inlet of the motor group could be bigger and still have an aerodynamic shape. Furthermore, the aesthetics of the cabin could be improved again.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
Further iterations of the structure, wing attachments and battery positions. The wing is spitted in half for transportation and handling reasons and the batteries are shifted forward to account for the weight balance.
It is noticed at this point that the plane may still be too tail heavy due to further approximations. The batteries are shifted forward again and the wing is shifted backwards to solve this problem.
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Evolution: The development of the concept
The wing attachment structure is also changed. Now is more similar to many general aviation airplanes. This design puts less stress on each single beam and looks less critical to manufacture and assembly mistakes or imperfections.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
It is noticed that there are still unuseful surfaces in the rear of the motor group that do not contribute to the structure or the performance.
If the cylinder was to be part of the structure and carry a significant load, it would need a substantial increase in thickness. Giving the upper beam the responsibility for the tail loads, and extending the attachment points of the cylinder backwards, the cylinder is now not structural anymore and can be manufactured as a thin aluminum sheet. Only some circular reinforcement may be needed in the front edge of the cylinder. In one last iteration, the upper bean is optimized for torsion using a pure circular cross-section.
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Universidad Europea de Madrid
Evolution: The development of the concept
Alejandro GarcĂa Soto
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
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Consolidation: Final check and geometry details
Consolidation
Final check and geometry details Throughout the whole evolution process, the geometry was constantly assessed regarding weight balance and performance both on ground and airborne. However, it seemed of no purpose to include each and every examination. Now, with the last iteration of the design so far, weight, aerodynamic surfaces and clearance on the ground is checked.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
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Universidad Europea de Madrid
Consolidation: Final check and geometry details
Alejandro GarcĂa Soto
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
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Universidad Europea de Madrid
Consolidation: Final check and geometry details
The wing area is chosen by similarity with aerobatic planes within the weight of Trasgo: In the range of 6 to 7 m².
Light aircraft stability and control by Gordon Robinson
The tail size is analyzed using the volume coefficient. No similar aircraft volume coefficient was obtained, so it was compared to a typical general aviation value of 0,5 for the horizontal stabilizer and 0,03 for the vertical stabilizer, expecting bigger values because of the aerobatic nature of the plane.. Due to the V shape, the projections on each plane are used.
The values are in the expected range: 1,47 vs. 0,97 for the horizontal projection and 0,52 vs. 0,44 for the vertical projection. Due to the obvious differences in the configuration, further refinement is expected using empirical and virtual models as a reference.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
http://www.extraaircraft.com/330SC.php http://www.airbus-group.com/airbusgroup/int/en/news/media.19bf802f1fad-4ce7-b61c-b5d6eab6b51d.-E-Fan+Brochure.html
In this preliminary stage of the design, only several weights are known: The batteries, the motors and the average pilot. The rest of the airplane has to be roughly approximated. Two planes are used in order to have a numerical figure. The Efan for being electric and one pilot; and the Extra330sc for being an aerobatic plane and one pilot. E-fan Empty weight of 500kg. Unknown weight of motors and batteries. Extra330sc (Empty weight 585kg)- (motor weight 200kg) = 385kg Taking into account that these planes are made out of carbon fiber and allowing some margins for the complicated motor attachment in Trasgo, an approximation of 600kg of empty weight is taken.
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Consolidation: Final check and geometry details
Without batteries and motors, the weight is approximately 450kg. Comparing it with the Extra, it seems a reasonable approximation to start with. Follows the detailed calculation for the center of gravity:
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
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Conclusion
Findings and future work With the exploration done so far, it is clear that there are many opportunities for the electric airplane, not only in the aerobatic sector. New designs on solar and transport airplanes may arise and develop. This bachelor project was restricted to the conceptual design: to explore many possible alternative configurations and render a viable concept solution. The future objectives of Trasgo are double: keep working towards the manufacture of a prototype and join the open design community. In order to continue the developmen of Trasgo, the next step is to calculate both the structure and the aerodynamic performance and simultaneously manufacture a radiocontrolled prototype to check assembly and accessibility issues. In order to join the open design community, a web page will be built in order to share all the drawings and design solutions. Videos and images will explain the features, challenges and opportunities of this concept. Future involvement of universities and open source enthusiast will be promoted. The aim of this project is to develop on many different directions within the personal and educational sectors, maintaining the open source contract of sharing alike, so other people may be able to work further on the ideas.
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Preliminary design of a light, electric and aerobatic aircraft: Trasgo
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References Reference for specific data is provided close to it in the document body. Further references and sources of information: •Lecture notes from Raul Carlos LLamas Sandín on Aircraft design course on Aerospace engineering, UEM. •www.flightglobal.com •en.wikipedia.org •www.extraaircraft.com •www.airbus-group.com •www.nasa.gov/
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