The Solar Impulse is a revolutionary concept that will push back the limits of our knowledge in the field of materials, energy management and the man-machine interface. It is an aircraft with an inordinate wingspan for its weight and of an aerodynamic quality that to this day has not been equaled, capable of tremendous resistance, despite its light weight.
From the solar captors to the propellers, it is all about optimizing the different links in the propulsion chain and integrating an environment that is as hostile to the materials as it is to the pilot and of course to respect the weight and resistance constraints. An exercise in high flying.
The construction calls on the most advanced technologies and stimulates scientific research in the field of composite structures, the so-called intelligent light materials, and the means of producing and storing energy. It will be possible to use these results as much in the construction of the aircraft as, subsequently, in numerous other applications useful to society.
The design of the aircraft, pure and futuristic, will itself be the symbol of the spirit of the project in the sky.
The question of energy determines the whole project, from the structure's dimensions to the extreme weight constraints. At midday, each m2 of land surface receives the equivalent of 1000 Watts, or 1.3 horsepower of light power. Over 24 hours, this averages out at just 250W/m2. With 200m2 of photovoltaic cells and a 12 % total efficiency of the propulsion chain, the plane's motors achieve no more than 8 HP or 6kW - roughly the amount of power the Wright brothers had a available to them in 1903 when they made their first powered flight. And it is with that energy, optimized from the solar panel to the propeller by the work of a whole team, that Solar Impulse is striving to fly day and night without fuel!
At the conference Future of Electric Vehicles in San Jose Dec 7-8, electric aircraft will be covered by NASA, ETH Zurich and PC-Aero and many of the new components for EVs that are to be announced by other speakers will be suitable for aircraft. In addition, Hawkes Ocean Technologies covers Deepflight underwater aircraft-like submarines.
The construction of the prototype is the fruit of intense collaboration between the Solar Impulse team, charged with the plane's design, the materials suppliers, the components producers and other partners. It is only by wrestling with the specifications and fully exploring everyone's potential that totally new aeronautic solutions came to light. In the final stages, 50 employees were joined by more than 100 experts and advisers to create an explosive synergy.
Multiple forms of energy have to be managed and their conversion phenomena understood and optimized:
- photic - the mechanics of solar radiation
- electrical - in the photovoltaic cells, the batteries and the motors chemical - inside the batteries
- potential - when the plane gains altitude
- mechanical - through the propulsion system
- kinetic - when the plane increases speed
- thermal - the various losses (friction, heating...) to be minimized at all costs
Efficiency and storage capacity
The 12,000 photovoltaic cells are in 130 micron monocrystalline silicon, selected for its capacity to combine lightness and efficiency. Their efficiency could have been higher, following the example of the panels used in space, but their weight would then have penalized the plane during night flight.
This phase being the most critical, the main constraint of the project today lies with the batteries. Still heavy, they require a drastic reduction of the weight of the rest of the plane, so as to optimize the whole energy chain and to maximize the aerodynamic performance provided by a large wing span and a wing profile designed for low speeds.
With an energy density of 200W/kg, the accumulators needed for night flight weigh 400kg, or more than ¼ of the total mass of the plane. Improving battery capacity would eventually allow a second pilot, a smaller wingspan or a higher flight speed.
The on-board computing system gathers and analyses hundreds of flight management parameters, giving the pilot information to interpret for making decisions, transmitting key data to the ground team and, above all, providing the motors with optimal power for the particular flight configuration and battery charge/discharge status. In this way the plane can self-correct and minimize its energy consumption.
Under the wings are four pods, each containing a motor, a polymer lithium battery consisting of 70 accumulators, and a management system controlling charge / discharge and temperature. The thermal insulation has been designed to conserve the heat radiated by the batteries and keep them functioning despite the -40 °C encountered at 8,500 meters. Each motor has a maximum power of 10 HP. A gear box limits the rotation of each 3.5 metre diameter, twin-bladed propeller to 200-400 revolutions/minute.
Structure and materials
To attain a 61m wingspan with the necessary rigidity, lightness and flight controllability and with just 1500kg take-off weight is a challenge which has never been achieved until now. Solar Impulse is constructed around a sort of skeleton in a carbon fibre-honeycomb composite using a sandwich structure. The undersides of the wings are covered with flexible film and the upper surface with a skin of encapsulated solar cells. One hundred and twenty carbon fibre ribs placed at 50cm intervals profile these two layers and give the body its aerodynamic shape.
- Wingspan: 63,40 m
- Length: 21,85 m
- Height: 6,40 m
- Weight: 1 600 Kg
- Motor power: 4 x 10 HP electric engines
- Solar cells: 11 628 (10 748 on the wing, 880 on the horizontal stabilizer)
- Average flying speed: 70 km/h
- Take-off speed: 35 km/h
- Maximum altitude: 8 500 m (27 900 ft)