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Solar Impulse Flight by Sun

Credits: ©2009 Solar Impulse

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 with respect to weight and resistance constraints. The craft weighs only as much as a midsize car. Its array of 12,000 solar cells mounted on the wings is designed to pump renewable solar power to the plane's electric motors. Those solar cells also energize the Impulse's batteries during the day, eventually allowing it to take flight at night. Two men, both pioneers and innovators, both pilots, are the driving force behind Solar Impulse. Bertrand Piccard, who made the first non-stop round-the-world balloon flight, is the initiator and chairman. André Borschberg, a fighter pilot and a professional airplane and helicopter pilot, is the CEO.


Solar Impulse Airplane in Hangar

The Solar Impulse is made of carbon fiber concentrate, looks like a glider, and has a wingspan of over 60 meters. The wings are covered in almost 12,000 solar cells which store excess power in over 400 kilograms of batteries. These solar cells power four ten horsepower electric motors, which propel the plane into flight. ©2009 Solar Impulse

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!

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

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.

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.

Solar Impulse Full Specs
Maximum altitude 12,000m

Outside temperatures + 80°C to -60°C

Maximum weight 2,000 kg

Average speed 70 km/h

Wingspan 80 metres Slightly more than the Airbus A380, in order

to minimise induced drag and to provide a

maximum surface area for the solar cells

Power of the engines Max. 40 kW
The average engine power made available

over a 24h period by the sun is comparable

to that used during the first flight by the

Wright brothers in 1903 (12 CV)

Environmental control

and life support system

Elimination of CO2 and humidity

generated by the human body

1 single pilot

Man-machine interface device Under development To provide the pilot with more detailed

information about the airplane's flight

characteristics than normally available on

traditional airplanes. This information could

be derived by other senses than sight and


Essentially constructed from

carbon fiber-sandwhich structure
Using very thin materials with the lowest

possible densities

Batteries lithium , weight of 450 kg,

from 200 Wh/kg battery capacities

Solar cells monocrystaline silicon, 130 micron

thickness, about 250 m2 surface,

min 20 % photovoltaic efficiency

Ultra-thin and integrated in the wings

Human parameters - Sleep management, MMI

Energy parameters
Capturing and channelling of the

energy, battery, engines

Trajectography parameters - The met, hours of sunshine
Several hundreds, even thousands of

parameters to coordinate in order to

develop a machine evolving in an area of

flight still unexplored today. In order not to

penalize the needs of propulsion, success

can only be achieved through optimizing

output and reducing overall weight.

Safety parameters - Reliability

Mechanical parameters materials, mass

Aerodynamic parameters - Quality of flight, loads, performance,

aeroelastic phenomena

Thermic parameters Radiation


Bertrand Piccard's Solar-Powered Adventure Video Link

Solar Impulse Video Link

Solar Impulse Airplane

Solar Impulse