Solar Impulse
HB-SIA

With its huge wingspan equal to that of an Airbus A340, and its proportionally tiny weight – that of an average car - the HB-SIA prototype presents physical and aerodynamic features never seen before. These place it in a yet unexplored flight envelope.

 

Carbon fiber structure, propulsion chain, flight instrumentation, everything has been designed to save energy, to resist the hostile conditions facing airplane and pilot at high altitudes and to marry weight restraints with the required strength.

 

It was not built to fly round the world. Its purpose was rather to demonstrate the feasibility of the program by making the first ever whole day-and-night flight without fuel, a task that it accomplished brilliantly in July 2010. The lessons learned by the team are now being applied to the construction of Solar Impulse 2 (HB-SIB), which is due to circumnavigate the Earth in 2015.

The question of energy defines the entire project.

At midday, each square meter of land surface receives, in the form of light energy, the equivalent of 1000 watts, or 1.3 horsepower of light power. Over 24 hours, this sun energy 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 an average power of 8 HP or 6kW.

That’s roughly the amount of power the Wright brothers had 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, that Solar Impulse managed to fly day and night without fuel!

Seven world records:

  • Absolute height: 9235 m (30300 ft)
  • Height gain: 8744 m (28690 ft)
  • Duration: 26 hours, 10 minutes, 19 seconds
  • Free Distance along a course: 1116 km (693.5 miles)
  • Straight distance, pre-declared waypoints: 1386.5 km (861 miles)
  • Free distance: 1506.5 km (936 miles)
  • Distance along a course, pre-declared waypoints: 1487.6 km (924 miles)
Légende photo

HUMAN RESOURCES

The construction of the prototype would have been impossible without the intense collaboration between the Solar Impulse team, charged with the plane’s design, and the materials suppliers, the components producers and other partners. It was only by wrestling with the specifications and fully exploring everyone’s potential that totally new aeronautic solutions were found.

ENERGY RESOURCES

Multiple forms of energy have had 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 gains speed
  • thermal – the various losses (friction, heating…) to be minimized at all costs

EFFICIENCY AND STORAGE CAPACITY

The 12 000 or so photovoltaic cells of 145 micron monocrystalline silicon combine lightness and efficiency. Their efficiency could have been even higher, like the panels used in space, but their weight would then have been much too high, penalizing the plane during night flight. This phase being the most critical, the main constraint is the one imposed by 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 using a high aspect ratio wing with a low-speed profile. With an energy density of 240Wh/kg, the lithium polymer accumulators weigh 400kg, or more than ¼ of the total mass of the plane.

STRUCTURE AND MATERIALS

Reconciling rigidity with lightness and controllability in flight, Solar Impulse is constructed around a skeleton of carbon fibre-honeycomb composites in a sandwich assembly. The upper wing surface is covered with a skin of embedded solar cells and the lower surface with lightweight flexible film. Between these two surfaces, one hundred and twenty carbon fiber ribs at 50cm intervals give the wing its aerodynamic profile.

PROPULSION SYSTEM

Four pods are fixed under the wings. Each contains a brushless, sensorless electric motor, a polymer lithium battery consisting of 70 accumulators, and a management system controlling the charge and temperature thresholds. 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 output of 10 HP. A gear box limits the rotation of each 3.5 meter diameter, twin-bladed propeller to 400 revolutions per minute. Over a 24-hour period the average power used is 8 hp, or 6 kW.

CENTRAL INTELLIGENCE

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 via telemetry to the ground team and, above all, providing the motors with optimal power for the particular flight configuration and battery charge/discharge status.

How does one prepare for a 20-hour flight in a solar airplane?

Flying a solar airplane is no easy task especially when one considers the size of the cockpit, which amounts to a meager 46 ft³ (1,3 m³)! Today’s Phoenix to Dallas flight, which will last approximately 18 hours, will also allow André to beat his previously established absolute distance world record in the solar ...

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Flying a solar airplane is no easy task especially when one considers the size of the cockpit, which amounts to a meager 46 ft³ (1,3 m³)! Today’s Phoenix to Dallas flight, which will last approximately 18 hours, will also allow André to beat his previously established absolute distance world record in the solar airplane category, achieved in 2012 during the flight from Payerne (Switzerland) to Madrid (Spain). The current flight will total 868 miles (1396.8 km).

But how does André stay focused and awake now that he’s only half way? Apart from being in great physical condition, the pilots also have to prove an extraordinary level of concentration and mental strength necessary to maneuver the unique and extremely lightweight solar aircraft during such long legs. In a cockpit devoid of an autopilot and dependent on the Mission Control Center (MCC) for flight route updates, André and Bertrand have to maintain a heightened level of focus throughout each flight.

The only way André could feel anxious before a flight is if he considered himself insufficiently prepared. Aware of this, every aspect of the flight is analyzed to the smallest detail. For example, the day before getting in the cockpit he closes his eyes and visualizes the first hours of the flight from take-off onward. He imagines all possible developments with different “what if” scenarios, even emergency ones, until he feels comfortable with the indications the Flight Director and his team have briefed him on that day. A final, shorter briefing is always given early in the morning the day of the flight to update the pilot on any potential changes.

André actively engages in Yoga, respiratory exercises and meditation. Yoga keeps his muscles flexible and comes in handy in the cockpit to maintain blood circulation. He regularly controls his breathing for many different reasons. For example, when he’s feeling a little tired he will start breathing faster to increase his energy level – an exercise he likes to do the day before a flight. But he will also control his breathing to relax. He does this by inhaling seven times and exhaling seven times bringing his heartbeat to a regular rhythm and allowing him to relax during calmer moments in the air. Finally, meditation is particularly handy the day before a flight, just before going to bed, to clear his mind from unnecessary thoughts ensuring a deep and restful sleep.

The Borschberg family is known to be fond of good food which explains why André doesn’t particularly like the few days preceding a flight. Don’t get me wrong, he’s ecstatic to know he’ll be soon in the cockpit again! The only downside is that he’s required to pay attention to his diet by completely cutting out aliments rich in fiber, like fruits and vegetables (see Brian’s funny video here to understand why). Even during the flight, André only brings snacks like cereal bars, nuts, dried fruits and maybe a sandwich, accompanied by water or a sports beverage.

With a growling stomach at the time of landing, transforming more into a lion’s roar during the ensuing press conference, it’s not surprising that André likes to indulge in a hearty meal – don’t forget to bring out those veggies!

A blessed take off

Solar Impulse has gracefully lifted off of Arizona’s soil at 04:47AM MST (UTC-7) on its way to Dallas Fort Worth International Airport. Landing is expected to occur tomorrow, Thursday May 23nd around 01:00 AM CDT (UTC-5).

Piloted by André, the solar airplane will be attempting to break the world distance ...

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Solar Impulse has gracefully lifted off of Arizona’s soil at 04:47AM MST (UTC-7) on its way to Dallas Fort Worth International Airport. Landing is expected to occur tomorrow, Thursday May 23nd around 01:00 AM CDT (UTC-5).

Piloted by André, the solar airplane will be attempting to break the world distance record in the solar airplane category, a record previously achieved by the same couple – Solar Impulse and André – when connecting Payerne (Switzerland) to Madrid (Spain) in 2012. The total estimated distance is 868 miles (1396.8 km). The third leg of the Across America mission, Dallas (TX) to St. Louis (MI), will be piloted by Bertrand.

What made today’s departure so unique was the presence of a Native American Medicine Man, Delmar Boni, of the San Carlos Apache Tribe. Dressed in traditional clothing with a beautiful crown of feathers partially covering his face, Delmar circled around the pilots, thumb painted yellow, while chanting a pleasant tune.  

He blessed today’s pilot, André, and the wings that will carry him on his journey to Texas. He also blessed his partner and project co-founder, Bertrand. There was something very soothing and mystical about this ritual. The tone of Delmar’s voice and the comforting flow of words he murmured, although incomprehensible to our ears, couldn’t be mistaken for anything other than a powerful blessing.

Today’s flight will be challenging because of the length - the longest distance ever flown by this aircraft - but it’s off to a good start: accompanied by America’s history and Native traditions, Solar Impulse is on its way to explore the future.  


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