Paris Air Show

Paris 2011: Solar Impulse To Perform Le Bourget's Greenest Flying Display (but only if it stops raining)

 - June 23, 2011, 4:30 AM
Solar Impulse

The world debut of solar-powered Solar Impulse airplane could hardly have happened in less auspicious weather conditions.

First, it faced a battle against strong winds to get to Le Bourget from its most recent port of call in Brussels. And then, as of press time, heavy rain and shifting winds had prevented the solar-powered aircraft from making its planned flying display appearance here at the Paris Air Show.

In a pre-show interview, André Borschberg, company CEO and pilot, said that the Solar Impulse’s flying display is planned to last about 20 minutes. But he admitted that the aircraft would stay grounded in the event of rain and convective turbulence. On the runway, a seven-knot wind is acceptable, but crosswind is limited to four knots.

The single-seat HB-SIA is just the prototype of HB-SIB, a larger airplane that has the goal of flying around the world in 2014. But this is more than just the personal flight of fancy for program co-founder Bertrand Piccard, whose real goal is promoting renewable energies.

After flying last year and taking a winter break, Borschberg resumed flying in the spring from Solar Impulse's base in Payerne, Switzerland, to expand the flight envelope.

One of the flights focused on turbulence. So the team went looking for it on purpose, to see how the plane would handle it.

“When the plane is flying in calm conditions, it’s a real pleasure to pilot it,” Borschberg said after a two-hour sortie. “She’s docile and gentle. But when she gets into turbulence, she’s something of a shrew, and much harder to control. If a “bubble” of warm air only touches one wing, the plane will become unbalanced. Then the pilot has to fight to correct the plane’s trim.

The solar-powered airplane’s first international flights were from Payerne to Brussels and then Brussels to Paris. The aircraft was displayed in Brussels last month as part of a week of events organized on the occasion of the European Renewable Energy Council's annual conference. The European Commission is a sponsor of the project.

The flight to Brussels was a big technical challenge for the Solar Impulse team. Flight director Raymond Clerc and his team had been preparing for months.

“To operate in environments as complex as the international air traffic network and the taxiways of Brussels airport, we needed to anticipate and study every eventuality, so as to be able to react quickly if unexpected circumstances arise,” Clerc explained.

Next year, the HB-SIA will attempt longer international flights.

The second aircraft in the project–HB-SIB–is in the design phase with a goal for manufacturing the first parts this summer. Rollout and first flight are slated for 2013, one year before flying around the world with a few stopovers.

The first airplane’s main feat so far has been to fly through the night. Flying over his native Switzerland, Borschberg managed to stay in the air for 26 hours and nine minutes in July last year. When the aircraft took off, its batteries, weighing close to 900 pounds in total, were charged at 75 percent.

The objective during daytime was to accumulate as much energy as possible. Electrical energy was stored in the batteries, thanks to 11,628 solar cells on the wing and horizontal stabilizer. In addition, Borschberg climbed the airplane to 28,000 feet to gain potential energy.

During the night, he focused on keeping a minimum sink rate. This translated into a 23-knot airspeed. The sink rate was high at the beginning of the descent, close to 100 feet per minute, because of the thin air. It decreased and came eventually close to zero at the end of the night.

There is no autopilot in the HB-SIA and so no sleep for the pilot. Borschberg used yoga techniques–breathing, exercising muscles without moving and maintaining concentration. He also had a very simple accessory–a water spray for his face.

At dawn the sun rays were powerful enough to recharge the batteries, in addition to powering the motors. At this point, the aircraft could have started a new day/night cycle–had the pilot been able to stay awake indefinitely. HB-SIA landed with several hours of flying power in its batteries.

HB-SIA has a wingspan of 208 feet, a length of 72 feet and a weight of 3,500 pounds. Solar energy is captured by 11,628 solar cells, of which 10,748 are on the wing and 880 are on the horizontal stabilizer. The solar energy is converted to electricity, which powers four 10-horsepower electric motors and is stored in batteries. HB-SIA takes off at 19 knots and cruises at an average speed of 38 knots.

As proven by Borschberg’s overnight flight, the airplane’s endurance is limited only by the endurance of the pilot.

Solar Impulse by the Numbers

  • Wingspan: 208 feet
  • Length: 72 feet
  • Weight: 3,500 pounds
  • Power: four 10-hp electric motors
  • Solar cells: 11,628 (10,748 on the wing, 880 on the horizontal stabilizer)
  • Average flying speed: 38 knots
  • Takeoff speed: 19 knots