The return of Light Flight

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Solar Impulse in flight

Andy Pye revisits the Solar Impule project and finds out how circumnavigation by aircraft was ultimately proven to be possible without using a drop of fuel.

Solar Impulse has completed its round-the-world flight without using any fuel. On 26 July, the Solar Impulse 2 (Si2) aircraft landed in Abu Dhabi, the original starting point of its 17-leg, 43,000-kilometer journey. By landing back in Abu Dhabi after a total of 23 days of flight, Si2 has proven that clean technologies can achieve the impossible.

Taking turns in the single-seater 3.8m3 cockpit, Bertrand Piccard and André Borschberg crossed Asia, the Pacific Ocean, the USA, the Atlantic Ocean, the Mediterranean Sea and the Middle East.

A few hours before touching down in Abu Dhabi, Piccard spoke to UN Secretary-General Ban Ki-moon live from the Si2 cockpit: “Solar Impulse has flown more than 40,000km without fuel, but with an inexhaustible supply of energy and inspiration. This is a historic day for Captain Piccard and the Solar Impulse team, but it is also a historic day for humanity,” said the UN Secretary-General. “You may be ending your around the world flight today, but the journey to a more sustainable world is just beginning. The Solar Impulse team is helping to pilot us to that future.”

“This is not only a first in the history of aviation; it’s before all a first in the history of energy. I’m sure that within 10 years we’ll see electric aeroplanes transporting 50 passengers on short to medium haul flights. But it’s not enough. The same clean technologies used on Solar Impulse could be implemented on the ground in our daily life to divide by two the CO2 emissions in a profitable way. Solar Impulse is only the beginning, now take it further!” said Piccard.

An entrepreneur and skilled aviator, Borschberg took on the technical challenge of developing the solar aeroplane and making it fly. More than taking turns at the controls of Si2 in the air, the first ever Round-The-World Solar Flight is also a tandem achievement on the ground: while Piccard developed the project outreach to promote clean technologies, Borschberg pulled together the team that designed and constructed Si2 as well as organized the flight missions.

“Flying one leg with a completely new type of aeroplane is difficult enough, but flying around the world is a real challenge. More than a demonstration, it’s the confirmation that these technologies are truly dependable and reliable,” emphasised Borschberg. “There is so much potential for the aeronautical world: while 100% percent solar-powered aeroplanes might take longer to materialize, electric aeroplanes will develop in the near future because of their tremendous advantages such as energy efficiency.”

Based on experiences with the HB-SIA, the Solar Impulse 2 has been given a longer wingspan of 71.9m, slightly less than that of the Airbus A380. Even with such a massive wingspan and all of the batteries required (633kg), it weighs little more than an average automobile (2300kg). 17,000 solar cells line the top of the wings, and energy-dense lithium-ion batteries are used to sustain night-time flying.  Three ABB engineers joined the Solar Impulse team where their work includes improving control systems for ground operations and enhancing the charging electronics for the plane’s battery systems.

“We have flown 40,000km (around the world) without fuel, but there is still a lot to be done to encourage a worldwide implementation of clean technologies and to motivate everyone to reduce their dependence on fossil fuels in their daily lives, hence the creation of the International Committee for Clean Technologies,” added Piccard.  “Solar Impulse is of course very well positioned to contribute to the next generations of manned or unmanned electric aircrafts. By capitalising on the engineering skills and expertise gained over the past decade, we will continue to work to encourage concrete innovations and disruptive technologies.”

To create its solar-powered aircraft, the Solar Impulse design team used “Engineered to Fly” on Dassault Systèmes’ 3DEXPERIENCE platform. Applications for 3D modelling of complex structures and composites, digital simulation and full data traceability enabled them to virtually experience the aircraft in its operating environment before it embarked on its record-setting voyage, and be successful with the first attempt.

“What an extraordinary experience to have witnessed the culmination of this 12-year design project and collaborative effort to build and fly an aircraft that many thought impossible,” said Bernard Charlès, Vice Chairman & CEO, Dassault Systèmes. “Daring to dream, pushing the limits of aviation, echoing the imaginative spirit of past pioneers … congratulations to pilots André Borschberg and Bertrand Piccard and the entire Solar Impulse team for this milestone achievement—not just in aviation, but in demonstrating sustainable solutions for the future of our planet. Passion for innovation makes possible the impossible.”

The 3D geometry was fed into the Siemens Femap and NX Nastran finite element analysis systems using the STEP or IGES common formats. Analysts initially used the geometry of the wing’s outer surfaces to create a simple analysis model. Later, they added 3D solid elements representing the Kevlar aramid paper honeycomb core for more detailed analyses, such as local and global buckling.

FEA also played a crucial role in minimising the cockpit weight. It is tiny (3.8m3), but is still three times larger than in HB-SIA, yet still weighs less than twice as much as the original (60kg vs 42kg)..

The wing structure is a third area where FEA has contributed. The new wing is a Kevlar honeycomb core covered with carbon fibre. Because the operational plane flies faster than the prototype, its wings had to withstand greater loads. Analysts went from a material weighing 100g/m2 to one weighing 25g/m2. Similarly, the motor gondola has to carry a heavier load, achieved by changing from a framework structure with fairing to a sandwich structure.

Décision, based in Ecublens, Switzerland, produces innovative composite structures and is the official supplier for the Solar Impulse. The company was responsible for the production of the main carbon fibre composite structures for both the HB-SIA prototype and the current Solar Impulse 2 aircraft, including the wing’s longitudinal beams, the fuselage, vertical and horizontal stabilisers, the engine gondolas and the pilot’s cockpit.

Décision and North Thin Ply Technology (NTPT) collaborated to build Solar Impulse 2’s parts using NTPT’s Thin Ply Technology carbon fibre prepreg. This technology enabled the production of extremely lightweight, high performance composites structures.

North Thin Ply Technology (NTPT), headquartered in Penthalaz, Switzerland, produces a range of weight-saving prepreg materials, including UD tapes of 30-600 g/m2, conventional prepregs, multiaxial preforms and machinable carbon fibre blocks. The company also produces highly uniform composite tubes and automated tape laying (ATL) machines. NTPT’s products are used in numerous high-performance composite applications in the marine, motorsports, industrial, aerospace, sports and luxury goods sectors.

Advanced plastics from Covestro, formerly Bayer MaterialScience, were also used in the construction to make the aircraft simultaneously extremely lightweight and strong, and protect the pilots from extreme outside temperatures.

Rigid polyurethane foam is one of the best insulating materials. It is both lightweight and strong and is suited for extreme temperatures ranging from +40 to -40°C. It is ideal for a solar aircraft without air conditioning or heating that must be as light as possible.

Transparent, extremely strong, freely formable, a good insulator and only half as heavy as glass. These are the properties that make the high-performance plastic polycarbonate the perfect material for the cockpit glazing of the Si2.

A large portion of the aircraft’s hull is covered with a shiny silver coating formulated with polyurethane raw materials that reflects the sun and is impervious to the wind. Thanks to new process technology, such paints and coatings can be produced with less solvent, require less heat to dry or are even self-healing.

Latest posts by Andy Pye (see all)

About Andy Pye

Andy Pye is a graduate of Cambridge University and has had a high profile career in the technical press as well as being a pioneer in web publishing.

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