Our Solar Impulse engineers gave some incredibly good news last week when they announced that Decision, our key supplier of large carbon parts, completed the second airplane’s new wing spar.

In July 2012, during the final structural test of the wing spar, the central part succumbed to the load and broke. This was a vivid reminder that pushing the limits is no easy task and that sometimes, when you’re right on that thin border, you may fall overboard. However, what might have been dramatic in July last year has become an advantage today; the construction delay of HB-SIB brought many new and exciting opportunities such as this year’s Across America mission.

The wing spar is pretty much the solar airplane’s backbone and most important part. It’s the central structure of the wings and, in Solar Impulse’s second generation airplane, it is much larger for a plane meant to fly faster. Consequently, the wings will be subject to more loads by a factor of two. During last year’s failure only the central part of the spar broke and, after thorough testing, the two outer sections of the spar were spared. Nevertheless, the entire spar was rebuilt for consistency reasons, a process that took 10 months, as the design and the production process have since improved. The leftover outboard sections will be kept as a backup.

The wing spar looks like a long rectangular box. It’s fully made out of carbon and it’s glued – or bonded, as the engineers say - together via a very special chemical process, including 20 curing cycles, in a gigantic oven. It takes 64 minutes to bond the parts together and 88 minutes total for the final bracing and cleaning. These time limits must be strictly respected to avoid an uneven process, which can be fatal for the structural integrity of the entire wing spar.

Everything went according to plan thanks to positive collaboration between Decision and our engineers, both working hard to achieve the best and most precise results.  

For more information about the construction of HB-SIB, check out the Making Of or HB-SIB timeline. 

The mood in Dübendorf hit a slump after the disappointing result of the final spar test last week. Despite this setback, an overwhelming amount of encouragements and support have been received by Solar Impulse after the fact.

A flow of emails were sent by partners and supporters voicing empathy for what happened, reaffirming their support for the project, offering their expertise and confirming their confidence. This has certainly been a great booster to the overall mood at Solar Impulse. After the initial shock, the team has gathered to identify everything that can be learned from the problem.

The team of engineers has also come to the conclusion that we will have to build a new spar in order to avoid carrying over the damages from the broken one, but it is still too soon to give an estimate about the delay this will cause in the construction of the second airplane. However, it is also the occasion to look at the problems from a different angle by trying to identify the opportunities that can be seized by the additional time available by this delay. We will of course keep you informed of future developments.

This is the first time a final structural resistance test has failed in the history of the project, but knowing how many people are there to support Solar Impulse is a great reminder of how pushing the limits can also come with some setbacks. Instead of letting those events discourage us it has proven, on the contrary, that is strengthens the team spirit, teaches how never to renounce and allows us to rise up again for innovation.

While the HB-SIA prototype continues its mission in the Kingdom of Morocco, its big brother is beginning to take shape as the Solar Impulse team in Dübendorf slowly brings to life this next generation aircraft.

In the construction of our second aircraft, the spar is the structural backbone of the airplane. Earlier this week we undertook the second structural testing of the spar, the 72 meter long carbon structure that will be the wing’s internal frame. This testing is a key milestone and one of the most crucial moments as we developed a new set of special technologies. It represents full validation of the design and construction technology employed.

A first attempt was made a week ago but, to the horror of the team of engineers, an unsettling cracking sound was heard triggering them to immediately stop the testing activity and verify the problem.

Just like during the testing of the motor gondola, the structural loadings are initially simulated in a computer modeling program before they’re done in reality. The program is used to study the resistance of the wings to the most extreme atmospheric and meteorological conditions the airplane might experience in flight. Although such conditions might not ever take place, it’s a way to ensure the wings will be prepared to resist in any given scenario. In fact, this is very similar to other transportation aircrafts which are also tested before put into operation.

The wing spar is set upside down, as can be seen in the photograph, and lead weights are distributed along the wooden frames, simulating aerodynamic forces. For the spar to be safe for flying, it needs to survive the structural test without any consequences, but as soon as the cracking sound was heard, the team turned pale-white fearful that irreparable damage was done to the spar. Although I am not an engineer, I can imagine how terrible it must be to think that months of hard work could be thrown down the drain.

Thankfully in this case what will most likely come out of the workshop won’t be a damaged spar but a new set of jokes. In fact, it turned out that the terrible screeching wasn’t from the spar or the testing equipment, but from the hangar’s roof! The weather never gets tired of playing bad jokes on Solar Impulse, does it? During the first test, the thermal conditions were so extreme, including a mix of heavy rain, strong winds and intermittent sunshine, eventually leading the tin roof to cry out for help and resulting in great agitation and cold sweats within the Dübendorf team. But all is well that ends well: during the second attempt, the structural testing unfolded smoothly to the joy and excitement of Solar Impulse’s engineers.

So when will we hear a new series of jokes? I guess we should ask the Dübendorf boys…

Now that we have completed the "Rivages" project of building Bernard Stamm's new 60 foot monocoque, better known by the name of "Cheminées poujoulat", which is due to take part in the forthcoming Vendée Globe 2012, we move on at the beginning of July to our next project, building the S2, code name for the new Solar Impulse airplane.

Once again we go from boat to airplane, but with our expertise in the use of composite materials as the common factor.

Even before the first flights of HB-SIA, our team at Decision SA were thinking about innovations that could be implemented in the new plane. We reached the stage of working on the lightest carbon fibres weighing only 90 g/m2 (by comparison the sheets of paper we all use daily weigh 80 g/m2) and we wanted to find solutions that we could put to the Solar Impulse design team to make the plane even lighter wherever possible.

The way forward came by taking recent developments in yacht sails by the company Createx of Cossonay and combining them with a new technology called TPT, or Thin Ply Technology. With this technology, we succeeded in laying down on our vacuum tables complex multiple layers of 25 g/m2, plotted by a robot guaranteeing absolute precision in the angles and positioning of the fibres. Incidentally, the CNT (Carbon Nano Tube) included in the new epoxy resin was developed and supplied by Bayer, one of the main sponsors of Solar Impulse.

To validate this new process we worked all through spring and, thanks to the intensive and productive collaboration among all those involved (the Solar Impulse Design Team, Createx SA, Empa and Decision SA), we came to the conclusion that this highly innovative process should be used in building the S2.

So here we are at the end of the year, with all the tools for manufacturing the wing spar completed, and in the process of making the braces for the spar which, when assembled, will be more than 70 m long.

The initial stages of this construction epitomise the mindset of those driving this marvellous project forward: a constant effort to find innovative solutions, to establish their reliability and then to implement them.

Hence the pleasure we all have at Decison SA in building the structures of this extraordinary airplane.

The electrical cabling has been checked one more time and the flight controls adjusted. The seriousness and precision with which the engineers and technical staff go about their tasks in the workshop are very reassuring.

Over the past weeks, we have successfully tested the flight controls - spoilers, wing flaps and the air-brakes - that are built into the wing. These have been mainly functional tests along with load testing. The flight controls on our prototype are mechanical and activated directly by the pilot using push rods.

Despite their huge surface area (the wing flaps are the size of the entire wing of a very large glider), the flight controls are activated with almost zero friction, and operate with the accuracy of a watch movement over the entire 60 metres!

We remain on schedule for the first tests on the Dübendorf airstrip.