Softly, softly (left) and truly, madly nuts (above). In the end it took Solar Impulse 2 some 16 months to lap the planet with several months spent in Hawaii repairing thermal damage to the lightweight batteries during the five-day leg from Japan. Two pilots alternated for the voyage. Cruising speed was 34-54mph and to date the project cost is $165million+. Or, to get there even faster if you fancy your chances, there is Yves Rossy the Jetman. One to four engines and at least 125mph; as Bertrand Cardis says, ‘He is still alive!’
back to that large electric aeroplane… ‘Of course Solar Impulse was not our first air- craft, we had done a lot with other aircraft manufacturers before this project,’ he says. ‘And it was very far from our craziest
aircraft too – remember that we built the Jetman for Yves Rossy.’ Ah yes, Yves Rossy, the man who first strapped on a powerful jet engine and set off to chase the birds… ‘What can I say about this man,’ says Cardis now. ‘I think the best thing to say is that (fortunately) he is still alive.’ Compared to the spectacular Jetman
project Solar Impulse is of course a sober venture in pursuit of low-energy transport solutions. ‘This project began originally as a modest feasibility exercise in conjunction with EPFL [the world-famous Swiss institute of technology in Lausanne],’ says Cardis. ‘I am a graduate of EPFL and still know
a lot of people there, so it was an easy col- laboration. At EPFL I studied mechanical engineering and hydraulics, however, not materials which at the time was a narrower subject than it is today. They were inter- ested in us for this project because of some of the more advanced work we’d done with Alinghi – boats like the big cat, which showed we could take on and deliver com- pletely new concepts at a very large scale.’ The Solar Impulse project was triggered
by the first successful round-the-world balloon flight that Bertrand Piccard had just finished with Brian Jones. ‘We were involved with the structure of his giant balloon (from Cameron Balloons in Eng- land) which was around 80m across,’ recalls Cardis. ‘But a balloon on the ground is completely unmanageable. Where do you put it! If there is a big storm it is not an easy thing to put in a hangar! ‘So later on Jones contacted us saying,
“Next time I want to fly around the world in an aircraft, but using only renewable energy.” And this led to Solar Impulse.’ Little was Bertrand to know that even
the scale and complexity of building Solar Impulse were to be dwarfed by the work required to complete the finished design.
‘The initial feasibility study for Solar
Impulse was quite positive and so the backers asked us to propose some struc- tural solutions to build the plane. We had ideas already, though they were not very “standard” for an aero structure as we had not done a complete airframe before; we just applied our own ideas without too much consideration of what had gone before. ‘Having offered a number of engineer-
ing solutions they invited us to build some of the smaller pieces of the airframe to see how we did. Soon we were asked to build some of the bigger components and in the end for the round-the-world plane we built the entire structure, the wings and wing spars, the tail planes, all the stabilisers and control surfaces, even the engine gondolas. ‘In between we built test beams for the
huge wing spars and some early proto- types of other larger elements. From there things grew somewhat – by the time Impulse left here for her round-the-world trip we had put over 70,000 man hours into the design.’ This project was split broadly into two
parts: a design team and a building team. ‘At first the two teams are effectively one but once you have an idea of how you wish to build the main structure then the two proceed separately. You set out the build method you want to use, then the design team take this information away and start to calculate and design the object; then it comes back to us with some initial drawings, including some very early laminate specs and so on. In fact, this project began soon after we had completed the successful 2003 Alinghi ACC America’s Cup campaign so in practice much of the process rolled straight on from that. ‘Gradually a similar design loop estab-
lished itself, much like any other good project, including the Alinghi ACC yachts. A design idea feeds into an engineering solution which in turn is catalyst for the next step in the design process. ‘And so it goes on and hopefully the
project evolves successfully. Usually it does in our experience. ‘But this was a very big project for us. It
eventually ran on for double the time we originally expected. Remember, we were expecting to fly around the world in 2008 or 2009. But first we built the prototype and then we moved onto the final aero- plane; at every step there was an enormous amount of testing, data analysis and numer- ous changes that always needed to be made. ‘It takes a lot of time to build two planes
and to then make changes after every test flight; the post-flight analysis run by the team was at the extreme end of complex. ‘That said, the final aircraft is actually
quite a simple structure, but it is only simple because each element has been so highly optimised. For example, there is a single carbon wing spar running across the 30m span of the aircraft. To this beam are attached numerous very light carbon ribs to give the wind its correct profile. Then over the upper surface of the ribs where you would normally expect film, similar to the America’s Cup boats, there is effec- tively one giant solar panel – the cells are applied to the film laminate which is then applied to the wing. The bottom surface is normal transparent film.’ All of which did of course lead to new
manufacturing methods. The first plane was built in conjunction with ACG in the UK (now part of Solvay). For that proto- type Décision dipped under 100g/m2
for
some of the airframe, sometimes going as low as 90g/m2
, which at the time was con-
siderably lighter than normal for an appli- cation of this type. In fact, by the time they were ready to start on the second aircraft Cardis’s team knew that in many places they would go even lighter still. ‘As the design improved we understood the loads in the aircraft better,’ says Cardis, ‘and many of them reduced dramatically. [An unusual challenge optimising an aircraft like this is that many of the loads in fact decrease with lift-off]. ‘So for the second plane we approached w
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