Solar Impulse
HB-SIB

HB-SIB: Solar Impulse’s second generation aircraft currently under construction in Dübendorf (Switzerland). Designed to fly around the world in 2015, the testing phase is slowly coming to an end while completion of the airplane is scheduled for the end of 2013. Flight tests will start early 2014.

HB-SIA: Solar Impulse’s first generation aircraft. Initially designed to prove that a solar airplane can fly day and night, it has exceeded expectations by flying across countries, connecting continents and now flying coast-to-coast across the United States from San Francisco to Washington D.C. and New York City.

This week Solar Impulse engineers are busy testing the cockpit fairing at the RUAG Large Subsonic Wind Tunnel in Emmen (Switzerland). It’s one of the largest structures in Europe and is used for a variety of tests including aircraft development, automotive research, rain tests and boat aerodynamics. The maximum blowing speed is of 68 m/s. The wooden mock cockpit will be used, enclosed in a polyurethane foam shell with covering material on the external surface. These tests are meant to verify the airworthiness of the fairing, the cabin door, and simulate the behavior of the materials during flight.

The key of this test is to see the ability to jettison the cockpit’s door during an emergency situation but also to see the overall behavior of the structure when winds are blowing from different angles. During a similar test on HB-SIA, the cockpit was mounted on the roof of a car and driven around the runway. This time Solar Impulse has decided to use a wind tunnel because all the variables that need to be tested are easier to control. In fact, the wind tunnel has a turn table module which allows tested objects to be rotated around their axis. This function allows the engineers to control the wind direction to the nearest degree and consequently study the behavior of the cockpit and materials in different situations such as during certain manoeuvers, cruising speed as well as emergency situations.

Still, this is not the only test that’s taking place at Solar Impulse. The Iron Bird, testing of HB-SIB’s electric system, is ongoing with electric engineers and electricians hard at work to ensure that all of the cockpit’s control systems are working properly when connected to each other. And finally, preparations are being made to setup the landing gear test which should take place in the coming weeks.

While HB-SIA continues its journey Across America, and certainly its last adventure, HB-SIB is slowly coming to life impatient to become the next solar airplane and discover the world. 

Photo: Courtesy of RUAG

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. 

As we’ve seen in the previous episodes, each of the airplane’s components are first conceptualized, designed and then structurally analyzed. But the only way to truly know if something is fit for flight is to test it with the appropriate loads. Both HB-SIA and HB-SIB are experimental, prototype aircrafts and although everything is simulated, calculated and designed in 3D before construction, it’s only an approximation of reality.

This is when the Testing team comes in. Run by David Oldani, these four “testers” have a direct relationship with the Structural Analysis team who provide them with the loads to be applied to a given part. David then has to set up the test in a way to best simulate reality.

That’s in fact the greatest challenge of testing: simulating reality. While in flight, the weight of the aircraft and the loads applied to it are partitioned differently. It’s like when you jump into a swimming pool, you feel extremely light while in reality your weight is still the same. David’s job is to find a way to counterbalance the weight of a part to simulate the flight load cases in the best way possible.

Essentially there are two kinds of tests, the destructive and the nondestructive ones. It’s this team’s job to decide which part should be tested to its fracturing point or not. Why such extremes? When a part is stressed to the maximum it provides valuable information about its limit resistance and ultimate breaking point.

Testing might be the final step in the production process of a part, but it’s also the tensest. Underneath the seemingly relaxed and nonchalant attitude of the engineers hides suspense and an overall judgment day feeling; and I don’t blame them. Everything is optimized to the limit and literally handmade making the transition process from computer to reality similar to a translation from Japanese to Italian. The engineers’ worst nightmare came true just last year when the aircraft’s central part, the wing spar, succumbed to the loads and fractured right through the middle.

Building a solar aircraft of this size (72m wingspan) and light weight (2400kg) is an incredible feat, something the normal aviation industry doesn’t have to face on a day-to-day basis. “A normal civil, certified airplane can be built rapidly and the testing procedure thereafter is insignificant because it has already been proven that it can fly as opposed to our prototype aircraft that needs to undergo numerous structural and flight tests before it can be certified,” acknowledged David.

Solar Impulse is not only pushing the limits of what’s possible, it’s also proving, every step of the way, how innovation, perseverance and faith can challenge our common perception of the world. 

In the photo from left to right: Paul Metzler, Yves Heller, David Oldani, Jens Menzel. David Oldani (here above).

Follow the series here: "THE MAKING OF A SOLAR AIRPLANE"

I’ve always associated batteries to an expensive tool that I never have at hand when needed (usually realizing I’m out of stock when the stores are closed). The fact that most modern-day electronics (cellphones, mp3 players) can now be charged through a laptop, has been such a liberation. But although standard AAA or AA’s are being used less by the average consumer, the utility of this special energy storing device is expanding in many other areas.

Think about the automotive sector and hybrid or electric cars; or maybe about our inseparable friend, the laptop, to name a couple. This type of battery is the so-called secondary battery (rechargeable) to be used multiple times and the kind that is sparking the interest of large chemical companies because of their potential to resolve our dependency on fossil fuels.

What is so particular about Solar Impulse’s batteries is their weight, efficiency and lifespan ratio. In HB-SIB’s case they are particularly revolutionary. The batteries, made by the Korean producer Kokam, required extensive research in order to further push the limits. The key lies in the complex chemical formula which has improved the oxidation issue. Just as an apple gets dark and rots when peeled and left outside, batteries age faster and lose efficiency when oxidized. This technology is two years ahead of the industry, but it’s the most we can unveil as the rest remains a well-kept secret. Ameliorating this usual ageing process allows Solar Impulse to have batteries able to guarantee 2’000 flight hours, versus 500 in HB-SIA’s case.  

I really enjoy the metaphor the electric engineer gave me: batteries are like human cells and that’s the reason why they’re so difficult to study. Each of HB-SIB’s batteries consists of 70 lithium-polymer cells. Like in humans, each cell is different from the other. Also, like in the human body, cells do not like stress. Extremely hot or cold temperatures aren’t their forte and some days a cell is more efficient than others because of certain parameters which were applied the day before, just like ours. Think of it, if you physically exert yourself one day, you most likely will have sore muscles and be less efficient. This explains why, when we say that each lithium-polymer cell can be charged up to 4.35 Volts (V), it’s because it’s full potential equals 4.35V but it doesn’t mean that on Friday it will exploit its full potential like it did on Thursday. All this is dependent on the temperature conditions as well as on the type of charge and discharge.

To ensure that the lithium-polymer cells are fit for the aircraft, they have to undergo numerous tests. The tests are done not only to study the cells’ behavior in extreme temperatures or how much energy they can store for how long, but they are also done to better understand their reaction to different situations. The challenge is finding the optimum balance between lifespan and energy; factors dependent on temperature, cell voltage and current.

Knowing the cells allows Solar Impulse engineers to better prepare for mission flights. For example, it was found that keeping a constant temperature of 25°C inside the motor gondolas provides the ideal environment for greater battery efficiency. And I must admit I don’t blame them for Switzerland’s trying weather conditions in the winter don’t necessarily appeal to my own cells either…

Picture: HB-SIB batteries

Just like the eternal to and fro between civil engineers and architects to find the best balance between design and a physically viable structure, a constant discourse goes on between the Design and Structural Analysis teams at Solar Impulse. The difference is that they’re all engineers, so no need to fix wacky structures that can only exist in cartoons.

Everything that’s designed has a purpose but every part needs to fit in the greater scheme of things while also abiding to the strict lightweight guidelines. Led by Geri Piller, the Structural Analysis team consists of 4 engineers. The Design team has the concept, but it’s up to Geri’s team to decide which and how many materials to use for a given part in relation to the load that it must carry.

Geri once gave me a 101 Structural Analysis course (I would have certainly done better at reading a Chinese newspaper though), but I did manage to retain something: every material reacts differently to loads (for example, steel reacts to stress differently than carbon) and this is crucial when building a part.

For reasons of weight, the majority of HB-SIB’s structure is made out of carbon, a very peculiar material. Carbon is extremely resistant in the direction of its fibers, but extremely frail in the other. The Structural Analysis team has to decide in which direction the fibers must be placed, how thick each layer has to be and how many plies are needed. This results in complex manipulations with a specialized software (FEA finite element analysis) where the structural engineers manually input the characteristics they want and subsequently observe how the part reacts to the expected loads applied to it.

It’s not a linear process (it takes two to tango). It’s a constant back and forth between structural and design engineers, an ongoing discussion to reach perfection because, once the design and structure make the perfect match, the part is finally sent to the producer; a joint effort that gives birth to a new part. Because of the unique nature of this aircraft, every part is literally handmade. Consequently, some information can be lost in translation when transforming the software design into a manufactured part. That’s why every part needs to be tested thereafter; a crucial process Geri and his team actively engage in. Stay tuned for information about the Testing team coming soon on our blog!

In the photos: from left to right: Björn Müller, Stefan Pfammatter, Geri Piller and Dominik Dusek (adjacent), Geri Piller (bottom), FEA (top).

Follow the series here: "THE MAKING OF A SOLAR AIRPLANE"

Before building anything there needs to be a concept and a design. Like when you build a house: you think about its location, its size, its architectural design and, most importantly, the thickness of your wallet. You obviously can’t start with the roof without a foundation and you certainly can’t install the bathroom without first connecting the drain pipe to the grid.  

Once the parts are designed, who puts the puzzle together in a timely and coherent manner? Someone has to see the big picture, juggling deadlines, budget and production. At Solar Impulse, that role is attributed to Robert Fraefel, Head of Airplane Development.

Röbi, as the team likes to call him, is the man behind the scenes, the choreographer of it all. He motivates the engineers, encourages them when obstacles arise and stimulates them to pick up the pace when a deadline is approaching. He’s also somewhat of a public relations figure, meeting with dozens of suppliers and producers to seek their collaboration and ensure their deliveries meet the strict quality requirements.

An engineer with a Formula 1 background, Röbi immediately found himself in his element working for a project that pushes the boundaries; but the parallel ends there. At Solar Impulse it’s hard to plan because “we don’t know how long it takes to develop a part until it’s ready for production”. At Formula 1 “you already know, more or less, how many parts you need, how they will look like”; you always have a starting block as opposed to Solar Impulse.

When I think about the number of factors and variables involved in the construction of such a unique aircraft, I can’t help but wonder how Röbi’s hair hasn’t turned grey yet. Between the engineers in Dübendorf and Payerne, he has to regularly commute between the French and German-speaking regions of Switzerland to ensure everything is evolving smoothly.

“You have to start with something even without knowing the whole story and just go step by step.” Revealed Röbi during one of our conversations, “we tend to want to know everything from the beginning, but we have to just take the first step, advance a little and then go forward to the next keeping the target in your head.” That’s Solar Impulse’s philosophy: step-by-step and we eventually get there. 

Just as when André and Bertrand were already thinking of flying a solar aircraft without even having the hardware, Röbi started working on the project 7 years ago not knowing if it would actually be able to fly; but step by step…

Follow the series here: "MAKING OF A SOLAR AIRPLANE"