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"

During the last step of the Mission 2012 between Madrid and Payerne via Toulouse, many of you were the one day official photographer for Solar Impulse. The entire team was very touched about the interest many of you showed and the number of picture which was uploaded during this event. Believe me André and Bertrand had a hard time choosing the 2 winners, as the pictures were original and creative.

Congratulations to Olivier Rapin (picture on the left) and to Hakan Erbuke (picture below) who were chosen for their pictures. They will receive a Solar Impulse Cap, an official Team t-shirt and the HB-SIA’s book signed by the 2 pilots.

Have a look below to the 2 albums where you can find all the pictures taken during the contest.

Since I started working at Solar Impulse two months ago, a lot of people have asked me “but how does it fly at night?” It might seem like a miracle, but it’s actually a simple game of physics and energy maximization.

When HB-SIA is on the runway ready for take-off, the batteries are typically charged with solar power to minimum 50% for pilot safety. Aside from take-offs and landings where the aircraft speed is increased to 30 knots (approximately 55km/h) for maneuverability, the solar aircraft is always flown at 25 knots (approximately 45km/h), its design point for minimum energy consumption. The entire flight cycle revolves around energy savings and optimization. The aircraft essentially makes use of electric and potential energy. Electric energy or – to be physically correct, chemical energy is collected in the batteries. Potential energy is stored in the aircraft height. For example, a football on a hill has latent potential energy. As soon as it gets a slight push, it will roll down converting its potential energy in kinetic energy (speed) and eventually comes to stop because in real life, every motion is accompanied by losses.

So in order to fly with the utmost efficiency, the Solar Impulse airplane needs to juggle the energy storage between height and battery to find the best equilibrium.

But what really happens during the flight? You have already seen how the energy production cycle works in the previous article (From Sunlight to Flight), now I will show you what happens to HB-SIA day and night, also illustrated in the image.

During the day, the pilot slowly ascends to a higher altitude in thinner atmosphere to avoid turbulence and cloud formations. Interestingly, the solar generators also convert more energy at altitude. Sun radiation is partly absorbed by Earth´s atmosphere before reaching the ground. The higher Solar Impulse is climbing, the more sun power is available and can be stored in the batteries. In fact, for the highest possible solar power generation, HB-SIA should be in outer space; but that’s a little too far for the time being.

As the sun begins to set on the horizon, solar power obviously decreases. Once the available solar power is not sufficient to support level flight anymore, the pilot reduces the motors and initiates a gentle descent (about 0,4 m/s) to a low night loitering altitude of 1000-1500m meters. Out of its maximum altitude of 28000ft (8000m), the prototype can glide for 4-5 hours consuming almost no electric energy. When the lowest altitude is reached, usually long after sunset, the motors, now powered by the batteries, are used to maintain a level flight at 25 knots until the morning. As the breathtaking tones of the sun on the horizon start filling the sky with warmth, the aircraft can once again begin its ascent, and the cycle begins.

What is most incredible is that this revolutionary aircraft could practically fly perpetually into infinity if it weren’t for the human side of the pilots. So how do we make humankind perpetual? Well, I think that’s another story.