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Perpetually inhuman: HB-SIA’s flight cycle

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. 

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 ...



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