Perhaps you’ve heard about Solar Impulse, the solar-powered aircraft able to fly both day and night without the help of fuel. Striking out on an unprecedented flight across the United States, the single-seater plane hopes to further its “Clean Generation” initiative to further the development and acceptance of renewable energy as a viable source of power.
Solar Impulse’s flight from San Francisco, California to New York City will alternatively carry its proud founders Bertrand Piccard and Andr Borschberg stopping at five check points including Phoenix, Arizona, Dallas, Texas, Saint Louis, Missouri, Washington D.C., and New York City.
The physics of flying at night
One of the most popular questions pertaining to the actual flying of Solar Impulse may be how it continues to have enough energy to fly during the night. Viktoria Dijakovic of the Solar Impulse team describes it as 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. . . . 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.
Solar Impulse’s month long journey will further the founder’s primary objective to show that with today’s technology, flying day and night powered only by solar energy without using any fuel, nor producing polluting emissions is possible. Their next flight, to fly around the world with a second generation aircraft currently under construction, seems to foster an even deeper message about the pioneering human spirit and its unstoppable quest to discover new and better innovations for the way in which we live.