Solar Impulse
Posted on December 9th 2009
To fly a plane around the globe sounds like a grand idea. To fly a plane around the world without fuel seems a paradox, if not impossible. And that is the grandiose claim of the Solar Impulse project.
The Solar Impulse project is the brainchild of Swiss balloonist, psychiatrist and inventor, Bertrand Piccard. Piccard was the first to circumnavigate the globe in a balloon and within the next 5 years hopes to go one better by flying a solar powered plane on an approximate equatorial route round the Earth.
The project faces immense challenges if it is to succeed and sees its goals as unachievable without pushing back the current technological limits in all fields. The result of tackling these challenges is the prototype plane, HB-SIA. Construction of HB-SIA started in June 2005 and it undertook a successful maiden flight on 3rd December 2009. The design goals of HB-SIA are to:
- validate the design and material choices;
- test the planes unique design characteristics;
- to gather and store sufficient energy to allow a 36 hour flight.
This will pave the way for a later design, HB-SIB, to undertake the full circumnavigation of the globe.
HB-SIA is a large plane, in appearance similar to a large glider. Its physical likeness to a modern day jet lies only in its wingspan, which is comparable to an Airbus 340. It has a tiny 1.3 cubic meter cockpit which houses a single pilot and instrumentation. Beneath its 63.4 meter wingspan are mounted four gondolas, each housing a 10 HP motor which each drive a 3.5 metre dual blade propeller. These engines provide the plane’s sole forward thrust and will keep the plane at a steady speed of 75 km/h. At 75 km/h there will be sufficient balance between the energy gathering mechanism, its storage and its use to allow the plane to fly the required periods under its own power. Each propeller is fitted with a governor which limits its rotation speed to the range of 200 to 4000 rpm.
The gondolas also house a thermally insulated lithium polymer battery set and computer control circuitry. At the anticipated flight altitude of 8500 meters, the ambient temperature of about -40 degrees C will impact battery functionality unless controlled.
One of the few constants in the design specification is the amount of energy available through sunlight. Over 24 hours, the incident sunlight falling on a square meter of the earth’s surface amounts to about 250 Watts. By employing 200 square meters of photovoltaic cells with a total efficiency of about 12 %, the power available to the plane’s rotors equates to about 6kW, or 8HP. This is comparable to the power available to the Wright brothers when they made their first powered flight in 1903. Although the energy efficiency of the PV cells could have been increased, there would be an accompanying increase in the weight of the plane. The weight increased would have significantly penalized the plane’s flight during non-daylight hours.
The take-off weight of HB-SIA is about 1600 kg. In order to optimize the plane’s weight, the skeleton of the plane is constructed as a honeycomb of lightweight carbon fiber. The underside is spanned with flexible film while the upper, sun-facing areas are covered with solar paneling. In order to maintain stability, 120 rigid carbon fiber struts bind the honeycomb interior. To maximize the available solar panel areas, a wingspan of 63.4 meters is required.
Current solar planes do not store energy for later use. They use the energy derived from the sun as it is gathered. This marks the current technological limit for solar flight. Solar Impulse will collect energy continuously and store it for use during non-daylight hours.
A significant obstacle encountered in the energy generation chain is battery design. The present energy density of 220 Wh/kg means the lithium polymer storage units will weigh about 400 kg for night flight, which amounts to greater than 25% of the plane’s total weight. Reduction in the battery weight would afford major design changes, namely a smaller wingspan, higher flight speed or potentially a second pilot.
Piloting the Solar Impulse presents it own series of challenges. Solar Impulse’s unique combination of size, low weight and high-rigidity are unlike any plane ever flown before. Repeated wing charge and vibration tests have led engineers to through a series of optimizations. Likewise, a flight simulator has enabled pilots to become familiar with the handling and control of such an aircraft. Pilots have already undergone tests during a 25 hour virtual flight in a 1.3 cubic meter cockpit.
On December 3rd, 2009, after almost 4 years in the design phase, the Solar Impulse prototype plane made its maiden test flight. HB-SIA, was powered along the strip at Dübendorf Airfield in Switzerland to its take-off speed of 35 km/h (22 mph) then raised from the ground to a height of one meter. HB-SIA was flown for 30 seconds, covering 350 meters, before it was safely lowered and brought to rest. The close proximity of Zurich International airport prohibited more extensive flights.
Although nicknamed a ‘flea hop’ by project leaders, the smooth maiden test flight came as great comfort to both engineers and designers as the plane mirrored precisely the behavior they had predicted in simulations.
Now, HB-SIA will be disassembled and taken to the Payerne air force base in Switzerland where it will be refitted with solar paneling. 11,628 low-weight mono-crystalline silicon cells, each 150 microns thick, will be mounted across the back of the plane’s wings. Then the plane will undergo its next major challenge: its maiden solar-powered flight.
After HB-SIA’s series of test flights, the follow-on design, HB-SIB, will incorporate the design and technological improvements gleaned from HB-SIA’s life cycle. HB-SIB will be slightly larger, with a 80 meter wingspan (larger than an Airbus 380) and possess a fully pressurized cockpit which will allow cruising above HB-SIA’s ceiling of 8500 metres. Additional gains made in the field of battery design may enable HB-SIB to be lighter and therefore allow two pilots and a full non-stop circumnavigation of the Earth.
